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PigletJohn, help needed re boiler capacity and BTU panic

(4 Posts)
ChunkyHare Thu 21-Jul-16 18:16:46

We are having a new boiler and radiators replaced in our house very soon.

The company doing it came out, measured everything up and recommended a Worcester 24i boiler. All good.

But, we are buying and supplying our own towel radiators for the bathroom and en-suite. The BTU calculator suggests 1600-1900 BTU but due to wanting to hang 2 bath sheets on it daily we needed a tall one. These are coming out with much higher BTUs namely 2914. Each.

I have a 13 and 10 year old boys who smell shower daily and so we are having dual fuel towel radiators which require 600w elements to dry the towels in the summer.

Is the increase in watts for the central heating going to be an issue for the new boiler? I can roughly work out radiators in watts and I think my hot water tank is 210 litres because it is a 4 bed house with a bathroom and an en-suite but only 1 shower at the moment (electric shower will be fitted into the bathroom shortly)

Do you think I need to ring the company and tell them that the towel radiators are of a higher BTU than they would have imagined or would they have calculated a margin of need that far exceed the maximum output of the boiler?

It would be around 600-700 watts higher. blush Damn those lovely towel radiators.

PigletJohn Thu 21-Jul-16 19:41:57

you probably mean something like this which has a 24kW output (it can be adjusted downward at installation if necessary) which is ample for quite a large old house. It is unlikely that your house needs more. It is quite unusual for a modern heating system to run flat out even in very cold weather. Modern boilers modulate the flame size and fan speed according to heat demand. An extra 600W here or there will not be noticed.

For example I have a 24kW boiler and a hw cylinder, the calculated heat loss of the house is actually about 15kW when freezing outside, so the boiler mostly ticks along quietly and rarely turns itself to full power. I have fitted oversized radiators so it can heat the house faster from cold if it has been empty all week, though with a modern programmable thermostat it is less necessary.

Of course you will be having TRVs on all your radiators except the one in the room with the wall stat, so none of the radiators will draw more heat than is needed to maintain the room at a comfortable temperature, even if the rads are oversized.

For interest, calculate the nominal heat demand of your house here
Unless it is large and old, I'm sure 24kW will be plenty.

BTW 100,000 BTU is round about 30kW (near enough for your estimates)

BTW2, if your cylinder is adjacent to one or more of the bathrooms, you can have the pipework plumbed so that the towel rail(s) heat up during and after every bath or shower (i.e. whenever the cylinder is being reheated) which is very convenient. The installers will know how to do it but might grumble about extra work and charge extra. Only very posh people use this method.

PigletJohn Thu 21-Jul-16 19:57:53


The SI has been developed since 1875, and a generally agreed version published in in 1960, so unless you live in a backward part of the world (Burma, Liberia, or the United States) your plumbers should have been using it since 1980 or earlier. The only reason I can see for using BTUs is that the numbers are bigger so sound more impressive than kW.

ChunkyHare Thu 21-Jul-16 21:26:48

Thank you so much for this it really helps.

It was a worry as I have 15 radiators and didn't want to kill a brand new boiler with my ridiculous oooh so pretty towel radiators grin which are replacing 2 of the 15 radiators.

The towel radiators break it down into watts and BTUs at 50 and 60 degrees but I had no idea what temperature the water in the radiators runs at.

I have seen a video on YouTube of a chap who plumbed his towel rail the way you describe which we did think was genius. Sadly I need that wall to put the shower on as that is where the bath is.

Again, thank you so much for your help.

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