Central heating using log burner - plumbed in wrongly? Hot water going the wrong way?

[rgledhill]

Calling all plumbers, calling all plumbers... I have inherited a weird system in my house! It was put in 30 years ago and consists of a monster log-burner, standard cylinder, standard pump etc. The pump runs at medium speed and I've tried to bleed all the radiators but there seems to be virtually no pressure in the system.

If I turn the pump up to full, I get two things - when I try to bleed the rad nearest the pump, it SUCKS air into the system and pumps the hot water straight up the pressurisation / water supply pipe, into the expansion tank!! I'm guessing this shouldn't happen :-/

I've spent some hours now trying to figure out how it's connected together, and now that I have, I think I can see why it happens. The expansion tank is fitted AFTER the pump, before the radiators, so naturally if the radiators are a bit harder to push water through (e.g. some thermostatic valves have turned off etc) then it's pushing the water back up. Surely this is wrong?

Also there seem to be two circuits - one for heating the cylinder (which I don't need as I have an immersion heater for that) and one for the radiators, but they're connected up in different circuits.

All advice very gratefully received! All I can think of is to put a non-return valve in the expansion pipe, and allow it to expand up the venting pipe (?)...

I'm not a plumber but am an engineer so forgive my lack of experience, I'm just trying to understand what should happen next

I've put a picture below - hopefully this can be scaled up to see how it appears to be connected...

Thanks everyone!
Richard

 

[R1CKY]

Re: Central heating using log burner - plumbed in wrongly? Hot water going the wrong

You can't put any form of valve on the feed or expansion pipe! My knowledge is a little rusty on solid fuel, but if I remember correctly you should have an injector tee where the return from the heating meets the return from the cylinder. Ideally, the flow for the heating and hot water should be off seperate tappings straight from the log burner.

Fitting a non-return valve on the return from the cylinder AFTER the tee from the expansion cistern is an option, but it needs to be specifically for this type of system, anti-gravity valve I think they are called!

 

[ne1ls0n]

Re: Central heating using log burner - plumbed in wrongly? Hot water going the wrong

fit a valve on the expansion and then the vent becomes blocked = ticking timebomb waiting to go off!

 

[snowhead]

Re: Central heating using log burner - plumbed in wrongly? Hot water going the wrong

Don't try and bleed with the pump running.

 

[rgledhill]

Re: Central heating using log burner - plumbed in wrongly? Hot water going the wrong

Right, thanks, no NRV in that pipe then! (Now I explained it out loud, I can see how that would be a bad plan ;-))
Yes, the log burner has two sets of pipes on it but in the previous owner's infinite wisdom (or rather, laziness) they just tied it all together and just use one inlet/outlet.

If I try and bleed without the pump running, there's virtually no pressure in the system so no joy there either, although the radiators seem reasonably full upstairs. (The two downstairs ones dribble a bit from the bleed valve indicating just enough pressure to push out air in those).

In fact, digging a bit deeper, it seems to be basically an open vented central heating system with gravity-fed hot water heating... am I right? Similar to this one (thanks to the diyhowto website for the picture):



But (crucially) it seems like the pump and the radiators are the wrong way round, so to speak. I.e. the pump should pump water INTO the radiators, not suck it OUT of them. So if I can effectively swap the positions of the pump and the radiators in my original diagram... would that work?

Cheers
Richard

 

[R1CKY]

Re: Central heating using log burner - plumbed in wrongly? Hot water going the wrong

Yup, it is pretty much a gravity hot water and pump heating system. It is normal practice to have the pump on the flow, but for some reason on this sort of setup, the pump always seems to be on the return, which seems wrong to me as most of the system is under negative pressure.

I'm sure someone with more experience of this system will be along to explain why it is done like this!

 

[snowhead]

Re: Central heating using log burner - plumbed in wrongly? Hot water going the wrong

You've either got blocked bleed nipples, blocked downstairs radiators or blockages in pipework.

If you only have 2 pipes at the burner you can't have the pump in the flow, unexploded time bomb again.

 

[AWheating]

Re: Central heating using log burner - plumbed in wrongly? Hot water going the wrong

agree the nipples may be blocked up. safety pin time.

 

[rgledhill]

Re: Central heating using log burner - plumbed in wrongly? Hot water going the wrong

Maybe having the pump this way round stops the problem of the hot water in the cylinder convecting round into the radiators even when the pump is off, seeing as it would be pushing its way through the pump the wrong way? Or is that nonsense :-/

It's quite possible something's got blocked as the entire system was drained last week to fit a new radiator, replace some valves with TRVs (all set to max for the purposes of testing) and fix a minor leak at the back of the log burner. Also the expansion tank is open (no lid) and there's a lot of glass fibre insulation nearby and above it; looking at the state of the gunk in the bottom of the tank, I guess some of the fibres have found their way into the system.

Any suggestion how to clear these blockages? Repeated draining? An additive?

Meanwhile the system seems to just about operate ok with the pump at medium speed, with all the radiators getting hot (eventually), but I want to run the pump faster as the log burner is capable of producing more heat than the system can take out of it at the moment.

Thanks the advice everyone, it's really appreciated.
Richard

 

[tamz]

Re: Central heating using log burner - plumbed in wrongly? Hot water going the wrong

With a layout like this the pump needs to be on the return so your system will always be under negative pressure when it is running.
Put it on the flow before the rads and it will draw air constantly. Bleed your radiators with the pump off. It may take a while but it will happen.

You can't do away with the hot water side of the system whether you desire it or not. It is an intrinsic safety design of the system.
The only thing wrong is the heating return should have an injector tee. This is purely to minimise the effect the pump has on the hot water gravity circulation.

Btw radiators on a soild fuel system never heat well unless you are burning the correct fuel at full pelt. They are more of a background heat. Always take manufacturers outputs with a pinch of salt. They are unrealistic for normal use.

Also keep your pump set to the lowest setting it works at.There is no benefit in pushing water through the system faster than it needs but there are a lot of problems if you do.