Cylinder heat loss, via convection back to boiler

[Harry Bloomfield]

I've suggested this might be the case, and I think these readings confirm it. Does anyone agree, and agree with what I might do to fix the heat loss?

Open vent system, cylinder mounted in the first floor airing cupboard, Vaillant 418 boiler, downstairs, high on kitchen wall, some 12 feet away, horizontally. MOMO type, 3-port valve, which remains parked, in the last configuration used - in this case, open to HW cylinder. All the main pipe work, is 22mm

I have this week added an ebus adaptor, and Rpi, to download, and decode the data, which produced this, some four hours after the boiler last needed to fire....

harry@Rpi5:~ $ ebusctl read -v flowtemp
bai FlowTemp temp=35.12;sensor=ok

harry@Rpi5:~ $ ebusctl read -v returntemp
bai ReturnTemp temp=40.12;tempmirror=64893;sensor=ok

The cylinder was heated to 65C, but without any water being used, it was gradually still dropping, quicker than I would expect.

Those readings, suggest to me, that heat is being lost from the cylinder, back to the cylinder coil, along the return pipe, to the boiler, where it is cooled, then along the flow pipe, via the 3-port.

So I am again looking at ways to stop the thermo syphon, maybe a 22mm anti gravity valve, and where I could manage to squeeze one in. It cannot be in the return close to the boiler, the mag filter is squeezed in there. Can these things be mounted horizontally, in the long horizontal return run, or would it work in the flow, equally well?

 

[Madrab]

Can you check the return pipe when the system is off and cooled down, if there was thermal backflow then it probably would be warm/hot. I could certainly see it in a gravity system hence an anti gravity valve (single check valve) but not so much a fully pumped. In a C plan it was more to stop higher positioned rads from warming up through thermosyphon in the return when only HW was being called for.

Your case may be compounded by the 3 port being open to the cylinder but I wouldn't have normally expected there to be thermosyphoning from the cylinder down the return, if it was thermosyphoning then that should only occur if say the boiler was higher than the cylinder.

 

[Harry Bloomfield]

This mornings readings, with boiler unused since around 3/4pm yesterday. The return is still slightly warmer than the flow, as also confirmed by separate clip on digital sensors. Both are now much closer to ambient, but still well above the 5C ambient outdoor temperature.

What do you think?

This was at 9am this morning...

harry@Rpi5:~ $ ebusctl read -v returntemp
bai ReturnTemp temp=17.75;tempmirror=65251;sensor=ok

harry@Rpi5:~ $ ebusctl read -v flowtemp
bai FlowTemp temp=16.62;sensor=ok

harry@Rpi5:~ $ ebusctl read -v DisplayedHwcStorageTemp
f47 DisplayedHwcStorageTemp value=44.0

 

[Madrab]

As far as temps are concerned, unless the primaries are heading outside then the outside temp shouldn't come into it? They should settle around the indoor ambient? I wouldn't be overly worried about a 1deg differential though, if it was reverse flow and drawing heat out of the cylinder I would expect the differential to be much greater.

If there is thermal circulation from the coil down the return to the boiler then there's very little that can be done to stop that as that's the normal flow direction, putting in a check valve would stop the return from flowing the way it should be/needs to. With that in mind then the only real way to stop circulation down the return, off the top of my head, would be to drop a 2 port into the primaries somewhere and have that shut down when there's no longer a call for HW if the 3 port's default pos is open to HW.

Still can't get my head around a reverse thermal circulation down the return to the boiler though - that kinda goes against all the rules, as heat rises.

 

[Harry Bloomfield]

As far as temps are concerned, unless the primaries are heading outside then the outside temp shouldn't come into it? They should settle around the indoor ambient?

I was thinking - air flow cooling the heat exchanger of the boiler, via the flue.

 

[fixitflav]

I was thinking - air flow cooling the heat exchanger of the boiler, via the flue.

With the hot cylinder higher than the cold boiler, I don't see how comvection would happen.

 

[Madrab]

I was thinking - air flow cooling the heat exchanger of the boiler, via the flue.

TBH - I wouldn't think that would have any influence on the system water temps.