Advice on ball park radiator sizes please

marknw · 2023-08-08T22:54:39Z

Hi We're about to have an ASHP and rads fitted, and I'm concerned that the rad sizes recommended are excessive. Obviously it's hard to give definite advice without seeing the property, but if anyone has any thoughts I'd be grateful... The house is a 3-storey mid-victorian mid-terrace in the North Pennines, floor area in the order of 150m². Main rooms around 4m x 4.2m, 2.8m high ceilings. Stone built, we've added 100mm Kingspan internal insulation to all external walls and to the roof spaces, in addition to the usual roof insulation; as well as insulating under the ground floor. New triple glazing throughout. We're going for underfloor heating for the ground floor, new rads for the first and second floors. Installer has recommended a 8.5Kw Mitsubishi ASHP and a 210l DHW cylinder, which I presume will be adequate? But the suggested rad sizes (calculated for a 45℃ flow temperature) look very big to me: A quick play with thiswhich I found elsewhere on this forum (sorry - don't remember where) seems to suggest that these are about 50% too big, but I freely admit I don't really know what I'm doing, so I may have missed something... Could anyone comment on these rad sizes please? And also advise, if we install oversized rads will that have adverse consequences (apart from losing precious wall space!)? Many thanks

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Rusty

(@rusty)

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10/08/2023 2:08 pm

Posted by: @derek-m

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I'm afraid that I do not follow your logic, since heat loss is heat loss and is not dictated by the radiator size or type. Different size or type of radiators will merely require a higher or lower water temperature so that the thermal energy output matches the present heat loss.

Sorry, I obviously didn’t explain that very well, and wrong to boot. As you say, the room/house heat loss is fixed and determined by the fabric and outside temperature, etc. the radiators would then be chosen to provide that output at the desired flow temperature and flow rate, again as you state. I was speculating that the values might be for the standard 50C but without checking the actual radiator specifications and so misled myself on the figures, the Stelrad k3 700x2000 for instance looks like it outputs 5424W at delta-t 50 according to the data sheet, which is considerably higher than the values given. Please ignore my mistaken intrusion.

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marknw

(@marknw)

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Posts: 6

Topic starter 17/08/2023 7:05 pm

Thank you all - so grateful for your advice.

I went back to the company, and they forwarded my query to another branch (the original person being away for 3 weeks). The person there confirmed there was an error - apparently the original guy had read the wrong column of figures or something...

So I now have a revised set of sizes, which are much closer to what I anticipated:

Room	Watts required	Suggested radiator	Radiator type

Living room				UFH
Shower room
Kitchen
Dining hall
Hall
Landing (1st floor)	700	600x600	K2
Study	1050	600x600	K2
Sitting room	1600	600x1000	K2
Bed 1	1500	600x1000	K2
Bed 2	1600	600x1000	K2
Bed 3	1550	600x1000	K2
Landing (2nd floor	700	600x600	K2

Somewhat different to the originals!

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Derek M

(@derek-m)

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Posts: 4427

17/08/2023 7:36 pm

@marknw

I would suggest that you ask the company to provide a full set of heat loss calculations for your home. Also ask them to clarify what is meant by 'Watts required' in the table, and to confirm their calculated Leaving Water Temperature (LWT) coming from the heat pump.

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