Direct or indirect?

[WendyH]

Hope this is the right board for this question ... I'm planning a simple hot water system for an off-grid agricultural building. Nothing fancy. Straightforward vented hot water cylinder of about 90 litres' capacity. It's to supply hot water to 2 sinks and a washing machine plus underfloor heating for one room in winter only. Pipe runs will be short and straightforward. There are two heat sources. Solar and a 16-litre heat exchanger built into the flue of a wood-fired cookstove. The solar will be indirect as it makes most sense in the context. My question is, would the wood-fired heat source make more sense plumbed direct or indirect? I'm thinking direct, since there is far less chance of boiling a larger volume of water, though with the heat exchanger on the flue, rather than in direct contact with the fire, the chances of it getting excessively warm are much less. I would love to hear opinions on this ... Thanks.

 

[Ermintrude]

Indirect for me. Solar exchanger would be clogged after a few weeks. Hope the hex in the flue isn't something u have dreamed up ! Cooling flue gasses cause them to fall rattler than rise. You will end up with very sooty flue and high levels of co. Surely you would be better off with an old Rayburn or solid fuel back boiler? Or even a gasification log burner.

 

[WendyH]

Thanks. I mentioned already the solar would be indirect, so it's just the question over the wood-burner source. I'm aware of the consequences of cooling flue gases and it's taken into account in the design of the stove.

 

[Nostrum]

What's the heat exchanger made of? Ie do you want rusty water coming out f your taps? I'd think about the above as well, cooling a solid fuel flue is going to lead to tar deposits and possibly
chimney fires, condensation and the risk of a smokey room!

 

[SimonG]

Solar power for two sinks and a washing machine, with a bit of ufh in the winter? Bit overkill, especially with some cobbled together solid fuel heater. Why not just put pv on the roof, electric undersink water heater and a couple of oil filled rads.

 

[WendyH]

Solar power will be the sole heat source in summer, so no, it's not overkill in the slightest. It's an essential part of the system. With all due respect to your skills and all, you're not aware of the context and the reasoning that's gone into the design of the system. I'm not looking for people to suggest alternative set-ups here but just to address my question: direct or indirect for the wood-fired heat source?

 

[WendyH]

Nostrum - just noticed your reply. It's steel, so valid point and worth considering. Thanks.

 

[snowhead]

What material will the feed and expansion tank be made of supplying the Flue heat exchanger?

 

[WendyH]

Snowhead - stainless steel for the feed. Expansion tank unspecified as yet because it depends on whether the heat exchanger is plumbed direct or indirect.

 

[Reg Man]

Use a twin coil indirect open vented hot water vessel. Storage and header cisterns above, The header cistern MUST NOT be the ordinary Plastic type. Need one suitable for solid fuel. The flow and returns to the Hot water vessel would normally be gravity fed, as you require at least one gravity circuit. Another thing to look out for, is that not all modern twin coil vessels are suitable for gravity circuits. Look at your solid fuel appliance very carefully and find out if it has any formal approval, if not look at using a proper wood burner with back boiler. Always use a HETAS registered engineer, they will give you best on site advice for your requirements.