Solid fuel gravity/pumped CH circulation problems

[Oldhouse]

Hello - hope someone can help me with this query...

Bought an older house that came with a solid-fuel Rayburn as the heat source. The boiler only has two tappings, but there is a gravity DHW loop and a pumped CH loop.

When we first arrived, there was no injector tee on the system so when the CH pump was operating (it's on the return side of the boiler), it would drive the water backwards through the DHW loop and cool the cylinder. Having installed the injector tee, only the rads nearest the boiler on the 15mm CH loop will warm, and the 28mm DHW loop seems to receive the vast majority of the energy from the boiler and circulation from the pump.

There seems to be much more back-pressure from the CH circuit, making the pump 'choose' to circulate mainly through the DHW loop, and fail to drive the water all round the CH circuit.

FYI: the cold water feed tees off the return side of the cylinder coil, and the expansion tube tees off the feed side of the cylinder coil.

Is it possible to install a zone valve that isolates the DHW loop when the cylinder is warm enough? My worries are:

a) Can I install it without putting a valve or pump in the cold-water feed or expansion paths?

b) I've seen a diagram showing a valve between the feed side of the cylinder coil and the expansion tee. If that valve was closed, wouldn't the pump then drive the hot water into the expansion tank?

Any help gratefully received...

 

[Norcon]

http://www.rayburn-web.co.uk/raytech/chtg2.htm

Looking at Rayburns schematic and I assume the system is installed this way, I see no reason why a zone valve could not be fitted on the branch pipe to the cylinder.
The main priorty is that the venting and heat leak rad arrangements are in place.

 

[Oldhouse]

http://www.rayburn-web.co.uk/raytech/chtg2.htm

Looking at Rayburns schematic and I assume the system is installed this way, I see no reason why a zone valve could not be fitted on the branch pipe to the cylinder.
The main priorty is that the venting and heat leak rad arrangements are in place.

Thanks Norcon.

Yes, I've looked at that diagram quite a few times myself! I just wonder what might happen at the tee just above the high-limit stat on that diagram when the zone valve on the cylinder branch pipe is closed. If the back-pressure from the CH loop is very high, might the pump just drive all the water up and into the expansion tank? Or am I just being over-cautious?

 

[Norcon]

If you have a good head height on the vent pipe I don't think you will have a problem.
You could try fitting a balancing valve to the cylinder first and see what happens.

 

[Oldhouse]

Thanks for that - very useful.

And I guess I can easily test whether I've got sufficient head by simply temporarily capping the feed to the cylinder.

Solved. Cheers.

 

[Norcon]

Just looking at the schematic again (which may be different to your system) I think its important to remember that the injector T "induces" flow around the cylinder circuit due to the "venturi" priniciple back at the injector T.
Fitting a zone valve will only have the effect of controlling the temperature in the cylinder.

 

[Oldhouse]

Just looking at the schematic again (which may be different to your system) I think its important to remember that the injector T "induces" flow around the cylinder circuit due to the "venturi" priniciple back at the injector T.
Fitting a zone valve will only have the effect of controlling the temperature in the cylinder.

Our set-up's broadly similar to that shown in the diagram. I get the idea behind the injector tee, and since I fitted one, the DHW loop works well when the pump is operating, as well as by convection when it's not. And as I understand it, it's when the pump is operating that the injector tee becomes necessary - largely because you have to have the pump on the return side of the boiler so it's doesn't block the expansion path.

However, where our system does differ slightly from the schematic is that the feed exiting the boiler goes into some kind of ancient cast iron flow splitter tee that channels about two-thirds of the output flow to the DHW loop and one-third to the CH loop. So when the pump is operating, the DHW circulation is no longer powered solely by convection - it's actually pumped. Since I fitted the injector tee, it does at least circulate in the right direction.

But with this kind of system, I'm sure it would still be pumped when the pump was running to some extent, even if the DHW loop simply tee'ed off the main feed from the boiler; because it's likely that the back pressure will always be low enough that some of the pumped flow will take that path.

The idea of fitting the zone valve is to prevent the flow through the cylinder when it's up to temperature, and hence force the pump to more effectively push the water around the CH loop.

If that all makes any sense...??

 

[Norcon]

Have you anything like this .......

http://www.oldhouseweb.com/how-to-advice/gravity-hot-water-heating.shtml

or this ............
http://www.oldhouseweb.com/how-to-advice/gravity-hot-water-heating-continued.shtml

 

[Oldhouse]

Have you anything like this .......

http://www.oldhouseweb.com/how-to-advice/gravity-hot-water-heating.shtml

or this ............
http://www.oldhouseweb.com/how-to-advice/gravity-hot-water-heating-continued.shtml[/QUOTE]


No - it's not really that kind of system. Though I have had a good read-through of all that stuff and it's very enlightening.

 

[Oldhouse]

The system is very similar to the schematic on the Rayburn site you mentioned initially, but I reckon the pipework on the DHW loop and the CH loop are poorly matched to work together.

To give you an idea of what we're up against, the bloke who initially 'installed' the system had a second woodburner randomly plumbed into the CH circuit with no control or neutraliser whatsoever. I decoupled that fairly sharp'ish when we arrived, but looking back on it, I'm surprised the house didn't blow up years ago! All I'm trying to do is make the system work as best we can without having to rip up every floor and re-lay every inch of pipework.

 

[justlead1]

Hi,Having fitted a lot of solid fuel systems in the 60is i seam to remember a gravity control valve (cyltrol valve i think) femal 1" BSP it screwed directly to the primary return on the cylinder, and worked off the return temp. This had the effect of restricting the primary circulation thus allowing more forced circulation to heating circuit. However the problem you describe reads like the heating circuit needs looking at. One other point we made the injector tee's on site by filling out the stop on a 1" 1/2 "1" No.26 and then extending 1/2" copper some 4" inside the return, jumping up to 3/4" after the tee. This small length of restriction on both primary and heating circuit had no effect on performance. You could try reverse injection of flow? Good Luck

 

[Oldhouse]

Hi,Having fitted a lot of solid fuel systems in the 60is i seam to remember a gravity control valve (cyltrol valve i think) femal 1" BSP it screwed directly to the primary return on the cylinder, and worked off the return temp. This had the effect of restricting the primary circulation thus allowing more forced circulation to heating circuit. However the problem you describe reads like the heating circuit needs looking at. One other point we made the injector tee's on site by filling out the stop on a 1" 1/2 "1" No.26 and then extending 1/2" copper some 4" inside the return, jumping up to 3/4" after the tee. This small length of restriction on both primary and heating circuit had no effect on performance. You could try reverse injection of flow? Good Luck

Fear you may be right about the heating circuit, but definitely going to give this a go first. I made up the injector tee I installed in a very similar way to what you describe. It works a treat.

This looks like the valve you're talking about http://www.uk-plumbing.com/acl-drayton-invensys-cyltrol-valve-1bsp-angle-p-1377.html and it looks perfect for the job to me. I'll let you know how I get on.

Thanks for everyone's help. Much appreciated.