Backflow from cylinder to central heating boiler

[Old_Badger]

My central heating boiler is close to my hot water cylinder and above it. Consequently there is a gravitational flow from the coiled pipe inside the cylinder back up to the boiler. This occurs when the pump turns off. The result is that the water in the lower part of my cylinder cools down.

This only happens in summer. In winter my 'y' valve effectively stops this back-flow.

I think the problem might be solved by putting a non return valve in the pipe to the cylinder. But it would need to be one that opens with very little pressure. Has anyone tried doing this and is it possible to buy a suitable non return valve (22mm, preferably with compression joints)?

 

[45yearsagasman]

Has this always been a fault with you system or a recent occurence?

 

[Old_Badger]

It has been like this from the start. The possibility of back-flow never occurred to me when I decided my layout.

It is a fairly minor problem. The energy loss is probably less than one kilowatt hour per day for 100 days each year. So not hugely expensive. It would just be nice to solve it.

 

[heytesto]

What you are looking for is something called a featherweight check valve.
They used to be fitted on gravity HW/pumped heating systems to stop the upstairs rads getting hot in the summer.

You should find one on the web.

 

[Old_Badger]

Thanks for your suggestion about a featherweight check valve. I'll see what I can find.

 

[Norcon]

If you have unwanted gravity circulation then a valve with low resistance is not going to prevent it from happening. (unless reverse flow?)
You could try a spring loaded valve fitted upstream of the circulator which will apply a little more resistance. Most merchants supply them.
The closest to a featherweight valve is the Acaso Termobac which is designed to allow gravity circulation to take place.

 

[Old_Badger]

The flow I would like to stop is a reverse flow. The water is pumped into the coiled pipe in the cylinder from the top. So when the pump stops the water tends to flow backwards up to the boiler.

So maybe a normal non return valve would be better than a featherweight one. But how much pressure is my pump likely to apply in practice to make such a valve open and can I get a non return valve with a sufficiently low minimum pressure?

 

[Onetap]

What you are looking for is something called a featherweight check valve.

You could also convert the system to S-plan ( two 2-port zone valves) so that the cylinder is isolated when not being heated. You'd need to ensure the valve did not isoalte the cold feed or open vent, if any.

The mid-position valve remains open to the hot water port when the power is off, as you've found.

I tried a swing-check valve for the same problem, but the gravity circulation could push it open; it was ineffective. There was one storey between the thermal store and the cylinder, so the force applied was greater than in your case. Spring-check valves have been used sucessfully, I believe.

 

[Nige F]

Thanks for your suggestion about a featherweight check valve. I'll see what I can find.

Make sure you google feather Weight - not Fetherlite