Heat pump radiators

• Heat pump radiators
  • Using gas boiler and original radiators
  • Replacement radiators for 55°C flow
    • Radiators needed using my model heat loss figures
    • Performance of above radiators
    • Radiators needed using Octopus heat loss figures
    • Performance of above radiators
  • Comparison of  radiator upgrades

Using gas boiler and original radiators

Using the gas boiler at a radiator temperature of 70°C (e.g. flow 80°C and return 60°C) with the existing radiators produces the following according to my model. A radiator temperature of 70°C requires a radiator correction factor of 0.974 for 21°C room temperature, and 1.053 for a room temperature of 18°C.

TODO: update table

The only real problem is the study. Increasing the study radiator rated power to 665W would raise the temperature to 21°C.

Surprisingly, the lounge radiator seems to be just adequate.

Replacement radiators for 55°C flow

The Stelrad radiator prices below are from the Stelrad website.
The Flomasta prices are from Screwfix.
The Homeline prices are from Wickes. This range is made by Stelrad.
The prices include VAT, which an installer wouldn't pay.

Radiators needed using my model heat loss figures

Possible replacement radiators, assuming a radiator temperature of 52.5°C (55°C flow and 50°C return) and my model heat loss figures.

This is for a design outside temperature of -2.2°C and design room temperatures of 21°C (lounge, dining area, back room, study, bathroom, hall, ensuite and 18°C (bedrooms, kitchen). Note the use of 21°C as the hall design temperature - this is so that the hall radiator can assist with lounge and study heating. This gives ΔT = 31.5°C (derating 0.548) for lounge etc., and  ΔT = 34.5°C (derating 0.618) for bedrooms etc.

Room	Heat loss W	Required rated radiator output for 52.5°C W	Possible radiator	Radiator output at 52.5°C W	Radiator price £
Lounge	1862	3398	Stelrad Vertical 1800x600 2376W	1302	304
Dining area	634	1157	Stelrad K2 600x1200 2078W *Flomasta Type 22 600x1200 2051W	1139 *1124	136 *78
Lounge + dining area	2496	4555		2441 *2426
Back room	890	1624	Stelrad K2 700x900 1764W Flomasta Type 22 700x900 1739W *Homeline Type22 700x900 1767W	967 953 *968	175 124 *123
Front bedroom	171	277	Use existing radiator K1 400x700 470W	258	-
Back bedroom	29	47	*Use existing radiator K1 400x700 470W Flomasta Type 22 400x700 852W	*258 467	*- 60
Ensuite	232	423	Use existing radiator 150W	82	-
Back bedroom + ensuite	261	470		348
Study	662	1208	Stelrad K2 700x450 959W *Homeline Type 22 600x500 982W	526 *538	102 *66
Bathroom	338	617	Use existing radiator Towel 1200x600 415W	227	-
Hall	752	1372	Stelrad K2 600x800 1385W *Flomasta Type 22 600x900 1538W Homeline Type 22 600x800 1398W Flomasta Type 22 600x800 1367W	759 *843 766 749	118 *73 78 68
Total	5569			5600 5479 *5600	835 *644

Performance of above radiators

Performance of the Stelrad radiators listed above according to my model (homeW55). '*' next to a power indicates that the radiator is at maximum output.

Lounge and study temperatures are a bit low. Increasing the dining area setpoint to 21.4°C gets the lounge temperature up to 20.7°C; using a bigger dining area radiator would allow a higher setpoint.
The study radiator really needs to be bigger - a rated power of 1215W would do - but there isn't a suitable bigger radiator, without using a K3.
Adding 50mm polystyrene or mineral wool insulation to the study/garage wall would reduce the study heat loss to 478W and allow a 21°C temperature. A Flomasta K2 500x600 radiator with a rated output of 883W and cost of £54 would provide 484W.

Radiators needed using Octopus heat loss figures

Possible replacement radiators, assuming a radiator temperature of 52.5°C (55°C flow and 50°C return) and Octopus heat loss figures.

Octopus used a design outside temperature of -2.2°C, and a flow temperature of 45°C (which I think meant a radiator temperature of 42°C). They called the study 'Inner Hallway', and used a design temperature of 18°C for it.

This is for a design outside temperature of -2.2°C and design room temperatures of 21°C (lounge, dining area, back room, study, bathroom, hall, ensuite and 18°C (bedrooms, kitchen). Note the use of 21°C as the hall design temperature - this is so that the hall radiator can assist with lounge and study heating. This gives ΔT = 31.5°C (derating 0.548) for lounge etc., and  ΔT = 34.5°C (derating 0.618) for bedrooms etc.

Performance of above radiators

Performance of the radiators listed above according to my model (homeWO55). '*' next to a power indicates that the radiator is at maximum output.

The major problem here is the study. The lounge would also need supplementary heating.

Comparison of  radiator upgrades

To satisfy both sets of heat loss figures we would have replace 7 radiators (i.e. all except the towel radiators) at a cost of £1046.