System sizing based on actual consumption - a better way?

[JamesPa]

From my reading on here, listening to Heat Geek videos, personal experience with prospective installers, and comments and evidence from another forum Im becoming increasingly convinced that poor system sizing (generally oversizing) is one of the key factors to root out over-engineering leading to poor performance, significant excess cost and significant unnecessary disruption. 

 

The fact is that, in a typical retrofit, there are real unknowns due to:

 

• invisible fabric upgrades (eg wall insulation)
• the unknown performance of double glazing, which can have a U value anywhere between 2.8 and 1.4 
• later extensions of a different construction to the original building
• the unknown ventilation rates

 

It is thus perhaps inevitable that theoretical calculations err towards oversizing, in my case and others of which I have heard, by a factor of two. 

 

However the destructive effects of this are quite devastating, in terms of cost, disruption and the negative effects on performance.  Its all very fine blaming installers/surveyors (as I have done in spades because they have ignored what I have told them about the fabric), but, realistically, when they cant verify the key parameters of the key calculation, what else are they going to do if they are anxious to avoid 'im too cold' service calls?

 

The more I consider this problem the more convinced I become that this cannot continue and at the same time lead to satisfactory outcomes.  Heat pumps with a high modulation depth help, but don't fix the consequent overengineering of the system.  Range-rated heat pumps would similarly help, but again don't fix the consequent overengineering of the system. 

 

Of course there is an alternative, namely experimental measurements of whole house heat loss.  With a smart meter, half hourly meter readings are available and can readily be correlated with average OAT/degree days to get the required load (and with a bit more manipulation the 'thermal mass', which could help with setting up control parameters).  If no smart meter is present then annual consumption plus some information about heating patterns could, at worst, be used as a 'sense check' on the calculated figures.  In another thread there are references to companies that make special measurements to determine experimentally the heat loss which are a further alternative

 

Of course some room by room calculations will be needed to size radiators, but getting a radiator too big is nothing like as serious a problem as getting the system too big.

 

Discuss!  Has anyone got a better idea of how we might practically deal with the real fabric uncertainties in a typical retrofit?  If not then how else do you suggest we square off the problem that we simply do not, in many cases, know, with any degree of accuracy, the key parameters to do the theoretical calculation that is the basis of almost all much heat pump system design.

 

 

Edited May 24, 2023 by JamesPa

[sharpener]

Was going to start a new thread but this is quite germane.

 

Yesterday I had two different HP install firms round to quote. I would like to double check their answers against a black-box calculation of total heat loss but I have not found a complete table of the relevant U-values

 

The calculator at https://www.omnicalculator.com/construction/heat-loss is the most appropriate I have found but doesn't cover my exact materials (and may not account for air changes), the nearest approximations give about 6kW for each floor. This is consistent with several other rough estimating methods.

 

Assuming the following parameters for our 1996 barn conversion:

Two floors each 5 x 20 m with 2.2 m ceilings

My target temps are 20C for ground floor and 18C for first floor, relevant OAT location is Plymouth.

Stone (Devon slate/sandstone) walls 600mm thick, total perimeter 50m.

Pitched slate roof with 150mm of insulation on floor of loft

Ground floor has 1996 underfloor heating over 3/4 of the area so I assume some kind of insulation below the slab

One first-floor gable end is dry lined with 50mm sheepswool/50mm ventilated cavity/foil-backed plasterboard

MVHR to first floor. 6kW WBS. AGA.

 

can someone point me at accurate U values for this construction/suggest a better calculator (not a massive spreadsheet)/do the sum for me please?

 

Neither installer was here more than an hour or covered all the points I would expect. In particular neither checked the loft insulation or made any measurements beyond what was on the plans I had sent in advance. Both think the existing rads will do even though a few are manifestly under-sized. Both want to replace the HW cylinder, but one will consider adding a circulating pump instead. Only one considered noise vis-a-vis the neighbouring property. However the other one did look at my intake and correctly spotted the TT earthing, the 16mm^2 meter tails and the spare way in the CU - but doubted I would qualify for BUS, without explanation.

 

Both definitely say they will quote, on the basis of a Vaillant 12kW Aerotherm Plus (which is more than 0.6 m^3) and a Daikin Altherma HT ?16kW respectively. It remains to be seen what they come up with and whether it includes any heat loss calcs.

 

Separately I started a new online enquiry with Octopus and this time they have come back to say given my EPC rating (E on account of the walls)

"Based on your estimated heating and hot water energy requirements contained within your EPC, I'm afraid we're not quite ready to install a heat pump in your home just yet. It looks like you may require a larger heat pump or a more customised system design than we offer right now. "

 

Good Energy quote is in abeyance as I was waiting until I had got an EPC without recommendation for additional loft or cavity insulation (which mirabile dictu I have), not sure I will proceed with them, since @markocosic didn't like the proposed 12kW Midea as it would be working very hard at a rubbish CoP.

 

What a depressing experience this is proving to be!

[JamesPa]

1 hour ago, sharpener said:

Was going to start a new thread but this is quite germane.

 

Yesterday I had two different HP install firms round to quote. I would like to double check their answers against a black-box calculation of total heat loss but I have not found a complete table of the relevant U-values

 

The calculator at https://www.omnicalculator.com/construction/heat-loss is the most appropriate I have found but doesn't cover my exact materials (and may not account for air changes), the nearest approximations give about 6kW for each floor. This is consistent with several other rough estimating methods.

 

Assuming the following parameters for our 1996 barn conversion:

Two floors each 5 x 20 m with 2.2 m ceilings

My target temps are 20C for ground floor and 18C for first floor, relevant OAT location is Plymouth.

Stone (Devon slate/sandstone) walls 600mm thick, total perimeter 50m.

Pitched slate roof with 150mm of insulation on floor of loft

Ground floor has 1996 underfloor heating over 3/4 of the area so I assume some kind of insulation below the slab

One first-floor gable end is dry lined with 50mm sheepswool/50mm ventilated cavity/foil-backed plasterboard

MVHR to first floor. 6kW WBS. AGA.

 

can someone point me at accurate U values for this construction/suggest a better calculator (not a massive spreadsheet)/do the sum for me please?

 

Neither installer was here more than an hour or covered all the points I would expect. In particular neither checked the loft insulation or made any measurements beyond what was on the plans I had sent in advance. Both think the existing rads will do even though a few are manifestly under-sized. Both want to replace the HW cylinder, but one will consider adding a circulating pump instead. Only one considered noise vis-a-vis the neighbouring property. However the other one did look at my intake and correctly spotted the TT earthing, the 16mm^2 meter tails and the spare way in the CU - but doubted I would qualify for BUS, without explanation.

 

Both definitely say they will quote, on the basis of a Vaillant 12kW Aerotherm Plus (which is more than 0.6 m^3) and a Daikin Altherma HT ?16kW respectively. It remains to be seen what they come up with and whether it includes any heat loss calcs.

 

Separately I started a new online enquiry with Octopus and this time they have come back to say given my EPC rating (E on account of the walls)

"Based on your estimated heating and hot water energy requirements contained within your EPC, I'm afraid we're not quite ready to install a heat pump in your home just yet. It looks like you may require a larger heat pump or a more customised system design than we offer right now. "

 

Good Energy quote is in abeyance as I was waiting until I had got an EPC without recommendation for additional loft or cavity insulation (which mirabile dictu I have), not sure I will proceed with them, since @markocosic didn't like the proposed 12kW Midea as it would be working very hard at a rubbish CoP.

 

What a depressing experience this is proving to be!

Im not sure you will find a comprehensive table of U values and anyway there may still be unknowns.  For composite layered structures I use this tool https://www.changeplan.co.uk/u_value_calculator.php which also has built in quite a few values for elemental materials.  And I ask - because I think this accounts for a 3kW discrepancy in my case  - do you actually know your air change rate so you can calculate your ventilation loss? 

 

I am guessing those who have 'quoted' you so far will expect to do a full 3 hr survey for which you will be charged, and may change the quote (if my experience is anything to go by upward not downward).  But there is absolutely no guarantee that they will use the right data (again based on my experience.)

 

Have you got any real-world info about your consumptio9n that would allow you to do a sense check?

 

[sharpener]

1 hour ago, JamesPa said:

do you actually know your air change rate so you can calculate your ventilation loss?

 

Thanks for calculator link, will try it.

 

No idea. No trickle vents. MVHR on first floor (also extracts from kitchen) so we have all the windows shut (which OH doesn't like). Ground floor has WBS with external air supply, oil boiler ditto, AGA which sucks air from somewhere. N

 

1 hour ago, JamesPa said:

am guessing those who have 'quoted' you so far will expect to do a full 3 hr survey for which you will be charged, and may change the quote (if my experience is anything to go by upward not downward). 

 

Well tbf they haven't "quoted" as yet, but neither have they said it will be conditional on a paid-for survey.

 

1 hour ago, JamesPa said:

Have you got any real-world info about your consumptio9n that would allow you to do a sense check?

 

Yes, from a cold spell 15 months ago. Average output from boiler + AGA was 6kW round the clock, calculated from calibrated sight-glass readings.

But with major zoning in use:

2 bedrooms first and last thing, living room 1630 - 2230.

At other times we live in the kitchen/dining area.

DHW from PV topped up as necessary by boiler.

 

I am thinking this is not inconsistent with a 12kW ASHP set to heat the whole house all the time at least to setback levels.

 

Incidentally both firms yesterday said the Honeywell Evohome wireless TRV system would work fine and in one case was recommended, they just need the Call for Heat contact closure which it provides. So contrary to the received wisdom that HP works best heating the entire house all the time.

 

Heatgeek has a quite persuasive video which says that room to room heat losses make the radiators that are on work much harder so destroying the CoP, see e.g. this discussion (which is where the 9C brine temp came from). I guess it is a matter of degree, also you would think the thermal mass of the walls would even things out a lot but in practice although the living room never gets truly cold the rads lift it by 3 - 5 deg in half an hour. Which is why I put them in, the UFH was too slow.

[JamesPa]

1 hour ago, sharpener said:

Yes, from a cold spell 15 months ago. Average output from boiler + AGA was 6kW round the clock, calculated from calibrated sight-glass readings.

But with major zoning in use:

2 bedrooms first and last thing, living room 1630 - 2230.

At other times we live in the kitchen/dining area.

DHW from PV topped up as necessary by boiler.

 

I am thinking this is not inconsistent with a 12kW ASHP set to heat the whole house all the time at least to setback levels.

Based on my own experience I wouldn't assume this.  From a physics standpoint there is little or no thermal separation between rooms, so the whole house loss might not be much less with zoning than without.  My measurements on my own house tend to support this.

 

If you can bear to wait, run your current heating next winter at low flow temp 24*7 with at least most rooms heated.  You may be surprised.  This last winter I did that and consumed 10pc less energy than has historically been the case.  Take frequent measurements or get a smart meter.

 

I have gone from 16kW calculated by 2 MCS installers, to 10.5 calculated by me, to 7.5 actually measured during the very cold patch last December (when it was lower than the 'design' temp for five days and nights in a row).  This is consistent with a regression analysis of my half hourly meter readings and the maximum three hour average for the entirety of the 2022-2023 heating season.  I am now planning to fit a 9kW pump.  Of course I may end up cold, but if I need a bit of supplemental heating and as need a bit of supplemental heating on a very few days then that's probably a good trade off.

[SteamyTea]

5 hours ago, JamesPa said:

With a smart meter, half hourly meter readings are available and can readily be correlated with average OAT/degree days to get the required load (and with a bit more manipulation the 'thermal mass', which could help with setting up control parameters).

Something like this?

 

 

With the heating off.

 

 

With the heating on.

 

Edited May 24, 2023 by SteamyTea

[sharpener]

13 minutes ago, JamesPa said:

If you can bear to wait, run your current heating next winter at low flow temp 24*7 with at least most rooms heated.  You may be surprised.  This last winter I did that and consumed 10pc less energy than has historically been the case.  Take frequent measurements or get a smart meter.

 

At this rate I may not have any option. DIY - or any install with the 12kW Vaillant - will require full PP. The oil boiler will not require servicing this year as it is happy on alternate years, so I am thinking the market may be more competitive in the R290 space in 2024.

 

BTW congrats on the analysis @SteamyTea.

 

Edited May 24, 2023 by sharpener

[SteamyTea]

1 minute ago, sharpener said:

The oil boiler will not require servicing this year as it is happy on alternate years

Spend the service cash on a flow meter/logger and make a temperature logger.

[sharpener]

Or on Veritherm, their answer would be quicker and require less effort on my part?

  

17 minutes ago, JamesPa said:

Based on my own experience I wouldn't assume this. 

 

Don't follow @JamesPa. Which was "this"?

 

 

Edited May 24, 2023 by sharpener

[SteamyTea]

2 minutes ago, sharpener said:

BTW congrats on the analysis @SteamyTea.

Just the descriptive stats, not really done any analysis on it.

It should be possible with the 60 MB of data I have collected this year and the 500 MB of data I have collected in previous years to tease out what is really happening.

[sharpener]

3 hours ago, JamesPa said:

Im not sure you will find a comprehensive table of U values and anyway there may still be unknowns.  For composite layered structures I use this tool https://www.changeplan.co.uk/u_value_calculator.php which also has built in quite a few values for elemental materials. 

 

Well at a first stab I get this for the loss through the walls:

 

600 mm sandstone U = 2.504 W/m^2K x 250m^2 x 20C = 12.502 kW

 

Less demand because of the MVHR and internal heat sources, IIRC @SteamyTea allowed 3 deg rise for that. AGA gives off 1.5 to 2 kW 24/7.

 

Losses through the insulated loft will be about 4% of walls so neglible. D/G windows will be better than the walls. Haven't got an estimate for the floor yet but if there is any insulation at all there it will be small compared with the walls.

 

So the big unknown is the air change rate.

 

 

Edited May 24, 2023 by sharpener

[JamesPa]

2 hours ago, sharpener said:

I am thinking this is not inconsistent with a 12kW ASHP set to heat the whole house all the time at least to setback levels.

Apologies, this is the statement that I would not assume to be true.

[JamesPa]

Below is my plot of daily consumption vs degree days, calculated daily from half hour smart meter readings. 

 

Note the maximum daily total consumption is 165kWh ie average 6.9kW.  This is for a house that the MCS bods rate at 16kW.  Observations of my smart meter in the cold period (when it was 3C below the 'design temp') indicate 7.5kW and, if I would out the 3 hour average over the entire period it maxes out at 9kW, just once during the whole period, on a day when I turned the heating off at night.  So for certain 9kW is sufficient in my house and 8kW would very likely do. 

 

Furthermore actual observations indicate that most of the time my house sits at around 4kW.  Thus, if I were to do what the MCS bods want me to do and install a 16kW unit or more, I would be into cycling most of the time with the typical modulation ratio* of about 3, a COP disaster.  (*excluding units like Samsung which, according to @markocosic, deploy somewhat dodgy techniques to make the modulation ratio appear better whilst actually wasting energy and, in fairness, some of the 'better' units like Vaillant which appear to have a genuine modulation ratio which is greater than about 3)

 

The current way to calculate heat load is, I am fast concluding, pseudo-science because it depends on factors that, in the real world, are not well known (other than in a brand new building) and which make a significant difference to the result.  

 

This thread will hopefully stimulate constructive criticism of this view (based on the factors I have stated in the first post).  If I am right then the industry method of calculating heat load for retrofits needs to be upended.  I am genuinely looking forward to a robust debate on this matter.

 

Edited May 24, 2023 by JamesPa

[sharpener]

3 hours ago, sharpener said:

Yes, from a cold spell 15 months ago. Average output from boiler + AGA was 6kW round the clock, calculated from calibrated sight-glass readings.

But with major zoning in use:

2 bedrooms first and last thing, living room 1630 - 2230.

At other times we live in the kitchen/dining area.

DHW from PV topped up as necessary by boiler.

 

I am thinking this is not inconsistent with a 12kW ASHP set to heat the whole house all the time at least to setback levels.

 

So are you saying that in fact because the zoning does not have much effect - since the rooms are not thermally isolated - then the overall heat loss might be nearer the 6kW figure than the 12kW? I would be nervous about this without getting Veritherm in, I see they charge "from £500 plus VAT" https://veritherm.co.uk/testing/ but that is of the same order of magnitude as a couple of abortive installers' paid quotes. And for me it would be preferable to a long time spent logging lots of stuff.

 

The error bars are not inconsiderable though:

 

 

But I suppose the big question is how well is it viewed by the installation trade? If they believe in it and it reduces uncertainties and therefore the risk of an unhappy customer there is something in it for them.

 

So I will ask the two firms what they think when they get back to me. I will also ask them for references for similar properties to mine.

 

Is this the way forward? Has anyone here had Veritherm in?

 

 

Edited May 24, 2023 by sharpener

[JamesPa]

8 hours ago, sharpener said:

So are you saying that in fact because the zoning does not have much effect - since the rooms are not thermally isolated - then the overall heat loss might be nearer the 6kW figure than the 12kW?

I don't know a way quantitatively to estimate the effect of zoning, it depends on construction, shape and location of the zones.  Effectively the unheated room (and its walls) are just another layer of insulation on the heated room.  But the 'extra' insulation might be two pieces of plasterboard 100mm apart, and a roomfull of air.  Convection currents in either will suffice virtually to eliminate the insulating properties of the air and two pieces of plasterboard offer negligible thermal resistance. 

 

A house that is essentially a cube will benefit much less from zoning than a long, thin house where the zones are at either end.  You can get some sort of a qualitative feel by asking yourself this question:  when its cold outside (say 0C) what temperature are the zoned off rooms at.  If they are at 0C then zoning is having an effect.  But more likely they are at 13-17C in which case its not having anything like the effect you think it is.

 

Its impossible to tell whether the loss is nearer 6 than 12kW, I wouldn't be surprised if the loss were ~8kW but, as I know very little about your home (and even if I did don't have anything other than a qualitative idea of the effect), this is pure speculation.  The qualitative test in the paragraph above might tell you a bit more but not enough to make a decision IMHO.

 

So does this matter?  Again it all depends.  If the MCS bods fit a 14kW poorly modulating HP on the back of a 12kW calculation and your house turns out o be 8kW and require, say, 3-4kW through most of the shoulder season, you will get rubbish COP.  If they fit a 12 or 14kW unit that can modulate reliably down to whatever is required in the shoulder season the COP wont suffer significantly.  However even in the latter case you may still be forced, possibly unnecessarily, to do one or more of the following

 

add a buffer tank (cost, space, bad for COP unless properly set up, which it likely will not be)

upgrade primary feeds to the CH (cost plus disruption)

upgrade primary feeds to the DHW cylinder (cost plus disruption)

replace the DHW cylinder (cost plus disruption)

replace more emitters than is necessary (although thats not necessarily so bad because you will likely, as a result, to be able to run at a lower flow temp than designed)

 

In addition you will likely end up with a 2 fan unit not a 1 fan unit, which may therefore require express planning consent and, depending on your LPA and surroundings, this may cause problems or render the project impossible. 

 

Again without more detail its impossible to know, and even with more detail it depends on which particular preferences the individual installers have.

 

Only you can decide where the trade offs lie.  For my, essentially cubic, house with patchy and often invisible fabric upgrades, the knock on differences between the 16kW that the MCS bods (after a full three hour survey - well 2 actually) deem necessary, and the 7.5kW I actually measure, makes the difference between a horrendously disruptive, visually obtrusive and  almost certainly poorly performing project for which I would struggle to get planning consent, and a rather simple and relatively unobtrusive one which could be completed within PD rules if it weren't for the pesky requirement to use MCS installers to benefit from PD rights.  However mine may well be an extreme example and houses with a more consistent fabric will likely have a smaller discrepancy (having said that ventilation loss, which MCS by default assumes to be rather, likely accounts for about 3kW of the difference between calculated and measured). 

 

I have yet to find any MCS installer who will take any notice of my measurements (I do have one lead from another forum - but Im 'out of area' so its unclear if the individual will help), but I'm certainly not going to bow to the pressure and pay 15-20K (after the grant) for an installation that most probably wont work well and will certainly cause lots of unnecessary disruption and cost.

 

From what you have said so far I would, if I were you, seriously consider postponing a year, running your existing heating in 'heat pump mode - ie flow temp as low as you can get it whilst remaining comfortable and on most of the time' through the winter without zoning, and taking loads of measurements of consumption.  Also make sure you have access to outside temperature data (you should, its available) or record it.  If you have a smart meter get the half hourly readings, if not or you are on oil then find a way to measure consumption (logging flow meter? heat meter?).  However I stress that your house is different to mine so I really don't know.  But even if you do it and conclude 12kW is correct, you will at least have peace of mind.

 

Edited May 25, 2023 by JamesPa

[PhilT]

18 hours ago, JamesPa said:

It is thus perhaps inevitable that theoretical calculations err towards oversizing, in my case and others of which I have heard, by a factor of two.

Sorry if I missed this in your other posts but what system and spec did you go for, and how is it performing vs design etc?

[S2D2]

11 hours ago, JamesPa said:

Below is my plot of daily consumption vs degree days, calculated daily from half hour smart meter readings. 

 

+ @SteamyTea

It doesn't really matter for calculating peak load but for interest I was able to get a much better regression fit by adding the following variables to a multivariate regression:

 

Solar energy

Temperature delta

HDD*Windspeed

 

R2 = 0.93

[billt]

Have you looked at the Michael de Podesta YouTube video?

 

 

It seems at least as accurate a method for heat pump sizing as those used by commercial specifiers.

 

The heat punk calculator is a fairly simple interactive calculator that will give you heat pump size from building dimensions and construction and lets you play with emitter sizing and flow temperatures to see the effect on performance. You have to set up an account but it's free. Fairly time consuming initially but very informative.

 

https://heatpunk.co.uk/home

 

 

 

[SteamyTea]

13 minutes ago, S2D2 said:

HDD*Windspeed

Interesting one.

Windspeed on its own is a very weak correlation for my house.

 

Generally windspeed correlates nicely with OAT.

Mainly because we have high windspeeds from the SW, and low windspeeds from the NE.

Does mean we get caught out occasionally, Beasts from the East.

 

Edited May 25, 2023 by SteamyTea

[S2D2]

26 minutes ago, SteamyTea said:

Interesting one.

Windspeed on its own is a very weak correlation for my house.

 

Generally windspeed correlates nicely with OAT.

Mainly because we have high windspeeds from the SW, and low windspeeds from the NE.

Does mean we get caught out occasionally, Beasts from the East.

 

It's by far the least significant of the three but had enough of an impact for me to leave it in. If I get around to the known airtightness issue jobs I imagine it would become even less significant. Good airtightness and insulation would see it removed entirely as an input variable I imagine.

[JamesPa]

2 hours ago, PhilT said:

Sorry if I missed this in your other posts but what system and spec did you go for, and how is it performing vs design etc?

I haven't.  Currently I'm unable to find an MCS installer who will quote for a satisfactory system.  To resolve this  I would like to go non-mcs, but my LPA wants to impose a noise spec way beyond that would apply if the installation were under PD rights and which is not achievable.

 

Since PD installations require MCS and the LPA are being unreasonable so I don't have, and possibly can't get, express permission I'm currently snookered.  I am following up a couple of angles so haven't given up yet...

Edited May 25, 2023 by JamesPa

[SteamyTea]

24 minutes ago, JamesPa said:

Since PD installations require MCS and the LPA are being unreasonable so I don't have, and possibly can't get, express permission I'm currently snookered.  I am following up a couple of angles so haven't given up yet.

Write to your local MP, and the Energy Secretary.

And the BBC.

This nonsense has to be stopped.

[JamesPa]

1 hour ago, SteamyTea said:

Write to your local MP, and the Energy Secretary.

And the BBC.

This nonsense has to be stopped.

I will at the right time. 

 

I need to get to the end of the road with  my second application for planning consent (the first was consented with impossible conditions, they are saying that they will do the same with the second but I am pushing back harder and as yet I haven't had a final determination). 

 

I also need to exhaust the local political options, not least because the LPA is now under the control of a Green/Lib Dem alliance, with the Greens in the majority.  This is the case only since the local elections a couple of weeks ago, and I am expecting that they might take an interest once they get their feet under the table.    I'm already in touch with my ward councillor (Conservative) and a randomly chosen 'Green' councillor who are both showing an active interest.  The Council has published their cabinet line up so I will shortly be writing to the (Green) member in charge of planning!

[JamesPa]

4 hours ago, billt said:

Have you looked at the Michael de Podesta YouTube video?

 

 

It seems at least as accurate a method for heat pump sizing as those used by commercial specifiers.

 

The heat punk calculator is a fairly simple interactive calculator that will give you heat pump size from building dimensions and construction and lets you play with emitter sizing and flow temperatures to see the effect on performance. You have to set up an account but it's free. Fairly time consuming initially but very informative.

 

https://heatpunk.co.uk/home

 

 

 

Thanks for this.  The video is basically advocating a specific variant of the general principal I am suggesting (ie actually measuring the heat loss rather than, or in addition to, calculating it from theoretical assumptions about the fabric).

 

I registered for Heatpunk, its basically a same old same old MCS assumption based estimator, which will probably work fine if you have a house that is

 

a) of a consistent construction

b) hasn't had fabric upgrades, and

c) has the MCS assumed ventilation rates (are there any such houses?)

 

However it will get wrong, in some cases by a massive amount, houses which have been extended to different specs, have had partial fabric improvements or fabric upgrades to variable specs, or are not as leaky as MCS assumes.

 

Newer houses should, in principle, fare better as they are less likely to have variably-specified fabric upgrades.  However they may also be less leaky than older houses so I'm not entirely sure that overall the result will be that much better.  The heat pump manufacturers, at least those with web connected monitoring, have the stats on this if they chose to look.  It would be really interesting to find out what they know!

 

Im surprised that nobody has yet come onto this thread to disagree with the basic suggestion with which I kicked this thread off.  Where is the (preferably constructive) criticism?

Edited May 25, 2023 by JamesPa

[PhilT]

1 hour ago, JamesPa said:

Im surprised that nobody has yet come onto this thread to disagree with the basic suggestion with which I kicked this thread off.  Where is the (preferably constructive) criticism?

What, that oversizing is a major problem? My experience is quite the opposite. Billt's video rule of thumb suggests a 5.6 but I have an 11.2 and so far (2yrs) it's performed way better than design. I think modern scroll comp R32 or R290 heat pumps are so much more efficient at low output than they used to be. Looking at my graphs for early May there wasn't much comp cycling. Most of the"cycling" is simply the pump running for shorter periods and the room stat switching the pump off for longer periods when target room temp is met.

Edited May 25, 2023 by PhilT