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Plumbing Design – Part II



This is the Part II roll up of a couple of earlier blog posts and forum topics which provide the groundwork and context.

in summary, so far into commissioning and early use, everything is at least achieving our expectations and the house might in fact perform better than my predictions.  The key design points that I listed in part I seem to be spot on.  I want to compare a figure that I gave in the modelling topic with a corresponding plot during commissioning and testing to underline this:

image.png.e558f16cbd3b03fa4e5a343aa87b1a4d.png   5a0dbd5c77170_UFLheating13-16Nov.png.198c5d9a778f2c4e8ff7204e2b48d447.png


The first graph is a theoretical model based on a few simplifications, and the second live data, warts and all, and complete with hiccups as I test and restart the control system.  The bottom line is that the slab is reacting exactly as I modelled in overall behaviour, though one of the parameters is different.

  • The UFH pump at its medium setting is under half the modelled flow rate, increasing the delta temp between out and return from 2°C modelled to  5 °C measured. 

However, I decided to stay with this setting because the pump is almost silent at its medium setting, and the system and its subcomponents are still operating  well within specification at a delta of 5 °C.


So in my view, if you are building a house with near Passive performance (wall, and roof U values < ~0.15; windows < 1 and not too much area; well sealed warm space and MVHR; decent insulated slab), then you should expect heat losses of less than 40kWhr / day in worst winter months.  You therefore need to put roughly the same into the house.  You only need to input the net top-up, because your occupancy, normal electrical consumption and solar gains all contribute to this input; this net is going to be 1kW or less on average.  Given that a cheap and simple Willis heater can provide 3× this,  using something like a gas boiler capable of 16-20 kW is just crazy, in my view; in our case even the economic case for considering an ASHP is marginal at best.   


Yes, in terms of running costs, the electricity unit cost per kW is more than that for gas, but you also have to factor in other running costs such as boiler maintenance.  In our case, the British Gas boiler maintenance contract in our old house is less than our total expected heating cost in the new house so unit price comparisons are irrelevant to us.


As I commented in the Boffin's thread, you need to limit the heating going into the slab: one way (the one Jeremy currently uses) is to throttle back the heating rate right back (e.g. using a buffer tank and an accurate thermostatic blender) ; the other way is to use a chunking approach and simply heat the slab in one (or possibly two) chunks per day.  In the chunking case you instead limit the total heat injected into the slab per heating round (that is the integral of the power rather than the power itself).  Doing this might seem awfully complicated, but in practice you can let the slab physics do this maths for you.  You can use any moderate heating source that has a reasonably consistent but limited heat output; this could be an inline heater like a Willis heater or an ASHP with the flow temperature and rate at present set-point giving water at, say, 30°C. The slab itself slab acts as the buffer, so no additional buffer tank is needed.  The algorithm is simple:

  • Turn on the heating at a fixed time.  This could be the start of E7 or in the window of peak power if you have PV installed.
  • Turn it off when the average return temperature from slab reaches a specific set-point threshold.

The actual set-point (which in my house is going be around 27°C in winter) does vary by season because what you are doing is control the total heat put into the slab, and it will need trimming for any specific house and heating scenario,  but it is largely self correcting for short term temperature variations in that if the house gets a little colder due to greater heat loses in a cold snap, then the slab will require more heat to  reach the set point.


At the moment we are using a twice a day heating cycle.  This is settling down to ~6 hrs overnight during the E7 window about £1.50 and a couple of hours top up during the day (another £1).  This being said, we are still warming the house from a pre-commissioning temperature of around 13°C to a pre move-in target of 20°C as as you can see from the graph, we are currently increasing the house temperature by ~0.5°C / day on top of the sustain heat losses.


This in itself takes a lot of energy as we have approximately

  • 17 tonnes of slab,
  • 5 tonnes of plasterboard,
  • 11 tones of wood

inside the heated zone of the house and

  • 8½ tonnes of cellulosic filler in the insulation

Plugging these numbers and their  Cp's, it takes roughly 25 KWhr to raise this fabric by 1°C, or 4 hrs of Willis Heater to raise it by ½°C.  So at the moment roughly half of the heat input is maintaining heat loses and the other half is slowly raising the temperature of the house fabric .  This maintenance heating element is less than the JSH spreadsheet estimated for current average outside temperatures.


So another way of thinking about this is that if we do without heating for a day, then the house will drop in temperature roughly ½°C to compensate for heat losses.  The daily ripple in temperature with a single heating chunk will be less than this. 


If we only heat the slab during the E7 time window, say from 2 - 7AM, then the house temperature will peak roughly 3-4 hours later late morning and then fall by maybe ½°C during the rest of the day.  I feel that a ripple of ½°C will be barely noticeable to the occupants, and given that the heating during the E7 window is effectively half price, it is better to accept a midday peak (and possibly set the target temperature half a degree higher) than to pay double for an afternoon heating top-up to reduce the ripple.  

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Very interesting @TerryE.  Fantastic results and I'm sure very satisfying knowing that your house seems likely to perform better  than expected.  It's a very good point that you make about keeping things simple, there is an elegance to using a Willis heater which is simple and cheap to maintain/replace.  The downside of course is the higher running costs of direct electric.  I would have loved to adopt the same elegant approach, but having sat down and worked out the combined capital, maintenance and running costs, I found that such an approach was going to cost us far more than the ASHP route we eventually went. 


For us, the tipping point was 2500 kWh/yr in heating requirement.  Up to that level, your approach would have been the one we adopted.  Unfortunately, our heating requirement (due in part to our house design with its large external surface area, and our exposed location) does exceed that tipping point by some margin.  It's still very low compared to the vast bulk of the UK housing stock, just not low enough for such an approach.

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Even if you have gone the ASHP route because your house is at a different sweet spot, I would still suggest that the chunking approach is a lot simpler than having a buffer tank unless you also want to use it for DHW preheat.


I would also suggest that you include a Willis heater, simply because this fall back means that your ASHP is no longer a critical element. If it fails, then even if you take two weeks or a month to repair / replace it, then you will still have a warm house, albeit at the cost of a few £s a week extra electricity. So spending £300 a year on  maintenance contract becomes optional.  You can do the routine maintenance yourself and fix when broke.

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Agreed. We don't have a buffer tank, the ASHP just heats the slab directly in medium chunks (2 or 3 a day in cold / windy weather)  We don't circulate the UFH constantly as others have / plan, it just doesn't seem to be required.


Whist my system does have a back up heaters for both DHW and heating which would cover ASHP outdoor unit failure, it's not quite the same as a secondary fail safe Willis Heater which would in retrospect, been a very sensible idea.  

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I’ve not decided what to do with a backup but I’m considering a short immersion in the buffer tank so that if the ASHP does pack up we have something else. It may be that this goes in sooner rather than later to let me start putting some heat into the slab. 

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6 hours ago, PeterW said:

I’ve not decided what to do with a backup but I’m considering a short immersion in the buffer tank...


Peter, given that you have a buffer tank, this is functionally the same.  We did the sums and it made sense for us to use E7, but even if you are on a single rate tariff then putting 6-7hrs of heat a day @ 3kW will still make a big difference in warming up the house and the slab, and it represents a small start-up cost even at £3 per day.


11 hours ago, Stones said:

We don't have a buffer tank, the ASHP just heats the slab directly in medium chunks (2 or 3 a day in cold / windy weather)


Jason, surely you still need to have a method of sizing the chunks and linking these to the incremental heating need.  Yes, you can use a timed heating interval, but how do you decide the interval?  Putting digital thermometer on the return from the slab and using a threshold is a simple and robust method.


11 hours ago, Stones said:

We don't circulate the UFH constantly as others have / plan, it just doesn't seem to be required.


I think that Jeremy is correct here.  Because I monitor the circuits separately I've noticed that they aren't quite balanced so one zone is about ½-1°C hotter than the others by the end of the heating cycle.  Yes, I need to balance the zones, but after a couple of hours of circulation the difference between all the zones is less than 0.1°C, so the circulation really does rebalance the heat through the slab.  The pump is barely audible in the toilet at a medium setting and the 40W ultimately ends up as another kWhr of heat going into the slab.  OK. it might not make sense running the pump continuously on days when we are not heating the slab, but I think that running a 30mins on, 90mins off cycle is still probably worth it to keep the slab balanced and redistribute solar gain hotspots.         

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Sizing the chunks is down to however the adaptive system in the ASHP controller works.  I'll be honest and say I've not looked into how the controller makes such decisions / what algorithims it uses.  All I can say is that it works.


I haven't measured the temperatures of each individual zone / loop.  I have set the pump to over-run after heat input has ceased for 30 minutes and this does seem to keep all parts of the house at the set temperature.  Again, all I can say is that it appears to be working for us.   The next couple of months will confirm this and dictate whether I need to make any changes.  

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On  a separate note, @Pete and his wife came to visit our house and look at our CH / DHW design.  After talking to him, it struck me that you could always do my basic slab heating implementation microprocessor-free with programmable timer to supply the on trigger, and a remote thermostatic switch to supply the on-temperature break and a 3 pole 240V / 240V contactor to do the latching circuit logic.  Some to discuss over dinner tomorrow night, I think.

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As an aside, and possibly off at a tangent, I have a few hard-one points to make with regard to CH control and sensing.


The first is that, if you are naturally drawn to a technological solution, for the fun of designing, programming it and building it then it's a fun thing to do.  You will spend  a lot of time deciding on kit to use, fitting sensors and writing code to make the whole lot work.  All told it's fun, and it's how I initially envisaged making our system.  I don't regret the time spent doing this, but it was pretty time consuming, as first I had to try and model the house response under various conditions, then work out a control system that would work.


The killer for me was time, both the time spent developing the code, but more importantly, the time take for the house to respond to any change.  What started out as a slightly morbid thought, induced by my frame of mind at the time, came to dominate my thinking.  I realised that if I wasn't around, there was no one who could just step in to maintain or repair things.  That revelation caused a step change in my thinking.  I cant rely on home brew software, short life cycle hardware and interface obsolescence (all those with phone operated controls probably face a complete system change in less than five years).


I traded my natural enthusiasm for designing, building and developing stuff to only applying it to monitoring stuff.  That's been invaluable in fine tuning an off-the -shelf control system, and that just about satisfies my desire to design something novel.

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Guest Alphonsox


17 minutes ago, JSHarris said:

 I realised that if I wasn't around, there was no one who could just step in to maintain or repair things.  That revelation caused a step change in my thinking.  I cant rely on home brew software, short life cycle hardware and interface obsolescence (all those with phone operated controls probably face a complete system change in less than five years).


I traded my natural enthusiasm for designing, building and developing stuff to only applying it to monitoring stuff.  That's been invaluable in fine tuning an off-the -shelf control system, and that just about satisfies my desire to design something novel.


Completely with you on this Jeremy. I've had a couple of "life experiences" with close relatives over the last few months, as a result I have striped the control of the DHW and CH system back to absolute basics - A room thermostat, an actuator and a timer - all of which are standard off the shelf devices easily replaceable by just about anyone following a visit to Screwfix. The RPi is reduced to monitoring only . I honestly don't see a current requirement for a complex control system and I don't see them as maintainable over the long term. Just because you can doesn't mean you should :)

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I do think that we are converging in our thinking.  If you go back a year or 18 months,  you (Jeremy) and I were having a debate about whether or not a buffer tank and what I would call micro blending is necessary.  Is having complex sensing in the slab necessary?  I've reach my own conclusions in 3 three broad steps:

  1. Researching your and other's experiences and designs and looking at them critically.  This lead me to the tentative assertion that a buffer tank isn't really necessary, nor is a power limited control necessary.  You can just treat the slab as a 17 tonne storage heater which you warm up a few degrees overnight.
  2. I then did some reasonably detailed modelling of the time varying dynamic of the slab, and this was enough in my mind to confirm these assertions, but modelling is modelling and also needs to be validated by real-world data.
  3. Hence my decision to add reasonably extensive instrumentation to see if my heat chunk approach where I essentially use my slab as a storage heater behaves as I modelled it.  In a nutshell it does.

It has always been my intent, as documents on eBuild and here, to run my system experimentally for the first year with extensive instrumentation to characterise its performance, and then to implement a final "production" control regime based on this experience.  But by doing this process, my final system won't be an act of faith, but an evidence-based and informed implementation.  Other's can copy the end-design if they want and skip 1-3, because someone else has done it.  In my case I can happily treat my slab as a storage heater.  I just have to be a little careful to put the right amount of top-up heat in each night.


In terms of the relay version, I think that it would look something like this:



The alternative would be drive all of this logic at 24V DC with low voltage switches and a 2P/NO relay and use it's output to drive the SSD.

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@TerryE if I was picking one thing up from your drawing I would suggest the Live feed to the contactor and Willis doesn’t go through the manual on switch but I expect that’s just how it’s drawn ..?? 

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I will expand this is Part III when I get time to post it including a fixed copy of that circuit @PeterW, but I am a bit busy at the mo.  We've now been using the heating system for two months, including an interim NodeRED controller.  I still have to put in an upper level of control, but at the moment all I am doing is having the Willis on 00:00-07:00 (our E7 slot) and a top-up of 2 hrs in the late afternoon.  This leaves a residual ripple of around ½°C over the day (as @JSHarris has discussed previously), but other than that the house is amazingly stable.  If the external temperature climbs to around 10°C then the house slowly warms up by about 0.1°C per day on the overall ripple, but for an overall average of 4°C or so, this small delta seems to wash out -- to the point that putting in any more advanced control algorithm just isn't a priority.


As we get into lat Feb / Mar if it does steadily warm up then I might have to trim back the afternoon heating to 90 mins or an hour and by the end of March we should have stopped it entirely.     But here is a typical day's usage:



The two top hats are heating the slab.  The other overnight spikes are the SunAmps, washing machine and dishwasher.  The other base ripple is our general electricity usage for cooking, lighting etc.  The main variation is the SunAmps -- you can tell if someone has had a bath because the spikes are fatter, and the type of cooking going on that day.


So our total energy bill per day for our large 4 bedroom house is around £5, and this should fall to around £3.50 by end March and around £1.50 by May.   You can see why the cost benefit case for adding an ASHP is marginal.


One other point that we've realised: a "comfortable" temperature varies according to your activity level: maybe 19°C if you are working; 20°C if you are up and mobile; 22°C if you are sitting at a desk or watching the TV, etc..  Getting too hot is a real PITA because you need to start stripping off to avoid sweating; so we now keep the house at ~21°C dropping to 20½°C by the evenings, and there is just so much thermal inertia in the house that it's just easier to live with this so get the comfort by putting on a gilet in the evenings. 

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