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Need some help with choosing the right Rads

Air Source Heat Pumps (ASHPs)

Last Post by Toodles 2 months ago

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Renewables345

(@renewables345)

Active Member Member

Joined: 7 months ago

Posts: 9

Topic starter 24/02/2026 12:36 pm [#2847]

Hi all,

I've got an installation coming up of a Warmflow Unit, and I am having some trouble with the radiators for the bathroom.

The heat loss is around 1.9kW. I plan to have one electric towel rad and a standard central heating rad. Does anyone have any recommendations?

Thanks in advance

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JamesPa

(@jamespa)

Illustrious Member Moderator

Joined: 3 years ago

Posts: 4634

24/02/2026 12:53 pm

Posted by: @renewables345

↑

Hi all,

I've got an installation coming up of a Warmflow Unit, and I am having some trouble with the radiators for the bathroom.

The heat loss is around 1.9kW. I plan to have one electric towel rad and a standard central heating rad. Does anyone have any recommendations?

Thanks in advance

Wow, thats a big (or very poorly insulated) bathroom. Are you sure its that much?

As I'm sure you know radiator output and thus selection depends on the design flow temperature so without that its impossible to say. For example a radiator that is rated at 1kW at the standard 50C difference between rad and room (eg flow temp 75, return 65), will emit only 360W at a more typical heat pump flow temp of 45 and return temp 40.

Whats your design FT? You may struggle to find a radiator, other than a large type 33, that will do 1.9kW at 45. I had a similar requirement in my lounge area and ended up with a (very satisfactory) fancoil because a passive radiator with that capacity just wouldn't fit.

This post was modified 2 months ago 2 times by JamesPa

4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.

🧐

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Toodles

(@toodles)

Famed Member Contributor

Joined: 4 years ago

Posts: 2694

25/02/2026 5:53 pm

@jamespa In case it may be of any interest (or help!), our bathroom is ~2m x~2m and 2m high with just one external wall with triple glazed window. The radiator is 600H X 700W K2 COMPACT and the dual fuel towel rail above is 760H X 600W. Our flow temperature was calculated at 50 C. but 40 - 45 is normal for most temperatures in the last three winters. We like an elevated temperature in our ablutting area of ~25 C. or so and this is achieved easily. The door is only shut over in the evenings ahead of shower time but does benefit from solar gain in the day as it faces SSW. Regards, Toodles.

Toodles, heats his home with cold draughts and cooks food with magnets.

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