thermal store on wood fired back boiler, any good?

Hi all,

I am looking for some advice, positive or negative.

We have moved into an ex local authority 3 bed semi in North Yorkshire, double glazed, cavity wall and loft insulation all in place thanks to previous owners. The CH system has been running off an old coal fired Grant back boiler, which has now been retired. So at present no CH system. We have been looking at a number of options, but really want some non 'I will try to sell you this system' advice.

Thermal stores - are they worth the extra outlay over a std vented system and associated tank?

We have been quoted, as an requested option, to fit a 500ltr thermal store running from a 8-15kw (3-5 to room) stove that in theory would supply all CH ( 4 big rads, 3 smaller and a heat dump in the bathroom) and HW. I have read that in some cases people are strugling to meet HW requirements. Does anyone have one installed on a wood burning boiler and how have you found it?

Many thanks for reading

Matt and Rosie

 

My brother has something like that - but he also has some solar thermal panels on the roof and a gas boiler for backup.

I think it would be unwise (unless you are really keen on chopping wood) to plan for at least one other reliable heat source. If the weather is warm then you probably won't want to light the stove just for hot water. If the weather is very cold then you may find the wood consumption means the novelty of chopping wood wears off quite quickly.

Having an open vented store does mean you can easily accommodate multiple heat sources and loads without complicated plumbing or safety devices - and I suspect that without an open vented heat store of some sort, the other items needed might soon make a dent in the cost difference.

 

With a thermal store you need to feed everything into it eg solar panels wood berner and gas or oil for backup in cold weather but you need a big rad to waste heat as a safety feature

 

Hello indigofish ,

Can you give an up-date on is I'm looking to go down the same road.

Kind Regards
CJ

 

This will be the heat dump that I already quoted about but you didnt like

 

ianmcd - Could you be a little more constructive an maybe try to input something a little more positive. You may or may not have a vast knowledge on all things heating, where as I have very little and trying to understand what seems to be a very complicated system to me.

 

I was actually being very constructive, but as usual when someone asks for advice on here, they are given advice that is not what they want to hear, better you ask someone else that will say what you are trying to do will work and you will go and try it, if you can not control the energy(heat) given out by the source(with gas or oil or electric you can control the source) with a wood burning stove you can not turn it on or off, it is on or it is off so if it is on and your water is up to temperature the heat has to go somewhere hence the heat dump

 

As I have said in my first post:

It is a very rural and isolated no gas - electric is unreliable oil far to expensive, wood is the only real option. If you know of a better way round this please or explain how I can make this work.

 

What people are telling you is that burning wood or other solid fuel is not a simple thing when you take into account the hazards.

A well designed log burner can be closed down to very low heat output but it will still be producing heat until the fuel completely stops burning. If your heat storage water is close to boiling point then the heat, even though minimum output, may take the water over 100°C and produce steam under pressure, That hazard will have to be vented.

One or more radiators may be able to dissipate enough heat to prevent stored water reaching dangerously high temperatures, but you will need some means to ensure the pump is able to circulate water round those radiators even during a power cut. ( very rural, power cuts more likely ).

To ensure safety it may be necessary to have a store of cold water which can be used to displace the over heated water in the hot water storage tank until the heat source has stopped producing heat. That hot water will have to be dumped safely.

All this depends on some one being there and knowing what to do if ( when ) the heat store goes over temperature and becomes a safety hazard. Or a well thought out and reliable automated control system has to be in place and tested regularly .

Slamming shut the dampers on a log burner in an emergency shut down can result in the production and release of carbon monoxide into the room.

 

As long as the store is vented, then there should be no risk from pressure buildup - but you may well start boiling off water. Your header tank MUST be capable of holding boiling water for this reason. There have been a small number of very tragic cases where a plastic tank has softened to the point where it's dumped boiling water through the ceiling onto the occupants sleeping below - IIRC in one case a young child died from scalding.

There are various methods for managing an overheat - which is best depends on your specific circumstances.
When I was researching thermal stores a few years ago, at least one manufacturer had a safety valve that fitted into a boss near the top of the store, and if the store exceeded 95˚C then it's thermostatic valve opened and allowed cold mains water to pass through a coil built into the unit to remove heat from the store. Relies on reliable source of cold water.
If your physical arrangement allows it, then you may be able to get away with a simple radiator and thermostatic valve to allow thermo syphoning from the top of the store, round the rad, and back to the bottom of the store.
Or, again if your specific circumstances allow it, you might accept simply boiling off water - but that would have to be very carefully considered.
Or you might arrange that an oversized (metal) header tank is capable of dissipating enough heat. So if the store boils, boiling water and steam are carried up the vent pipe and condense the steam in the header tank. Again, something to be done only after very careful consideration of the risks.

And finally, you need to consider "other users". It may be fine to think "I know what I'm doing" - but what if you are not keeping an eye on things and someone else stokes up the fire ?

Also, while you say wood is the only option, I'd consider an alternative heat source - it may be as simple as an electric immersion heater. In warm weather you may find it "a bit of a nuisance" having to fire up the wood burner just for the hot water. And unless you have a good supply of your own wood, and don't value you time harvesting and processing it, it's actually quite an expensive form of heating. So while you've ruled out oil as too expensive, a small oil tank and boiler may well be a good investment long term - for warm wether when you don't want a fire in the living room, you just CBA to get some wood in, ...

 

I have spent a total of £380 on LPG (bottles) since last October, and my house has been very toasty.
LPG isn't as expensive as some would think.

 

Hi indigofish have a look at Heating innovations Ltd H2 Panel have a chat to Mike smith a really nice guy with 30 plus years of solid fuel knowledge.
Hope this helps.
Andy.