Heating project: Planning and design

This has been a mammoth project (and it's not finished yet), and blogging about it seems almost as great a task. To avoid this post becoming unreadably long, I'm breaking it down into stages, starting with all the thinking and planning that occupied me for several months as I tried to figure out how to get the heating system I want.

It's been quite a while since I last wrote about the heating project, and there's a good reason for this: We did very little work on it for over four months (hmm, notice a similarity with the last post?) Although not much physical progress was made during that time, I did do a lot of thinking and planning and yes, learning about central heating systems.

To recap, the plan is to replace our standard gas-fired combi boiler that heats water for taps and radiators, on demand, with a wood burning stove with back boiler, the water from which is stored in a tank that in turn provides hot water to taps and underfloor heating. This tank will also have input from solar panels (thermal, not electric).

The solar heating is obviously thoroughly green, as the sun's energy is freely available, endlessly* renewable and totally clean. Wood is slightly more debatable as burning wood obviously releases CO₂ in the same way as burning coal, gas or oil. However, more wood can be grown in a relatively short timescale (relative to making more fossil fuels), which then reabsorbs the CO₂, so provided that enough trees are planted, wood doesn't add to the greenhouse gases in the atmosphere. Also, it's renewable, meaning it won't run out like fossil fuels. It's debatable whether it's sustainable when you consider the amount of land required to grow enough trees to heat one house (i.e. we don't have enough land for everyone to heat their houses this way**) Finally, burning wood isn't very clean in the sense of smoke and particulates. The prettier the flames, the less efficient (clean) the burn. I'm sorry, but I do like pretty yellow flames in my fireplace.

Back to the project. So far, we have a stove:

Second hand wood burning stove

We were at the local hotel one day and spotted this outside on the pavement, where it had previously been in the bar. We asked how much they wanted for it, and got told we could have it for nothing if we'd take it away. Needless to say, we didn't need telling twice! It needs new glass in one of the doors, a new handle on the other door, and a new grate, but it was definitely working before they took it out. It has an integral boiler, which we wanted, that was linked up to radiators in the hotel. I reckon the space heated by the stove and those radiators is about the same size as our house, so the stove should be about the right size for us.

Part of our research involved finding the information plate on the front of the stove and contacting the manufacturer. We now know it's a Charnwood 40B (the B is for Boiler) and they were kind enough to track down the installation instructions of this obsolete stove, so we also know that its nominal output is 3.2 kW to the room and 7.5 kW to the water, which is pretty much ideal. They also sent us a price list for spare parts, and since they were so helpful, we ordered replacement door glass from them, even though we could have got it cheaper elsewhere. The grate bars were over £100 for a set, so we passed on those. We don't want to burn coal anyway, and wood prefers a solid bed, so we scrounged a couple of fire brick from old storage heaters (from the hotel again) which fitted nicely.

We contacted Thermoboard requesting a quote for underfloor heating. We were thinking of their pipes-in-polystyrene system, to be fitted under the floorboards. There's excellent access to the underside of our floor, but getting the pipes in the right place would involve a lot of drilling through joists and fiddling long lengths of uncooperative pipe through the holes.

When the quote came, it wasn't for that system at all, but for pipes embedded in chipboard flooring. At first I thought they'd made a mistake and quoted for the wrong thing, but the more I thought about it, the more it seemed like the right solution. We had wanted to polish the floorboards instead of covering them with carpet or whatever, but if I'm honest, they're nothing special. I'm sure they're not as old as the house (which is nearly 200 years old) and probably much more recent. I'd had a feeling for a while that this project wasn't quite right and that Something had to give. Maybe that something is the floorboards.

So, we have the stove, give or take a few repairs, and the specification for the heat distribution. I was glad to see that the requirements of the underfloor, namely 5.2 kW, were in the same ballpark as the output of the stove. What we needed next was to specify that tank to link the two together. Even if we didn't want input from solar panels, we'd still need a tank to store the heat from, e.g. lighting the fire in the evening through to warming the house for getting up in the morning. Having the heat stored also allows more control of temperature, which is necessary as the underfloor heating operates at a relatively low temperature - relative to what leaves the stove, that is.

Through the 'ish forum, I got in touch with a couple who are installing a similar heating system and have done a great deal of work investigating exactly what they need. As luck would have it, they live about twenty miles from us, so we arranged to visit and pick their brains. Over tea and cake, we looked at the design for their tank. It has a couple of features to solve problems that had been bothering me. 1. If you have input from the solar panels in the bottom of the tank, it will preheat the water for the stove when you have the stove on, but what about in the summer when you're relying on solar alone and only need hot water, not heating too? Won't you end up with a big tank full of luke warm water? Solution: A chimney type arrangement running up the middle of the tank. This takes hot water from the bottom of the tank straight to the top. It also has holes, so when the water at the top is hot, it starts to diffuse into lower areas of the tank. Brilliant! 2. What do we do if we go away for a week or two in the winter and need to keep the house from freezing? Solution: Have a second immersion heater in the bottom of the tank (you have one halfway up anyway, for hot water when the summer sun is being a bit Welsh), but above the chimney thing. This produces the tankful of tepid water that we were trying to avoid when we want a shower, but is exactly what we need to keep the frost off the house.

Our new friends also recommended their supplier, Newark Copper Cylinders, who are very helpful and will make a tank to whatever specification you want. Their prices aren't too bad either. This was an enormously helpful meeting (and it was very nice to make new friends, too!) which solved a couple of problems and saved me a lot of work researching tank manufacturers.

I still had a couple of questions regarding the tank***, namely where to put it and how big to get. 1. Where? I want the stove-to-tank system to run without electricity, i.e. on the principle that hot water rises, so I'd originally assumed we'd need to put the tank in the loft. This puts serious constraints on the size of the thing, especially considering that it would be over our heads, and loft joists aren't always particularly strong. Alternatively, we have plenty of space below the stove (in the workshop) but that would require a pumped system. If we had a power cut, we'd then have to put out the fire in case the water in the back overheated and it exploded. 2. How big? In general, the advice for a thermal store (the kind of tank that stores heat for future use) is, The bigger the better. That's not very helpful if you'd really like to put the tank in the loft and want to know how small a tank you can get away with.

There is an online space I know of where experts dwell and talk in detailed technical terms about designs for green houses (that's eco-friendly houses, not places to grow plants). I've been aware for some time that the answers to all my questions may be found here, but scared to venture to that place, for fear of not understanding a word that's said. It's like mastering just enough of a foreign language to ask a question, then not having a clue what the answer means. That place is the Green Building Forum.

Having got my tank problems down to two questions, I plucked up courage and posted my questions on the Green Building Forum, with suitable pleas not to talk in jargon - and they didn't! I got some really helpful advice on positioning, namely the suggestion that I consider putting the tank on the same level as the stove. This was something that hadn't occurred to me, as I'd assumed that a thermosyphon system (i.e. working on the heat rises principle) would require the tank above the stove. Apparently not. After some discussion, we came up with a plan to put the tank in the airing cupboard. This is on the same level as the stove with the kitchen in between. The flow pipe from stove to tank goes up over the kitchen ceiling and down (a little) into the tank, then the return pipe runs from the bottom of the tank down under the kitchen floor and back up into the stove, so we have a circle of pipes around the kitchen, with stove on one side and tank on the other. As the hot water rises from the back of the stove, it sets a current flowing round the circle, drawing cold(er) water into the bottom of the stove via the return pipe. As the tank is halfway round this loop it gets filled with hot water on its way round.

Another piece of advice I got from the Green Building Forum was to get in a professional plumber. Apparently designing a central heating system is not a job for an amateur. This posed a bit of a challenge. We had tried to get quotes from plumbers earlier in the year. One was very enthusiastic, insisted that what we needed was a pumped system with the tank downstairs, and sent us a quote that was several thousand pounds over our budget. Another was very reluctant to give advice and kept asking what we wanted to do, which didn't inspire confidence. A third seemed convincingly knowledgeable, but never actually quoted at all.

Since the third plumber seemed to know what he was talking about, I got in touch with him again. He remembered coming to visit and was quite embarrassed about not quoting. He'd been busy and by the time a few weeks had passed, assumed we'd have found someone else. Well, we hadn't and this was three months later - would he quote please? And would he mind if I sent him all the information I'd amassed in the meantime? He agreed, but even so, it took several weeks and more phonecalls before he eventually sent us a quote, though before he did, he called to discuss some of the details, which I felt was a good sign. We accepted the quote and booked him in for the last week in October, giving us less than a month to order the underfloor heating and tank, and lay all the heating boards ready for him to connect up the pipework.

At the time I called the plumber, I still didn't have a clear answer to the question of how big the tank should be. Advice from two contributors to the green building forum was, "Standard rule of thumb gives 400 litres," and, "There's no way that's going to be big enough." Our friends from this 'ish forum had said, "I wouldn't want to risk anything less than 450 litres," and when I spoke to the plumber he said, "With this sort of system people would usually have 800 to 1000 litres." All this was pointing to a very big tank, but I still wasn't convinced.

There are two approaches to determining tank size; big enough to absorb all the heat you might generate or big enough to supply all the heat you're likely to need. I had a feeling it was the first approach that was leading to the very large estimates, whereas the second approach seemed more sensible to me. Surely if the tank's full of hot water we can just burn less wood? There was also some of the second approach, but assuming we'd light the fire fairly infrequently and want the tank to keep us supplied with a day or more's heat between lightings. We really only want a couple of hours' worth - after that we'd be happy to light the fire again if need be.

I did various calculations for how much water we'd need, came up with various answers, and generally got stuck halfway through. Eventually I found an approach that convinced me, if no-one else. The quote for underfloor heating included a figure for flow rate, which I multiplied up to the two hours we wanted, then multiplied by 1.5 for the rule of thumb that heating uses two thirds of the tank and hot water one third. That gave me 350 litres and that was what I ordered, ignoring all advice to go for bigger, and also ignoring advice from the tank supplier to go for smaller!
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* Yes I know the sun will burn out one day, but that's really not a timescale that concerns me.

** This is the argument I've heard. I have no idea what the figures are, but it sounds plausible.

*** I know I should call this a cylinder, especially if I'm talking to a plumber and trying to explain where a problem is, but it's a big vessel containing water. It's a tank, OK?