Thermo-syphon

[Harry Bloomfield]

You may have read, a week or two ago, I bought a pair of cheap digital temperature displays, with remote sensors, displaying with an accuracy to 0.1C, with the intention of utilising them as pipe thermometers.

After a while assessing there accuracy - they are spot on, I have clipped them on the flow and return of my boiler, using pipe insulation. What I've noticed since my Vaillant system was installed, is that the boiler display almost always shows several degrees above ambient [1], even when it has been several hours since there was last a call for heat. I have a OV cylinder, and a MOMO 3-port valve. MOMO leaves the valve setting at what ever position it was last called to serve - HW or CH.

I am not at the moment needing to run the boiler for space heating, it only needs to fire up to maintain the HW in the cylinder. Triggering the heating, to increase the room temperature, thanks to my fancy Vaillant predicted control system, the flow temperature only hits 34C, prior to shutting off, with a return of 27C. Which seems fine.

With the boiler cold, having last served HW, there is a temperature difference across the boiler of around 5C, due to the thermo.

Thanks to the temperature displays, I think I have now worked out the reason for [1] above - thermo-syphoning. If the MOMO stops having just served the heating of the cylinder, the valve will remain open to the coils in the cylinder. Once the boiler shuts down, thermo-syphoning, will take effect, circulating heat back from the cylinder, down to the boiler, and cold air around the boiler's heat exchanger, will dissipate that heat.

It's not necessarily a problem just of MOMO type valves, because even the spring returns default to being open to the cylinder, when there is no call for heat. At least if there is a call for CH a MOMO will remain set for CH, until the next call for HW.

 

[JonathanM]

You may have read, a week or two ago, I bought a pair of cheap digital temperature displays, with remote sensors, displaying with an accuracy to 0.1C, with the intention of utilising them as pipe thermometers.

I was going to ask how they'd worked out. Thinking of getting the same.

 

[Harry Bloomfield]

I was going to ask how they'd worked out. Thinking of getting the same.

Well worth the money, I cannot fault their accuracy.

 

[Johntheo5]

With the boiler cold, having last served HW, there is a temperature difference across the boiler of around 5C, due to the thermo.
It's not necessarily a problem just of MOMO type valves, because even the spring returns default to being open to the cylinder, when there is no call for heat. At least if there is a call for CH a MOMO will remain set for CH, until the next call for HW.

That's very interesting, wonder how much a cylinder would loose overnight, ironical really, as there are various tricks to get that devilishly cunning but very ingenious mid position valve to revert to DHW even if CH was its last port of call.

 

[Harry Bloomfield]

as there are various tricks to get that devilishly cunning but very ingenious mid position valve to revert to DHW even if CH was its last port of call.

You are going to need to explain that?

 

[jeff the gasman]

anti gravity valves

How does an anti gravity valve help me save money on my central heating? What is an anti gravity valve? An anti gravity valve is a non return valve which works to prevent thermosiphoning. They ar…

dolantethole.wordpress.com

 

[Johntheo5]

You are going to need to explain that?

I will try and find a link to it later but think it involved a extra relay or two, obviously a anti gravity valve will solve one problem but if CH is the last port of call at night's end then the motor will be running in a stalled state for a extra 8 hours or so a night which probably reduces the motor life. Its no good programming the HW to be programmed on last, for say 5 minutes at the end, as the cylinder stat would have to be calling as well.

 

[fixitflav]

What I've noticed since my Vaillant system was installed, is that the boiler display almost always shows several degrees above ambient [1], even when it has been several hours since there was last a call for heat.

Is your boiler higher than your HW cylinder Harry? If so, convection would explain it, but with the more common setup of boiler lower, I don't see how it could happen.

 

[Harry Bloomfield]

but if CH is the last port of call at night's end then the motor will be running in a stalled state for a extra 8 hours or so a night which probably reduces the motor life.

No, as said, it's a MOMO valve. I swapped the spring return 3-port for a MOMO 3-port, a couple of years ago. MOMO's are only powered, when they need to actually move, then once in position power down and remain in the last position called, until called to change.

The thermo-syphon flow is in reverse - the return pipe, is a few degrees warmer than the flow at the boiler.

 

[fixitflav]

No, as said, it's a MOMO valve. I swapped the spring return 3-port for a MOMO 3-port, a couple of years ago. MOMO's are only powered, when they need to actually move, then once in position power down and remain in the last position called, until called to change.

Does MOMO have a mid-position?

 

[Harry Bloomfield]

Is your boiler higher than your HW cylinder Harry? If so, convection would explain it, but with the more common setup of boiler lower, I don't see how it could happen.

No, the top of the boiler is about 2 feet below the ground floor kitchen ceiling, cylinder is in the airing cupboard on the first floor, with a 10 foot horizontal run of 22mm.

I am not suggesting it is a fault in my system, because the thermo- syphoning will happen with any system with a HW cylinder - there is nothing to prevent it.

 

[fixitflav]

No, the top of the boiler is about 2 feet below the ground floor kitchen ceiling, cylinder is in the airing cupboard on the first floor, with a 10 foot horizontal run of 22mm.

I am not suggesting it is a fault in my system, because the thermo- syphoning will happen with any system with a HW cylinder - there is nothing to prevent it.

In theory it shouldn't happen AFAI can see. The warmer less dense water is higher, and ought to stay there.

BTW where is the post about pipe thermometers? I can't find it and I might want to buy a couple.

 

[Nozzle]

In theory it shouldn't happen AFAI can see. The warmer less dense water is higher, and ought to stay there.

This. It doesn't happen in reverse. So once the boiler heatex has cooled to the same temperature as the coldest part of the cylinder, then any natural circulation / convection stops.

 

[fixitflav]

This. It doesn't happen in reverse. So once the boiler heatex has cooled to the same temperature as the coldest part of the cylinder, then any natural circulation / convection stops.

That's what I'd expect. So after several hours the boiler h/ex should be at ambient temperature. If this had come up a few weeks ago I'd have had a feel of my boiler, but now the CH may not be off for the several hours needed to be conclusive.

 

[Johntheo5]

I have seen it happen with a maxipod thermal store located a meter or two above a stove (gravity circulation), the cure was to extend (loop) the stove flow pipe up over the maxipod and back down.