Plumbing wood burner to hot water tank

[Annpan]

OK... this has now become the official saga of JohnM and annpan.

We are having our wood burner (a 14kw clearview with baffle boiler - 40000btus?) fitted on Thursday however I climbed up to the loft today to have a look at the hot water tank and it looks very odd indeed. It is old, looks abit like an oil barrel and the top 3rd is seperated from the rest of it, their was a lid that lifted right off and all I could see was the over flow, the ball-cock with tap attached and a pipe that came out of the lower section of the tank and curved round and down again (not touching the surface of the water)... it looked like some kind of percolation device. We do not have a cold water tank.

Anyway we had thought about using the same tank as we have run out of money but now it looks as though we will need a new tank. We are not opposed to plumbing in a new tank ourselves but are keen to not waste money or time.

We have been looking at accumulator tanks, I am not sure if we could install one ourselves or if it would be a specialist plumber thingy... I hate all tradesmen at the moment, not content on taking several days to do a few hours of work, it seems that some of them enjoy pissing off for a few weeks too

Any thoughts?

Sorry for all the long posts, as I said it is a saga.

[Muddypause]

What you have is a hot water tank with its own built in cold water header tank.

They are not very common, and are usually used in situations where there is no cold tank and space is limited.

The mains cold water fills the top part of the tank and is controlled by the ball valve; it has a pipe leading from the bottom of the header to somewhere near the bottom of the main (hot) tank (this may be hidden inside). Hot water to the taps should come from near the top of the bottom part of the tank, IYSWIM. There may also be a small electric immersion element inserted near the bottom.

The pipe you see that "curved round and down again" - do you mean it curves back and overhangs the top (cold) tank? This would be the vent pipe; it is there to allow the hot water to expand, and if necessary vent safely into the cold tank. If you had a conventional cold tank which supplied a hot tank, the hot tank would still have this pipe going back up and overhanging the cold tank, and may be linked to the pipe that leads to the hot taps.

This sort of tank is probably obsolete by now, superseded by things like combi-boilers, or mains pressure hot water systems. But providing it is still sound, and not too caked up with limescale, it should still work as intended. All the ones I've seen have no insulation, so it would be a good idea to lag it as much as possible, even with old blankets, or what have you.

I'm guessing an 'accumulator' tank is what I know as a 'thermal store', and is quite an interesting way of storing hot water for both radiators and taps in the same tank. They differ from conventional hot tanks inasmuch as the main body of water is heated by the boiler (and circulates through any radiators), and water to the hot taps passes through a coil in the tank, where it picks up heat from the stored body of hot water. This means mains pressure (and potable) hot water, amongst other advantages. Disadvantages include not being suitable for use with a condensing boiler, but I guess that wouldn't affect you.

I think the only issue with installing a thermal store that would (technically) prevent you from doing the work yourself would be if the tank forms part of an unvented system (a vented system would have a cold water header tank involved somewhere). You would also need to understand how they differ from a conventional tank when you come to connect it. Any boiler installation (including back-boilers) also has to conform with current building regs.

[johnM]

Thanks Muddy, I'm AnnPan's OH.

Eventually moving to the new unventilated Accumulator / Thermal store tank is probably still the plan but would it be likely that we could connect our existing tank to the new stove and boiler?

Not sure how relevent it is but the new boiler is 27,000 BTUs, the existing tank is maybe around 80 litres.

Thanks again for your help, we've found a few bits and pieces online but it's best to hear it from someone.

[Jack]

Gidday

John, whilst I don't understand about some of the things up top, I do know that I have had al sorts of arguments with plumbers about how a wood burner should be hooked to the hot water storage, what ever sort it is.

The main thing to remember is that hot water will always rise so the hot water from your fire should go into the top of the storage, and return to the wood burner from the bottom. Many plumbers out here want to put the hot water into the tank only about a third the way up. If this is done, the water then has to heat the entire volume of water in the top two thirds of the starage, and therefore take longer to give you hot water from scratch. If it takes longer it must be less efficient.

I have plumbed up a wood burner to a hot water tank and managed to get water hot enough for a shower in 15 minutes from dead cold.

[Muddypause]

...would it be likely that we could connect our existing tank to the new stove and boiler?

From this distance, I don't see why not, though obviously it's hard to be definitive over a forum. I'm assuming that there was some sort of boiler heating it before now, and so the new stove will simply replace that.

But the lack of any other cold water tank makes me wonder if it is a 'direct' system - that is to say that the water in the tank is heated directly by circulating through the boiler, and the same water is what comes out of the taps. This would be pretty archaic, though was probably quite common at one time. You shouldn't do this if you are running radiators from the same boiler, and it can lead to furring problems in hard water areas.

More conventionally these days, the water that is heated by the boiler circulates through a heat exchanger in the tank, where it gives up its heat to the main body of water there, before circulating back to the boiler again. This is a separate loop of water (the 'primary circuit') and is not the water that comes out of the taps. I doubt you have an unvented primary circuit as this has only been allowed in the past five-ish years in this country. A vented primary circuit would have its own small header tank and vent pipe of some sort.

However, direct or indirect, as long as the circuit that flows through the boiler is properly vented, there is little risk of anything terrible happening. I have limited experience of wood burning stoves, and none at all of ones with a back boiler in them, but I guess there must be a way to control the heat output of the boiler, so that the water doesn't just get hotter and hotter until it boils. Obviously with a gas boiler you can just shut the gas burners down, but I'm not sure what you do with a wood stove. Presumably there will be some sort of installation guide and user's manual with it to give you the low-down.