Selecting radiators for my boiler / BTU calculation question

4 Sep 2007
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United Kingdom
Hi Guys,

I have recently bought a little 2 bedroom mid terraced house. It's quite small and I have arranged for a corgi engineer to come and install a new boiler and radiators. (It only had night storage heaters in it before)

I am going to supply all parts but I need some help selecting the radiators...

I have chosen the Potterton Gold 24HE (24kW / 81000ish BTU) boiler. This should be more than adequate - Which? best buy - Good reviews - seems like a good option.

I have read quite a few things online that you need to do various calculations when choosing radiators and this has just got me really confused.

There are various BTU calculators where you can enter room dimensions, window area, type of room, type of walls, number of walls etc and it comes out with a Watts/BTU required rating for that room. Do you just select a radiator with that output? Is that all there is to it?

As the boiler is 24kW - do I need to spread the 24kW between the radiators for the boiler to work efficiently? :confused: That might be a problem with 4 radiators...

I thought I would use TRVs on all the radiators - is this a good idea?
Do I need to have a room thermostat in the lounge - is this necessary

1) because of building regulations?
2) if I use TRVs?

If I do need one, are wireless ones any good? (I'd rather not have one if I can get away with it.)

Is there anything else I need to think of?


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Firstly do not believe all you read in Which about good boilers!

Vaillant, Worcester or Alpha would be better.

You need to size your combi to your hot water requirements, not the heating requirements. I personally never fit 24Kw combis as in the mid winter you could be ages waiting for a bath to fill.

If my customers have majority showers then I fit a 31Kw combi, if they have nothing but baths then I suggest a 37Kw, as this will fill the bath quicker, as long as you have adequate cold mains flow and pressure in your house.

The heating side of the combi can then be range rated down to minimum which is usually around 9Kw, to make it more efficient.

TRV's should be fitted on all rads except the one in the room where the room thermostat is.

Yes the wireless stats are good and save a lot on installation time.

Most professioanls would use an industry standard rad calculation program such as Stelrad STARS, or the traditional Mears calculator. 5 minute job to size your rads.
Thanks for the reply Dave.

Another bonus with the Potterton is that it comes with a 3 year parts and labout warranty, so they must be pretty confident in the product. A friend has the larger 33kW version and it is great. This one will do 9.8l/min which should be fine.

Where you said,
The heating side of the combi can then be range rated down to mini mum which is usually around 9Kw, to make it more efficient.

Does this mean that you would need to have a radiator setup of at least 9kW or in my case 7.4kW? (In the sales blurb it says it modulates the output min 7.4kW (condensing) 6.8kW (non condensing).)

So I could get any size radiators as long as combined they are over 7.4kW?

Does it matter which side of the radiator the TRV goes? Would I just use a standard valve on the other side?

Sorry about all the questions - I just want to make sure I get it working as best possible.
Just size the rads for each room. The Potterton will modulate down as required and the room stat will turn the ch off on the boiler when temp set is reached.

Traditionally the trv's are fitted on the right of the rads, but this is not set in stone and would depend on access or whether they are subject to drafts etc.

You can buy the trv/locksheild packs as a matching pair.

I think you have covered all the basics needed, any deeper delving and you will just confuse and worry yourself.

Make sure you have the boiler serviced annually and installed correctly and power flushed as otherwise the small print in the warranty may invalidate the 3 year warranty.

If you wanted to spend a bit more why not go for the Baxi Platinum. Same boiler but costs more because Baxi add another 2 years warranty to it.

The 3 and 5 year warranty is just a numbers game to Baxi/Potterton.
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So as long as the radiators are greater than the minimum BTU/kW required in my calculation I have nothing to worry about...?

I'll have a look at the Baxi Platinum too.
Don't worry if the minimum boiler output is greater than the heat output of the rads. The boiler will just turn off sooner via the room stat and/or boiler stat.
Be in no doubt that you are paying for the 3 years, most boilers come with a standard 2 year anyway now. Get a 28kw boiler at least for combi’s bigger is better, 24 is only good if you live by your self

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