Thermo-siphoning through boiler from thermal store ?

[www.GymRats.uk]

The summer project of adding a multi-fuel thermal store, new gas boiler & Wood Burner boiler is now providing hot water and heating, Wood burner still to fit but I believe the store might be thermo siphoning through the boiler causing a temp. drop from 60 degrees to around 50 degrees overnight when the boiler is deactivated.

Due to it's location the new Intergas heat only boiler is close to the ceiling so the top of the boiler is higher than the top of the thermal store by about a metre. Flow and return pipes exit the boiler at the top so they run over the flue next to the ceiling then drop down the wall and across to the thermal store tap-off points. Flow is right at the top of the store and return is around

2/3rds of the way down the store where it goes into boiler pump then back up to top of boiler.

All formerly hot pipes have been lagged as they were leaking heat from the store but the boiler case seems to remain warm. (around 30 to 35 degrees, much the same as the insulated pipes.

THe boiler thermostat is the same height as the return pipe to the boiler and set at 60 degrees which still gves more than enough DHW for 2 people mixed down to 55 degrees.

The Intergas boiler heat exchanger is a big slab of Aluminium which the water wiggles throughfrom bottom to top so I'm now suspecting that the heat exchanger is working in reverse when the pump isn't running, transferring heat from the non-pumped system water to the inside of the boiler.

Does this sound plausible? I've found references to a "London Loop" where the DHW pipe on a hot water tank is taken down the cylinder before returning back up into the loft or wherever.

Am I being overly concerend? Let's say for example the boiler and connecting pipes have become "part" of the thermal store, and heat is leaking from the boiler heat exchanger into the case of the boiler and potentially out through the flue, is this still likely to happen if I dropped a loop in the flow and return pipes down to the floor before going back up to the boiler?

From my understanding, although thermal siphoning by convection might be reduced by a downward loop heat can still travel downwards through the water by conduction both of the water and the copper pipe all be-it at a slower rate.

Other than that the store takes around 20 to 30 minutes to recharge to

60 degrees after 2 full showers with the boiler set to an output temp. of 70 degrees.

Right, more importantly for me now is to get the woodburner/boiler plumbed in and a flue fitted before gas prices go up again!

Thanks for your thoughts as always (those that I can see). Cheers - Pete

[www.GymRats.uk]

<snip>

A picture paints...

I've just added a picture to G-Drive. You can see the Flow/Return pipes exiting the top and 2/3 way down the T.S. (pumped retrun) both meeting up with eachother before crossing to, and up the wall then across to the boiler.

The silver "wrap" insulation is over foam insulation and the black patch is just electrical tape for taking a temp. reading by I.R. thermometer.

Or shorter: Cheers - Pete

[Tim+]

Is this not a situation for a simple non-return valve?

A quick Google suggests that there’s a lot of confusion regarding check valves and non-return valves regarding when to use which type and it looks like sellers get it wrong too!

Unless I’m mistaken, this is a non-return valve, not a check valve, and is the type you’d need to stop thermo-siphoning.

For info about the difference:

Tim

[Roger Mills]

You need a valve designed to prevent thermo-siphoning. It's a sort of check valve with a spring-loaded flap which needs a finite pressure to open it. It will open to pumped flow but not to thermo-siphoning. I had to install one in my first house over 50 years ago. That had gravity HW but pumped CH, but the upstairs radiators were getting hot due to therm-siphoning, without the pump running. The valve - which we affectionately called a fu-fu valve, cured it.

[Fredxx]

Does the gas boiler and wood burner have it's own pump?

In the past, I coupled 2 boilers in a similar configuration, where each had a pump and a non-return valve.

[Rob Morley]

Fit a zone valve activated by the pump power?

[www.GymRats.uk]

That was another thing I'd considered Rob.

I suppose the consideraton is whether the store is actually thermo-siphoning or whether it's simply transfering heat by conduction through the "static" water and pipework in which case a blockage in water flow might not make a difference.

Going to pick up a cheap dual sensor thermometer to keep an eye on pipe temps to see if I can spot anything going on.

[rick]

My thermal store has a 2 port valve which closed when

1) no call for Heat 2) no pump action to 'stir' the store.

This is to prevent Thermal Cycling Its a simple motorised 28mm 2 port vavle

[www.GymRats.uk]

Thanks Rick. That idea had certainly been on the list of preferred options. I've got to drain down the system one final time once I've connected the wood burner and given the extra pipework a rinse through so that might be someting to look at... that said, I've just remembered the flow/return pipes are very close together (just enough for insulation) so would need to do more jiggery-pokery with pipe runsfor which the faff and fiddle to cure what might even be an expected and acceptable heat loss might not be purposefully usefull...

Was going to get a cheap multi-channel temp. guage to see what flow and return pipes are actually doing after a number of hours.

Cheers - Pete

[www.GymRats.uk]

Having a few leaking joints to replace I did the drain-down on Friday and chopped a 2-port zone valve into the boiler flow pipe about 6" from the cylinder inlet (right at the top of the thermal store).

Well I can happily report that thermo-syphoning back through the boiler has been completely cured.

Pipes now cool and boiler case is a normal temp. rather than becoming a passive radiator bleeding off valuable heat to the room and out the flue.

Thanks all for suggestions and input. Looks like this years major heating project is drawing to a close just in time for winter. (once I have the Wood burner flue fitted!)

Cheers - Pete