Low-Temp ASHP Dilemma: New vs Used - what would you have on your shopping list if you were to do it again?

[Pablo Blowfish]

We recently had an Aira chap round, (long warranty). Told us a 6kW ASHP would be ok. We're not eligible for a grant because the previous tenant had one installed (even though the grant was £750 over 10 years ago). (We removed the 2012 ASHP—it was a noisy machine, and we’re after something quieter and more efficient - (is this the right thinking?) it's still in the garage).

 

My partner is all for buying new (with a long warranty) I'm up for taking a chance as there's some good value machines out there. - What system would you have if you were to do it again ? 

 

Current thinking is 2 zone 60ish m2 under floor heating 16mm pipes 100mm centers  + radiators up stairs - aiming to have a low flow temp and take advantage of the thermal mass of the thick walls and floor

 

Looking forward to hearing your thoughts, especially considering the low flow temperature target and potential ASHP options (new vs used) - what should we have on our shopping list - ?

Background

We bought an old (1850 stone built ) drafty damp house with an ASHP that needed a complete overhaul / insulating / damp problems solving... it's been quite a journey but we now have windows.. so thats nice.. 

Current Setup Plan:

• Heating Zones:
  • Ground Floor: ~60 m² underfloor heating with 16mm pipes at 100mm centres.
  • First Floor: Radiators.
• Aiming for a low flow temperature to maximise efficiency and leverage the thermal mass of thick stone walls and the floor.

Heat Loss Calculations:

I did some rough heat loss calcs based on U-values from ChangePlan's U-Value Calculator.

Heat Loss Breakdown:

Area	m²	U-Value	Temp Diff (°C)	Heat Loss (W)
Extension Walls	44	0.179	25	196.9
Stone Walls	48	1.26	25	1512
Windows & Doors	20.38	0.82	25	417.79
Old Roof	71	0.14	25	248.5
New Roof	34	0.147	25	124.95
Old Floor	56	0.22	25	308
Extension Floor	29.8	0.163	25	121.44
Total Fabric Heat Loss				2929.58 W

Ventilation Heat Loss:

Volume (m³)	ACR	Constant	Temp Diff (°C)	Heat Loss (W)
365.5	1	0.33	25	3025.46

Total Heat Loss (including 20% buffer):

• Fabric: 2929.58 W
• Ventilation: 3025.46 W
• Total: 7364.44 W (7.36 kW)

 

[ringi]

On 08/12/2024 at 13:12, Pablo Blowfish said:

Current thinking is 2 zone 60ish m2 under floor heating 16mm pipes 100mm centers  + radiators up stairs

 

There is no good way of mixing normal radators with UFH, as either the radators have to be huge to run at UFH temperatures,  or the flow temperature needs a mixing system to reduce it for UFH.   But the efficiency of the heatpump is defined by the emitter that has the highest required flow temperature.

 

Maybe you are happy with bedrooms at below standard temperatures, so will get most of the heat from the rooms below.

 

Fan convector "radators" are one option.  Before thinking of UFH for upstairs, remember no UFH work well if it have floorboard over it.

 

If your heatlose was a lot higher I would go as far as saying two independent heat pumps to seperate UFH from radators.

[JohnMo]

I really wouldn't bother chasing the lowest possible temperature, you just going to end up with heat pump cycling. I could run my boiler at 36 degs or below and it cycled every 6 minutes on then off.  6 mins on, 6 mins off. The lower I went the shorter the cycle. Raised the temperature to 40, would run all day every day, never cycling on or off. Just steady running flow temp varied by 0.2 degrees.

 

Same is true with heat pump. You need a differential between flow temp and room or you cannot dump the heat quickly enough.

 

But as said above 100mm centres and radiators is a bit of a waste of time. You may a well do 150mm centres and have an easy life.

 

On 08/12/2024 at 13:12, Pablo Blowfish said:

thinking is 2 zone 60ish m2

No do a single zone, I would be trying to single zone the whole house and then run weather comp.

 

If buying again I would look to go out and just buy a Panasonic R32 monobloc ASHP.