[another] going greener with Viessmann 200W / efficient heating system design questions

ABrass

Are we talking about solid walls with 25mm Polystyrene insulation and a target of 200mm in the roof?

That's pretty poor, especially the loft as you should be looking at at least 270mm for modern specs.

With a Victorian build you should have suspended floors and probably no under floor insulation. What would you be putting in if you go for UFH?

Plus the basement will suck heat out. Insulating them is especially ticky as you need to plan for condensation.

Have you got quotes for doing external solid wall insulation?

Reed_Richards

Screwdriva said:

Reed_Richards said:

I'm all in favour of the sentiment but I think you ought to be a bit more scientific and try to match the size of the radiator to the heat loss from your room.  What you are trying to do is to run your boiler at as low a temperature as possible in order to achieve maximum efficiency so you should size each radiator (or set of radiators) to keep each room at 20 C when it's, say, - 4 outside and have the boiler heating the radiators to, say, 40 C input temperature.  Choice of input numbers may vary but the point is to do some calculations rather than just finger-in-the-air radiator sizing.

Every single heating engineer (there have been a few) I have spoken with about the subject, including the Heat Geek who designed our system, have said the following "there is no such thing as too large a radiator for low temperature heating". 

Reed_Richards said:

At current fuel prices the running cost of a heat pump should be about the same as that of a gas boiler but an electric boiler, however cheap to install, would cost about 3x as much to run.  You can look at how much you currently pay per annum for gas and multiply that by 3 for an electric boiler.

I haven't crunched the latest numbers but I understand that Octopus Gas tracker averages around 4p per kWh. Their Agile incoming is currently ~.20p or thereabouts. Not sure if that factored into your calculations of 3X the cost of running but currently I believe gas is 4X cheaper than electricity, if not more. 

I think the OP may have left the building, however:

You can always throttle a radiator to balance your system and, in effect, cause it to behave like a smaller radiator.  In which case you will just have a radiator that cost more than it needed to and which occupies more wall space than it needs to.  If you don't mind either of those two factors then I agree with all your heating engineers and with Heat Geek.

I don't have a Smart Meter and could not get one last time I tried (there wasn't enough space for it to fit).  Therefore I don't pay much attention to Octopus and their fancy tariffs for those that do.   I was basing my figures on SVT pricing (and I haven't looked at that for a few months).  In fact last time I looked, heating oil was the cheapest fuel you could get. Perhaps the more salient point is that fuel prices are all over the place at present and any decision based on today's prices could easily be invalidated tomorrow.

P.S.  I looked up Agile on the Octopus website which says:
In Winter '22-'23, a typical household on Agile paid around 35p/kWh on average (a little more than the average variable tariff).