"Lean" Design

[MarkA]

5 hours ago, pdf27 said:

One of the potential problems I personally have is that my wife is extremely sensitive to variations in temperature, and as a result we will need cooling in summer. Extending the UFH circuit upstairs will solve this, and make the heat pump's job a little easier too - I'm still trying to work out if there are cheaper ways of doing the same thing though

 

@pdf27We are intending using small fan coil units for cooling upstairs (driven by ASHP in cooling mode).  Then have option of doing heating as a backup as well.

[MikeSharp01]

17 minutes ago, MarkA said:

We are intending using small fan coil units for cooling upstairs (driven by ASHP in cooling mode).  Then have option of doing heating as a backup as well.

Why not do it via your MVHR, as is our plan, and put the heating / cooling fan coil unit next to the MVHR unit - I agree this cools everywhere but by working with the MVHR systems distribution boxes you might even be able to control that, by restricting flows to downstairs at night time. 

[MarkA]

@MikeSharp01, when we looked into it, it seemed that the fan coil units would deliver greater cooling capacity.  We have a lot of south facing glazing so are concerned about overheating potential.  Planners have been extremely unhelpful to the point of being obstructive when it comes to practical shading solutions.

Edited September 3, 2018 by MarkA
typos

[Jeremy Harris]

I can confirm that cooling using the MVHR isn't that effective; the volume of air moved just isn't great enough.  A fan coil unit recirculates room air through the cooling coil and can have a much higher air flow rate, and hence much greater cooling effect.

 

Our MVHR has a built in air-to-air heat pump, that can be used to deliver pretty cold air, but the maximum cooling power is only around 1.5 kW for the whole house, which isn't much on a day when there is a lot of solar gain.  We've mitigated the solar gain as much as we can, both practically and in terms of what the planners would accept, but the greatest cooling influence on the whole house is just cooling the slab by reversing the ASHP and using the UFH as under floor cooling.  That is far more effective than having the MVHR on full boost delivering cool air.

[MikeSharp01]

45 minutes ago, JSHarris said:

using the MVHR isn't that effective

No, I know that, it has been discussed here before several times - I just want to have as many options as I can for heating / cooling, we have a passive stack system designed in as well an ASHP coolable slab. No south window shading to speak of but will have film and interstitial blinds on the 'rectangular' windows. The one thing I have not found is a passive house qualified Louver system which I would like to put at the bottom of a couple  of the north facing walls that can open with the passive stack roof lights and draw the cooler north wall air into the building and pass it out through the roof lights in the cooling season.

[ProDave]

2 hours ago, Ed Davies said:

There are two things in Passivehaus like that but neither are that:

 

1) 10 W/m² as the peak heating load in the design-worst case conditions. So you're 49.6% over that if -10 is your design worst case.

 

2) 15 kWh/m²/year as the total energy use for space heating.

 

Both are, I'm fairly sure but not certain, final energy use. You get to choose, you can meet either of these criteria, you don't have to meet both.

 

That's 15W / m² of actual heat.

 

That heat is derived from an Air source heat pump into wet UFH so assuming a COP of 3, that's 5W of electricity / m²

 

You can do a lot with figures and statistics.

[MikeSharp01]

23 minutes ago, ProDave said:

assuming a COP

but can you?

[TerryE]

8 hours ago, pdf27 said:

The 20 W/m² is what the underfloor heating would have to deliver at the design cold condition in a two storey house if it is only fitted into the slab

 

Thanks for the clarification. That would be around 1.4kW sustained in our case.  We would need the outside temp averaged over something like a 4-day period to be < -5 °C to hit that sort of number, and we have a 3 floor house.  Have a look at my blog posts on the 3D modelling of the slab-heat flow.  Last winter was particularly cold and we only went over the 1kW figure over two months.   Because we have a slate floor we get a good 7W / K / m² off the UFH, and as I said using 1 or 2 strategically placed Dyson's can double that if there is the odd week when you need it.

 

IMO, in terms of heat output and achieving overall heat balance we have a very comfortable margin in the GFL slab UFH. 

 

8 hours ago, pdf27 said:

I personally have is that my wife is extremely sensitive to variations in temperature, and as a result we will need cooling in summer.

 

I don't know if you (two) have lived in a house of this class, but what we and all of our guests note is that there is no material variation anywhere in the house; no snug feet away from the cold utility.  The only noticeable variation is that ~1°C drop going upstairs in winter.  Because we haven't fitted our ASHP yet we do our bulk heat input using E7 overnight into the slab and the overall thermal inertia is high enough that we still only get about a 1°C ripple over the day, that's < 0.1°C/hr so you don't really notice it changing.

 

Jan and I have decided that we will install an ASHP probably late next spring but I can only make a cost-benefit case if I do this myself.   And yes paying for pumped kW heat at a third of the cost will impact our time of day heating profiles and the use of active cooling in the winter.

 

Upstairs UFH seems a complex and costly solution to a tiny issue.  Like my ensuite towel rail, if you do have a passive class house, then you[ll only use it because you've got it, and not because you need it.

 

As @mike2016 suggests we are also considering an inline heater in the MVHR as an option.  I suspect that the main reason for the imbalance upstairs is that if our exhaust is at 21°C and the outside temperature is ~0°C then the inlets into the bedrooms is at ~18-19°C and this is enough to bring the temperature down.  We don't want to use this to provide bulk heating so I don't see @JSHarris concerns as being material in this usecase; this is more a trim control, and even a 1kW heater would be more than enough for this.  However, I will do the maths and the cost benefits before doing this. 

     

[Jeremy Harris]

10 minutes ago, TerryE said:

 

As @mike2016 suggests we are also considering an inline heater in the MVHR as an option.  I suspect that the main reason for the imbalance upstairs is that if our exhaust is at 21°C and the outside temperature is ~0°C then the inlets into the bedrooms is at ~18-19°C and this is enough to bring the temperature down.  We don't want to use this to provide bulk heating so I don't see @JSHarris concerns as being material in this usecase; this is more a trim control, and even a 1kW heater would be more than enough for this.  However, I will do the maths and the cost benefits before doing this. 

     

 

For clarification, trim heating with the MVHR would be fine.  We don't use it as we don't seem to need it.

 

Cooling is where the MVHR falls over, as we still seem to be able to get more than 1.5 kW of incidental heating plus solar gain in hot weather, and so the MVHR just can't draw heat out of the house fast enough on its own.  Bearing in mind that we probably have around 300 W of background incidental heat gain (occupants, things that are running in the house etc) plus one or two kW of short period heat gain from cooking, it doesn't take much solar gain to start to cause the house to warm up.  This Summer the MVHR would have been turned off at times, if it wasn't for the fact that it has an air-to-air heat pump that can cool down the fresh air delivered to the rooms, as with that heat pump off there were times when the MVHR would have been delivering air at over 30 deg C into the house, which would not have been welcome.

 

I'm currently (well, not right now as I'm busy shifting junk around...) making a combined pre-filter and ASHP run pre-cooler for the MVHR intake, just to cool the incoling air when the MVHR is on 100% bypass in warm weather, and to save running the MVHR heat pump as well as the main ASHP.  There should be an energy saving in doing this, as the additional demand on the ASHP should push it up into a slightly better operating region, so the incremental increase in energy that it will use should be less than the energy used by running the MVHR heat pump.

[pdf27]

4 hours ago, MikeSharp01 said:

Why not do it via your MVHR, as is our plan, and put the heating / cooling fan coil unit next to the MVHR unit - I agree this cools everywhere but by working with the MVHR systems distribution boxes you might even be able to control that, by restricting flows to downstairs at night time. 

Provided the water is not too cold (say 19°C) that shouldn't be a problem. The major concern is over-ventilation, but that's mostly a winter rather than a summer problem so it might well be acceptable (since bedrooms being a bit cooler in winter isn't a real problem).

[pdf27]

3 hours ago, ProDave said:

That's 15W / m² of actual heat.

 

That heat is derived from an Air source heat pump into wet UFH so assuming a COP of 3, that's 5W of electricity / m²

 

You can do a lot with figures and statistics.

Yes. That's why I've been very careful to reference everything to a standard. It might not be the right standard to build to (an entirely different question which I'm trying to avoid in this thread), but it means that I can get much closer to a solution-neutral problem statement. In any case, the 10W/m² is a comfort rather than energy-efficiency criteria, and the 60 kWh/m²/year for everything is the energy efficiency one.

 

52 minutes ago, TerryE said:

Thanks for the clarification. That would be around 1.4kW sustained in our case.  We would need the outside temp averaged over something like a 4-day period to be < -5 °C to hit that sort of number, and we have a 3 floor house.  Have a look at my blog posts on the 3D modelling of the slab-heat flow.  Last winter was particularly cold and we only went over the 1kW figure over two months.   Because we have a slate floor we get a good 7W / K / m² off the UFH, and as I said using 1 or 2 strategically placed Dyson's can double that if there is the odd week when you need it.

I've read the blog posts several times - I don't agree with all the design decisions but I think your logic and thermodynamics are both excellent.

Is the 7 W/m².K a measured value? That's rather a lot higher than I would have expected, and is worth remembering.

 

55 minutes ago, TerryE said:

I don't know if you (two) have lived in a house of this class, but what we and all of our guests note is that there is no material variation anywhere in the house; no snug feet away from the cold utility.  The only noticeable variation is that ~1°C drop going upstairs in winter.  Because we haven't fitted our ASHP yet we do our bulk heat input using E7 overnight into the slab and the overall thermal inertia is high enough that we still only get about a 1°C ripple over the day, that's < 0.1°C/hr so you don't really notice it changing.

 

Jan and I have decided that we will install an ASHP probably late next spring but I can only make a cost-benefit case if I do this myself.   And yes paying for pumped kW heat at a third of the cost will impact our time of day heating profiles and the use of active cooling in the winter.

 

Upstairs UFH seems a complex and costly solution to a tiny issue.  Like my ensuite towel rail, if you do have a passive class house, then you[ll only use it because you've got it, and not because you need it.     

My cousin has what I **think** is a Passivhaus in Hamburg, where we've stayed several times - we both like it a lot.

 

55 minutes ago, TerryE said:

As @mike2016 suggests we are also considering an inline heater in the MVHR as an option.  I suspect that the main reason for the imbalance upstairs is that if our exhaust is at 21°C and the outside temperature is ~0°C then the inlets into the bedrooms is at ~18-19°C and this is enough to bring the temperature down.  We don't want to use this to provide bulk heating so I don't see @JSHarris concerns as being material in this usecase; this is more a trim control, and even a 1kW heater would be more than enough for this.  However, I will do the maths and the cost benefits before doing this.      

Question: has anybody on here got a house with UFH in the slab and a radiator in the UFH duct running off the same water temperature as the UFH? It won't deliver much heat with the water at 25°C, but there might only be a tiny amount needed:

At 10W/m² there would be a ~25°C temperature difference between inside and outside. Reducing that to ~23°C upstairs means that the heat load is only 9.2W/m² - meaning a 0.8 W/m² supplemental heat requirement is needed to bring the temperature up to 20°C.

Assuming 0.3 air changes per hour with a 2.4 m ceiling means that there will be 2.4 x 10^-4 kg/sec of air movement, assumed to be 3-4°C warmer than room temperature. That's a heating power of 3.5 x 2.4 x 10-4 x 1010 = 0.85W/m².

That's spookily close - it's a very crude calculation since it doesn't allow for ventilation losses, etc. but it does suggest that a wet duct heater running off the UFH circuit might well be enough to make up the difference between upstairs and down almost completely, at least in cold weather when over-ventilation is a problem. In summer you can probably just turn up the flow rates and accept the slightly higher ventilation losses.

 

30 minutes ago, JSHarris said:

I'm currently (well, not right now as I'm busy shifting junk around...) making a combined pre-filter and ASHP run pre-cooler for the MVHR intake, just to cool the incoling air when the MVHR is on 100% bypass in warm weather, and to save running the MVHR heat pump as well as the main ASHP.  There should be an energy saving in doing this, as the additional demand on the ASHP should push it up into a slightly better operating region, so the incremental increase in energy that it will use should be less than the energy used by running the MVHR heat pump.

Why not leave the MVHR in standard mode and just post-cool the inlet air with the main heat pump? If the inside of the house is cooler than the outside you still gain from the heat exchanger, and it makes the MVHR heat pump redundant.

[Jeremy Harris]

 

 

6 minutes ago, pdf27 said:

Why not leave the MVHR in standard mode and just post-cool the inlet air with the main heat pump? If the inside of the house is cooler than the outside you still gain from the heat exchanger, and it makes the MVHR heat pump redundant.

 

Because I can't physically fit a post cooler and condensate drain in the space available between the MVHR and the fresh air distribution plenum.  Ideally that's where I'd put it, but it wasn't something I'd thought about when installing the MVHR, and almost all the ducts running to that distribution plenum come up through the floor right next to it, so it can't now be easily re-positioned.

 

The reason I decided to fit a heat exchanger in the intake is driven by another slight problem I want to fix.  The external air intake is up under the eaves of the house and regularly gets clogged up with cobwebs, fluffy seeds, etc in summer.  Also, the F7 intake filter on the MVHR gets very badly fouled by lots of coarse stuff, small flies that get through the intake grill etc.  Luckily the intake is on the rear of the house, the side that just faces our big retaining wall, so adding an insulated external duct extension down the wall to a lower level where I can more easily get at it to clean it isn't going to be a problem at all. 

 

The plan originally was to just fit a large, coarse, washable, foam pre-filter on a new intake at a lower level, so I could easily just lift it out and wash it, with the hope that this pre-filter would help prevent the main pollen filter from getting clogged up so quickly.  It was the hot weather this summer, combined with me looking at the best way to run the extended intake duct down the wall, that led to me noticing that it was fairly close to the ASHP.  Running a couple of extra flow and return pipes, in parallel with those going through the wall to feed the UFH etc, would be easy, as would adding an extra cable through the existing ASHP cable duct with a control signal for a motorised valve.  I already had a "cooling on" relay, with plenty of spare capacity to run another motorised valve, so I started thinking about modifying my new filter box to include a heat exchanger, motorised valve and a condensate drain tray.  This all looks easy enough to do (I just need the time to do it).

 

In practice, cooling the "wrong" side of the fresh air feed won't make any difference anyway, as any time that the cooling will be turned on the MVHR controller will have already moved the MVHR bypass valve to 100%, so the MVHR heat exchanger will be bypassed anyway, and the intake duct will be directly connected (via the filter) to the fresh air distribution ducts.

 

[Ed Davies]

6 hours ago, ProDave said:

That's 15W / m² of actual heat.

Yes, you're right. Getting confused by previous discussion with @JSHarris where he said it was primary energy which I was pretty sure it wasn't but somehow got it in my head that it was therefore final energy. Well, it is final final energy, sort of ?

[Stones]

3 hours ago, pdf27 said:

 

Question: has anybody on here got a house with UFH in the slab and a radiator in the UFH duct running off the same water temperature as the UFH? It won't deliver much heat with the water at 25°C, but there might only be a tiny amount needed:

 

 

We had a low temp radiator system fitted in our last house and very effective it was.  The main 'issue' is that radiator need to be oversized compared to a conventional radiator system - all our radiators were probably the equivalent three to four times the size they would have been with a high temp system.  We did initially have a towel rail in our bathroom but ended up swapping it out for a (designer) conventional radiator with a significantly higher BTU rating.  Our flow temps were low, max 33C at 0C ambient, most of the time running mid 20's.  Raditors never felt warm but if you did turn them off, you would notice the drop in temperature of the relevant room.

[TerryE]

On 03/09/2018 at 16:21, pdf27 said:

Is the 7 W/m².K a measured value? That's rather a lot higher than I would have expected, and is worth remembering.

 

We had a long thread about this a couple of years back.  @JSHarris and @SteamyTea were two of the other main actors.  The figure drops out of Stefan–Boltzmann as the radiant component dominates heat losses at the small delta T.  There are minimal conductive losses on an solid / air interface and a few °C is too small to generate enough instability to create any convective flow in a passive house.  In our case we have a mat slate floor which is rough at a micro level and about the perfect radiant surface.  The figure would be lower for a carpeted floor,  but not as much as you'd think because whilst carpet is a good insulator, the surface is almost fractile and so the effective radiant surface is a lot larger.  Certainly looking at the slab temps and our rate of heat loss, this is in the right ballpark.

Edited September 4, 2018 by TerryE

[pdf27]

43 minutes ago, TerryE said:

 

We had a long thread about this a couple of years back.  @JSHarris and @SteamyTea were two of the other main actors.  The figure drops out of Stefan–Boltzmann as the radiant component dominates heat losses at the small delta T.  There are minimal conductive losses on an solid / air interface and a few °C is too small to generate enough instability to create any convective flow in a passive house.  In our case we have a mat slate floor which is rough at a micro level and about the perfect radiant surface.  The figure would be lower for a carpeted floor,  but not as much as you'd think because whilst carpet is a good insulator, the surface is almost fractile and so the effective radiant surface is a lot larger.  Certainly looking at the slab temps and our rate of heat loss, this is in the right ballpark.

Hang on a second, is that floor temperature or flow temperature? Floor temperature would make sense, flow temperature disagrees rather violently with several sources (e.g. this one from John Guest). That has a 40/30°C tiled/screeded floor (i.e. 15°C dT between water average and air) providing 60 W/m², i.e. just over 4 W/m².K: switching to carpet gives you about 50% of that heat output.

[Jeremy Harris]

17 minutes ago, pdf27 said:

Hang on a second, is that floor temperature or flow temperature? Floor temperature would make sense, flow temperature disagrees rather violently with several sources (e.g. this one from John Guest). That has a 40/30°C tiled/screeded floor (i.e. 15°C dT between water average and air) providing 60 W/m², i.e. just over 4 W/m².K: switching to carpet gives you about 50% of that heat output.

 

Floor surface temperature is the standard parameter used for UFH design.  The formula for heat output per m² is (8.92*Δt)^1.1 if you want to work it out reasonably accurately, where Δt is the differential between the floor surface temperature and the room temperature.

 

I'd treat guesstimates for flow temperature from UFH suppliers with caution, as they tend to fall over as Δt falls to passive house levels.

[TerryE]

Sorry if there is any confusion: floor temperature.  The relationship between the flow temperature and the floor temperature is highly non-linear both in terms of space and time.  It took me ages to get my head around this and it wasn't really until I did the modeling that I got a good feel for it.   You just can't use steady state or linear approximations validly.   I explained this in a little more detail in:

 

Instead of tightly controlling the flow temperature and therefore putting in a very variable power input, I limit my power input.  The slab heats steadily over about 4 hrs radially from the pipe centres before it even starts to approach an equilibrium where the floor temperature has risen enough for the heating to start to tail off.  On a 7 hr @ 3kW  heat, the final inflow temp is around 29°C and the outflow temp at 26°C, but the inflow temp collapses within a couple of mins of turning off the heat -- as the radial gradient collapses.  The floor reaches max temp around 11:00 - 12:00.

 

Conventional Gas and other boiler systems are simply not designed to output a steady power of < 1 kW.  You have to add a lot of additional hysteresis to avoid constant start / stop and knackering mechanical components.

 

As to Jeremy's formula IIRC, it is a curve fit from BRE for conventional UFH systems which typically need to output maybe 3× or more than a passive house UFH system. The exponential component is a fit to the convection effects that you will see if your floor is 5° or more than room temperature, but my floor never gets more than a couple of degrees warmer.  I ain't going to argue 7 vs 8.  They're the same ballbark as far as I am concerned.  Maybe J still has the BRE doc ref.

 

 

Edited September 4, 2018 by TerryE

[pdf27]

9 hours ago, TerryE said:

Sorry if there is any confusion: floor temperature.  The relationship between the flow temperature and the floor temperature is highly non-linear both in terms of space and time.  It took me ages to get my head around this and it wasn't really until I did the modeling that I got a good feel for it.   You just can't use steady state or linear approximations validly.   I explained this in a little more detail in:

 

Instead of tightly controlling the flow temperature and therefore putting in a very variable power input, I limit my power input.  The slab heats steadily over about 4 hrs radially from the pipe centres before it even starts to approach an equilibrium where the floor temperature has risen enough for the heating to start to tail off.  On a 7 hr @ 3kW  heat, the final inflow temp is around 29°C and the outflow temp at 26°C, but the inflow temp collapses within a couple of mins of turning off the heat -- as the radial gradient collapses.  The floor reaches max temp around 11:00 - 12:00.

 

Conventional Gas and other boiler systems are simply not designed to output a steady power of < 1 kW.  You have to add a lot of additional hysteresis to avoid constant start / stop and knackering mechanical components.

 

As to Jeremy's formula IIRC, it is a curve fit from BRE for conventional UFH systems which typically need to output maybe 3× or more than a passive house UFH system. The exponential component is a fit to the convection effects that you will see if your floor is 5° or more than room temperature, but my floor never gets more than a couple of degrees warmer.  I ain't going to argue 7 vs 8.  They're the same ballbark as far as I am concerned.  Maybe J still has the BRE doc ref.

Yeah, it's pretty non-linear - fortunately I have an ANSYS license at work (quite a lot of what I do is thermdynamics and heat transfer, albeit at very high power densities), which is pretty much designed for exactly this problem so modelling it is actually pretty easy, at least in a steady-state condition which is what you care about for the design worst-case sizing condition. That isn't important with a Willis heater since you're power limited only and want to run on E7, but with an ASHP it's rather more important.

If I'm understanding things correctly, you essentially control your heating by putting a fixed power in and turning it off when the return temperature rises to a value you've empirically found to be a good match for your desired comfort levels. What I'm contemplating is very similar indeed - running a small ASHP with the flow temperature turned down as low as I can, turning it on when the room temperature drops and relying on the return water thermostat to turn it off when the dT starts to drop off as the concrete starts warming up. That isn't going to lead to short-cycling - in your case the ASHP would be running for at least 2 hours at a time - and allows me to make use of the fact that the floor temperature will be very close to the desired air temperature to control overshoot extremely well, particularly if it is nearly purely radiative at which point the T⁴ term will really have a huge impact on power .vs. temperature. That, and running at very low flow temperatures has a major positive impact on power consumption - throw in the use of the SG Ready terminals to turn up the thermostat when PV is available and I think I could get the imported power values very low indeed.

[Jeremy Harris]

I tried slab temperature control initially, as I was convinced (still am) that controlling the slab was a better way of ensuring the heat input to the house was accurately controlled.  After many iterations of the control system, I gave up.

 

What I have now is a constant temperature flow into the slab when the room temperature stat is calling for heat (it's a low hysteresis stat, +/-0.1 deg C) that just turns the UFH valve on or off.  If the buffer tank (used for DHW pre-heat) needs heat the ASHP stays on until the buffer tank stat is satisfied.

 

This simple control works exceptionally well, and holds the house at a very even temperature, much better than I could get my slab control system to do.  I was measuring flow and return temp with the slab control system too (still am, for data logging, as the sensors are still fitted).

 

The key to getting the simple control system to work well was to use a low hysteresis room stat, and more importantly, to carefully control the UFH heat input.  I found that conventional thermostatic mixers weren't great when turned right down to low flow temperatures, but luckily there is an off-the-shelf electrically actuated valve that does the job superbly.  I have it fitted to the return line from the UFH manifold to the heat pump, and it acts as both the UFH on/off control valve and as a way of controlling the Δt between the slab flow and return.  It is a motorised valve with two clip on temperature sensors that fit on the flow and return pipes and tries to maintain a 5 deg C differential.  I've found this works surprisingly well, as it keeps the flow temperature down below 25 deg C all the time, and I found, by experiment, that allowing the flow temperature to exceed 25 deg C tended to lead to a higher than desirable room temperature overshoot after the UFH had turned off.  It was my desire to try and prevent this "heat soak" overshoot that had prompted me to try using slab temperature control.

 

I'm now using off-the-shelf controls, too, so when I'm gone someone else can still maintain and repair things if they go wrong, something that I think is worth thinking about.

[TerryE]

@pdf27, Assuming that you have a conventional "double-back" layout for your zones and also have access to the Ansys heat flow modelling, then you can easily set up the heat flow model that I did, which is to approximate the slab as a concrete tube the length of a zone run and the radius set by the slab thickness and pipe spacing.  This radial symmetry makes give a 2 spacial and one time dimension model which is computationally solvable over 10s of hours.  OK, the radial symmetry assumption breaks down in reality because the UFH pipes are not in a cylindrical medium but set in a slab concrete with a insulating surface below and a radiant one above. Even so the model still gave an excellent prediction of the time response of the slab and the heat-off impulse response in the both the model and the actual slab shows that the radial component dominates the heat flow during heating: it's an extremely useful model.

 

To me what this all underlines is that the slab itself is the biggest heat capacitor in the system, so there is little point in adding complexity of external smoothing using TMVs and buffer tanks.  So long as you are pumping enough heat per day into your slab then a passive class house with warm slab + UFH + cellulosic filler will stay comfortable.  In my experience your divide the year into three broad zones:

• No active heat management is required (roughly 6 months / year) because the intrinsic heat excess is enough to keep the house at a comfortable equilibrium and MVHR exchange / bypass gives adequate trim.
• One per day heat adjustment is sufficient (roughly 3-4 months / year). This is my overnight top-up / cool-down.  Again so long as this adjustment gives enough bulk heat-balance, the MVHR exchange / bypass gives adequate trim.
• One per day heat adjustment is insufficient (roughly 2-3 months / year).  Here for the mid winter months a single heating period starts to give a daily heat ripple that is noticeable, so you need multiple heating periods per day.  For the UK climate range IMO you will never need more than 3, but at 4 the ripple will be less than 0.1°C.

At the moment I use the day-to-day average temperature as a control feedback to compute the total amount of daily heat (and in future cooling) needed to apply.  Because I use a fixed input heater this maps directly to heating time.  If you are on E7 it does make sense to have an asymmetric cycle with a bulk heat overnight, but the tops can be spread through the day.  It sounds like you are going to adopt similar ASHP heating  scheme to the one that I plan to and @jack has done, which is to set the ASHP output temperature at a low setpoint (in my case around 27-28 °C).  You then need to control the mark/space ratio to maintain the overall daily thermal balance.   And the CoP at this set point is excellent.  (Though if you have kids and use lots of HW and want to use your ASHP to preheat this then this would greatly complicate this approach.)

 

There are a number of strategies here, eg. use a fixed cycle (say 6 hours) and then control the on time or use a fixed on-time and control the off-time to give a variable cycle, but IMO these are all based on macro thermal balance.  This will work well for my house, but if you have "acres of S facing glass" then day-to-day highly variable solar gain will become an issue that you need to factor in.

 

Jeremy and I started out at very different design conclusions from a very similar problem analysis and our solutions are conditioned by historic investment decisions.  Even so we have significantly converged in our approaches.  Jeremy uses a single internal datum, I daily average a couple of DS18B20s measuring room temperature.

 

To be honest, if mid-winter heating was my only concern then I'd stick with the largely E7 Willis approach.  Yes, the running cost is maybe  £300 p.a. more than using an ASHP,  but I have no complex mechanical systems to maintain and to replace every 10 years or so, so there is no cost benefit case here.  The real issue that makes me plan to introduce an ASHP is the summer cooling one: for about a month a year, I need to dump heat from the house actively to keep a comfortable internal environment, and I can't do this by an additive heat solution.

 

One last comment.  I've mention the impact of solar gain which can throw a "big spanner in the works".  You can get very sunny days in December and if you have a large south-facing area of glass, this can be a big pulse of kWh into the house.  The other one that causes us fun is visitors.  When our kids+family or others come to visit,  then just the heat and activities around hosting these guests adds environmental control challenges.  For example, 6 people sitting in a room will cause it to start to warm up noticeably!

Edited September 5, 2018 by TerryE

[Jeremy Harris]

14 minutes ago, TerryE said:

One last comment.  I've mention the impact of solar gain which can throw a "big spanner in the works".  You can get very sunny days in December and if you have a large south-facing area of glass, this can be a big pulse of kWh into the house.  The other one that causes us fun is visitors.  When our kids+family or others come to visit,  then just the heat and activities around hosting these guests adds environmental control challenges.  For example, 6 people sitting in a room will cause it to start to warm up noticeably!

 

We get pretty much the same - often the worst case solar gain for us is late autumn to early spring, when the sun can penetrate deeper into the house. 

 

We also have exactly the same issue with visitors.  A while ago we held an open weekend, showing groups of around half a dozen people around every couple of hours.  It got so I could pretty much predict when the MVHR would switch to cooling more (it's slightly audible on full boost daytime cooling) and that would be around 10 to 15 minutes after the visitors arrived.  People are surprisingly effective at warming up the air in the house.

[jack]

The overheating effect isn't so noticeable in our house, as it's very open plan and has a decent volume with concrete floors.

 

The one place we really feel it is the TV room, which is small and carpeted. We can't sit in there as a family with the door closed for very long in summer.

[Dreadnaught]

Very interesting discussion. 

 

@jack, with the benefit of hindsight, would you have changed anything about your TV room to correct for that propensity for summer overheating? Asking as its relevant for the design of my future house.

[TerryE]

22 minutes ago, Dreadnaught said:

would you have changed anything about your TV room to correct for that propensity for summer overheating?

 

In my old house, you closed doors to keep the "coziness" in and the cold drafts out.  In my new house, we rarely close doors because the rooms can overheat when containing a high body count, and start to feel a little stuffy. (You get so used to the "fresh air" feeling of an MVHR house that it is noticeable if the warm breathing body count exceeds the planned airflow and so the CO₂ count starts to rise even slightly.)  So we often compromise and leave them ajar.

 

The one mistake that we did make was the positioning of our bedroom MVHR inlet, which is diametrically opposite the door into the hall.  The issue here is that we have an ensuite which has its own extract effectively on the oppsite side of the same wall as the inlet, so the flowpath between outlet and inlet bypasses the body of the room.  I leave the bedroom door about 1" ajar which cures the issue, but this does mean that the hall nightlights do shine through the  crack into the bedroom.

Edited September 5, 2018 by TerryE