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Buddy Posted on 11/6 12:31
FAO gas boiler experts

Three questions:

1. At what point does a gas boiler heat the water? I presume it is on the way into the tank? So if I run off all the hot water, presumably the boiler won't be able to heat the new stuff unless I use the immersion first? (That's all one question by the way)

2. This is actually another sub-question from (1) I suppose - which heats a tank of water faster, immersion or gas boiler?

3. What might the problem be if the "hot water" function on my boiler is heating the radiators?

trodbitch Posted on 11/6 12:34
re: FAO gas boiler experts

Ooooh quiz!

I think the answer to part 3 is that you have a valve that switches/routes the hot water from the boiler between the heating and the hot water but it is not switching.

Buddy Posted on 11/6 12:37
re: FAO gas boiler experts

Would that suggest that it might not be heating the water at all? Only I can't run that experiment without the answer to (1).....

trodbitch Posted on 11/6 12:40
re: FAO gas boiler experts

Disclaimer: this could be wrong.

If your rads are heating up when you have the hot water on, the boiler is heating the water and it is being pumped around the heating system. I suspect the immersion is not even being used as that water does not even go anywhere near the tank. What should be happening is the water is heated and stored into the tank with your immersion being used to give you a boost as and when required.

It is possible, your hot water is all going into the central heating and only the immersion is being used to heat up the tank.

I think I'll sensibly stop there as its probably a case of the blind leading the blind!

Jesus_Swept Posted on 11/6 13:10
re: FAO gas boiler experts

Buddy, I have exactly the same problem with my system.
Up until a few weeks ago, all we used to have switched on was the central heating. This heated the rads and gave us all the hot water(how, I don't know).
Since we had a problem with leccy and our lass switched the whole system off, we no longer get hot water from the CH. If we put hot water on , all of the rads a scalding hot. Switched to using the immersion instead but not sure I should be using this all the time.
Now I sound like Jim Royle, going round telling out lass to turn the immersion off !!

Buddy Posted on 11/6 13:18
re: FAO gas boiler experts

That was my thinking trod.

I worked out it was cheaper to run the immersion for an hour than the boiler, something like 36p per hour for the boiler at full burn and 30p for the immersion, based on the rate at which each meter was going round. If that's the case then I should only really need to use the boiler if I want heating, UNLESS the boiler heats a tank-full 20% quicker than the immersion does.

I'm sure there used to be some people on here who knew about this stuff.

trodbitch Posted on 11/6 13:42
re: FAO gas boiler experts

Well chboro is the expert on this. He works a lot longer hours though. He'll be around tonight and I'll hoof it for him, see what he says.

Buddy Posted on 11/6 14:47
re: FAO gas boiler experts

Much appreciated.

chboro Posted on 11/6 19:03
re: FAO gas boiler experts

1. At what point does a gas boiler heat the water? I presume it is on the way into the tank? So if I run off all the hot water, presumably the boiler won't be able to heat the new stuff unless I use the immersion first? (That's all one question by the way)

The water is heated through the pipes that go through your cylinder (hot water tank) - flow and return like a radiator, but it goes through a coil of pipes inside the cylinder If you run off all the hot water you don't need to use the immersion - the heated water rises to the top of the cylinder when heated through the pipes from the boiler.

2. This is actually another sub-question from (1) I suppose - which heats a tank of water faster, immersion or gas boiler?

Your immersion will give you a more direct heat so would heat it a little quicker for a small amount, but the hot water from the boiler heats from the bottom so you would get a full cylinder hot quicker from the boiler side unless you have a really long immersion heater.

3. What might the problem be if the "hot water" function on my boiler is heating the radiators?

Either bad design of system, or the valve or valve head is bugger - sometimes it is just the motor but that depends on what make it is.


I worked out it was cheaper to run the immersion for an hour than the boiler, something like 36p per hour for the boiler at full burn and 30p for the immersion, based on the rate at which each meter was going round. If that's the case then I should only really need to use the boiler if I want heating, UNLESS the boiler heats a tank-full 20% quicker than the immersion does.


You haven't taken into account the fact that the boiler modulates - the gas rate lessens as the boiler gets warm so it isn't running at full rate for the full hour.

trodbitch Posted on 11/6 20:18
re: FAO gas boiler experts

I didn't even hoof this! How does he know?

trodbitch Posted on 11/6 20:20
re: FAO gas boiler experts

"You haven't taken into account the fact that the boiler modulates - the gas rate lessens as the boiler gets warm so it isn't running at full rate for the full hour."

I'm impressed. You geek.

chboro Posted on 11/6 20:49
re: FAO gas boiler experts


Buddy Posted on 12/6 9:13
re: FAO gas boiler experts

Excellent work. Final question then: if I turn all the radiators off individually, but leave the central heating set to "on" (not through choice but because as mentioned it's buggered), will the central heating system use any gas at all, or will all my gas usage be attributable to the hot water?

chboro Posted on 12/6 17:43
re: FAO gas boiler experts

If your stat is down, and all the rads are turned off individually then the gas used should only be for heating the water, but the pipe may get hot but not the rads depending how it is piped. Not enough to worry about.

Why don't you just get it fixed?

chboro Posted on 13/6 9:43
re: FAO gas boiler experts

Any more questions while I am around?

Buddy Posted on 13/6 9:53
re: FAO gas boiler experts

Nope, that should cover it, thank you for your assistance.

The landlord can fix it when he likes, it was more questions 1 and 2 I was interested in really, but I have to worry about the radiators as I can't just switch the heating off.

chboro Posted on 13/6 10:02
re: FAO gas boiler experts

The landlord has to fix things within a time scale after faults have been reported if they have anything to do with water, gas and electric nowadays I think, may be worth getting onto him since it's been costing you extra money on electric and gas due to the fault.