This is just a quick post to mention that I’ve now ordered a new gas boiler and solar hot water cylinder, ready for the renewal of my central heating system this summer.
Boiler sizing turned out to be a bit tricky. My current boiler, installed by the house’s previous owners in about 2001, is a Potterton Osprey CL150 which has a rated output of 44kW (yes, 44 – there I’ve said it!). As I might have mentioned previously, it always made an impressive ‘whumph’ when firing but last winter, despite us living in pretty cold conditions (e.g. ice on the insides of some of the windows) we still had a gas bill heading towards £1000.
So I’ve done a room-by-room heat loss calculation, followed by some whole house estimates (EST and SEDBUK) as a cross-check. The really difficult figure is determining the U-value for my 18″/450mm solid stone walls (limestone with rubble core). Estimates vary wildly from 1.05 to 2.13 so I’ve ended up taking the average of 1.6W/m2K – I suspect soft Dorset limestone is at the higher end of this range really. By my calculations, with the insulation as it is now, I need 23kW peak and, with the improved loft and some upstairs internal wall insulation planned for this summer, this falls to 19kW. This is based on an estimate of 3900W for ventilation losses (by volume) and 2000W for hot water. As a final sanity check I looked at our gas consumption last winter (~27,000 kWh over 6 months) and the 6 hours per day the boiler was running, and it conveniently came out at 25kW average.
My short-listed manufacturers were:
- Worcester-Bosch
- Vaillant
- Viessmann
- Broag-Remeha
As you might guess, I’m keen to install reasonably sophisticated controls to maximise the amount the boiler is condensing, as well as to be able to control which rooms are heated, to what temperature and when.
After much deliberation I’ve bought the lowest output Viessmann Vitodens 100-W (WB1B) system boiler which is rated at 26kW. As an aside, the staff on the Viessmann technical support line are very helpful. This is still rather lower than the current 44kW rating but I’ve convinced my plumber that I’m happy to risk a smaller size on the basis that:
a) the maximum power required is based on a steady state with an outside temp of -5C, inside 20C (South West): in extreme weather we could always top-up rooms with an electric heater or, (one day!) a wood burning stove,
b) I want to improve the insulation of the house and in so doing will reduce the power required – if it’s still cool then that will be even more of an incentive..,
c) I won’t blame him if the house is cold!
The boiler has a modulation range of 9-26 kW at 50C flow/30C return, i.e. a ~3:1 ratio which seems typical for entry-level ‘premium’ boilers (higher spec’d ones like Vitodens 200 or the Remeha Avanta typically have closer to 4:1, and I expect this range will increase over time).
Note the boiler’s 80/60C rating is 8.2-23.7 which shows how you lose nearly 9% efficiency when the boiler is not condensing, emphasising the importance of having radiators large enough run at a low return temperature. 
This neatly also leads onto one nice 100-W feature which is that it has basic weather compensation built in – to activate it you just need to buy an add on cable/thermistor (part no. Z006506) for under £40 (hmmm, there’s some margin in there I think!). You might also like to note that the guidelines for the Part L building regs, due to take effect in October this year, appears to make weather compensation controls mandatory (p16 Table 1) on new gas domestic boilers anyway. I’ll probably write an article about weather compensation another day.
So that’s all for now. The other topics I’m researching are:
- I am replacing radiators as part of this exercise so need to calculate how much to oversize radiators by to reduce return temperatures and optimise condensing (I’m de-rating standard BS EN442 radiator output by 40% at the moment).
- Almost all of my radiators are under windows or on external walls – I’m trying to work out whether some should be positioned on internal walls to lower the temperature gradients on the windows and so reduce heat loss (a contentious subject!).
- Solid wall insulation: my current plan is to use Gyproc Thermaline SUPER phenolic insulation-backed plasterboard (another contentious subject!), fixed with BG’s system RF (dots of sealant adhesive with seals at edges) but only in some first floor rooms.
If you have any thoughts on any of the above I’d appreciate your comments!
December 27, 2010 at 2:17 am
Good stuff. Take a look at the Camden eco house. They are tackling similar issues, there are some useful hints. I visited it as part of London Open House weekend a couple of years back. I found a video here http://www.camden.gov.uk/ccm/content/environment/events-and-initiatives/twocolumn/low-energy-victorian-house-towards-zero-carbon-dwellings.
But I did argue with the architect about the cost effectiveness of solar hot water. Presently in the UK by my calculation, the payback isn’t there (net present value considering how long the heat collectors will last, the efficiency of modern boilers, particularly when you consider that the cost is incremental – unless you have a “passivhaus”, you have to have a boiler as well. One does it for environmental reasons.
December 27, 2010 at 11:55 am
Thanks for the comment Richard.
That’s an interesting site you mention (http://www.levh.org.uk/) – it’s good that there’s so much more information and documentation on improving the performance of old houses since when I started this blog. There are still challenges though – moisture control in insulated solid walls (ground floor most critically) is still not an area we have long term information on – I see LEVH have embedded humidity sensors but can’t see any published results from them yet.
Yes, I agree that solar thermal payback can be tricky. It depends a lot on your location/roof, hot water demands (time and volume), when the system is installed, what products you buy and who installs them.
For example, for me I needed a new cylinder and wanted mains pressure hot water (of course one can argue the merits of low pressure, aerated showers). The house is well suited to solar (has a dinky SSE facing valley roof with access window) and I’d ruled out a combi. The marginal cost of a larger, twin coil solar cylinder was probably about £150.
If you go down the DIY route you can buy all the parts from Navitron for £1000 or so. If you have a family, especially if you’re not on mains gas, I expect you could pay that back in well under 10 years. Of course then there’s still the dilemma of shipping products from the other side of the world, but that applies to a high proportion of what we consume.
For professionally installed systems, which are likely to cost £4000+, the purely commercial payback is more tricky particularly if you’re on mains gas and have an efficient boiler. It will be interesting to see what happens with the Renewable Heat Initiative.
There are many DHW details to be optimised too – the amount of water in dead legs as you wait for a tap to get hot, the standing losses in a cylinder, thermosyphons in connecting pipes… plenty more to blog about when I get time!