Plumbing automatic woodfired boiler

[AJH]

I had the job of helping out a client who is converting to a renewable heating system, I was just erecting an exterior ss insulated flue for him.

I am not familiar with dhw plumbing other than a few natural convection back boilers fitted to stoves. These generally have a header (feed and expansion) tank with a large bore pipe direct from the boiler to a vent over the header tank, Teed off to supply the hot water cylinder. This is so any boiling can vent itself directly into the header.

The wood boiler he has imported is designed for a sealed system, our understanding is that UK regs preclude solid fuel appliances from being fitted sealed because of the requirement for an atmospheric vent. This boiler does have a pressure safety valve.

He has plumbed it in "conventionally" with the circulation pump flow through the heat exchanger and then via a 3 way diverter valve to dhw, ch or both with the 22mm vent between the boiler and the diverter valve. This results in circulation venting to header no matter how slow the pump is set.

Apart from increasing the height of the vent above the header tank, to oppose the head of the pump working against the restriction of the pipework downstream of the diverter valve, I cannot see an obvious solution. Putting the vent on the suction side of the pump would increase the force any boiling water would need to overcome on its way to the vent.

Gas Boilers installations I have seen only appear to have the vent Teed off the 15mm fill pipe but I think solid fuel boilers require a separate vent direct to header.

He'll be contacting the manufacturers on Monday but I'm guessing they'll not be very helpful.

I'll soon be facing a similar problem when I attempt to incorporate my salvaged solar hw panels into my house and install a back boiler to complement my existing gas boiler so would like a few pointers.

AJH

[Aidan]

See the downloads on

understanding is that solid fuel boilers should be installed on an open-vented system. I also understand that solid fuel appliances have CORGI-like restrictions in that they should only be installed by HETAS registered installers. The website above should have all the details.

It is possible to pipe an open-vented pumped system; in fact, most systems were open vented 20 years or so back. The only difficulty is in ensuring that the pump will not push water out of the open vent under any conditions (variable system resistance with TRV, variable pump speeds, zone valves in various positions, etc..). It wasn't a problem with cast-iron boilers, the waterways were large and hydraulic resistance was low. High efficiency boilers had higher resistance and smaller waterways, so the differential pressure across a boiler was too high to have the CF on the return & OV on the flow.

You could pipe the system with a secondary pump, i.e. primary pump circulates water through the boiler, flow to secondary heating zones is taken from/returned to the primary circuit through two closely spaced tees. Too many details to expound here.

[AJH]

Yes this is as I stated in my query.

They've not quite got it sown up as well as Corgi, HETAS registered fitters can avoid having a visit by a building control officer. I think HETAS is simply a non profit making company (which bitter pill is made sweeter for staff and directors by various emoluments) which maintains a register of competent fitters. I registered with a similar scheme and am still "provisionally" approved, I have done insufficient installations so far and thus have never become fully approved. In this instance the boiler will be commissioned by my employer, who is HEATAS registered and who I was working for installing the flue, on Tuesday week, once the circulation is sorted, it adds ~15% to the install cost though, such are restrictive practices. Self stoking wood burning boilers and flues are already hideously expensive before this additional expense.

OK as I said we have tried to slow the pump, the zone valve is fixed by previous gas installation.

Yup, that's the problem. I understand OV as Open Vent, expand CF (combined flow?)

Do go on, although these seem expensive alternatives to a higher vent. Anyone know typical heads these circulation pumps have? 2m of extra

22mm copper looks like a cheap solution that fits all legal requirements.

AJH

[Aidan]

CF = Cold feed. The water won't pump-over into the feed & expansion tank unless the differential pressure head between CF & OV exceeds the height of the OV above the water level in the tank.

Pumping-over problems can usually be avoided by having the CF & OV close to each other, so that there is no significant differential presure between the two connections.

[AJH]

Thanks Aidan, it becomes obvious once someone puts it into simple words :-).

I see the mistake now, the previous gas boiler had the cold feed to the suction side of the pump, teed to this in the loft was the 15mm vent.

In replacing the boiler the client had fitted it exactly as the gas boiler had been plumbed but replaced the vent on the cold feed with a

22mm vent direct to F&E tank from the hot side of the boiler heat exchanger. This made a circuit from cold feed to tank when the pump was running.

Simplest solution seems to be to cap the current cold feed and provide it via the new 22mm vent pipe, I think.

AJH