Weather test happening in the UK on ASHP design and installations.

[Marvin]

This weather should indicate if the ASHP assumptions, calculations, designs, installations, running  and power connections are up to requirement!

 

Outside temp 5.4C   Buffer tank temp aim 39.4C  Internal temp 22C

I wonder how Norfolk is doing after -12C over night...

[ProDave]

Only a mere -8 here last night and not been above 0 for several days.  Snow well over 12" drifted deeper in places.  I shovelled the snow off the balcony today as I was getting concerned at the weight.  Still able to get in and out.  The gritter made an appearance today followed not long after by the postman, but yesterday's bin collection did not happen.   Local mountain pass shut and A9 north of Helmsdale shut then open then shut......

 

Still toasty warm inside.  But this is "normal" winter weather here so no surprises been here before.  A power cut now would not be fun, I would be back and forth to the wood shed frequently if that happened.

 

No sign of an end to the cold snap for another week here.

 

As to be expected, I have not seen the ASHP need to defrost while it stays below 0.  RH inside house starting to drop, about 32% now.

[DamonHD]

A couple of visits to -5C here in sunnyish London have passed without incident for our ASHP.  Flow temp got up to 47C...  (I have 50C@-7C for one end of the WC 'curve'.)

[SteamyTea]

Well the dewpoint temperature hit 0°C.  RH is 81%.

 

 

 

[JohnMo]

1 hour ago, Marvin said:

Outside temp 5.4C 

Last time we saw that temp was 23 Dec. Plenty of defrosting going on, so CoP took a hit, but other than no issue. Just ordered my smaller heat pump, so no concerns.

[Marvin]

Hi @JohnMo We live nearer the equator than you so more sun...

[JohnMo]

9 minutes ago, Marvin said:

Hi @JohnMo We live nearer the equator than you so more sun...

🥵

[SimonD]

-6 last night with pump running at 32C flow temp most of the night with a peak of 34C after a defrost cycle. Indoor temp just above 21.5. I actually had 4 defrost cycles between 10pm and 9am.

[MikeSharp01]

1 hour ago, DamonHD said:

I have 50C@-7C for one end of the WC 'curve'.

 

Feels way to high for our floor. Got to get my head around this WC thing we need to keep our slab interface to the wooden floor maxed at 24^oC so a max water temp of 26 perhaps.

 

4 minutes ago, SimonD said:

-6 last night with pump running at 32C flow temp most of the night with a peak of 34C after a defrost cycle. Indoor temp just above 21.5. I actually had 4 defrost cycles between 10pm and 9am.

 

What was your floor surface temperature?

[SimonD]

3 minutes ago, MikeSharp01 said:

What was your floor surface temperature?

 

I'm all on radiators. Hindsight since getting the heat pump tells me UFH would have been a good idea and I could have run much lower flow temp. But isn't that self-build and learning for you? The rads aren't too intrusive thankfully. Quite pleased though as my calculated theoretical flow temp at -3 was 37C.

[SimonD]

21 minutes ago, MikeSharp01 said:

Got get my head around this WC thing we need to keep our slab interface to the wooden floor maxed at 24^oC so a max water temp of 26 perhaps.

 

Why as low as 24? Most of my commissioning of wooden floors over ufh say max 27C floor temperature and on gas systems this has sometimes required floor temp sensors.

[MikeSharp01]

3 minutes ago, SimonD said:

Why as low as 24?

Technically you are correct the flooring company say 27 but I set the max at 24 in my calculations to see if we could get the output, I think we can, and have a good safety margin - the floor is second only to the windows package cost wise.

[SimonD]

3 minutes ago, MikeSharp01 said:

the floor is second only to the windows package cost wise.

 

Nice floor then! 😁 What are you planning mean water temp wise v air temp as at 26 you'll need to have a very small DT?

[JohnMo]

56 minutes ago, MikeSharp01 said:

WC thing we need to keep our slab interface to the wooden floor maxed at 24^oC so a max water temp of 26 perhaps.

Flow temp isn't exactly related to floor temp, it varies by floor design, floor covings, pipe spacing, flow dT and pipe depth  The deeper the pipes, the wider the pipe spacing, the more the flow temperature is diluted as it radiates radially out from the pipe across the floor.

 

If your well insulated and have a floor surface temp 24, get the shorts and tee shirts out and the windows.

 

[MikeSharp01]

37 minutes ago, SimonD said:

Nice floor then! 😁 What are you planning mean water temp wise v air temp as at 26 you'll need to have a very small DT?

Hopefully! Yes the balance / trade off between flow rate and delta temp I have done calcs based on 3, 5 & 8 K against our limiting UFH (pressure drop wise) loop. Our pump can deal with 7m head and at 3K it will be running at 6.3m head drop.

[PhilT]

17 hours ago, JohnMo said:

Just ordered my smaller heat pump, so no concerns.

Out of interest why are you doing this? I can't see how this would save any significant amounts of money given your bill was already very low. What about recovery times - how often do you get power cuts and for how long? Impact on hot water heating time? Impact on defrost recovery time, headroom for other unexpected issues etc?

Edited 11 hours ago by PhilT

[JohnMo]

15 minutes ago, PhilT said:

Out of interest why are you doing this? I can't see how this would save any significant amounts of money given your bill was already very low. What about recovery times - how often do you get power cuts and for how long? Impact on hot water heating time? Impact on defrost recovery time, headroom for other unexpected issues etc?

Couple of motivations. First just bought a hot tub, and want to heat by heat pump, so need a 6kW heat pump. Which I have and it's too big for our house, main due to poor modulation with output being about 4kW, again a heat demand of 3.5kW at -9.

 

Due to cycling my overall SCoP is around 4. Even though when actually running the CoP is closer to 5 to 6. The new 4kW has a min modulation of 1kW at 12 degs, so will run continuously when needed, without any or very little cycling. Expect to see an uplift to a scop of 5. So a 20 to 25% reduction in the heating costs. Coming from less cycling and ability to run cooler flow temps, as a result of not having to compensate for cycling. Plus we need a heat source the hot tub, so a better heat pump for house covers both.

 

Unexpected issues we have covered by log stove, power cuts have battery and if that runs flat, a generator, that can power whole house including heat pump. Have an immersion for DHW heating on really cold days, but heat pump can do 4kW at -10 at 36 degrees flow and we have a heat demand of about 3.5kW for the odd day at that level.

 

 

[ToughButterCup]

19 hours ago, SteamyTea said:

Well the dewpoint temperature hit 0°C.  RH is 81%.

 

 

 

 

@SteamyTea , in the context of the posts to date, would you explain your post  please? For those of us who are hard-of-thinking about Dew points and RH stuff please

Thanks very much

[SteamyTea]

2 hours ago, ToughButterCup said:

would you explain your post  please

No.

I am busy at work.

But is short, even with relatively high temperature, the high RH will cause an ASHP to freeze up.

[MikeSharp01]

5 hours ago, JohnMo said:

The new 4kW has a min modulation of 1kW at 12 degs

Which model is that?

 

[JohnMo]

Hiaer R290 monobloc 4kW. Snip from the data book - even does 80 Deg flow if you ever needed it 

 

[Marvin]

On 06/01/2026 at 16:47, Marvin said:

This weather should indicate if the ASHP assumptions, calculations, designs, installations, running  and power connections are up to requirement!

 

Hi Everyone.

I felt it was a good time to see how these things are working. Seems to me not too bad!

 

[ProDave]

Yes, already proven here.

 

Very early on in the build I confirmed the heat loss calculation was correct by leaving a single electric convector heater of known power on and plotting internal vs external temperature for several days.  That gave me the Dt for a given power input, which tied exactly with the calculations.  In any event I bought a 5kW ASHP to satisfy a max demand of just over 2kW.

 

For me the only leap of faith was no heating in the bedrooms.  SWMBO likes a cool bedroom about 18 degrees.  Usually we have to keep the bedroom door shut to keep it down to that.  In this cold spell we had to open the bedroom door occasionally to let a little bit of heat in, and still achieved the required temperature with no actual heating there.

[FuerteStu]

On 06/01/2026 at 16:47, Marvin said:

 

I wonder how Norfolk is doing after -12C over night...

Still on gas here on the Norfolk/Suffolk border. When it stayed below -5c all day we burned through £7 of gas in 24 hours. 

Probably higher than it could have been because SWMBO likes it at 23c at all times

 

My one redeeming relief came in watching the snow melt on all the other bungalow roofs around me, but my loft conversion flat warm roof kept it's white later until it rained last night. 

 

Expensive times, could have been more so

Edited 2 hours ago by FuerteStu

[Michael_S]

I had been expecting posts here and MSE re units freezing up so it is very reassuring that there have not been any.

 

Our install is DIY using a second hand unit teed into a set of radiator tails so we have a bit of a flow issue (about 0.7m3/h).  My 'design temp' was 45c rad temp at -3 OAT.  IN reality due to the rad chosen for tee in we have 3 rads not working and flow temp of 45 at -3 with return at 36 so I guess average rad temp of 41ish but are still holding temp at 21-22C

 

[By my calcs that is about 7.5kw (45-36)*0.7/0.84 at -3 when the heat pump is using about 3kw so cop of 2.33.  When we got to -6 flow temp went up to 48, return 38 so 8.3kw at 3.8kw input 2.2 cop]