Hi,
I’m midway through the specification stage of an ASHP installation at my house with Octopus.
My actual heatloss seems to be on the cusp between two HP models (a blower door ATT soon will give some more data points compared with the CatA MCS defaults used in the survey) but at the moment, the installer prefers the larger HP and from what I’ve been reading in these pages and elsewhere I think the smaller monoblock may be preferable.
Within limits John Cantor’s video isn’t categorically too stressed about having a slightly oversized HP, but I’m wondering if there is a Defrost angle in this?
So, my question is: When deliverling the same heat output in a period, would a smaller HP running continuously be more or less susceptible to icing up than a larger HP running intermittently?
My present assumption is that sizing the HP so it just about manages to keep up at design temps is optimum (because it provides the most useful downward modulation for milder times). But if this is at the expense of the HP icing up on cold days then the argument fails.
It makes sense that a larger evaporator will help during defrost conditions but whether this will be an issue with the smaller heat pump it’s difficult to tell without knowing more.
Could you share with us some details about the system being proposed?
What is the heat loss the installer has calculated?
What is the capacity of the proposed heat pump?
What is the make and model of the heat pump?
It seems that some heat pump makes and models are affected more by over-sizing than others, so this question of if you will see a performance penalty with the larger heat pump is quite make model specific…
The house is 180m2, newly triple glazed, re-doored and care taken with other draughts and leaks wherever possible in 1980s fabric.
Installer’s heat loss is: Fabric - 5,089, Ventilation - 4,551
Proposed system is 11kW Daikin EDLA11DA3V3
Assuming 180 m2 x 2.35m ceiling height = 423 m3 building volume.
Air change rate = 4551W / (0.336 x 423 m3 x 23K) = 1.4 ACH average.
Given that you have new windows, doors and have taken care with draughts etc that suggests a fairly air-tight building, do you have any special ventilation beyond intermittent extract fans?
Sounds like it might be plausible that the air change rate is half of this maybe 0.5-0.7 ACH, you could calculate what your minimum ventilation requirements should be based on part F building regs and cross check this with a blower door test if you wanted to delve into it deeper… You might be interested in reading the thread here: Measuring air change rates with CO2 sensors for heat pump heat loss calculation.
0.7 ACH would give 2275W reducing total heat loss to 7365W.
@ColinS’s experience with defrost performance with a EDLA11DA3V3 (ASHP sizing for Freezing Conditions) is that you cant rely on more than 7.5 kW from this unit (without the backup heater). It doesn’t give the 10.8 kW or so at -3C as quoted on the MCS database. Octopus are likely aware of this and are probably suggesting this unit so that you have capacity to cover periods below zero.
The 9kW EDLA09DA3V3 has the same compressor as the 11 kW as far as we understand and so the low end modulation is not necessarily improved until you get down to the 8kW model… I dont think we know what the real world capacity of the 8kW model is under defrost conditions but if it was similar to the 11kW model it would likely leave you a fair bit short?
Do other Daikin owners think this is a fair reflection of our understanding?
There’s a chance this is not fully maxed, we’d need to be sure the max flow temp target was a fair bit higher than the actual flow temp, but the shape of the flow temp increasing suggests it might be close…
Thanks very much for these replies and pointers Trystan. It has helped to see other perspectives.
But I’m still uneasy about what may be over-emphasis on the part of the supplier to comply completely in their contractual relationship with MCS “rules” about how warm I need my house to be every day of the year. I understand that in the future some other family living here may prefer the house to be warmer, but we have been here 30 years and don’t have any intention of moving any time soon. We haven’t heated our house as high as 21C for years! Our priority is to burn as little gas as possible and our interest in heat pumps is to come off gas entirely.
I mentioned at the top of this thread that I think our house may be on the cusp between needing the smaller Daikin unit or the larger one. Why should MCS methods dictate a system design that is more expensive for us to run whilst we are still living in the house? We are quite happy to have the house a couple of degrees cooler during extreme weather for a few days a year and in-extremis we, like you, can spark up the log-burner.
On the one hand, there is extreme reluctance from the supplier to deviate from MCS “rules” about using default values for Air Changes based only on year of construction (even when I offer them the results of a blower door test)…
…Whist simultaneously on the other hand you suggest the supplier is deliberately re-evaluating downwards the MCS published outputs of heat pumps. Surely MCS is equally authoritative throughout every page of its publications?
In this place we are all about accurate measurement and analysis. This supplier’s approach of applying some of their own black magic in the back-end calculations might end up giving me the most appropriate system, but that’s not a transparent journey that I can follow easily.
It might be more consistent for them to accept more complete ventilation heat loss analysis where available and also add a correction factor to derate the output of the proposed heat pump during challenging weather.
Although I’m not advocating oversized heat pumps, there is an advantage in slightly oversizing in that it gives you more options to operate flexibly on different tariffs. By doing that on Agile, my heat pump operating cost is about half the cost of running a gas boiler. Thats a feature of the thermal mass/inertia of the building, but to take full advantage you need to be able to pre-warm and re-heat a bit quicker after riding through high cost periods.
I sometimes think we have tunnel vision on achieving the highest sCoP when that might not deliver the lowest opex.
A heatpump is oversized most of the year so there is often plenty of capacity to loadshift.
Most analysis on here shows most heat pumps are oversized compared to design conditions also.
I’d suggest that you would be better off with undersized with electric backup for the small periods the heat pump can’t meet the heating demand requires from a cost point of view. Cheaper unit, and better efficiency and running costs in milder weather.
We have saved a load on agile, but working out the savings of loadshifting and dropping efficiency Vs running as efficiently as possible is really difficult. And we would have saved without any loadshifting anyway as agile has performed so well below price cap recently.
Loadshifting on agile is especially problematic as 30 min price shifts can really drop your COP if it causes start/stops.
From an unrepresentative sample of on over 1 winter, I would probably opt for the higher output version.
The calculated heat loss for our house is about 6.5kW at 21/-3. The heat pump is rated at 8kW. It just about manages to maintain internal temperatures at the design conditions but has no spare capacity. It also defrosts continually at low OATs which increases electricity consumption and reduces the energy available for heating.
On this day the defrosts were happening about every 50 minutes, they cost about 6% of output and the drop from 8kW to 6kW over the frosting cycle reduces the available heat output.
I’m pretty sure that the next size heat pump up, which has a larger heat exchanger, would have worked significantly better under these conditions.
The alleged inefficiency of over sized heat pumps seems to be associated with short cycling. I’m pretty sure that that can be obviated by proper system design. My system never short cycles (understandably) but increasing the size from 8kW to 12kW isn’t going to change the likelihood of short cycling much.
It sounds like you are quite keen to go for a smaller unit @dmajwool and I guess that means the 8kW unit rather than the 9kW which we understand as being the same as the 11 kW at least in it’s ability to modulate. Lets assume that’s capable of 6.8 kW in reality. (Yes I agree MCS should have authoritative data on this but that’s not been our experience on @ColinS’s daikin, Daikin also insists it should be able to deliver 10.8 kW but we are yet to see it and everything else about Colin’s system is as it should be to deliver this capacity).
It might be worth reviewing all of the heat loss assumptions, If you can do your own e.g using https://heatpunk.co.uk/home or our heatlossjs tool https://openenergymonitor.org/heatlossjs/ and think carefully about all the U-value assumptions alongside the air change rates that could give us a bit more clarity on how much wiggle room there is?
The 21C in livingroom and 18C in bedrooms usually works out to a whole house internal average below 20C, when I put in 20C for a whole house in my own heat loss calcs it’s actually more demanding than the MCS temperatures.
I agree you should be allowed to dictate the design conditions for your own home, it’s frustrating that MCS dont allow customers to sign off non standard changes as long as the implication of those changes have been communicated clearly by the installer… but that is unfortunately where we are and so we have to find other ways around it I guess…
Do you have detailed gas consumption for this last winter? anything that you can go off there in terms of maximum heat input over a 24 hour period?
Have you also considered any other installers? You can choose from a wider range of heat pumps that way. If you want high performance with relatively little optimisation effort Vaillant and Viessman are good, but you do end up paying quite a bit more of course…
There are also quite a few oversized Vaillants and Viessman’s on heatpumpmonitor.org and oversizing does not seem to affect their performance half as much as what we’ve seen on some of the Daikin’s.
Performance is also even more significantly effected by emitter sizing and pipework of course, it’s not just the heat pump.
@meatballs with my day-job hat on (I work for the Electricity System Operator on future whole energy strategy and planning) undersizing heat pumps and topping up with direct electric is a concern because its very likely to add to system peak demand which is the main cost driver for both wind-drought generation installed capacity and network investment. Its one of the reasons I’ve been experimenting with load shifting.
Simple load shifting on the current agile tariff is easy and doesn’t need to cause frequent starts and stops because the pricing algorithm adds 13p/kWh between 4pm and 7pm everyday, so you know that by avoiding some or all of that period you can have a significant impact on weighted average cost.
I believe you also have to consider that the actual number of days when the temperature drops well below your design will be very few and you have to accept that you will have more defrost cycles on those days but having a more efficient heatpump for the remainder of time. By using an ovesized heatpump you will have short cycling and really opposite to the above.
The only advantages of having a larger heatpump is you are able to expand the system with more or larger emitters because potentially you have a bigger flow rate window which would enable you to run low and slow keeping you deltaT at optimum.
I see repeated assertions that heat pumps become inefficient when oversized.
I don’t believe it unless you are describing extreme cases - say a gas boiler drop in with a flow temp of 55C and on/off control.
If the system is reasonably well designed with adequately sized emitters and good control strategies I can’t see a good reason why a 50% over-sized heat pump would be significantly less efficient than a “correctly” sized one.
Even if the part load efficiency drops a bit it doesn’t actually matter.
When the load is low the OAT will be high so the COP will also be high. Combine a low energy demand with a high COP and the primary energy used isn’t a lot.
Conversely, when the load is high and the heat pump is running continually (apparently in a high efficiency mode) the COP will be low as the OAT is low but the energy demand is high and you will be consuming a lot of primary energy.
Examples from my installation. The worst day this winter produced 148kWh of heat energy with a CoP of 2.6. Yesterday the system supplied 32kWh of heat with a heating CoP of 5.9. Electrical input was 57 kWh versus 5.7 kWh. 1/10th of the energy for 1/5th of the heat.
In other words the vast majority of the energy needed is on cold days so that is when you should aim to optimise performance. It doesn’t matter a great deal if the performance deteriorates a bit at times of low demand (not that I think that it does deteriorate.)
I would agree with that. The problem seems to be more when the particular heat pump control strategy is lacking, e.g does not handle mild weather cycling well or takes a while to get back up to a decent efficiency as we’ve seen with some Daikin’s. Then there is the question of at which point does oversizing become an issue e.g 50% might be fine for some heat pumps but we’ve seen systems oversized by 100% or more.
It would be great to do more of a detailed data analysis of this question. I think the findings will not be a simple one rule for all heat pumps but something more nuanced.
We have a 1950s 190m2 home and our measured door test was 320m3 per hour, giving <1ach.
The installation design was willing to use this figure for air leakage loss, even without the certificate.
In hindsight we should have paid a bit extra and got the certificate and then there would be no debate on it.
I highly recommend the blower test since you will also get the chance to find where your residual leaks are and then fix them. Our ventilation is via an Mvhr system and so the efficiency loss on that came to only 2% of our total heat loss.
Nothing I’ve read on the Daikin unit would induce me to have one other than the 8kW if our total heat loss was low enough but it’s not.
Daikin is nowhere near as good as its badge and the larger ones can’t turn down well.
I have a 16kw Daikin unit that was installed with no actual heat loss calculation or testing. For winter, it is very oversized. I think it may be close to 200%. Granted, we have not had very cold days this winter but I think the most I have seen it produce is maybe 200kwh in a single day, so about 8.3kw average through the day. Coldest month was December, and it produced 5.2kw on average, with 1.35kw of draw.
It has been a long winter of tweaking, but with good results as you can see below (data from MMI).
KwH Water used
Kwh Water Generated
COP HWT
KWh Heating used
KWh Heating Generated
COP Heating
Total heating + water
Nov-23
114,00
365,00
3,20
458,00
1986,00
4,34
2381,00
Dec-23
160,00
500,00
3,13
825,00
3356,00
4,07
3886,00
Jan-24
121,00
421,00
3,48
735,00
3291,00
4,48
3741,22
Feb-24
120,00
422,00
3,52
510,00
2409,00
4,72
2842,23
Mar-24
132,00
454,00
3,44
401,00
1906,00
4,75
2370,16
What I have found that, at least for my installation (100L 4 port buffer tank), the oversizing does not appear to have too bad of an effect because the 16kw modulates quite low (not sure how much in %, I have seen as low as 700w). I want to change the piping to turn the buffer into a volumiser in the return and see how that works, but you can see that I have somewhat improved the COP by a lot of tweaking (I am sure there is more to gain but I am slowly learning).
I have a minimum flow temp of 28, with 3deg overshoot to get longer cycles. I also have Madoka cut off if too hot.
I wish I had hardware monitoring to really see what is going on, but so far my oversized Daikin ‘monster’ appears to perform rather well, at least in winter.
Regarding de-frosts. At night with LWT of around 35 it did it every 40 mins or so when between -4 and 1 deg. I tried limitting the power and it defrosted more often, which I found weird.
Your output figure for the EDLA08 would agree with what I was told; which was that Octopus wouldn’t fit an 8kW to our property as the output in worst conditions was only 7kW.
