# Total allowable radiator BTU for gas Combi Boiler Query

Posted 29th Jan
Property has 7 radiators. 1 newish one and 6 rather old imperial ones.

Could I effectively use 7 radiators each with a BTU of 10,000? As that will be lower than the maximum of the Direct Heat Output Range (32,800 to 81,900 btu/h taken from boiler details below?

I am not going to get 10,000 BTU replacement radiators because two rooms have small radiators. One of which due to its position can only have a Type 11 or Type 21.

Is there a rule of thumb that the total BTU from all radiators in a property should only be _ percentage of the max BTU of the boiler?

Some places give different BTU amounts for the same branded radiator. Due to measuring at a different temperature. How do I know what the maximum BTU is, to ensure I am staying within my boiler's capabilty?

As it is a Combi boiler it will switch from central heating to hot water and vice versa when running.

Boiler Info:

Maximum CH heat input
29.334.2kW
100,000 (Btu/h)

CH heat output range
9.6-24.0 kW
(32,800-81,900) (Btu/h)

Maximum DHW heat input
29.334.2kW(100,000) (Btu/h)

Maximum DHW heat output range
9.6-24.0 kW
32,800-81,900 (Btu/h)

DHW flow rate @ 35 °C rise
10 l/min
Mains water pressure required for max. flow rate
0.8 bar
Minimum water flow rate
2.7 l/min
Mains water pressure required for min flow rate
0.3 bar
Maximum inlet water pressure
10 bar

I am thinking bigger radiators will heat the rooms quicker. Meaning the boiler will have to work for less time as the same amount of water is going around the system.

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Oversized radiators are a good idea as it allows a lower CH water circuit temperature, this saves money and also helps the boiler stay in condensing mode longer.

Using oversized radiators hoping for quicker heat up time by keeping the flow temperature high will result is overshooting of the set point.
Edited by: "paulj48" 29th Jan
They will be quite large at 10000 btu if you want them too heat up quickly it's probably not best idea, or if you only have them on for short periods of time. But yes they system should cope with them.
Edited by: "MynameisM" 29th Jan
@paulj48 and @MynameisM Thanks for replying.

The current radiators heat up but don't seem to make the room rather warm or hot; in the case of the heating system being on for quite a while. The water in the system is must be hot and reached the correct water temperature because the boiler stops heating after a while. The boiler starts again after a bit. Must be when the water temperature in the system drops. Then it stops heating and on again after a while etc. Even if the heating is still on. Actually it seems like the boiler is not heating for most of the time. When the heating has been set to be on for 2 hours.

System was flushed the other year and I bleed the radiators.

I think the existing radiators just aren't up to the task of properly heating the rooms. My plan is to get the metric equivalent size for most. Only to upgrade to Type 21 or Type 22 radiators because most of the existing radiators are Type 11. I would like to think the upgrade should make a large difference overall to heating each room. I will try to fit Type 22 because they can emit the most heat out of the types I have mentioned. Although Type 22 radiators are dearer. It isn't hundreds of pounds per radiator more expensive.

Strangely I have two different Toolstation voucher codes to use. One for £5 off with a minimum spend. The other is £10 off. I am going to do two separate orders and get a couple of Kudox radiators. Hopefully they will arrive dent, chip and scratch free.

The newish radiator makes a big difference than the old radiator. It is a bigger and a Type 22.

Another issue is whether to get a radiator bent for the bay window. It costs quite a lot when I looked a few years ago to get one bent to the right curve. I suppose a straight radiator on deeper brackets could be fitted. Doing that will lose room space. The existing curved radiator has a very small height. Odd, when their is space for a taller radiator.
For anyone thinking of fitting Kudox radiators. The following information on their website is useful. As it gives the dimensions including depths and heat outputs:

kudox.com/dat…ts/
Another consideration if you're doubling the load is are your existing pipes are big enough to run this increase?
paulj4829/01/2020 21:21

Another consideration if you're doubling the load is are your existing …Another consideration if you're doubling the load is are your existing pipes are big enough to run this increase?

I think it's just standard 15mm copper pipping for the Central Heating System. The system will have the same amount of water in it though. As it is a closed system. The level specified by the boiler and recommended by gas safe people carrying out the annual service. It would mean the radiators may need to be balanced again. Especially the new radiators with new lockshields fitted. Currently using Hive and Tado. Partly due to me not gettting around to fitting the Tado Extension box.

My parents had a new boiler installed together with new radiators. The supplier did upgrade one or two rads from what was requested by the installation company. Due to not having them in stock. Their home appears to heat up quickly and can get a lot hotter than mine. Especially when comparing the same amount of time.
thetarget30/01/2020 14:22

I think it's just standard 15mm copper pipping for the Central Heating …I think it's just standard 15mm copper pipping for the Central Heating System. The system will have the same amount of water in it though. As it is a closed system. The level specified by the boiler and recommended by gas safe people carrying out the annual service.

Well if that's the case then you may have issues then, a rule of thumb is 15mm pipe can be fed to 15kw of radiators where as 22mm pipe can carry 40kw.

I doubt you only have 15mm pipe though as there should be 22mm coming out of the Bolier on the flow and return.
Edited by: "paulj48" 30th Jan
paulj4830/01/2020 14:28

Well if that's the case then you may have issues then, a rule of thumb is …Well if that's the case then you may have issues then, a rule of thumb is 15mm pipe can be fed to 15kw of radiators where as 22mm pipe can carry 40kw. I doubt you only have 15mm pupe though as there should be 22mm coming out of the Bolier on the flow and return.

Could well be 22mm pipe. I haven't gone and measured it. Existing boiler seems to be max 29.3kW input and max 24kW output. I certainly haven't had strange looks from the Gas Safe people. They have informed me my ancient boiler is functioning fine and all parts are still available for it. One did suggest the existing lockshields could be a bit iffy and may be causing an issue.