I'm trying to cut our enormous oil bill and heat our house in a more sustainable way. We currently use a Worcester Bosch Heatslave 26/32 Oil Combi providing hot water and heat to 12 radiators. I now want to introduce alternative heating sources into the mix. The plan therefore is to fit a vented thermal store system.

I would estimate we need around 10-12KW heating plus hot water.

Initially I'm looking at connecting a Clearview 650 with boiler (either the 27,000btu or 40,000btu unit). We have a free wood supply from our neighbours sawmill and our 6 acres of woodland. I would also at this stage be looking at connecting a conventional (condensing?) oil boiler to provide back-up. Eventually I intend to add solar, and ultimately an immersion run from a micro hydro system (producing around a constant 1KW).

What I'm finding most confusing is information regarding thermal store sizing. There seem to be two camps regarding this, one saying I need a smallish store of 250-350Litres, and another recommending a monster 1500Litre store.

What size of store do I really need?

The other thing!

Apologies if you have done this (don't know where you are on the 'curve') but you should aslo look long and hard at your levels of insulation and improving the airtightness of you house. There is no point installing a state of the art multi source heating system if you live in an uninsulated colander.

S.

Hi,
The UK stores tend to be quoted smallish as they tend only to think in terms of DHW rather than as a space heating buffer / accumulator. DPS for example would consider the 400-500 lit ones to be massive, whereas in Europe that’s a mere tiddler. One main reason for the larger European units is that with a wood log gasification boiler (which is more common) it’s loaded up and burnt hard and intensely. The majority of heat is stored as they would expect quite a low heat load due to excellent insulation. Then after the fire has gone out say by noon the rest of the day is fed off the 1000-2000 lit tank. These also may have a solar register, again they might be using more than the odd single panel we see here. The tank needs to have the capacity to take heat from many panels so that again gives a larger tank. This is because we tend to only see solar for DHW whereas in Europe tend to include it in the overall space heating system. Basically if it’s a UK thermal store its smaller, than if manufactured in Europe. These would tend to start at about 500-750; anything smaller would just be a DHW cylinder. Also the larger European accumulator tanks tend also to have the DHW exchanger as a very long stainless coil spirally up through the tank. This can contain 40-60 litres of water which is instantly available, after that you rely on the same heat transfer that you have in the Combi boiler (except that a plate exchanger)

That said insulate everything and draught-proof like crazy is cheaper and easier so do that first. Even if you do have a thermal store/wood fuelled system the reduced heat load will allow you to run for longer off of the tank before needing to refuel. Like wise if you have a back up oil boiler you can run this into the accumulator tank to charge it up, then sit back and meter the heat out from the tank in the course of the day, I know that sounds a bit like a storage heater (which we all hate) but that will allow an oil boiler to operate at the best efficiency.

Cheers, Mike up North

Correctly sizing the thermal store is a matter of feeding the right known values into a simple calculation. Any thermal store supplier should be willing to do this for you. If they can't/won't then you need a different supplier.

Lots of interesting replies already! I love this place... I'll try and answer some of the points:

Jeff B asked about our oil bill. It's horrific! We use 6000L a year at the moment. Points to note - we live in Scotland at 800ft elevation in a poorly insulated 5 bed detached house. We work from home so the heating is on 16 hours a day from September to April. I hope to save at least 50% through simple insulation and draught proofing. Because we have a free wood supply, we don't mind keeping the woodburner going all day, but I'd like the oil boiler to still be there in case of absence, illness or plain laziness.

Skywalker, I've looked at DPS before, but they seem pricey and I was thinking about a Navitron tank, which can also be tailored to specific requirements. I'm also worried about the size and weight of the thing, it will be sitting on the ceiling joists of our utility room.

Jeff B says he might keep his heat slave combi as a back-up, I didn't think that was possible because it can only handle a cold feed, therefore I couldn't connect it to the thermal store? Is there a way to make it work? If I could avoid having to budget for a new boiler that would be great.

So am I right in thinking that there's not much point in having a huge thermal store unless you have a wood gasification boiler or similar 'maximum burn' device? We just want something that will allow us to combine various heat sources simply, and to allow us to move further and further away from the oil-guzzling position we're currently in.

To clarify - The system I had envisaged works along the following lines: There is a thermal store which provides hot water via a heat exchanger heating mains water. The central heating is driven by a pump and uses the water in the thermal store. The operation of the pump would be controlled by a timer and thermostat. The woodburner would be the primary source of heat, backed up by the oil boiler which would kick in if the thermal store temperature dropped below a certain point. So if I was driving the central heating hard and having a shower or bath, both primary heat sources would be working, but crucially, the woodburner would reduce the amount of work the boiler has to do, and therefore I'd use less oil over the year.

Eventually I'd add the other inputs - solar and immersion, which would reduce further the need for the boiler to kick in.

I've been told that many combi boilers will not work with thermal stores as the water returning to the boiler is not cold, and the inlet valves can only handle cold water. Is this not true? How do you attach a combi to a thermal store?

Ah, I see what you're getting at, instead of using the hot water circuit of the combi which requires a cold feed, use the central heating circuit - which must be able to handle a warm feed. Does that work?

Are you installing your system yourself? It would be great to exchange notes, as our plans are very similar apart from the number of radiators we're trying to heat.

mitchino - basically your proposed system should work fine. Take a look at the DPS web site...

http://www.dedicatedpressuresystems.co.uk

We have one of their vented stores with a heat exchanger. Our store is fed from an oil boiler but they can make stores to
order with any number of extra coils for multiple sources - solar or GSHP etc.

Couple of points...

Thermal stores are normally operated at temperatures of around 50C, that's lower than the water temperature from a boiler so rads may need resizing/enlarging if they are marginal at present. We have under floor heating which needs lower temperatures anyway. The power of a rad is proportional to the temperture difference between it and the air rather than their absolute temperature so they might need to be nearly double the size. Switching singles to doubles might be easier than making them wider anyway.

Oil Boilers (eg My Grant Vortex) requires a minimium return temperature of around 40C to prevent corrosion... so "pre-warmed water", as someone said, is practically mandatory. This is achieved by fitting an automatic mixer on the side of the store to ensure the return temperature to the boiler is maintained above 40. In practice the store is above 40 most of the time so the mixer only seems to function when heating the store from cold.

These thermal stores aren't cheap. My guess is that a store + wood burner+ installation is could be £10,000 if done professionally. I doubt it will save you more than 50% unless you plan to feed the wood stove constantly. The payback period might be 10-20 years? Will you stay in the house that long?

Before you go any further check that your oil boiler isn't cycling a lot. When we first installed our system the boiler cycled a lot. On/Off for 1 min at a time. Calculations I did suggested the primary pump supplied with the store was too small for our boiler. The boiler flow temperature was reaching the temperature set on the boiler stat. Even with the boiler stat turned right up the boiler kept cycling. The numbers suggested we needed to double the size of the pump and I've just done that. Now when the store calls for heat the boiler runs continuously at about 65-70C until the thermal store is upto temperature. Then it shuts off until the store temperature falls. The boiler stat is set on max so it's not used. The boiler flow temperature is set by adjusting the pump speed and the return temperature. Operating this way should be much more efficient but we have yet to check the impact on our oil bills.

Colin

I have a different understanding of how my DPS unit will operate.

Thermal store temperatures are typically 75C (at least this is what it says on the DPS website & when I spoke to them they were clear that this could be upped a bit if wanted) & as the heating system is fed direct from the store (1/2 way down it in my case) you should get water out at roughly that temperature.

DHW comes out at around (just above I think) 50C as there is a Thermostatic Mixing Valve on the output from the heat exchanger to prevent scalding.

I have specified my store to dump excess heat into the heating system ramoving the need for a seperate heat dump so this is another option.

My store will also have a TMV for the boiler return water & this allows for quick heating of the top of the store & then switching over to full store heating.

I'm DIYing so my store/fire/new boiler/solar combination (including re-lining the chimney and new rads/plumbing etc) will end up at around £7k (probably less).

Payback is an odd concept as far as I am concerned. I am paying extra for a system that can be heated from almost any source (I get free/dirt cheap wood, sun is free, gas gives me current levels of flexibility & electric for when gas is too expensive/I can afford to pay for the green stuff - use of these should be fairly low anyway).

The rest I can't really comment on.

Our boiler will be only around 15kW max output and will modulate so I think we should be OK interms of flow & return temps (this is one of the reasons I have a proper plumber involved!).

No TRV's on the heating circuit, thanks for the advice I give it a 'dry run' when we get the back boiler going. Easy enough to fit a motorise valve on the gravity feed which is off when unpowered for emergencies.

We only have one bathroom with just a shower & a basin so opted for a single exchanger. We have long term plans for an extension but this will only add another shower & maybe a bath neither shower will be powered so the 80kw exchanger should keep up fine.

One radiator for a dump is all that is usually recommended but it needs to be able to be gravity fed so somwhere well above the level of the fire in the system is a must (depending on the heat output of your fire you might want to think aboout how big it is though). Many people put it in their airing cupboard but with solar there is the possibility of needing to dump excess heat in the summer so the loft is potentially good option (I hate the thought of wasting heat on a cold winters night in a power cut however - even if it just warms the towels up). If we go for TRV's (we are 50/50 on this at the moment) I will fit a 'spare' radiator directly into the gravity circuit from the back boiler to the fire on a motorised valve which opens when there is no power (I seem to have confused this above!).

S.

CWatters: I used to own a house where we did just this: The whole system heating (water+space) was based on a wood burner. The fun of relighting and cleaning wears off, I would say, after about 2 weeks.