UKC

Currently finalising the plans for the installation of a woodburner with back boiler to run a central heating system in our new (old stone) house.
We are looking at an open pumped system with gravity fed heat-sink radiator circuit to control the system in the event of pump failure / power cut.

We have all the specs for pipe sizing for the F&E tank and for the gravity loop.
However, what is the current thinking on pipe to radiator size?
What's the current thinking on microbore?
Receiving conflicting advice from a range of sources.

Info from any heating engineers / plumbers useful here, but feel free to chip in if you're not but you've had a system installed yourself.
ie. What was installed for you and how well does it work / have you had any problems.

In reply to interdit:

I have that system, wood burner + 1 radiator and hot water cylinder. The rest of the house is wet electric.

Mine is gravity only, run in 22mm with the rad in 15 mm. I wouldn't use microbore.

In reply to Philip:


Don't think you'd get away with microbore on a gravity system!
As far as I can see I need pumped as I'm needing several radiators spread across the same floor that the burner is on. My separate gravity circuit is purely a failsafe if the pump dies.

It was a heating engineer that recommended microbore, a part of keeping the volume of water in the system as low as possible.
I've read of the pluses and some minuses, but am keen to hear of as broad a selection of people as possible.

In reply to interdit: I worked for a leading woodburner manufacturer, and we recommended 15mm pipework, as, if the pump failed you would have a larger volume of water and heat sink, and the water would keep flowing for some time until it slowed to a stop. If the pump failed when the stove was running hard the single rad heat sink would probably not cope effectively and you would boil over the top into the header tank, which would cause a lot of condensation in the space where it was. Depending on the stove output, you could always put in a bigger heat sink rad, of approx 15% of the total boiler output, but we would still recommend the 15mm pipework to the other rads.

In reply to johncook:
That makes sense. I think microbore is popular with rural plumbers simply due to ease if installation in stone walled properties.

In reply to interdit: went through this one three years ago.two good online resources are "the green living forum" and"stoves on line" which although a commercial site is full of downloadable educational material.

We went for a dryback and Clearview seem to be actively playing down the blackboiler capability of their stoves.

Also lot of interesting discussion about blackboiler from folk who really. Know thieeir stuff over at green living forum

In reply to interdit: I have a Rayburn Supreme running 6 old cast iron rads through a 22mm pumped circuit with 15mm off to each rad and hot water cylinder off that with an injector T. Mine is a single storey cabin so getting the feed and expansion in and getting the gravity feed for the hot water cylinder was a headache. It just about all fits and seems to work well.
I have a single heat sink rad which will gravity feed itself but I'm still worried about power cuts and am considering adding a 110AH battery and inverter and a relay to run the pump in the event of a power cut for this winter.
I installed all of the system myself and it has been pretty trouble free (touch wood)

In reply to johncook & everyone else:

It was only the one heating engineer recommending microbore - he was quite evangelical about it, which I suppose is a warning sign in itself!

So far then, we're looking something like:

Two circuits using 4 cross-flowed taps on the back of the boiler.

Pumped circuit -
Looking like 22mm distribution with 16mm for the rads (I'm in France so it's 14mm or 16mm, not 15mm).

Heat sink circuit -
28mm gravity loop feeding 2 radiators in two rooms directly above the burner - Radiator capacity totals 10% of boiler.

Stainless steel F & E tank in attic (directly above the room above the burner) with copper ballcock. 22mm open vent.

In reply to interdit:

I'm just after an extra bit of insight from anyone that has a woodburning pumped central heating system.

During normal operation of the fire, when the fire is supplying heat to the rest of the house via the pumped circuit, how hot does the gravity fed heat-sink radiator run?

Does it overheat the room that it is in?

Cheers to everyone for all of the information so far.

In reply to interdit:
Depends on how hot the fire is, whether you have other rads on, and the size of the room that the heat sink rad is in. If the other rads are off and the fire has been blazing for a bit, the heat sink rad will get scary feckin hot hot hot since its the only escape for the heat (other than direct from stove to room, and/or boiling the boiler).

We have a similar system, back boiler on a stove to a heatstore. Oh, and also chose 15mm over microbore, also only recommended by one engineer!

In reply to interdit:

What size back boiler. I had 4 or 6 kw (can't remember which) and it was too small for our house, which is why I replaced it with a different system. As far as I can see, it difficult to get a big enough wood burner to heat the house with the ~40% that goes into the back boiler without the rest going into the room being too much.

When the pump is operating the gravity fed gets to the same temperature as the other rads.

If you haven't bought the rads yet I'd invest in some good quality models that work well at lower temp.

In reply to goosebump:

Thanks for that answer! Very useful.

The burner we are looking at has a thermostatic control, so if the water being returned is hotter than a preset we have selected (as you would expect if most radiators were turned down) then the fire self damps and uses less fuel - So hopefully visitors in the guest room with one of the heatsink radiators won't boil - unless the pump fails.

I've done further checking since my post above and 14mm appears to be the radiator pipe of choice in France (and therefore the 15mm 'equivalent' for UK installations) rather than 16mm.

In reply to Philip:

Thanks for the feedback Philip.

20kW (output to water) & nominal 3 to 10 kW to the room.

http://www.stratfordboilerstoves.co.uk/models/freestanding-stoves/eb20-he.h...

The problem a lot of people have is select a boiler with too much output from the fire to the room.

If my research is correct:
If you have an efficient burner/boiler combination that puts almost all of the heat into the water PLUS adequate radiator(s) in the room with the fire then you can balance the heating of all the rooms including the one with the fire in it without having to overfire the thing, or run it shut down and suffer the rest of the house being cold.


My mother in law has this same fire installed (only found out after we had made our selection) and complains that the main room doesn't get warm enough from the fire - The crappy 'engineers' that installed it didn't put a radiator in the same room as the fire - something that needs to be rectified, but the rest of her house is toasty.


Cheers.

In reply to interdit: this is a commercial site,butnever the less,a mine of good nformation downloadable as pdf

http://www.stovesareus.co.uk/catalog/help_and_advice.php

Also greenliving have forums where people discuss and disect stove related issues with the same passion and conviction that UKCers display on her

In reply to interdit:

They look very good stoves.

In reply to David Hooper:

Thanks for the links David.
Familiar with some of them (from when we installed a woodburner (no boiler) at the last house), but some are new to me. All useful.

In reply to interdit: What type of material is the piping going to be? Traditional rad and pipe sizing is based on the hydrostatic resistance encountered in the pipe runs. So basically unless you're installing pipework that has a lower coefficient of friction than traditional copper/GB/qualpex doing the system in micro-bore is just going to introduce additional friction into the system and ultimately make it less efficient.

I've lived in an old Welsh farmhouse for nearly 30 years, the walls are stone, three feet thick and full of cold air. I use an old Reyburn Royal in the kitchen(old barn adjacent to the house) and a Jotul 500 in the house. I have only used wood in both all this time. I've never been able to afford any other kind of heating. With a trend to greener fuels it is now harder to get adequate supplies of timber. A few point need to be borne in mind when considering woodburners. (1)Make sure you get a good quality Cast Iron stove,eg, JOTUL. Put max insulation in your loft space, (insulation is much cheeper than fuel). And remember you will need a lot of timber. For years I
havested trees marked for me by the Forestry Commission,gathering around 15 tons a year.Now due to the Health and Safty Executive I car'nt even pick up brushwood let alone go in the forest with a chainsaw. Now at over £150 a load and my being over 70 I'm thinking I'll have to go on to solid fuel.

In reply to interdit:
Hi,
you are right to choose an open pumped system no matter what kind of fuel you use
but for your system to work properly the hydraulic installations should be as follows-
1-your gravity fed heat-sink rad should be at least 500 mm higher than boiler, connected by 22 mm pipe
2-an arrow on the pump should face the lower flange of the boiler.
Mind that your old stone walls as well as ceiling and floor needs at least 50 mm rockwool insulation