Radiators are just radiators aren’t they .. apparently not, and especially if a ASHP is being used and minimising flow rate is important to maximise the efficiency (COP).
All most all radiators are designed to run in a standard Central Heating system where using a gas boiler means the water temperature is high which means that the Delta T (Difference between Flow Rate and Desired Room Temperature) is high. Radiators have a heat outage (watts) which is based normally on a Delta T of 50.
An ASHP uses much lower flow temperatures than a gas boiler so the Delta T is significantly lower. Our space heating design is based upon a flow rate of 35/30C and a room temperature of 21C so a Delta T of 11.5.
There is a standard calculation (correction factor) used almost universally to convert a radiator output from a Delta T of 50 to 11.5, the result as an example is that a Radiator output 2160watts in a standard Gas CH system, becomes 338watts when used in a low temperature ASHP system. Which means that either more radiators are used, OR, very big radiators are required (even though our heat requirements are very low).
We discovered Eskimo Radiators via searching for alternatives. There Column radiators are designed and built in the UK, and created specifically for low temperatures, Aluminium vs Steel, Finned, Low Water Volume, and have a very different and better correction factor creating vs double the output wattage at the same temperature. In the example a similar radiator would produce 520watts.
This may not sound much, but when the total space heating requirement is a peak of only 4Kw – with outside temperature -5C it enables us to specify smaller and fewer radiators and we think they look damn good.
Point North, Gloster Hill Self Build 