28 September 2021
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Low temperature radiators


Prunus
(@prunus)
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Joined: 6 months ago
Posts: 17
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I've put the room heat loss numbers I generated from the MCS Heat Pump spreadsheet into the 'Domestic RHI SPF Calculator' spreadsheet which has a radiator sizing tab... and all our radiators are too small. This is not a surprise as they're all single-panel from 1970s.

When upgrading the radiators, I can obviously go for type 22 (double panel) or 33 (triple panel), but there's a fairly brutal derating factor. The radiator sizing spreadsheet gives a number for derating the advertised heat output of a Stelrad radiator of 0.3 - in other words, you get 30% of the rated output. Given we're taking radiators designed to run at deltaT of 50C (average water temp 70C) and running them at deltaT of 20C (water temp 40C) this isn't very surprising.

So I wondered, are there any radiator designs which are better at emitting heat at low temperature? There are fan assisted radiators (which I might start another post about) but let's consider passive radiators for now. I could imagine you could do different things with the fins, channels etc to design for a lower temperature.

Any models that are worth looking at?


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Kev M
(@kev-m)
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Joined: 7 months ago
Posts: 226
 

We have standard Stelrad K2s and they work well at a maximum input temp of 50.  The fins make them emit heat/air out the top instead of the front.  They are a bit bigger than boiler hesting systems but not excessively so.  The K3s emit more heat per unit area but are thicker and also quite a bit more expensive. Have you tried running your existing rads at 50 degrees?


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Prunus
(@prunus)
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Joined: 6 months ago
Posts: 17
Topic starter  

@kev-m I haven't tried running rads at 50C as the old oil boiler is very unhappy if the output temp is set below 70C. I'm just curious whether there are rads that are better designed for low temps than simply oversizing conventional rads. If I'm doing a replacement anyway I would like to see if I can budget for a flow of more like 40-45C to improve the COP.


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