Thermal Syphon

[oldbutnotdead]

Finally going to connect the thermal store to the woodburner. Height differences are a bit marginal- about a metre vertical on the hot across 2 metres horizontal. I'll be using a pumped loading valve so under normal circs this won't be a problem but under abnormal circs which works best- a continuous (30 degree ish) climb from woodburner to store or a short (800 mm or so) vertical out of the woodburner then about 2 metres of very gentle (2 degree) rise to store?

 

[Jackrae]

30degree continuous. The 2 metres of almost horizontal will hinder thermo-syphon flow. What are you proposing for the return leg - it's routing is as important as the flow line.

 

[oldbutnotdead]

Same question really, again there's about 700 drop from store exit to burner. The general drawings show it flat but that also seems counter-intuitive.
Under normal circs the primary circuit will be pump assisted (tarty loading valve with pump triggered by flue thermostat) so I could live with reduced thermal syphon efficiency as long as the syphon will still function in any power cut scenario. The pipework is a lot easier and neater doing up & gentle (and down and gentle on the return).
Yes there is a heat loss rad as well, thats about 4 metres above so easy life.

 

[polesapart]

Up and gently across is how I'd do it; radiussed (swept) bends or better still pulled bends on any 90s. And min 28mm pipework obvs.

 

[oldbutnotdead]

Cheers both. Hopefully by the time I get to do it (about a fortnight) Jewson toolhire will be back in business. I'm not buying a 28mm tube bender for 6 bends (mind, when I actually set it out I may find that swept half bends are all I need)

 

[MeldrewsMate]

Glad @polesapart mentioned using minimum 28mm pipe for those primaries; insulate the flow too, but also insulate the return if your flow is good.
I'd go with the vertical first, then gentle horizontal, as the first flush of hot water will 'push' harder away from the boiler and get the circulation going more quickly.
I take your point that a gentle slope will fit better with your decoration (or for other reasons), and that's your choice to make.

Pumped circulation really is far more fuel efficient than gravity circulation, but I assume your 'abnormal cicumstances' means during a power-cut.

 

[oldbutnotdead]

Glad @polesapart mentioned using minimum 28mm pipe for those primaries; insulate the flow too, but also insulate the return if your flow is good.
I'd go with the vertical first, then gentle horizontal, as the first flush of hot water will 'push' harder away from the boiler and get the circulation going more quickly.
I take your point that a gentle slope will fit better with your decoration (or for other reasons), and that's your choice to make.

Pumped circulation really is far more fuel efficient than gravity circulation, but I assume your 'abnormal cicumstances' means during a power-cut.

Yeah that's exactly it- if I wasn't using the loading valve I'd have to put the store upstairs to get the gradient & I don't really want it up there (I'd need to beef the floor up, it'll take up a useful chunk of a bedroom, it may be noisy cos there's a pumped plate hex on it for mains presssure hot water).
Yes that was my initial thought that a vertical leg at the woodburner would help get the syphon going, likewise a short vertical drop on the return at the store.
And whatever I do I want it to be safe- tbh a few times I've overfuelled the woodburner & its started kettling but the dampers work fairly quickly and I'm not in the habit of leaving the house with a blazing fire on.
Ta for all the contributions. Glad I bought the 28mm last year- was £13/3 metres when I bought it, looks like about £18 now!