Non return valve on central heating

zeebests

Hi, I've tried to post a picture but can't seem to get it to work so I'll try to describe the situation.... I have a solid fuel boiler that we have used for years to provide hot water via a vented gravity fed copper cylinder. Just recently I added a radiator and circulation pump that runs to the other side of the house via a t junction. I put a non return valve between the cylinder cold return and the T at the boiler to try and stop the cold water from radiator going up to cylinder. Doesn't seem to be working very well. The pipes to the cylinder don't seem to be heating up now. Is the non return valve (spring type) preventing the gravity fed flow in the system? Am thinking of trying a swing type valve instead. Any thoughts?

zeebests

IMG_20200624_154953.jpg

John.G

There should be no valve in the flow or return in a gravity fed system (safety, even though you have a vent straight from the boiler to the F&E tank) but a poppet type NR valve will certainly prevent any hope of circulation due to the very low circulating force developed in any gravity system.
The pump is pulling water from both the boiler flow (OK) and the cylinder flow (not ok) creating reverse flow through the coil with cooler water from the rad.


I would suggest removing the NRV and relocate the pump to the boiler return with the cold feed upstream (before) the pump suction, there is also a cold feed to the cylinder coil; not sure why this is there but if you don't get any pump over into the F&E tank then should be OK. With the pump off then IMO gravity circulation will still take place as the pump impeller will create little or no resistance to flow, you will have a fully pumped system with the pump in service, you should install a pipe stat on the boiler flow pipe to stop/start the pump but you can operate the pump manually initially to test the system.

zeebests

Thanks for the reply. Yes, I figured spring nr valve was too strong and was interfering with the flow so I went and got a swing gate valve which I was planning to fit. The extra cold feed to the coil I put in place after putting in the nr valve in case I had trouble filling the system (airlocks). I also put in the safety valve so I could easily let out air at the top of the system.
I don't have any room to move the pump nearer the boiler as the stove is up against a wall and the T comes out pretty flush with another wall. I can see what you mean though about the return flow from the pump maybe interfering with the easy flow of water from the coil. I was hoping that the flow from the pump would feed through the boiler and at the hot T branch off also up to the coil to encourage flow this way. At least with the swing valve the cylinder should get to flow with the pump off?

John.G

Yes, I think you should get gravity circulation with the S.check installed in the horizontal.

zeebests

Even with the pump maybe pushing against the
cold return flow? I guess it worth a try except I'm getting pretty sick of draining the system and then fixing up petty leaks ��

John.G

Quote: "At least with the swing valve the cylinder should get to flow with the pump off?"

As you said above, you will only get gravity circulation with the pump off, IF you could t in the cylinder flow downstream of the pump then you might get a bit of gravity circulation with the pump off to trigger the pipe stat (when/if fitted). (No NRV fitted), you then essentially have a fully pumped system.

zeebests

Ok, so I had to go ahead and pull out the stove from the wall and fit a circulation pump to the cold return with a stat on the hot pipe above it. My problem is now that I'm getting over flow of hot water into the f&e tank via the vent. If I put an expansion tank on the end of the vent it will make it a closed system. I have already got a pressure release valve situated near the hot cylinder. Is there any problem with this plan?

zeebests

Oh, and I took out the swing check valve altogether

John.G

zeebests wrote: »

Ok, so I had to go ahead and pull out the stove from the wall and fit a circulation pump to the cold return with a stat on the hot pipe above it. My problem is now that I'm getting over flow of hot water into the f&e tank via the vent. If I put an expansion tank on the end of the vent it will make it a closed system. I have already got a pressure release valve situated near the hot cylinder. Is there any problem with this plan?

You cannot use a closed system with a stove. What is the pump make/model and your present setting?.

zeebests

Here is a pic of the pump. I think it's on the lowest setting

John.G

zeebests wrote: »

Here is a pic of the pump. I think it's on the lowest setting

Apoligies, I got mixed up with another thread, (you have no stove) yes you can convert that system to a sealed system with a expansion vessel, I will look at it later on and make a few suggestions.

zeebests

Actually, I do have a stove, it's a solid fuel Stanley. I just labelled it as boiler in the pic

John.G

You can't make this a sealed system so, I would remove that second cold feed that you installed which may stop that venting, the 4m pump on it's lowest setting should not normally cause venting.

zeebests

I didn't show it on the diagram but that 2nd feed which joins the cold supply to the boiler up near the cylinder actually has a tap valve so it can be turned off, which it usually is.

zeebests

From my pic of the pump do you know if I do in fact have the pump on its lowest setting?

John.G

Can't tell from the pic but you should be able to hear it when you change settings, obviously loudest at the highest.
Can you post another schematic of your system as confused as to whether using one or two pumps now?

zeebests

I'll try and put a revised pic of the system up tomorrow. But basically, I have the one pump and I moved it to the back of the stove on the cold return pipe.

John.G

Is the cold feed on the pump suction or discharge?.

zeebests

New pic

John.G

That looks like a typical set up so a bit surprised at venting especially with a 4M pump running on minimum speed if F&E tank in attic and cylinder below it. My own system (oil fired boiler) has the pump also on the boiler return but has a "combined vent and cold feed" where the cold feed is connected into the vent where it rises up past the F&E tank. Other installations have the pump on the boiler flow....from the boiler (flow pipe) you have vent then cold feed (no more than 150 mm from vent) then pump which basically is a combined vent and cold feed, more correctly, a close coupled system as the vent is separate from the cold feed.

zeebests

The cold feed is on the suction end of the pump. The vent is a 3/4" pipe that runs off the hot flow pipe at the boiler and is 60cm above the water level of the f&e tank

John.G

Are you getting pump over just running the pump with a cold boiler and cylinder coil or what?.

Generally, this set up should not pump over as there should be little or no frictional head loss in a stove (or oil fired) boiler as a high head loss causes a imbalance in the vent and cold feed leading to pump over.
Difficult to suggest a "cure" but long term would be to re locate the pump to the boiler flow side with the cold feed close to it (on return) and then the vent close to the cold feed, vent nearest to boiler.
Very short term would be to connect the cold feed to the vent just under the F&E tank, if no pump over then connect the cold feed to the boiler flow very close to the existing vent.
Overall though, even though a PITA, I feel that re locating the pump, vent & cold feed is IMO the proper way to go.

zeebests

Thanks for your replies and input John. I will experiment a bit over the next few days and see in what conditions the pump over occurs. By linking the vent to the cold feed I would be increasing the residual pressure in the vent pipe? And am I correct in thinking that you're suggesting I reverse the vent and cold feed positions so that the vent is on the cold return and the pump and cold feed are on the hot flow from the stove?

John.G

By combining (linking) the cold feed & vent there is no U tube imbalance effect so you cannot get pump over or pump back through the cold feed, the problem with this is that if the stove overheats and boils then steam is trying to go up the vent and cold water trying to come down the same pipe.

Other suggestion is to relocate the pump to the boiler flow side (pumping out of the boiler) then relocate the vent and cold feed so that you have from the boiler (flow side) vent, cold feed then the pump, that way if the stove overheats then the steam can escape up through the vent and cold water can come down its own pipe to replace it.

zeebests

Ah, so all the junctions would be on the upper flow pipe at the stove. And the vent would be on the suction end of the pump and thus as far away as possible from the extra pressure being created from the pump?

John.G

More or less, the main thing is to keep as little distance as possible between the vent & cold feed. (see my masterpiece)

John.G

You could try this without shifting the pump, I have yellowed out the un needed piping, in the top one you can just keep the valve shut, red is the new bit of pipe.
The vent and cold feed are then independent but very close together, you may have some other idea yourself.

zeebests

Just tested the system and water is pumping over with the system cold. As I turn the pump up the flow increases. One other change I made to the system recently is that I moved the cylinder further away and instead of continuing the 1" pipe to and from the stove I reduced to 3/4" for about 2m to the cylinder. Don't know if this makes any difference, don't see how it would. The f&e tank is about 6-7m above the stove btw.
The idea of tying the f&e feed to the vent... My feed is 1/2" and the vent is 3/4" - I don't suppose that makes a difference?
Also, I was thinking I could maybe just swap the vent and feed at the back of the stove? This would effectively put the vent at the suction end of the pump. I could also do this without pulli g the stove complety out again...

zeebests

The question is, would the pump then force water up the feed into the f&e. Not a problem I guess unless it overflows (there is an overflow pipe leading out through the wall).

zeebests

All other options would involve pulling out the stove and redoing a load of joints that I've already had a major headache getting leak free...

John.G

You have to have the vent on the boiler flow side and because the pump is on the return you cannot put the vent on the pump suction.
Re combined Vent & cold feed, 1/2 " is the correct size for the cold feed so no problems from that end, so perhaps try that as you only have to drain the F&E tank and a few more litres more as you should be teeing in just under the tank.

zeebests

By teeing the feed into the vent by the f&e tank I'll effectively providing a feed to the flow and feed pipes at the stove. I understand that this will increase the resistance to escaping steam/hot water but it remains to be seen whether this will be enough to prevent pump over or too much (as you said) to prevent safe release if excess pressure. I do have the safety release valve by the cylinder as a back up I guess

zeebests

Putting the feed after the vent within 115mm would be simple enough but I'd still have to pull the stove out again ��

zeebests

Putting the feed after the vent within 115mm would be simple enough but I'd still have to pull the stove out again 😖

John.G

That to me, is the option to go for and while I can't promise you 100% success it is should work, otherwise there is something very strange with the set up.

zeebests

Is there any point in teeing the feed to the vent or should I go straight to placing the vent and feed next to each other at the stove flow?

John.G

I would suggest going straight to the stove flow.

zeebests

Thanks for your advice John. I pulled out the stove and fitted the feed next to the vent as suggested and now there is no pump over even with the fire lit and the pump on full power ����. Happy days ��