Combi boiler and on-demand hot water

[DaveC]

Hiya

I've been wondering, to much confusion how the heat exchangers on these boilers work. I've drained my heating circuit, so I could remove a few radiators while I re-did some walls (stripping, skimming and painting).

Lookinga this diagram

can see that the on demand hot water is heated through a secondary heat exchanger, which looks to be fed from the actual water that runs through the radiators in the heating circuit loop.

Since I have no water in my main heating (radiator) loop, is it then a given that the secondary heat exchanger will not function, and the main heat exchanger will simply be heating air?

Right now it's all turned off, and I need to do a bit more work behind radiators, but can't live without hot water. But at this time of year can live without radiators.

I don't have a manual, just moved in the house recently, and there wan't one. Going to get it serviced or looked at first, just to make sure it's all ok. It was working when I moved in, but turned it off at the plug since to make sure that it didn't do anything stupid while I had the radiators off!

Any kind of advice would be greatly appreciated. I'm guessing all combi's use this kind of heat exchanger system?

Cheers again

Dave

[Christian McArdle]

Yes, you need water in the main loop. However, if you anticipate having the radiators drained down for some time, you could always fit a bypass (if one is not fitted) and cap off the radiator circuit. This will allow you to fill the combi up, provided that the expansion vessel, bypass, filling loop, pressure gauge and pressure relief valve are still in circuit.

Christian.

[Ed Sirett]

The over-whelming bulk of combi boilers need the primary side to be pressurized top function. many boilers will refuse to fire without pressure in the primary which is good. Otherwise the boiler will likely suffer a very good chance of being permanaently damaged (unsoldering the main heat exchanger is bad news).

Some Ferroli models _might_ allow you to run with no primary water but again you do run the risk of damage at least in principle.

However many models don't require that radiator circuit is connected up so you can isolate the boiler (there are likely valves near or in the boiler for this), fill it up, bleed the air and use the HW. Do check that this is permitted by the instructions though.

The manuals for many models are available online.