Wood burner and heat store on same level -advice please

[portised]

My apologies if this has been covered before but I have searched and could find no reference to this question before:

We live in an old listed cottage and are having a new heating system fitted, (at present all we have is a new just fitted wood burning stove with back boiler). This heating / hot water system will consist of a gas boiler, log burner with back boiler and an immersion heater backup. It will be a vented system using a 250 litre twin coil heat store and a separate expansion tank. Problem is a very low roof height and pitched ceilings upstairs means the heat store will be downstairs on the same level as the boiler and log burner. The heat store will be approx 9 meters away from the log burner and this log burner has a 6kw back boiler. The main boiler size is likely to be around 20kw or 24kw but not purchased yet. All the rads add up to total 11kw, with the upstairs rads being 4kw of this total. The circulation between the log burner and the heat store will have to be pumped as all the 22mm pipes are horizontal and no gravity possible, in fact they are under the floor already, so they go down, along and then back up again.

My question is how do I build in a fail safe for over temperature of the water in the log burner circuit in a power cut or pump failure? The heat store will have an overheat coil fitted and set at 95c dumping cold mains water through the heat store to a drain in event of failure, but it is the log burner boiler circuit between the stove and heat store that I am concerned about.

I am told it would be normal to dump all of the excess heat in a failure to the upstairs rads, but how would you do that in this set-up?

Any help or advice would be much appreciated.

 

[oilman]

My question is how do I build in a fail safe for over temperature of the water in the log burner circuit in a power cut or pump failure?

The heat dump MUST be gravity fed and have no active bits in. It must also be open vented, and piped in 28mm.

IMO a wood burner with a back boiler is a potential problem and should not be installed. A purpose built wood fired boiler is another matter, but back boilers are a disaster waiting to happen.

Why do you need a 20kW boiler when you have 11kW of rads? If you are using 22mm you can normally pass just under 14kW down it anyway.

A wood burner will be much more efficient if it is run dry. What make is it? You do not have to use it just because you have bought it, you can always get the right appliance and sell or otherwise dispose of the one you have.

 

[Norcon]

Have a look here also.
http://www.navitron.org.uk/forum/index.php/topic,4858.0.html

 

[BigBurner]

Look at this

http://www.bacwak.net:8080/public_view.htm?viewId=8#

My question is how do I build in a fail safe for over temperature of the water in the log burner circuit in a power cut or pump failure? The heat store will have an overheat coil fitted and set at 95c dumping cold mains water through the heat store to a drain in event of failure, but it is the log burner boiler circuit between the stove and heat store that I am concerned about.

In case of failure what mechanism dumps the the heat from the thermal store (correct terminology)? I think you mean thermal store.

 

[gordonspants]

I am not a plumber by the way.
As said, if you get a power cut, without control or a heat dump you're stuffed, which you know.

UK stuff is usually openvented gravity 19th century technology with a pump stuck on somewhere to make it high tech. The europeans do it differently, with belts and 3 sets of braces to stop the sides of their houses getting blown out when the lecky goes off.
So they have boiler air thermostatically controlled, a boiler quenching coil, a pressure relief valve as a last resort, and even use a UPS to keep the power to the pump going in a power cut.

First, the boiler on the stove needs to have thermostatically controlled air - look for a dial or thumbwheel somewhere - if the boiler water gets too hot, the primary air is throttled back. If you don't have that control, it may get tricky.
Next, the link to the navitron site mentions a honeywell quench valve that lets mains water into a cooling coil at 95 degree boiler water temp - but note this is for a clever german boiler stove that has the cooling coil within the boiler waterjacket on the stove itself. NOT a thermal store coil 9 metres from the actual heat source. In an emergency, cold water at mains pressure goes into the quench coil, takes the heat from the overhot boiler, and sends it to drain. This will keep quenching until boiler temp is reduced - which may be tricky without thermostatically controlled airflow. But one thing that's occurred to me is 'what-if' the stove glass breaks - woosh - no air control at all, superhot overfired boiler - the quench coil seems essential in any boiler I reckon, whether openvented or not.

With 9 metres of horizontal pipework and a pump, you're getting A Bit German in your design, which may be a bit advanced for the UK, but you will still be openvented. No gravity heat dump is A Bad Thing traditionally.
The full-on unvented european no gravity way may not be UK-approved - note your building control officer needs to be aware and happy with your plan - it's normal to find a hetas approved installer, who can install the lot and sign the work off instead - but a hetas bloke who'll get involved with something unusual may take some finding - and they are seriously rushed off their feet at the moment with every man and his dag wanting a woodburner. And mention no heat dump or gravity feed when you phone them, and they may hang up on you.

Pump is controlled by a stat on the boiler and a flue temp stat, in parallel, so either switches it on - belt and braces again.
Pump is part of another clever bit - as said above, UK boiler stoves are thought to be rubbish, as they cool the burn to the point of uselessness, tarring the chimney and stove. That's 'cos there's no proper temp or flow control on UK stuff with a few exceptions eg Aarrow ecoboiler, Charnwood etc.
Google laddomat and have a read - it keeps the boiler return temp correct throughout the burn so you don't get tarring or too much smoke, oh, and navitron is the place for this as above, as there's a section on their forum for wood burning.
If you need bits you can't find easily here, use ebay.de and then babelfish to translate it. German voltage is the same as here, so a termovar/laddomat should be plumb'n'play.
Best of luck - it's all good fun