Our self build is designed for SIP construction, so it has very good insulation and low air leakage. It's one and a half stories, room in roof with a footprint of approximately 100m2. The house is long and thin (17m by 6m approx). It faces east/west, with the living room at the north end featuring an inglenook with a modern woodburner. The layout isn't ideal for energy efficiency, but such are the limitations of the site and planning restrictions. We're a young family of four, so the house is occupied for much of the day and we have typical family useage - say a 2 sinks, 2 showers, 1 bath, one dishwash and a washing machine load each day.

Our primary heat sources will be solar hot water panels on the roof (flat panels are a planning pre-requisite) and an LPG combi boiler for backup (we bought a Glowworm 24cxi combi boiler for our caravan 18 months ago and intend to transfer it over to the house rather than buying another new boiler).

Now the question is, what's going to be the best way to design the heating in the house.

We think that it makes sense to warm our home with underfloor heating on the ground floor and a heat recovery ventilation unit to redistribute heat around the house. Upstairs we think we can live without direct heating, though will probably install underfloor heating (and towel rail) in the bathroom.

The living room is approx 18 foot by 13 foot, so from this page http://www.metaldevelopments.co.uk/woodwarm/calculator.asp we get an estimate of 3kW heating from the stove. That's quite a little stove for a big room. We could use a bigger stove and rely on the heat recovery ventilation to take some excess heat and transfer it to the rest of the home?

It makes sense to have a thermal store to combine at least the solar and LPG heat sources, and provide mains pressure hot water for showers. I'm really not sure on sizing, but various sources seem to suggest something like 450 litres will be more than adequate for the hot water load, whilst buffering the underfloor load to a lesser extent?

We could use the woodburner with a back boiler to generate hot water and space heating around the home. Some (such as this one http://www.wodtke.com/momo.html) have a 70/30 split, with 70% going to the boiler. That would make sense given the relatively low heat requirements of the living room. Unfortunately the thermal store will be sited some distance from the woodburner (10 metres or so horizontally). Common advice seems to be that stove heaters cannot be pumped safely. However, DPS suggests a scheme for pumping with a vented, gravity circuit for fail-safe operation. See Overheat Protection using a Plate Heat Exchanger nearly half way down the page - http://www.heatweb.com/Wood/index.htm

So, does that all make sense? There are probably a dozen different ways to do it, but this seems to work to me. We're not aiming for zero carbon heating - rather the goal is to reduce the load as much as possible (well insulated, airtight house) and supplement a reliable, low cost LPG heat source with opportunistic solar and wood burning.

Some questions that come from this are - what size panels should we use? Am I right in thinking we can run a thermal store off a combi boiler by plumbing it through the central heating circuit on the boiler? How do we get a heat recovery and ventilation system designed and installed at a sensible cost (best quote so far is £6K - though I think it would be quite practicable for me to install it myself, which would reduce the price by a useful amount)? What other stove options are there? Does the cost and complexity of plumbing in a wood burner outweigh the benefit it provides? With a young family and a demanding business to run, we're not likely to run the stove every day during the heating season, but this system seems to offer the best means of taking advantage when we can light it.

Any thoughts, comments, ideas, suppliers, warnings?

Crusoe - thank you for your comments. Having read your considered replies to other threads on similar topics, I was hoping to hear from you on our (mad-ish) plans.

As for LPG, well the bottom line is that in an already expensive and complex system, the added cost and complexity of a heat pump is beyond our already stretched budget at this point in time. Besides that, my thought processes are along the lines of:

1. Heat pumps appear typically to have a COP that brings their costs in line with gas/oil, but not so low as wood fuel. Whilst there are claims for higher efficiencies, I'm a little cynical and feel that we've yet to see that significant advance in the technology (it's essentially refrigeration, which is a mature technology). Given the cost of installation, the payback time of a heat pump compared with LPG, gas or oil is pretty long - especially as we already have a (new, efficient) boiler that we can use.

2. However much green electricity generation comes on line in this country, it's going to be minority and as such prices are going to track the fossil fuel generators. I'd expect significant price hikes in future, particularly as Britain has been tragically under investing in generation capacity over the last couple of decades and there are political issues over supply of natural gas from Russia. By contrast, LPG is a by-product of petroleum refineries, so whilst it will tend to track the price of crude, it will tend not to be so strongly affected by the nasty combination of infrastructure and political problems that are brewing for electricity.

3. I would have liked a biomass boiler, but both log and pellet boilers are exceptionally expensive, and present problems in terms of space required. Log boilers are quite demanding to run and require a much bigger thermal store to even out the generation - another space issue. Pellet boilers sound ideal but pellet supply is at best described as in it's infancy at present.

4. We will cook with gas, so we plan to have a supply on site. An LPG boiler provides reliable, automated backup for our other heat sources, so if we can reduce the demand as low as possible and make it easy to benefit from the other heat sources in the home, the actual level of LPG use should be minimal.

5. In five years time, if the situation or technology available has changed significantly, we can swap out our LPG boiler and use another heat source.

If you've got LPG on site and the boiler, then I think you're exactly right to do as you propose, if you can solved the vented/unvented issue. Stores work nicely with UFH. DPS open vented stores are great for many reasons, especially that if you need to top up the heat in the top of the store with a gas boiler you get fully hot water at the top in a very short time, this is potentially important if you have a larger store. This would not be the case if the boiler circuit had to be on a coil.

BUT 2 downsides. Solar coil is by default at the bottom: you might want to consider 2 switchable coils, one in the middle to provide less but higher temperature stored heat on cloudy days. Secondly the external heat exchanger pump is not speed controlled. So when a bathroom tap is turned on, you are effectively pumping fairly warm store water into the bottom of the store, "un-necessarily" diminishing the higher grade heat at the top. It's not the most important issue in the world and I think that these systems make up for it in all other ways, but it just niggles. Mike Up North mentioned a few months ago that some continental stores use a triac and temperature sensor to control the pump speed, it would be interesting if this were taken up in the UK.

Posted By: Johan

You can, of course, heat a TS from a combi on LPG.

That's true, but combi boilers are designed to run on sealed systems as far as I know so either you need to run your TS sealed (1.5bar sounds good to me" alt="" src="http:///newforum/extensions/Vanillacons/smilies/standard/smile.gif" >) or since you're talking about a woodburner with a boiler you might want an open vented TS. In which case a pump and a PHE for the combi will overcome the sealed system issue (DPS has got a picure of this setup on the page you sent the link to.

My understanding is that it should be possible to run the combi system with the thermal store being charged by a coil connected to the boiler's heating circuit and at pressure - the boiler is designed to pump and accept a higher return temperature on the heating circuit (and I'm presuming should be designed to run in condensing mode whilst doing so). The DHW circuit on the combi would then be disconnected/turned off.

I don't believe it's a requirement that the water in the thermal store is heated directly by any of the heat sources it utilises?

Tuna

put in a vented accumulator above the stove to catch to heat. pumped circuit (indirect) from that to the thermal store.

solar in the store

lpg to the store.

stats will control the delivery of heat and the controls will favour renewables before sticking the lpg on.

sizing of collectors - MCS accredited installers should use sap to do this, based on the internal floor area of your house..... so for example a 150m2 internal floor area, south facing roof - 3 Velux SO8's (flat plate in roof) would deliver 56% solar contribution. Considering higher occupancy in the above example, maybe 4 X SO8's depending on budget.

sizing of store - based primarily on the store's capacity to deliver DHW compared with the reheat capacity of your lpg boiler.

MarkK

Miked2714

Or coils...I agree Absolutely.

Tuna

It's not a requirement for direct connection - but heat flow will often be more efficient than a coil simply because the store will absorb as much heat as your combi will throw at it. Without the cycling you can get with a coil.

It may be worth checking (direct with mfr) if the heating side of your boiler will function on a low pressure (vented) system, or whether this will trip the low pressure switch. And whether this wd be needed in this scenario.

Crusoe

Posted By: crusoe
It's not a requirement for direct connection - but heat flow will often be more efficient than a coil simply because the store will absorb as much heat as your combi will throw at it. Without the cycling you can get with a coil.

Direct is always better, but a PHE is better then a coil as you can easily get >100kW heat transfer through one. As I said earlier DPS had a picture of a system using one together with a sealed boiler.

Posted By: Miked2714The pressure in an open vented system is naturally dependent on the height of the header tank, so it may be possible to get enough pressure at the boiler by ensuring the header tank is as high as possible. Pipe to header tank yet another route for heat loss but that's life!

It is usually around 0.5bar, about 16 foot. If the boiler is on the ground floor and F&E tank in the roof apex then you may do it.

Or use a stainless steel pressurised thermal store. Advanced appliances will do one. They do not require G3. However I would have a pressure relief valve on one (one is also on the boiler) and a high temp cut-out, heavy duty contacts, stat probe on the top, set to 95C. When this cuts out power is off to the heating appliances.

Posted By: Mike (Up North)Hi,
I had a quick look at the above mentioned stores in stainless. One thing that struck me was that the direct model (that’s with the boiler and store and radiators all connected through on the primary side) is not pressurised as per a sealed system. It has a vented Combi tank on the top as a feed & exp tank. You could of course have a remote F&E tank. I was expecting that being made in S/S it would be used on a sealed system as per many of the imported stores (our long list of accumulator buffer store is well known) which are made of usually mild steel and usually have a 3bar working pressure limit (some are 1.5 bar becyuse the steel is thinner)
There is very little to gain in having this in S/S, the water is within the coil so that’s not a reason, it must be above radiator height as the F&E is on the top of the unit, copper tanks do this. The only reason that I can see is a fear of corrosion on the primary side. There is a huge range and diversity of mild steel tanks already in use, and oxygen levels and inhibitors have been discussed at length on this site.
Cheers, Mike up North

They do pressurised thermal stores to order. Or any type to order. They will provide a plate heat exchanger too, but they prefer you assemble it. They make the pressurised Nu-Heat UFH thermal store, with an internal plate heat exchanger. This has no coils and is all direct. They do not sell this thermals store separately and Nu-Heat only sell the store to UFH customers.
http://www.nu-heat.co.uk/s.nl/it.I/id.394/.f
http://www.nu-heat.co.uk/core/media/media.nl?id=1371&c=472052&h=4ebe53df67820adf2817&_xt=.pdf
I don't like the idea of only having one cylinder thermostat, two is best for anti-cycling purposes.

I would rather have a stainless steel cylinder when pressurised. You mention a list of stores. Where is this? Is there a full list of suppliers somewhere? Thanks.