Boilers in tandem replacement question

15 Aug 2012
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United Kingdom
I've just moved into a fairly old and big house. The current setup is as follows:

Two Viesman ESS boilers running in tandem to heat up 22 radiators. About 70% of the radiators are double, with the rest being a mix of single or small doubles. Two thermostats control the heating. One is for upstairs and the other is downstairs. Rarely will all radiators be on at the same time (only in Winter most probably and only for a few hours). There's also an unvented water heater.

One of the boilers is totally gone. Totally corroded and I've been advised to replace both as they are fairly old. I've been informed that the piping etc is in excellent condition, so it should be a case of boiler replacement only. I've also been advised to go for a stainless steel replacement as these last longer.

I assume I only need a system boiler and not a combi.

Do I go for a single large capacity boiler or should I stick to having two boilers in tandem? Can anyone recommend what boilers to go for? I need boilers that will heat up the radiators quickly and are suitable for big rooms.
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I've been advised to replace both as they are fairly old. I've been informed that the piping etc is in excellent condition

Who gave you advice and information?
Use an online boiler sizing calculator like

A rough guide is usually 1.5kw per radiator so 35kw for 20 radiators.
One fairly large boiler will happily run it.

However with 22 radiators balancing may be a problem.
I would suggest depending on the piping two pumps to drive separate
sections of the house.

A boiler with weather compensation will also help.

My choice would be an Atag.
I'd go for just one.

Two would be more expensive and use twice as much gas and two to go wrong.
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thats a pretty easy diagnosis...piping if correctly sized and installed will out last most of us...

john cleese and his chronies excelled on this one..
Is it a system boiler or a conventional boiler that goes with an unvented water heater?

Edit: or heat only?
you can replace with 1 larger boiler or replace 2 smaller boilers the advantage of having 2 is that if 1 fails you still have 1 working so can still get some heat and hot water the downside you have 2 to maintain choice is yours
Do you really heat all your rads all the time? Is your gas bill about £3000 ?

If it was my home then I would fit two boilers because its inefficient to have an over rated boiler for most of the year when just one would be adequate.

The Viessmann 100 is a reasonable little boiler as is the Intergas and both have a five year warranty with the right installer.

Not all of them are heated. Only in winter will there be an overlap where both upstairs and downstairs rads are on.

Don't boilers step so they are only using gas as per the load required?
They do modulate to a degree.

But only down to about 33% of the maximum on most models. Then they have to control the heat output by on/off modulation which is relatively inefficient.

So I would fit two in my own property as I like to try to make things as efficient as possible.

Your actual boiler output requirements are probably no more than about 15 kW most of the time. As long as you start each zone at different times you will only need maximum power if you are heating all 22 rads in the -1 C outside weather.

Two 18 kW boilers will be my choice!

What is the power output of your current boilers? How well does just one manage?

They're 24kW I think. Presently, one is managing just fine as we only have either upstairs or downstairs switched on.

Do both boilers have to be the same make and model? I'm thinking of replacing the broken down one only and using the new one as the master.

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