AJH wrote:
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


See the downloads on http://www.hetas.co.uk/

My 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 wrote:

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


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.