Upgrading non-pumped hot water in CH system

[Keith Dunbar]

I have a pretty basic central heating system - oil fired boiler, four pipes - two flow and return to hot water tank (not pumped), two flow and return to radiators (pumped). I also have a woodburner boiler which also has four pipes - again two flow and return to hot water tank (not pumped), two flow and return to radiators (pumped). The woodburner pump is wired to a thermostat that takes the temperature of the return from the hot water tank, so it only comes on when the hot water side is hot enough. So far I haven't discovered how the pipes from the woodburner are connected with the pipes from the oil boiler - don't know if that's relevant or not.

The radiator in the bathroom is plumbed into the hot water circuit, not the general radiator circuit. I feel I need to upgrade so the hot water side of things is pumped - the hot water tank and bathroom radiator take ages to heat and never get very hot.

There is a thermostat on the hot water tank presumably going to the oil boiler but I'm not sure what that controls in a boiler. There is a room thermostat which presumably switches the pump on and off.

I'm not an expert plumber, and the arrangement of the pipes makes me think I would be better installing a pump on the hot water side of the oil boiler and perhaps also on the hot water side of the woodburner boiler.

Any thoughts?

Keith

[dom]

AIUI there needs to be a gravity only (unpumped) circuit on any heating device that isn't electrically controlled (e.g. your woodburner) - in case there's a mains power failure.

[robgraham]

Hi Keith You do need to either trace the whole system out or get someone to do it for you.

The first point as Dom says is that you must not pump a log burning stove as an electrical failure could block the heat path from it. So the log burner has to be gravity fed.

I have a similar set up to yours - oil burner and log stove. The problem with such a system is that if it is not designed correctly the system passes hot water from one heat source through the other which is wasteful and possibly not good for either heat source.

The solution I was given was to use a Dunsley Neutraliser (you can google for a source of those) which can be thought of as a miniature heatbank in that it is a mixer tank for both heat sources. The log burner gravity feeds into it, the oil burner is pumped into it and the CH/HW is pumped out of it with a switch over valve for the HW. So there are two pumps, one two way valve and this little mixer tank. I have appropriate thermostats and timers to switch the system and shut off the oil burner when the log stove is hot.

You could of course just go to the full size heatbank configuration which can be viewed in the DIYWiki where the HW tank takes the place of the mixer tank I have - I plan to go that way sometime as it would get over my lack of head for the HW system.

Anyway that is my solution - it may well be that there are others.

I can supply diagrams if you want.

Rob

[Keith Dunbar]

Ah - good point. Hadn't thought of that.

Mind you, the bathroom radiator barely heats up when the heating's on - though I guess it's probably pretty sludged up.

[Keith Dunbar]

Thanks for the offer, but I'm wondering if I need to do all that -we generally tend to have the boiler switched off when the woodburner is on.

I realise I've mixed up two issues here. My main question was really whether it's OK for me to get pumped hot water from the boiler by fitting a pump to the hot tank feed from the boiler rather than messing about with the pipe work and putting some sort of motorised valve in?

Keith

[The Natural Philosopher]

err. not strictly true. You CAN pump it as long as there is an open gravity fed loop *as well*.

[robgraham]

Keith It doesn't matter if you have the oil burner on or off - if the system isn't properly configured hot water from one source will be circulating through the other source which is not good design practice. If you put a pump in the H/W feed from the oil burner, how are you going to stop circulation through the wood burner when it is cold ? That is what the Neutraliser is all about.

You also should realise that a motorised valve in the gravity system isn't on either as it could end up in the shut-off position when the wood burner is on.

Rob