Two circuits for cooling?

[JohnMo]

Thing often missed in oversizing ASHP and then adding buffers is the huge impact on running cost. You have additional pump(s) to run, but more importantly is the ASHP cycling because just too big. If my heat pump was smaller or modulated better, I would be getting close to a CoP of 6 at the moment, but due to cycling it's just under 5 most days, a month ago it was closer to 3 (switching circulation pump off between heat cycles made a huge difference). Add multiple pumps running, a cycling ASHP and buffer that could easily drop to sub 3 CoP, doing the same job.

 

Heat / cooling circuits oversize to hearts content, all good, but ASHP no, not good for life of the heat pump, or your pocket.

 

Tried just about every combination of heating and cooling setup (including the one being discussed by OP), and paid the bills. By far the best performer and cheapest to run (by a huge margin) is a single zone system, no buffer, mixers or additional pumps. If I installed a 4kW heat pump my ASHP running costs would drop by another 10 to 20%.

 

Add a couple of additional pumps, an ESBE mixer and a big buffer, add 2 to 4k to the bill. And all they do is degrade performance and add to annual costs.

 

Sounding like a broken record now.

[SBMS]

I've tried to do a cooling calculation based on 30 degrees outside, 21 inside.  I have more rooms but those won't be cooled.

 

My absolute worst case peak cooling is around 21kW for the whole house (red columns). Appreciate this is unrealistic etc.  I've looked around and a fair bit of wisdom indicates designing to around 75% of peak and I have done this (orange columns).  This is around 16kW.  This exceeds the heat pump, but then again 6kW of this is for the kitchen diner, which, when it is very hot, we are likely to have all doors open in any event, so any cooling in here is likely to be fairly pointless. This drops the design to around 10kW which a 12kW heat pump should be able to provide.  

 

In actual fact, the important rooms are master bedroom,  living room, bedrooms 1 and 2 and the hallway, which only has a design cooling load of around 5kW.  It has a peak load of around 9kW.

 

I feel that an ASHP would deliver enough cooling for the house, but I do think that I'd need the FCUs on their own, cooler circuit - especially the cooling loads for the landing and halls which are driven by FCUs.

 

Thoughts?

 

Floor	Room	Orientation	Floor Area (m2)	UFH Available Area (m2)	Cooled By	Solid Ground Area - 0.10 (m2)	External Wall Area - 0.15u (m2)	Pitched Roof Area - 0.12u (m2)	Flat Roof Area - 0.11u (m2)	Window Area - 0.71u (m2)	Door Area - 1.0u (m2)	Velux Area - 1.0 u (m2)	Rooflight Area - 0.6 u (m2)	ACH	Design Internal Temp	Glazed Doors?	Cooling Required	G Value of Doors/Windows	Heat Gain - External Wall (watts)	Heat Gain - Pitched Roof (watts)	Heat Gain - Flat Roof (watts)	Heat Gain - Windows (watts)	Heat Gain - Doors (watts)	Heat Gain - Veluxes (watts)	Heat Gain - Rooflights (watts)	Heat Gain - Uncontrolled Infiltration (watts)	Heat Gain - Internal (watts)	Total Heat Gain (kW)	Wall Gain (Design W)	Pitched Roof Gain (Design W)	Flat Roof Gain (Design W)	Window Gain (Design W)	Door Gain (Design W)	Velux Gain (Design W)	Rooflight Gain (Design W)	Ventilation Gain (Design W)	Internal Gain (Design W)	Total Gain (Design kW)
Ground	Hallway*	South	22.73	19.00	UFH	22.73	11.70	0.00	0.00	2.73	2.73	0.00	0.00	0.50	21	No	Yes	0.346	15.80	0.00	0.00	755.66	0.00	0	0.00	8.223714	136.38	0.92	13.43	0.00	0.00	566.75	0.00	0.00	0.00	6.58	109.10	0.70
Ground	Living Room*	South	26.39	23.90	UFH	26.39	26.23	0.00	0.00	3.08	0.00	0.00	0.00	0.50	21		Yes	0.346	35.42	0.00	0.00	852.54	0.00	0	0.00	9.547902	263.90	1.16	30.10	0.00	0.00	639.41	0.00	0.00	0.00	7.64	211.12	0.89
Ground	Gym*	West	26.55	26.55	UFH	26.55	14.79	0.00	26.55	0.00	0.00	0.00	0.00	0.50	21		Yes	0.346	19.97	0.00	26.28	0.00	0.00	0	0.00	9.60579	318.60	0.37	16.98	0.00	22.34	0.00	0.00	0.00	0.00	7.68	254.88	0.30
Ground	Open Plan Living*	North and East	41.00	40.00	UFH	41.00	38.17	0.00	41.00	0.00	25.62	0.00	7.28	0.50	21	Yes	Yes	0.504	51.53	0.00	40.59	0.00	6456.24	0	1310.40	14.8338	410.00	8.28	43.80	0.00	34.50	0.00	4842.18	0.00	982.80	11.87	328.00	6.24
Ground	Kitchen*	East	38.00	25.00	UFH	38.00	4.03	0.00	0.00	0.00	0.00	0.00	0.00	0.50	21		Yes	0.504	5.44	0.00	0.00	0.00	0.00	0	0.00	13.7484	380.00	0.40	4.62	0.00	0.00	0.00	0.00	0.00	0.00	11.00	304.00	0.32
Ground	Utility/Boot Room	East	12.00	6.60	UFH	12.00	20.88	0.00	0.00	0.00	1.89	0.00	0.00	0.50	21	Yes	Yes	0.346	28.19	0.00	0.00	0.00	392.36	0	0.00	4.3416	72.00	0.50	23.96	0.00	0.00	0.00	294.27	0.00	0.00	3.47	57.60	0.38
Ground	Snug/Playroom*	East	14.39	12.70	UFH	14.39	7.93	0.00	0.00	0.00	3.15	0.00	0.00	0.50	21	Yes	Yes	0.346	10.71	0.00	0.00	0.00	653.94	0	0.00	5.206302	86.34	0.76	9.10	0.00	0.00	0.00	490.46	0.00	0.00	4.17	69.07	0.57
Ground	Study*	South and East	10.90	9.90	UFH	10.90	19.66	0.00	0.00	1.43	3.15	0.00	0.00	0.50	21	Yes	Yes	0.346	26.54	0.00	0.00	247.39	544.95	0	0.00	3.94362	65.40	0.89	22.56	0.00	0.00	185.54	408.71	0.00	0.00	3.15	52.32	0.67
First Floor	First Floor Gallery Landing*	South	28.00		FCU	0.00	11.70	0.00	0.00	6.70	0.00	0.00	0.00	0.50	21		Yes	0.346	15.80	0.00	0.00	1854.56	0.00	0	0.00	9.7083	168.00	2.05	13.43	0.00	0.00	1390.92	0.00	0.00	0.00	7.77	134.40	1.55
First Floor	Bedroom 2*		21.00		FCU	0.00	26.49	0.00	0.00	5.23	0.00	0.00	0.00	0.50	21		Yes	0.346	35.77	0.00	0.00	905.48	0.00	0	0.00	7.281225	105.00	1.05	30.40	0.00	0.00	679.11	0.00	0.00	0.00	5.82	84.00	0.80
First Floor	Bedroom 1*	North and West	20.00		FCU	0.00	27.04	0.00	0.00	5.36	0.00	0.00	0.00	0.50	21		Yes	0.346	36.50	0.00	0.00	927.97	0.00	0	0.00	6.9345	100.00	1.07	31.03	0.00	0.00	695.98	0.00	0.00	0.00	5.55	80.00	0.81
First Floor	Master Bedroom (incl gable)*	North	23.20		FCU	0.00	39.44	24.00	0.00	8.90	0.00	0.00	0.00	0.50	21		Yes	0.504	53.24	25.92	0.00	1121.40	0.00	0	0.00	8.04402	116.00	1.32	45.25	22.03	0.00	841.05	0.00	0.00	0.00	6.44	92.80	1.01
First Floor	W/W*	South	9.68		None	0.00	12.35	0.00	0.00	1.43	0.00	0.00	0.00	0.50	21		Yes	0.346	16.67	0.00	0.00	395.82	0.00	0	0.00	3.356298	58.08	0.47	14.17	0.00	0.00	296.87	0.00	0.00	0.00	2.69	46.46	0.36
Second Floor	Second Floor Gallery Landing*	South	25.40		FCU	0.00	9.90	39.67	0.00	0.00	0.00	2.14	0.00	0.50	21		Yes	0.346	13.37	42.84	0.00	0.00	0.00	471.68	0.00	8.806815	152.40	0.69	11.36	36.41	0.00	0.00	0.00	353.76	0.00	7.05	121.92	0.53
Second Floor	Bedroom 4*		40.00		FCU	0.00	29.39	62.47	0.00	0.00	0.00	4.02	0.00	0.50	21		Yes	0.346	39.67	67.47	0.00	0.00	0.00	884.4	0.00	13.869	200.00	1.21	33.72	57.35	0.00	0.00	0.00	663.30	0.00	11.10	160.00	0.93
Second Floor	Bedroom 5*		31.45		FCU	0.00	17.51	58.00	0.00	0.00	0.00	2.63	0.00	0.50	21		Yes	0.346	23.64	62.64	0.00	0.00	0.00	577.808	0.00	10.9045	157.25	0.83	20.09	53.24	0.00	0.00	0.00	433.36	0.00	8.72	125.80	0.64

[JohnMo]

Can you post the spreadsheet as it's coming out as a jumble of numbers

[SBMS]

43 minutes ago, JohnMo said:

Can you post the spreadsheet as it's coming out as a jumble of numbers

Have attached my full spreadsheet.  First tab is the Cooling Load, which is a filtered view of the third tab which is the above but for every room.  Cooling Load strips out room where cooling is not required.  Second tab is my heat loss calculations just for completeness.

Heat and Cooling Calcs v1.xlsx

Edited August 3 by SBMS

[JohnMo]

Internal heat gains at nearly 3kW seem excessive, 8kW heat gain through doors? Not sure what that is, as there are no calculations?

 

It all seems huge, zero credit taken for any shading, internal or external, how have you passed building regs if its that bad?

 

 

[SBMS]

9 minutes ago, JohnMo said:

Internal heat gains at nearly 3kW seem excessive, 8kW heat gain through doors? Not sure what that is, as there are no calculations?

 

It all seems huge, zero credit taken for any shading, internal or external, how have you passed building regs if its that bad?

 

 

By doors we have a large set of sliders and bifolds, master bedroom bifolds and two sets of glazed French doors and a utility glazed door. So the ‘doors’ are

actually mainly glass. Correct I haven’t attributed for shading and that’s a good point. Agreed on internal gains I used an estimated watts per m2 probably over estimated. 
 

I don’t know what you mean by How have we passed building regs? Our glazing is all high performance triple glazing average is 0.7 u value and 0.5 g value. What is ‘bad’ about the figures? 

Edited August 3 by SBMS

[-rick-]

Part O requires buildings to be designed to avoid overheating, so you need to take account of solar gain/shading. Have you got those calcs from your architect/others?

[SBMS]

4 minutes ago, -rick- said:

Part O requires buildings to be designed to avoid overheating, so you need to take account of solar gain/shading. Have you got those calcs from your architect/others?

Yes. We have passed under the dynamic thermal modelling process. 

[-rick-]

So what did the dynamic modelling show? I know that Part O isn't enough if you want to keep the place at a steady 21 but your numbers are suggesting >6kW gain into one room. Did it specify shading strategies to limit the gain? Have you accounted for that in your calcs above? 6.5kw is a lot of heat to put into one room (even if it is a big one).

 

 

[JoeBano]

I’ve been running my fan coil units all summer on 12-14c flow temperature. I get water beading on my copper pipes when it’s really humid outside which isn’t too bad to worry about. The water coming out of the condensation trap I’ve had to bowls under them! A insulated pipes and condensate trap pipe work is on the list for me. 

[SBMS]

2 hours ago, -rick- said:

So what did the dynamic modelling show? I know that Part O isn't enough if you want to keep the place at a steady 21 but your numbers are suggesting >6kW gain into one room. Did it specify shading strategies to limit the gain? Have you accounted for that in your calcs above? 6.5kw is a lot of heat to put into one room (even if it is a big one).

 

 

You’re right rick and 1/3 of that glazing is actually underneath a covered canopy. I need to update the model to reflect that. We could also look at an extendable shade if needed - it is north facing though.. it does have quite large roof lights which don’t help - but again we could shade those as well to reduce the cooling demand on very hot days. 
 

I’ve asked the SAP assessor for their full report as well, never thought of that!!

[-rick-]

Ok. that should help a fair bit. Also, I noticed your main room is on a north-east alignment. Assuming thats where the windows are you should be limited in solar gain coming from the east (morning) so peak solar gain will be much more limited in time than a south facing direction. Shading will make a big difference and limiting the duration of max gain also makes a big difference. Look at the average over the worst 4-5 hours, not the peak. The floor slab should provide some buffer for peak load also.

[JohnMo]

23 minutes ago, SBMS said:

north facing

So why do you get any solar gain, yes if it's Australia but not here.

 

Just totted up our glazing all south west facing and it 52m² plus some east and west and we cope with a 6kW heat pump, internal blinds and purge ventilation when required. Generally it's still well oversized even on the extreme days.

 

Sorry to be blunt, your spreadsheet is rubbish in rubbish out, your heat pump is well oversized, and you need to step back and think again.

[SBMS]

On 04/08/2025 at 11:22, JohnMo said:

So why do you get any solar gain, yes if it's Australia but not here.

 

Just totted up our glazing all south west facing and it 52m² plus some east and west and we cope with a 6kW heat pump, internal blinds and purge ventilation when required. Generally it's still well oversized even on the extreme days.

 

Sorry to be blunt, your spreadsheet is rubbish in rubbish out, your heat pump is well oversized, and you need to step back and think again.

Oversized for cooling or heating?

[SBMS]

Checked with our SAP assessor and the house does pass under dynamic thermal modelling. Frustratingly they don’t have cooling load/heat gain calculations in their modelling software 🙄

[SBMS]

I think the undercurrent point of mine here - despite some strong opinions on the rubbish job i am doing trying to navigate it - is that as we self builders spend more and more time on insulating our houses from the cold, there is disproportionate consideration given to overheating. I haven’t seen any DIY approachable method of coming up with a sensible cooling load model

or calculation - but plenty of opinions that what I am producing is rubbish.

 

I think as we hit hotter summers and temperatures increase this will become as important as the (much larger) emphasis on heat insulation and heat loss. I have noticed many other self builders that have overheating issues on here, and I do think it’s a problem that the part o overheating doesn’t fully address. I could just throw air con at it, but as anyone who has had an AC contractor round they (under their industry guidance) size for PEAK load. We’ve had two separate contractors who sized that we need a vrf system at 24kW output!! That is definitely madness. 

The answer as I see it now is

a. size ASHP for heating and it’ll ‘probably be alright for cooling’

b. Get an oversized sub contracted ac system at double cost

c. Pay for an independent cooling load assessment (if one exists) - that is more realistic than a peak load calculation!

 

🤷‍♂️

 

[JohnMo]

15 minutes ago, SBMS said:

Oversized for cooling or heating?

Both

5 minutes ago, SBMS said:

The answer as I see it now is

My opinion and will be on the threads I answer.

 

Is size for heating and DHW.

 

An over sized takes a lot to get your head round and plenty of fiddling to get it to efficiently. It is unlikely to do it out the box. Wouldn't do so again.

 

Heating is the dominant demand for the heat pump. Do UFH so you can do either heat or cool. If you are thinking radiators in bedrooms, don't put in fan coils. Or provision for fan coils pre run pipes and electrical to bedrooms that are in use, do you need them - wait until you have lived in the house.

 

You can always and should open windows when it's cooler outside than inside when you need cooling.

 

The other thing when cooling you don't need the house at 21 degs to be comfortable, 24 is fine. Our lounge is currently 23.6 and very comfortable, outside is 17, so also have a window open in lounge and bedroom. And because the windows are open the heat pump has switched it's self off, because it has no demand , because the windows are doing the job.

 

[Nickfromwales]

1 hour ago, JohnMo said:

If you are thinking radiators in bedrooms, don't put in fan coils

Don't put in fan coils, or don't.....put in fan coils instead? Reads both ways, so just for clarity plz  

[JohnMo]

8 hours ago, Nickfromwales said:

Don't put in fan coils, or don't.....put in fan coils instead? Reads both ways, so just for clarity plz  

If you think there will be a need to cool, don't install radiators in bedrooms. They do not provide cooling,  fan coils instead either wall mounted or ducted will provide heating and cooling. 

[SBMS]

10 hours ago, JohnMo said:

Both

I think I’ve been pretty scientific in the heat loss calculations. I agree the cooling ones are not realistic. 
 

How are the heat loss calcs wrong though and producing oversizing?

[JohnMo]

18 minutes ago, SBMS said:

How are the heat loss calcs wrong though and producing oversizing?

Didn't you say you had around 7 or 8kW demand at -10 (not a realistic design temperature for England) but installer was planning to install a 12kW Vaillant (which puts out 14kW at -3)? That is some amount of oversizing.

[-rick-]

13 hours ago, SBMS said:

I haven’t seen any DIY approachable method of coming up with a sensible cooling load model

or calculation - but plenty of opinions that what I am producing is rubbish.

 

It's probably more complex than this but I thought it was mainly:

 

1. Solar Gain

2. The temp difference between outside and inside (same calculation as with heating)

3. The amount of air infiltration/exchange (same as with heating though might be additional focus on humidity)

 

The solar gain is the difficult one which is why if you are mainly north facing with some east facing windows (that already have shading) you might not need to worry too much about this. If you are south facing with no shading its a huge deal.

[Nick Laslett]

I had a play last night calculating the Cooling required for my build using LoopCAD. The house is already in the software from the UFH loops and Heat Loss calculations. 
 

This is strictly US/Canada using Manual J. The tutorial is pretty good. 
 

https://www.avenir-online.com/AvenirWeb/support/tutorial/?productLine=loopcad

 

You need a North American location as Manual J is US/Canada only. I choose Victoria Gonzales, British Colombia. This is a whole topic of its own, where else in North America has weather similar to London. 
 

Anyway, you then need to configure the walls, windows etc. To make it quicker, I just used the best U-Value defaults in Manual J.

 

This came out with a peak cooling load of 4kWh, which is the same as my heat loss, whatever that means. 
 

Basically the two short heat waves we’ve had this year in my neck of the woods, all ready confirmed that my current approach to cooling delivers what I need. House is built so maybe a bit late for some remedies. 
 

Might have another play with the latest version of LoopCAD as I was using the 2020 release. 

[SBMS]

3 hours ago, Nick Laslett said:

I had a play last night calculating the Cooling required for my build using LoopCAD. The house is already in the software from the UFH loops and Heat Loss calculations. 
 

This is strictly US/Canada using Manual J. The tutorial is pretty good. 
 

https://www.avenir-online.com/AvenirWeb/support/tutorial/?productLine=loopcad

 

You need a North American location as Manual J is US/Canada only. I choose Victoria Gonzales, British Colombia. This is a whole topic of its own, where else in North America has weather similar to London. 
 

Anyway, you then need to configure the walls, windows etc. To make it quicker, I just used the best U-Value defaults in Manual J.

 

This came out with a peak cooling load of 4kWh, which is the same as my heat loss, whatever that means. 
 

Basically the two short heat waves we’ve had this year in my neck of the woods, all ready confirmed that my current approach to cooling delivers what I need. House is built so maybe a bit late for some remedies. 
 

Might have another play with the latest version of LoopCAD as I was using the 2020 release. 

What is your current approach to cooling?

[SBMS]

12 hours ago, JohnMo said:

Didn't you say you had around 7 or 8kW demand at -10 (not a realistic design temperature for England) but installer was planning to install a 12kW Vaillant (which puts out 14kW at -3)? That is some amount of oversizing.

It’s 9kW at -4 in total. Sorry - the installer originally sized 12kW but they werent accounting for heat recovery so had a higher heat loss. I thought you were saying the heat loss calculation was wrong. 
 

They don’t have the calcs yet for updated heat loss. Interestingly heatpunk which produced the same output picked the 12kW as well, not the 10… I guess I have to account for DHW

as well