Combi boilers - how is the mains water heated

[Tim Watts]

Idle wondering as I'm looking at combi boilers and have little working experience of such beasts...

Do the use a secondary plate exchanger running the sealed system HW through the plate X or do some run the mains water (to the taps) through the main exchanger in a separate water channel?

This is leading to:

Can you safely operate a combi with the mains water turned off - assuming the radiator system is pressurised correctly - ie if you leave the house for a couple of weeks in winter?

Never thought about it before - but if the HW is heated in the main exchanger, and you turn the water off and lets assume a tap is on and drains that side dry - would seem to be potentially bad for the boiler?

[Andrew Gabriel]

Yes.

No.

Yes.

It doesn't work that way, so you are OK.

[Tim Watts]

Thanks Andrew - that is really useful :)

[Andy Wade]

With one exception, namely the Intergas design - which seems to have a lot going for it:

[John Rumm]

IME the vast majority do the former. A flow switch (sometimes a diaphragm switch) detects flow through the DHW and switches a diversion valve to short circuit the primary water through the PHE. However I seem to recall there were a few that used the main HE instead.

Yes.

There would be no flow on the DHW, so nothing to trigger the switch to the PHE. Even if it did switch, all that would happen in the boiler would reach its limit stat on the primary circuit and cycle off.

I thought you had already selected a system boiler that you wanted to fit?

[Tim Watts]

Change of mind :) Having seen combis in action at friends houses, I decided there was mileage in the relative simplicity (simpler external, not simpler boiler).

[John Rumm]

Yup there is something to be said for that. Go for the biggest f.off power output you can find for the DHW side, and then they are ok. Its what I fitted at my last place and it was an upgrade on the crap gravity system it replaced.

I toyed with using one here as well, heating an unvented cylinder on the CH side for baths / showers, but having a nice short run to the kitchen from the hot water side. In the end I could not find one that that could do the combination of things I needed (weather compensation / split temperature operation), so went with a system boiler.

Having had both, I would go unvented if you have the space and a good cold water supply (pressure and flow rate) since the performance is way in excess of what any (non storage) combi will achieve.