Simple radiator sizing for heat pumps

There is a lot crap talked when it comes to sizing radiators for heat pumps. There shouldn't be, its really not that hard. But we work in heat pumps so we have made it as hard as possible, its just the heat pump way.

Back when the RHI was launched 8 years ago a hideously complicated guide called the heat emitter guide was written. If you followed the guidance the radiators were so huge you couldn't actually fit anything else in the room. Within days everyone ignored it. I think within a week I had automated the rad selection in my heat loss calculator so we didn't have to use this awful document. The problem with the emitter guide was at its heart they decided to do everything assuming worst case scenario.

It started with a simple, but not widely understood idea. If you buy a type 22 or double panel double convector rad, 300mm high by 3000mm long and run it at 50C average temperature (52.5 in 47.5C out) in a room of 20 degrees C you get 1564 Watts out of it. see Stelrad table below.

But if you double the height to 600mm and maintain 3000mm wide you would assume you would get double the heat out of it or 1564 x 2 = 3128Watts, but you dont. You get 2747 Watts, that's a loss of 381 Watts, or over 10%. It turns out the output of a radiators falls as they get taller. I assume its because as the air rises up the radiator it gets up to temperature so the top bit is less effective than the bottom.

In the heat emitter guide its like they took the idea to extremes and assumed every rad was turned on end and was 3000 mm high but only 300mm wide. This is why they recommended the rads are 3,4 or even 6 times the size used with a boiler. If you use these recommendations the rads are massive, expensive and you dont win the order.

Its seems madness now that anyone even wrote this thing, all we was a radiator table like the one above, to size you radiators, lets face it Stelrad , Myson etc know lots about radiators. Once we have the table you can quickly select the radiator of choice to match the heat loss of the room from the heat load calculator. Here's one I prepared earlier for a job I sized this week.

If you are a fan of apps or excel Stelrad even do a tool to make it easier.

Now many heat pump deniers will tell you you need massive custom fan assisted triple radiators for a heat pump, I'm afraid that's not true either.

Below I've selected a fairly standard radiator size 700mm high, 1000 mm wide. You can see the compact K2 rad (type 22 or double panel double convector) gives 1961 Watts when used with a boiler running the rad at 70C average in a 20 C room. they call this output dt50. (50 degrees difference between the radiator and the room temperature)

On a heat pump, the same 700mm high 1000 mm wide K2 rad (type 22 or double panel double convector) gives 1010 Watts when used with a Heat pump running the rad at 50C average in a 20 C room. Output dt30.

So its about half the output. and ..............

If you want to go super eco and run the rads really cold, at 40 degrees average radiator temperature in a 20 degree room you get 596 Watts at Output dt20.

Personally I think the future lies in higher flow temps with slightly lower efficiencies but less rad changes, here's the output with a 60 degrees C average radiator temperature.

1467 Watts.

So to conclude:

Its possible the radiators in the house you are working on already has radiators that are too big for the room, oversizing radiators is very common. In many cases radiators are installed to fit nicely under windows. With the table above or the radiator tools you can check them against the room by room heat loss. If they are big enough you do not have to change them.

Heat pumps do not need massive radiators unless you want to run them at very low temperatures.

Standard rads are fine on heat pumps, you dont need triples or fan assisted rads, you can use simple white rads, they just have to be the right size.

If you push up the flow temperatures you have to change fewer rads, so you can save a fortune on installation but have a slightly higher run cost.

If the radiators are to small the options are make them bigger by replacing them or make them bigger by raising the temperature. If you dont you will never get the room up to temperature on the coldest days.

Over the years the temperature we can achieve from heat pumps has risen, back in 2010 it was hard to get a flow temperature of 50 degrees C. Now 60C is possible from most machines. This will rise again with machines coming to market. The higher the temperature the higher the run cost, but the less radiators you need to change.

Raising the temperatures will make heat pumps cheaper and faster to install, the general public will embrace our technology if its more familiar. Trying to flog massive cold rads is something we need to stop doing.