Gravity feed wood burner system

[Gus]

I'm installing an 8kw wood burning stove with flue boiler, which should generate about 2.5kw to heat the DHW by gravity/thermosyphon. I'm also fitting a heat leak rad (old cast iron one rated at about 1.3kw).

I have a few queries though about the design and layout of the pipework. The HW cylinder is almost directly above the stove, so pipe runs are very short (not much more than 2m). I'm using 28mm pipe and can get a decent rise on the feed and fall on the return. The problem is the heat leak rad. In theory it should T off after the feed to the cylinder, but if the pipework has to rise to the rad, then it'll have to T off immediately after the T into the cylinder to have any rise at all. (it's positioned right next to the cylinder, but the inlet is only about 4" higher than the cylinder inlet) I was going to reduce the pipe from 28mm to 22mm after the cylinder, rising to the open vent, so the rad feed could also be 22mm. The question is:

1. Is this going to work efficiently (i.e. heating the DHW first and only heating the rad when it's hot) or will I end up heating the cylinder and rad together?! (I know I can't put a valve on the rad, as HETAS say it should be un-valved for H&S)

2. If I T off for the rad BEFORE the cylinder with 22mm pipe, would this make the situation even worse, or would the smaller bore pipe limit the flow of water to the rad giving reference to 28mm feed to the cylinder?

Hope this makes sense!!

 

[justlead1]

Hi Gus, The water will have different priories to yours. But there is a need to fit a rad. Experience suggests the hot water will range thro. 50 degrees to say 90, dependent on boiler / fuel / wind conditions. Fitt a 22 mm x 3/4" lock shield and wheel head, this will allow a modicum of system control. As will historic knowledge once up and running. What ever the temperature the addition of cold at the point of use will available to blend the required temp. Good Luck

 

[Gus]

Thanks for the reply Justlead...I wasn't sure I was allowed to fit a valve to the heat leak rad, just in case the tank boiled up and there was no way to lose the excess heat? I agree that this would give me better control over the system though. As you suggest, I probably won't really know how it's going to work until I try it and then perhaps modify it from the experience. I'm still not sure whether to plumb the rad in before or after the feed to the cylinder though? If I tee off the main 28mm primary feed with 22mm to the rad, is it likely the flow will be mainly to the tank?

 

[WHPES]

Just a quick question - you are HETAS certified to fit the woodburner aren't you? (because if you aren't you shouldn't be doing it!)

Just fit a lockshield at one end which will give you a bit of control. You could also fit further radiators and a pump controlled by a pipe stat on the return from the cylinder.

 

[Gus]

Thanks WHPES. No I'm not HETAS certified and at the moment I'm not sure whether I'm going to be doing the install or not...if I do, then I'll bring in building control to check it out prior to work and then sign it off.

 

[justlead1]

Hi Gus. The gravity circulation will favor that which is most vertically in line with boiler. Tee of the primary F and R run pipework so it will not trap air (without a pump there is nothing to purge air) Good Luck