Combi won't provide HW when CH is drained - normal?

[Lobster]

I had to part drain a combi system today (well, depressurise it and make enough space to fit in a litre of inhibitor - see my post of about a week ago!). Unfortunately I find I'm out of PTFE tape which I want to use on the cap on the towel rail which I had to remove; decided to buy some Monday morning and finish the job then; with no CH in the meantime. However, now the boiler won't deliver any HW. Is that normal; should I expect that? Isn't it possible to use HW without CH?

Boiler is a Friendly Format 100E - I have no instructions for the beast though.

Thanks David

[TMC]

Hi David

My combi is a vaillant but I guess the principle would be the same

The central heating part of the system uses water pumped through the heated matrix (not the proper term I'm sure) of the boiler. This water also passes through a heat exchanger to heat the hot water. If there is no water in the heated matrix as you have drained it down then there is nothing to heat the hot water. I would be surprised if the boiler even fired up

Regards

Tony>

>

[Dave Jones]

yep that's normal, They use the central heating circuit but divert it through a heat exchange to heat the hot water.

[raden]

In message , Lobster writes

If the CH pressure (primary circuit) is too low, then yes, the boiler won't fire up at all. The DHW is provided via a secondary (water to water) heat exchanger

[Paul King]

What you *should* have done (hindsight's great isn't it?) is close the CH isolators at the boiler *before* draining/depressurising. That way the boiler stays pressurised and will fire up so that you get DHW. That's what I always did with my Vaillant combi.