We’re fitting a new bathroom and wondering if we need to consider anything to allow the room to be ready for heat pump heating in the not too distant future. Our plan is for a marmoleum floor which will firstly prevent access under the floor and secondly i think is not suitable for underfloor heating. Will we be able to heat the room just with a bigger radiator in future? Should we leave the door open for underfloor heating in the future, or abandon marmoleum and get underfloor heating installed now ? What would you do?

You don’t need underfloor for a heat pump, but you do need radiators that are big enough to emit enough heat at lower flow temperatures, which will give higher efficiency. The radiator could be longer (or taller) and fatter - perhaps being double panel instead of single panel. So think about where the connections are - I had a problem with a potential finger jam in the door and had to get the radiator relocated.
And you need 15mm pipes connecting the radiator to the main distribution which in most homes needs to be 22mm if not more to allow enough flow without getting noise issues. So best to sort that out before you put down the new floor.

1 Like

Hi Jonathan,
we were looking at an air source heat pump on the green grant scheme. We were told we’d need space for a hot water tank.
Our boiler is currently in the bathroom - we will be replacing it with the water tank when we finally go ahead. I have been told we will need a larger cupboard than the one we have for the boiler.
I don’t think the hot water tank has to be in the bathroom - but it’s worth considering where in your house or loft .
Hope this helps

My understanding is that you just need to have a big enough heat emitter. Just as others have said you don’t necessarily need underfloor heating, though the points made about pipe diameters are also true. To get an idea of what you will need the best thing is to do some back of an envelope calculations on the BTUs required.

Steel rad (https://www.stelrad.com/wp-content/uploads/2014/08/stelrad_technical_info.pdf) quote a conversion factor of 0.3 for a delta T of 20 and 0.4 for delta T of 25. That is, for every unit of heat (BTU) produced by a radiator on a typical gas boiler system (70 degree flow temp) the same radiator on a typical heat pump system (40 - 45 degree flow temp) will produce 0.3 to 0.4 units. The heat given off is just proportional to the difference in temperature between the radiator and the room.

You can get an idea of what radiator you might need by calculating the heat demand of the room (there are a few calculators online) or just looking up the heat output from your current radiator. Then just divide that figure by the conversion factor (0.3 or 0.4). This will give you a figure you can use to look at how big the radiator or towel rail you would need is (the BTUs quoted in sale literature are basically always for a 50 degree temp difference)

For example; one online calculator suggests a 2.5m X 2m bathroom with one window and one outside wall would need 1319btu. 1319/0.3 would be 4396. A standard double panel rad for that would be 80cm X 60cm or 140x60 single panel. You can also get more fancy rads like the column ones or even a chunky towel rail with that sort of output.

As far as I know, if you wanted to put in a larger radiator now while you are doing the work, so the room is ready that would be fine as long as it has a TRV. If you had a radiator that big without a TRV running on a hotter flow temperature it would emit a lot of heat into the bathroom anytime the heating was on and the boiler would have to work harder to raise the heat in other rooms.

This has been my approach while we’ve been doing things in our house, it’s obviously not as good as getting a full heat loss survey but it will give you a bit of an idea about what bigger radiators might mean in that room.

Thank you everybody for your responses. All really useful, and some things I hadn’t thought of