"Oversizing" radiators

[Ant1]

I have a boiler than can provide an average of 45000 btus and planning to replace my radiators in the next couple of weeks but only plan to replace the boiler in a couple of years.

I would like to upgrade my 6 radiators to 8000 btus each, this mean I would actually require approximately 63000 btus (48000 for the radiators + 15000 for the hot water) which the actual boiler cannot provide. Using TRVs half way through on each radiators, will the radiators be able to get 4000 btus each from the boiler?

Thanks for your replies,
Ant

 

[doitall]

Don't worry about it you will be surprised how hot they get.

A boiler that has to work for a living is better than one that ticks over.

 

[kevplumb]

only difference you MIGHT notice is it takes a bit longer to reach temp

and don't forget once the water is up to temp you gain that bit extra output

 

[ChrisR]

It's all a lot more imprecise than that really.
Bigger rads mean condensing boilers are happier,
HW cyllinders are a much bigger heat load when they're cold than when they're hot, and modern ones can be 25kW.
If a bigger rad extends the full length of a window, then you get fewer draughts.
etc
etc

 

[D_Hailsham]

Bigger rads mean condensing boilers are happier

We don't know that the OP has a condensing boiler. If he is planning to replace the boiler, the chances are that his current boiler is not condensing.

I would like to upgrade my 6 radiators to 8000 btus each, this mean I would actually require approximately 63000 btus (48000 for the radiators + 15000 for the hot water) which the actual boiler cannot provide.

As the boiler output is less than the radiator size the water will never reach max temperature, so the rad output will be slightly lower than specified.

As for the HW load, the easiest way to deal with this is to heat up the HW at times of low CH demand; i.e set the HW timer to come on well before the CH. After all, the only time you would need the full 63000 btus is if you were heating the house from cold (-1C) and heating a tank of water from cold.

Using TRVs half way through on each radiators, will the radiators be able to get 4000 btus each from the boiler?

What do you mean by the bit I have put in bold?

 

[Ant1]

Thanks for your reply D_Hailsham.

You are correct saying my boiler is not a condensing boiler, I have a Baxi Bermuda 552 back boiler (yeah I know, it is an old beast which works pretty well, specially since I had to replace the valve when it broke down on Christmas).

By "half way through" I meant using the TRVs between setting 2 and 3 (ie 2.5) for TRVs with a maximum setting of 5.


I downloaded the manual which I found from http://85.189.44.185/Baxi/System/index.htm (pretty good site by the way). It says that my boiler can provide from 10 kW (34,000 Btu/h) up to 16 kW (55,000 Btu/h) so I guess 45,000 Btu/h is the average for this boiler.

As the boiler output is less than the radiator size the water will never reach max temperature

I understand that my radiators will not be able to provide 8,000 btus, but can you confirm that setting the TRVs between setting 2 and 3 will allow the raditors provide 4,000 btus each (which is what my current radiators are providing).

Thx for your help,
Ant

 

[kevplumb]

the heat output is determined by the heat input check the data plate you might have more to play with than you think

and i'm not gonna tell you how to do it you need a competent fitter

the norm is i seem to remember 35000 to 55000 depends on how its set up

most bbu s are left on full tilt BTW

edit you already said it (knewi'd seen it somewhere )

 

[D_Hailsham]

I understand that my radiators will not be able to provide 8,000 btus, but can you confirm that setting the TRVs between setting 2 and 3 will allow the raditors provide 4,000 btus each (which is what my current radiators are providing).

The setting of the TRV does not influence the heat output of the radiator.

A TRV is nothing more than a switch which closes when the temperature of the air flowing over the head of the TRV has reached the set temperature. If the air temperature is below the desired temperature, the valve is fully open. When the set temperature is reached the valve closes, water flow through the rad stops and the rad starts to cool down. If the air temp drops a few degrees, the valve will open and water will flow through the rad again.

The heat output of a radiator is determined by three factors (apart from size, construction etc): Temperature of water entering rad; Temperature of water leaving rad; Temperature of the air surrounding the rad.

 

[Ant1]

Thanks everyone for your comments, it makes things much clearer.

most bbu s are left on full tilt BTW

You are correct kevplumb, the boiler manual stipulates that it has been set to the maximum, ie 55000 btus

A TRV is nothing more than a switch which closes when the temperature of the air flowing over the head of the TRV has reached the set temperature.

Thx for the clarification D_Hailsham. Just one suggestion, as TRVs work with the air temperature, wouldn't it make more sense to use temperature as a scale on TRVs rather than a set of numbers from 1 to 5?

 

[D_Hailsham]

As TRVs work with the air temperature, wouldn't it make more sense to use temperature as a scale on TRVs rather than a set of numbers from 1 to 5?

In theory you are correct.

But I think numbers are used to protect the manufacturers. In a laboratory situation they will be able to guarantee that on a particular setting the TRV opens at, say 19°C and closes at 20°C. But in actual conditions things may be different. There could be furniture in the way of the rad draughts etc. All they will say is that, in general a particular number should give a room temperature of X°C.

You can just imagine the number of Sale of Goods Act court cases if they put actual temperatures.