TerryE

Modelling the "Chunk" Heating of a Passive Slab

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We've jsut been having some debates about slab UFH strategies on the relevant forum, but I wanted to do some sanity checks without getting into over-specified 2D or 3D heat flow equations, which I then need to modle.  Some of you have probably come across this type of graphic (source BuildingPhysics.com):

image469.png

 

(BTW the reason for the alternating hot and not-so-hot pipes is that if you think about it, this is house UFH loops are laid, using the double-back technique.)  This also looks to be a steady state solution, and in my designs we will nearly always never get near steady state. 

 

Anyway, I tried to find some decent UFH design docs and after a reasonably long search I could find lots of guides that go into the installation side and failed miserably :(  Most ship over this sort of stuff and just make passing comments like "use model XXX pump of the heated area < 250m² and model YYY otherwise."  Worse what I can find is US stuff in GPM, etc.  Great.  So I am left with the book that I used 30 years ago to design our last CH system until someone can recommend something better.

 

We have 3 loops of 16mm PEX, and from my old CH days, the max flow rate should be  <1½m/s so ×3 loops that's 0.9 l/s or (3.2 m³/hr) so a 3kW heater will raise this stream by ~3 0.8 °C.  A 100m PEX loop at 1½m/s is a pretty high head IIRC (just over 1 bar), and this 3.2 figure seems rather high for 3 loops so halving the flow rate will double the heating to ~1.6 °C with a head of roughly 0.25 bar or 25 kPa which is more typical of UFH pumps.

 

So if I assume this lower figure then the difference between the out and return sides of the UFH will be ~1½°C but how much is the return above slab ambient?  The answer is whatever is needed to pull 3kW into the slab, and that's going to be the stable flow and return temp.   Modelling this is complicated because its not a true steady state solution.  The return will start at slab temp and rise maybe 1°C per hour as the slab volume around the pipes is heated.  As I said modelling it really complicated.  Validating by direct measurement is easy.

 

So at the end of a long heat, say 3 hrs, the flow and returns will be ~9°C and ~3°C above slab baseline temperature.  One the heat source is removed, the heat will continue to transfer towards a uniform distribution throughout the slab some 1½°C above slab baseline.  This will then slowly fall back towards baseline over the next day or thereabouts as heat is transferred from the  slab into the rest of the house structure and the room environment.

 

Jan is calling me for pre-dinner drinks so I'll break this now for any general comments / feedback :)

 

Edited by TerryE
I'm an idiot I forgot factor in the SH of water so the calcs are 4.2× out

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14 hours ago, TerryE said:

So at the end of a long heat, say 3 hrs, the flow and returns will be ~9°C and ~3°C above slab baseline temperature.  One the heat source is removed, the heat will continue to transfer towards a uniform distribution throughout the slab some 1½°C above slab baseline. 

 

So the assumption here is that even after a 3hr heat cycle the floor is still able to absorb all the energy delivered by the 3kW heater. In reality the absorbtion is going to be some function of the loop lengths and distribution and the thermal conductivity of the floor. I would expect the return (and hence the flow) temperature to rise with time due to localised heating effects around the pipes. - As you say very difficult to model.

 

I'm not sure where the 1½°C value comes from, can you please clarify.

 

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@Alphonsox, Neil, simple one first: the 1½°C comes from some information that I discussed in A few ASHP / UFH bits of information and is simply the ΔT calculated by taking the mass of my slab × SG of concrete and assuming that the heat has uniformly spread throughout it -- which (thanks to the conductivity of concrete and all of the rebar in the thicker profiles) isn't a bad assumption:  within a few hours of driving heat sources being removed I expect that it would be within 20%, say of this average.

 

As to a better model, getting a totally accurate one is difficult because the thermal conductivity of all the rebar is 60 × that of concrete.   The simplest thermal model to consider is that of a PEX pipe embedded in concrete.  This has two spacial dimensions and time of course, so is solvable by HEAT2D or equiv, which I am considering doing, but this also has its limits since the concrete has a boundary and the out and return runs are interleaved in the slab. 

 

A simpler way to envision this is to consider the rough approximation of the heat being transferred to a cylinder of concrete 300m long (my 3 loops are within 10% of this maximum pipe length), and of radius r.  This is roughly πr²lρcpΔt and plugging in the numbers = πr²·300·2400000·0.75Δt J or 1.6E+9·r²Δt J or 470r²Δt kWh. So if the 9kWh was just transferred to the cylinder of concrete 5cm around the pipe, this would be raised by an average 7°C which is the right ballpark, but in reality the concrete immediately adjacent to the pipe will be at whatever the water temperature in the pipes is, and the heat will quickly diffuse into the main slab, at least.  Whatever the distribution is in practice, the temperatures in slab will be bounded by the inlet water temperature.

 

One Q is to ask whether a 3KW input is safe and the point to note here is that is factors less the input for a conventional UFH load for this size of slab, or per loop.  Whatever happens I will instrument all six of the inlet and outlet temperatures and record the real response.  I will design my initial heating strategy to keep the heating within safe operating margins.  This will be by computer (RPi) control but with a thermal fail-safe cut-out (at say 35°C) independent of the computer control, so there is no risk of damage to the UFH system even if I make a programming cock-up during development.

 

Unfortunately it's going to be another 3-4 months before I get to the point where I can start to commission the system and to collect real data.

 

 

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I am an idiot.  4.2 kJ of heat will raise 1kg of water by 1°C not 1kJ.  Hence the figures in my first post are now a lot more sensible.

 

I also see from Jeremy's recent Mayfly.eu post 44 that he has broadly came to the same observation.  It's now got to the point where I need to document my design properly.

 

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Spoiler alert: I have at last got my head around all this through a mix of simple analytic models and a simple 2D model of the slab.  I wrote the model in Fortran to appeal to the other old farts that might just want to look at it or have a play.  IMO, the bottom line is that using a buffer tank for a low-temperature UFH is entirely optional, though the controls and heating strategy are slightly different with and without.  If you already have a buffer tank then it's not worth redoing the system, but if you are about to install one then IMO you can leave it out.  (Note this primarily applies to an MBC / passive slab or equivalent house.)

 

I've got friends coming around for a drink in 15 mins, so I will have to write this up later.  But I'll dump a couple of graphs here for food for thought.

TempVsTime.pngTempVsRadius.png

The model outputs are entirely consistent with my analytic approximation.  I will explain more when I next post.  Until later ... (drinky time)

Edited by TerryE
Update first figure to add data discussed below

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I just love that you used Fortran! 

Last time I used it was Fortran 77 running on a DEC Vax, to model 3D weapon trajectories from an instrumentation pack that used three rate gyros and three orthogonal accelerometers, using the Runge-Kutta  method to resolve the iterative problems created by multiple double integrations to derive position, when the weapon was rolling, pitching and yawing, so changing the accelerometer references with every sample.  Oh what fun that was, when we realised that the model was unbelievably sensitive to the entered initial conditions.  It started a whole new project in acquiring very accurate initial condition data.

 

Your graphical results look promising, any chance of the source code?  I've been looking for an excuse to install GNUFortran and have a play............................

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What is happening at between 22.5 and 23 (I assume °C), the heating side (the left) looks right, but the cooling side (the right) should not have such a pronounced change in it.

How does ambient air temperature affect it?

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Replies to Qs

 

8 hours ago, JSHarris said:

I just love that you used Fortran!  Last time I used it was Fortran 77 running on a DEC Vax ... Your graphical results look promising, any chance of the source code?  I've been looking for an excuse to install GNUFortran and have a play............................

 

The GNU gfortran compile is part of the GNU gcc bundle so adding it takes seconds on a Linux (or Cygwin if you run Windows) if you haven't already got it installed.  It is Fortran90 and I use the F90 features, so it is a little different from the old F77 that we both used last time in anger.  I initially wrote the early versions in C but the C language just gets in the way of reading the code and understanding what is going on.  So I moved it over to Fortran as this sort of application fits it so well.  So I prefer this.

 

As to the GUI approach, all of the parameters are just that, a block of Fortran parameter declarations (one of the F90 additions and which the compiler treats as compile-time including things like real,parameter::pi = 2*acos(0.0) which is nice). If I want to change anything, just tweak the parameter block, save and recompile; it only takes a couple of seconds.  So no fancy input parsing; I leave this to the compiler.  As to the output I simply dump CSV data to stdout, and post process this with (LibreOffice) Calc. for example my main output code is:

    write(fmt2,'(A,I4,A,I4,A)') '(I5,1H,,I5,',2*nRings+2,'(1H,,F8.4),1H,,F12.4)' 
    do i=1, nSections 
      write(unit=*,fmt=fmt2)iter,i,Twater(i),(T(i,j),j=1,nRings),(W(i,j),j=1,nRings),Wair,Q(i)
    end do

(The first line is standard hack because you still can't provide a variable count in a format repeat group.)  Sometimes I might use a shell filter such as grep -P '^\s*\d*,\s*50,' /tmp/a.lst > /tmp/a1.lst as a post process. (This picks out all lines with the second field = 50, that is the midpoint in the 100m loop), to keep the Calc file smaller.  If such a filter is useful, then it's trivial to add it to the code as an extra line or two of logic. 

 

6 hours ago, SteamyTea said:

What is happening at between 22.5 and 23 (I assume °C), the heating side (the left) looks right, but the cooling side (the right) should not have such a pronounced change in it.

How does ambient air temperature affect it?

 

Nick I'll cover this in my extended discussion below.

 

Basis of the model

(This section can be skipped by normal mortals, but this in the Boffins sub-forum after all.)

 

My passive slab has ~73m² of concrete 0.1m thick – that is roughly 17½ tonne of concrete in total across the footprint of the house, but the slab also contains another 10 tonne of perimeter beams, cross bracing and steel beneath this which all adds to its overall thermal capacity. Some 55 m² of this is covered by Under Floor Heating (UFH) pipe placed roughly 50 mm below the slab surface in 3 loops, each just under 100m in length and laid in the standard “double back” spirals used in most UFH designs. There is 300mm EPS below and to the sides of the bulk of the slab, so the main (radiant) losses are into the living space above it.  Accurately modelling this type of design is practically impossible because of the uncertainties introduced by the steelwork within the slab, and of huge computational cost of executing a time-varying 3D spacial model. So what I wanted to do here was to focus on the time dynamic and heat propagation effects around the pipe run themselves, by modelling an approximation which is more easily simulated but which will have the same macro characteristics of the heat flow through a real concrete slab.

  • So what I am going to focus on in this model  is an approximation which is a 100m long homogeneous tube of concrete some 150mm in diameter with a hole running through the middle some 14mm in diameter.
  • Water is circulating through the centre hole at 1 m/s and is heated by a 1kW heater at one end.
  • The surface of the concrete tube is radiating heat into a black body at 20°C at 3.5W/Km².
  • The initial temperature of the concrete is 20°C.

My reason for choosing this radially symmetric geometry is a pragmatic one: that this type is it is radially symmetric and therefore computationally solvable with a 2-D approximation, (and its steady state can by analysed analytically). The 3 × 100m pipe runs have a total volume of 5.3m³ which is pretty much the same as the piece of the slab covered by UFH (albeit roughly half of the total mass). The total surface area of the 3 tubes is 141m², which is roughly double the radiating area of the slab, so halving the normal approximation of 7W/Km² will give a similar net heating effect as the actual slab. The 1kW × 3 tube arrangement is directly comparable to the 3kW inline heater that I plan to use initially. So overall this model is good enough to explore some of the issues and performance characteristics that I want to quantify.

 

I approximate the concrete mediums by a set of concentric rings at a uniform mesh interval. As previously mentioned, since the radial temperature gradient is nearly 3 orders greater than the axial gradient in practice I can practically ignore the cell heat flows though the concrete and assume that the only axial heat flow as as a result of the circulating water. Removing these terms also allows me to increase the ∆l (intervals along the pipe) whilst keeping the evaluation stable.

 

By assuming that each material layer is a fixed multiple a fixed thickness ∆xi and by using a fixed time step ∆t, the solution can be approximated on a fixed (∆x, ∆l) mesh using a n∆l × 1-D implementation of a formulation of a simple delta approximation to the 1-D heat equation. This formulation uses 2 material properties that are specific to each material in the wall:

  • VHC the (per) Volume Heat Capacity, which is just a product of the density ρ and the heat capacity cp
  • K the thermal conductivity

It also is expressed in terms of the heat flow, W, at each layer boundary and of the temperature at the centre of each layer, so for the jth section of concrete pipe, Tij is taken at midpoint between boundaries i and i+1 and conversely Wij is calculated on the layer boundaries between temperatures Ti-1 j and Tij. Note that a positive W indicates a left→right heat flow (that is T is decreasing in the direction of increasing i). The basic equations internal to a given layer are straightforward and documented in the source code.  I've left discussion of the heat equation itself to Wikipedia, etc.

 

Analysis of the Results and What This Tells Me

 

This modelling the slab as three pipes of concrete has some strengths and some serious limitations.  The main advantage is that this gives us some understanding how the slab reacts to a prolonged heating period followed by the heat source removal.  What it is missing is that this radial approximation will start to break down as the heat diffuses through the slab, and in particular in my slab where I have large internal and perimeter ring beams which add another 70% thermal capacity of the slab and I'll return to this point later.

 

When the 1kW heat is applied it takes a few minutes for the return to start to lift in temp because the slab is cold and sucking all of the excess heat out but over the first 15mins or so it then settles down to a quasi steady state where the 1 kW generates a lift in circulating temp by some 1.6°C so there is a steady thermal gradient of roughly 0.016°/m along the pipe.  The radial profiles then slowly rise and the whole temperate slowly increases to support pumping 1Kw/100 = 10W per metre into the concrete.  The temperature profile of an individual 1m segment looks very much like the analytic solution to the radially symmetric 1-D heat equation because the second order terms are so small.   (I've updated the first graph above to show the temperature profile of the water over time for the start, middle and end of the pipe, and which shows this.)

 

The ambient air temperature plugs into the outer boundary condition.  The external heat loss is represented by a radiance of 3.5W/Km² at whatever the external Δt is.  This is a bit of botch because the real value is nearer 7 but only half the area is exposed, so this gives the right ballpark for the BC.

 

As soon as the heat is removed, the slab rapidly transitions into a different mode.  Since the radial profile is no longer being pumped from the centre, the 1/log(r) type profile starts to relax back to a more uniform profile as the heat start to spread more uniformly through the pipe cross-section. The circulating water now acts to redistribute the heat created by the 1.6° gradient pretty uniformly along the concrete "pipe" and another hour or so the difference along the pipe is minimal.

 

It is really by this point, say 5 hrs, in that the radial approximation begins to lose its validity. The slab is acting far more as a uniform plate some 75mm thick, insulated below and radiating above.  In my case the heat energy will also started to get "cached" in the beam underworks partly assisted by the rebar acting as heat pipes. So whilst the general shape of this curve will remain the same, I think that my real slab will be more sluggish in dumping heat so the curve will be flatter. 

 

My Conclusions

  • You can use the slab as a buffer
  • If you want to pump 3 kW into the slab then you need to accept that the circulating water temperature will rise of its own accord maybe 5-7°C more than ambient.  Alternatively if you limit the Δt a few degrees than the rate at which you an pump heat into the slab will be limited accordingly.
  • Measuring the flow return temperature is the easiest way to instrument the overall temperature of the slab (so llong as you ignore any heating periods and for maybe 3 hours after any heat input.
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Overall, that's a good approximation to the way our (very similar) slab behaves (and thanks for the copy of the model - I foresee a weekend ahead of being distracted into playing with it!). 

 

One thing I mentioned before, that in our house seems to have a noticeable impact on the behaviour, is caused by the inevitable bunching of the UFH pipes where they approach the manifold.  In our case there is only one door way, around 1.5m from the manifold, and almost opposite it, so as a consequence the pipe density there is high, and we have an area in the utility room and a part of the dining area of the kitchen where the slab heats up more quickly, particularly when the Δt is relatively high (for us, flow temperatures above about 25°C , up to the worst case I've found so far at 28°C ).  Because of the layout of our house, and the fact that the kitchen/dining room is the room with probably the greatest incidental heat gain (including solar gain) we end up with that room getting noticeably warmer and the heat then spread around the house by air convection much as through the UFH.

 

I've been around with the IR thermometer and the thermal imaging camera, and the hotspot in the floor is very clear when the flow temperature is up around 28°C, but barely visible when the flow temperature is around 24°C .

 

Having said that, I did disconnect the buffer tank (easy for me - I just pulled out the valve relay) and it's clear that it's not really doing anything in terms of keeping the house temperature stable, the slab is doing that, together with the relatively high heat capacity of the plasterboard lining and the underlying cellulose insulation..  The only effect of having no buffer was to make the ASHP keep shutting down early, to the point where instead of just modulating down to its lowest level (which is around 1.2 kW output) and continuing to run, it went into anti-short cycle mode and shut itself down for 20 minutes, then restarted.  The result was a much longer period of ASHP operation (around 2 1/2 hours) rather than the hour or so it normally runs for every two or three days.  The cause was almost certainly the low Δt between flow and return without the buffer.

 

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1 hour ago, JSHarris said:

is caused by the inevitable bunching of the UFH pipes where they approach the manifold

 

1 hour ago, JSHarris said:

having no buffer was to make the ASHP keep shutting down early, to the point where instead of just modulating down to its lowest level (which is around 1.2 kW output) and continuing to run, it went into anti-short cycle mode and shut itself down for 20 minutes, then restarted.

Which is having the greater impact on slab temperature homogenisation (is that a real term, slab temperature evenness) and would a different sized buffer tank make a difference, or would it be better to have more control over the heat source (or what ever sort).

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7 hours ago, TerryE said:

My Conclusions

I am going to have to reread all that a bit more, and then a bit more again.

I had forgotten what was trying to be modelled, which makes a difference to how I will read it.

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1 hour ago, SteamyTea said:

 

Which is having the greater impact on slab temperature homogenisation (is that a real term, slab temperature evenness) and would a different sized buffer tank make a difference, or would it be better to have more control over the heat source (or what ever sort).

 

As far as I can tell, the ASHP never goes into short-cycle protection mode with the 70 litre buffer, so I think that's probably a good indication that it's big enough.  It still leaves the question open as to whether it's too big, but as a 70 litre buffer is pretty small and cheap, it probably isn't worth the effort of finding out.

 

I don't think the buffer has any significant impact on the evenness of the slab temperature, mainly because it always runs at a higher temperature than the slab flow temperature, so is really just a parallel heat source as far as the slab is concerned.  Either the buffer or the ASHP can provide massively more heat at any instant than the slab needs.

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3 hours ago, JSHarris said:

One thing I mentioned before, that in our house seems to have a noticeable impact on the behaviour, is caused by the inevitable bunching of the UFH pipes where they approach the manifold.  In our case there is only one door way, around 1.5m from the manifold ...I've been around with the IR thermometer and the thermal imaging camera, and the hotspot in the floor is very clear when the flow temperature is up around 28°C, but barely visible when the flow temperature is around 24°C .

 

We might have the same problem but in our case mitigated the worst of the symptoms by an unintended consequence of an earlier design decision: we originally had 4×90m loops but I asked MBC to remove the areas under the units and breakfast bar from the cover.  At this point it was just easier to drop to 3×100m so we also dropped the utility room -- except the feeder runs into the manifold which all run into it.

 

But I was also thinking of doing a plate model were I approximate the slab by say 500×500 tiles with a plate heater at 40mm below the surface and (1) treat the UFH circulation as a very high horizontal cp on plates where the UFH is routed and (ii) include the deeper ring beam areas, again with an increased cp to reflect the overall effect of the rebar.  But this is a lower priority at the mo.

 

3 hours ago, JSHarris said:

Having said that, I did disconnect the buffer tank (easy for me - I just pulled out the valve relay) and it's clear that it's not really doing anything in terms of keeping the house temperature stable, the slab is doing that, together with the relatively high heat capacity of the plasterboard lining and the underlying cellulose insulation..  The only effect of having no buffer was to make the ASHP keep shutting down early, to the point where instead of just modulating down to its lowest level (which is around 1.2 kW output) and continuing to run, it went into anti-short cycle mode and shut itself down for 20 minutes, then restarted.  The result was a much longer period of ASHP operation (around 2 1/2 hours) rather than the hour or so it normally runs for every two or three days.  The cause was almost certainly the low Δt between flow and return without the buffer.

 

Jeremy, if you want to remove the buffer tank then you need to trim your other control mechanisms: (i) I recall that @jack runs without a buffer and sets his ASHP output temperature down to around 25°C at which point it is outputing around 2kW (this will depend on the low rate), and (i) you may need to open the UFH TMV mixer threshold slightly, because it's the limits on the Δt that in turn limits total amount of heat that you can dump into the slab in one go.   

 

At the moment I am planning on a single dump during E7 low tariff period because I am using an inline heater, however if I was using an ASHP then it might be better to use smaller heat slabs 2 or 3 times a day, say equating to the ASHP running for 30-60mins.  More modelling :)

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A smaller ASHP would also probably get around the issue.  Ours is only a 7 kW max model because Glowworm, along with a few other boiler companies that were importing badge-engineered ASHPs, pulled out of the market for a time, because, I think, there were a fair few performance issues due to poor installation design.  That meant I was able to by an "old stock" unit from a former Glowworm installer for probably less than cost price, £1700, including delivery and the programming unit.

 

In reality, a 4 kW unit would be more than adequate, but the cheapest inverter drive 4 kW unit was around double the price I paid, so I accepted having something that's a bit over-size.  In the event it's probably a good thing, as it allows the buffer tank to re-heat quickly when it's being used to pre-heat the incoming water for the DHW supply, and provide a 6 to 7 kWh heat input most of the time that DHW is being drawn off, which significantly helps the pre-heat system.  When we're in fall-back DHW mode, with just the preheat and instant water heater, it means that we have a total of around 20 kW available to "instantly" heat the DHW; 7 kW from the ASHP, 3 Kw from the Sunamp PV (it turns it's heater on when there's a DHW demand) and 9.6 kW from the Stiebel Eltron DHC-E.

 

I'm happy to keep the buffer tank, as it does a very good job of pre-heating the DHW, and provides a thermo-syphon to the PHE upstairs, so the PHE is always warm, even before the flow switch kicks in on demand to turn the PHE primary circulator pump on.

 

I'll be interested to see how your direct inline heater works, as ages ago I concluded that, if you weren't bothered about the SAP score and were happy to use E7, then that was probably the best way to go.

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10 minutes ago, JSHarris said:

I'll be interested to see how your direct inline heater works, as ages ago I concluded that, if you weren't bothered about the SAP score and were happy to use E7, then that was probably the best way to go.

 

As far as I can see you need the SAP rating for Building Approval and some people need a A rating for some BSoc funding approvals, but after that it's either an ego thing, or a sales gimmick.  We're not planning to sell our new house, so I don't really care about the SAP rating, just the true operating parameters such as performance, operating and maintenance costs.

 

Jan's residual concern is about the summer peak and whether we will need the ASHP to provide active cooling. 

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I couldn't agree more about the SAP score, as long as it passes and you get the home you want, who cares?

 

Any summer overheating can be resolved by something like a cheap air-to-air heat pump, like a split AC unit, fitted somewhere central.  These things are remarkably cheap for what is, in essence, something that's as complex inside as any ASHP, with the sole exception of the water circuit, with it's different heat exchanger.

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1 hour ago, JSHarris said:

In reality, a 4 kW unit would be more than adequate, but the cheapest inverter drive 4 kW unit was around double the price I paid, so I accepted having something that's a bit over-size.  

 

For reference, our 5kW Panasonic Aquarea was £1760 (ex-VAT), using my electrician's discount at the local Senate/Rexel outlet.  That included the basic programmer, and (from memory) delivery.  Interestingly, the price is identical today, 18 months later!  

 

The basic programmer is a bit of a faff, but the unit itself is a corker in terms of COP and low temperature performance.  I made some graphs for our plumber to show that in most weather conditions it wasn't far off the power delivery of a 7kW (or was it 8kW?) unit from one of the more common ASHP suppliers in the UK (might have been Ecodan).

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As requested, I've moved this from the other thread.  Not sure how helpful it is here, but someone might find it useful:

 

DHW and UFH.PNG

 

I've left out inline valves, filling loops, cold feed and hot supply from the tank, etc.

 

The tank is a 250L UVC with 100mm spray insulation.

 

We also have an Immersun unit (not shown) driven by PV when available.

 

I need to go back and check my notes but from memory:

 

- The ASHP is controlled by the standard dumb/annoying controller that comes with it.  For DHW I've programmed it to come on between 4 and 6 every morning and heat the tank to 55 deg.  I haven't in the past noticed too much defrosting, but it could well be happening all the time in cold weather.  I don't have any desire to be out in the freezing cold before 6 am to check!  I've been meaning to put a temperature sensor on the supply pipe from the ASHP and seeing how its temperature changes between 4 and 6 in the morning  - might do that this weekend if I have some time.  [Edited: have just moved on of my 1-wire sensors onto the ASHP return.  Will report back on what happens tomorrow morning]

 

- The immersions are connected to, and driven by, the Immersun.  It's on a timer to give a one hour boost to the top immersion in the mornings once the ASHP has finished its work.  I need to get around to changing that boost time until later in the day (more chance of there being PV available, plus slightly lower losses due to not having hot water sitting in the tank all day). 

 

As to performance, with this arrangement, we almost never run out of DHW.  This week is the first time time we've actually come close - I had a shower at around 9 after coming home from the gym and the water was definitely getting tepid.  I think we had about 6 showers yesterday between us.  A couple of days before we had a similar situation so we hit the immersion for an hour before my wife had a shower. 

 

Re: heating, I was running the ASHP with weather compensation until recently: flow temp of 25 deg (minimum possible) for temps above 8 deg and a flow temp of 29 deg for temps below 0 deg, with a linear range between those two.  This gave temperature overshoots in very cold weather.  It's surprising how hot the house feels when the floor temp is at 23.5-24 deg compared to 22 deg!  My wife loved it but I really didn't like it.

 

I turned weather compensation off recently and the slab temp has slowly been falling with this long run of cold weather (regularly below zero for 10-15 hours).  Based on this, I think a reduced upper flow temp (say, 27 deg) and a lower ambient temp for the weather compensation to kick in (say, 5 deg) might work.  Needs some time allocated for mucking about with it, basically!

 

My main annoyance is the lack of ability to integrate control of the ASHP and Immersun.  My home automation system is smart enough to do this, but as is so common, both the ASHP and Immersun don't make it easy to control them externally.  

 

There is a bolt-on modbus module for the ASHP, but it's an insane several hundred quid for a plastic box with what I assume is a cheap microcontroller in it.  I'd then need a £200 modbus extension for my home automation system.  It's very hard to find any information about what the interface would actually give me in terms of data and control.  

 

I don't think there's an easy way of interfacing the Immersun with a home automation system.  In the future I plan to look into controlling the immersions with the home automation system and having the Immersun run in parallel.    

 

Integrated control isn't really a priority, if I'm honest.  I just want it to be plug and play as far as possible, with an easy interface to manually boost via the immersion(s) if needed.  Once I get the weather compensation right, I think it'll be good enough for the time being.

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1 hour ago, jack said:

 

For reference, our 5kW Panasonic Aquarea was £1760 (ex-VAT), using my electrician's discount at the local Senate/Rexel outlet.  That included the basic programmer, and (from memory) delivery.  Interestingly, the price is identical today, 18 months later!  

 

The basic programmer is a bit of a faff, but the unit itself is a corker in terms of COP and low temperature performance.  I made some graphs for our plumber to show that in most weather conditions it wasn't far off the power delivery of a 7kW (or was it 8kW?) unit from one of the more common ASHP suppliers in the UK (might have been Ecodan).

 

 

That's a very good price!  When I was hunting around, part of the problem was getting suppliers to quote a price, as there were very few giving online retail prices, and those that were were silly prices.  IIRC, we were quoted around £4k, supply only, for the 4 kW Kingspan unit, and that was the smallest I could find where I could also get a price.  There almost seemed to be a closed shop surrounding the prices of these things, with some suppliers putting a ridiculous markup on them, way more than the markup on something like a boiler.

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Makes my punt on EBay for a 4Kw kingspan unit for £750 feel even better?.

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Found the spreadsheet.  It was a Nibe 8kW unit (Nibe is a very respected Swedish manufacturer).

 

Note that these are a bit deceptive, as the x-axis isn't a monotonic value.  It gives you some idea though.

 

Capacity v outside.PNG

 

 

COP.PNG

 

 

The other thing to keep in mind is that I believe these figures assume dry conditions.  In damp, cold conditions, it's perfectly possible that the the design and/or control of the Nibe will result in relatively better performance.  I don't know enough to say what could influence that: a larger heat exchanger, for example?  Either way, both units will perform a lot worse in real world conditions when it's damp and around low single digit temps.

 

Still interesting, I thought.  Interesting that the "8kW" Nibe doesn't appear to actually deliver much more than 6kW at any point across these parameters.

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To pick up the earlier discussion about modelling the slab, essentially this slab has two broad works in one of two broad regimes:

  • Heat is being pumped into the slab.  If we ignore the very short transitions, the slab rapidly settles down into a quasi-static state, that is most of the rate dependent terms in the heat flow equations are small enough that you can almost ignore them.  So if you are pumping in N kW with a water flow rate of X ltr/s simple maths give that the water comes out Y°C hotter than entering the heater.  When you plug in the figures for my case where I am using 1kW per loop, this Y will be about 1.6°C, so the water entering the slab will be about 1.6°C hotter than when it leaves it and the temperature drops uniformly along the length of the pipe ad roughly 10W of energy is pumped into the slab for each 1m of pipe run.  As the slab heats up, the 1.6°C drops very slightly so the temperatures all slowly rise together.

    If you crank the maths, there is a solution for the heat equation in the radial heat flow case and this is Tr = Tstart - K log(r/rstart) than is if you plot the temperature vs distance from centre of the pipe and use a log scale for the temperature then the plots are straight lines.  So my model plots is as follows as it enters the slab for the three hour heating period.

   TvsRwhenHeating.png.8f94f65f6e92b7d7c306e599f3e7516a.png

  • Heat spreading through the slab.  As soon as the heat is turned off, the temperature of the water entering the slab now drops by the 1.6°C, and the hot region near the UFH pipe quickly start to disperse radially through the pipe and also the water circulation redistributes the heat along the pipe redistributing this 1.6°C gradient uniformly throughout the slab, so that after another 3hrs the temperature is pretty uniform (note that the first 10 plots are every minute, then every 10):

  TvsRwhenCooling.png.a99f4e51937018fde12e59aa5f09f0da.png

So I can pump 3kW heat into the slab starting at 20°C.  The hot spots in the slab will be where the UFH pipe enters the lab and here the temperature of the concrete immediately around the pipe will get up to just over 25°C after 3 hours of heating, but within 30 mins of the heating being turned off the heat will have redistributed itself pretty uniformly throughout the slab (with the residual variation less than ½°C).  

 

In our case the heat will also redistribute itself through the whole slab -- that is including the load bearing beams -- again dropping the overall temperature increase.  Whilst there will be some heat losses externally through the slab (largely mitigated by the EPS wrapping) the main heat loss will be through the internal floor level surface into the living space.

 

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One last post and I think that I've done this to death.  I will summarise my conclusions and observations in plainer non-boffin language as a blog post, because the general observations and behaviour of the slab are worth understanding if you are  designing a low temperature UFH system.

 

The previous post shows that as you heat the slab through the UFH pipework at this 3kW rate that I've used as a baseline, that the temperature in the pipe gets up to maybe 5°C hotter than room temp immediately around the pipe and this temperature rapidly drops back as you move away from the pipe.  As soon as the heating is turned off then the UFH pipe fairly rapidly acts to start spreading the heat uniformly around the slab.

 

So I decided to add a bit of analysis to histogram the heat in the slab over time.  There is a slight complication in that the simulation uses an evenly spaced mesh at radial intervals from the centre core, but as I am collecting heat distributions then I need to do this at even volume steps, so that each sample represent the same volume of slab.  I did this the simplest way, which was to resample the heat curves (using a simple  linear interpolation) so that each sample histogrammed represented the same volume of concrete.  A bit of a bodge, but good enough to see what's going on, and here is the result:

589eb1da61466_DistributionofHeatinSlab.png.2659d271436220e94f2fe15617175e82.png  

The first three histograms show the effect as the slab is heated.  The main bulk of the slab slowly rises in temperature and there is this very thin long tail (of hotter concrete very near the pipe),  so for example by 3 hrs into heating the slab only  3% of the slab is over 24°C.

 

The next four show what happens when the heat is turned off and the UFH pipe is now just circulating water.  As can be seen in the previous plots with hot core is quickly cooled as the heat is redistributed along the length of the pipe.  This in effect squashes the this tong hot tail back into the main peak .  Over the next three hours or so (that is by 6hrs into the simulation) the temperature in the slab is already so uniform that the 80% of the slab is within ±0.1°C of the average temperature.  

 

And the reason that the average temperature is slowly falling is that the slab is slowly radiating heat back into the room-space. 

 

@JSHarris, @SteamyTea, @jack, does this make sense to you?

 

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Being a former applied, rather than pure, scientist, I feel a bit of measurement coming on!

 

Next week I'll retrieve the portable data logger from the water treatment plant shed (where it's been checking to see if my frost protection system works OK since a couple of weeks before Christmas) and bond the remote temperature sensor as close to the centre of the slab as I can get, and take a week or twos worth of data and see how it compares.  The room air temperature sensor on the main house data logger is pretty close to the centre of the ground floor, and the sample times are more or less the same (give or take 30 seconds) so in a week or two I should have data on slab surface temp, slab internal temp (50mm down from the surface, the slab is 100mm deep), ASHP flow temp (shows when the heating is on), room temperature and outside air temperature. 

 

The loggers record every 6 minutes, so there will be a fair bit of data, and it should be a fine enough resolution to be able to see the response curves.  Unfortunately I don't have sensors on the UFH flow and return, and right now can't easily add them, as I've run out of DS18B20s, and although I have spare ports on the data acquisition unit downstairs I'd need to add some code to read a couple more sensors and add it to the data file on the SD card.  However, it would be a safe assumption to just assume the UFH flow temp is 24 deg C all the time the heating is on, perhaps adding a lag of a single sample period to allow for the opening time of the UFH thermally actuated valve.

 

Would such a data set be of use?

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