Use a combi boiler as system boiler?

henry1 · 7 Jan 2021

Is there any reason not to?
Just cap cold cold water in, hot water out.
There's then the option of getting rid of the hot water cylinder in the future and using it as a proper combi without having to purchase a new boiler.
System boilers are also more expensive, with less inside...

sxturbo · 7 Jan 2021

intergas and Navien have the option for this and can use an unvented cylinder, and i presume a lot of others as they have hot water tank storage options with them, however i dont know if they must have an unvented cylinder or not

ianmcd · 7 Jan 2021

Depends which combi, they are not all the same

henry1 · 7 Jan 2021

I meant just a normal combi. It would be like having a normal combi installed but not using the hot water facility.
Is there any reason you can't do it?

sxturbo · 7 Jan 2021

henry1 said:
I meant just a normal combi. It would be like having a normal combi installed but not using the hot water facility.
Is there any reason you can't do it?

as ian said it depends on the model, what you are asking can be done yes, but depends on the model and possibly manufacturer.

do you want it to connect to your existing unvented cylinder?

maybe the heating engineers on here could suggest some boilers that will suit what your after

ianmcd · 7 Jan 2021

Depends on the length of pipe that you cap off, you might need, a mini expansion vessel fitted on the cold water inlet if you want to do this, abetter idea would be to connect the nearest sink to the boiler Hot and cold and use the rest of the house from your stored HW

henry1 · 7 Jan 2021

ianmcd said:
Depends on the length of pipe that you cap off, you might need, a mini expansion vessel fitted on the cold water inlet if you want to do this, abetter idea would be to connect the nearest sink to the boiler Hot and cold and use the rest of the house from your stored HW

Why is this needed? No water would be going in the boiler, or out, for hot water. The pipes would be empty and not connected to the cold water supply...

henry1 · 7 Jan 2021

This is more of a discussion rather than me wanting it.
I could rephrase my question and ask why isn't everyone using a combi boiler as system boiler when the former is cheaper and more versatile.
There must be a reason why not, otherwise everyone would be doing it...

muggles · 7 Jan 2021

Intergas installers are doing just that - fit the combi, change it to a system boiler in the parameters, and away you go. It could affect the warranty with some other manufacturers though.

ianmcd · 7 Jan 2021

because most combis use the cold water main as the method of HW expansion, if you leave it empty the boiler will keep popping seals and some plastic hydro blocks will split

henry1 · 7 Jan 2021

ianmcd said:
because most combis use the cold water main as the method of HW expansion, if you leave it empty the boiler will keep popping seals and some plastic hydro blocks will split

But no hot water is going to be used.

kidgreen61 · 7 Jan 2021

Depending on the boiler, it may be necessary to have cold connected in order to fill the heating system, obviously there are ways around that though.

ianmcd · 7 Jan 2021

as @muggles says Intergas can be used for this, not many others can

Terrywookfit · 7 Jan 2021

A good few years ago whilst we were on a training day... The Vaillant guy running the course Told us that the system and combi boilers were identical and that it was fine to use a combi as they were much cheaper. He also told us to remove the surplus plate he, diverter and aqua sensor for spares !!! This was the Ecomax !!

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Use a combi boiler as system boiler?

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