Listen to Barry about 3 mins into the latest podcast (awkward buildings) - he installs in Scottish retrofits and basically seems to be saying that the U Values and ACH values we use in the fabric spreadsheets are just wrong. It seems we aren't alone. he is also advocating stepwise upgrades as an option, not necessarily the 'big bang' that the industry more or less insists on. He is a breath of fresh air!
All the existing radiators are 450mm height and either P1 or K1 apart from two column radiators in the refurbished kitchen/diner. They were the biggest available, so I may need a third. I wonder if I should take a closer look at R290 heat pumps to see if I can limit the need for radiator changes
If most of your rads are K1s then upgrading to K2 double finned should bring the required flow temp down materially without compromising appearance. The lower you can get it the better.
Attached as promised by radiator upgrade business case spreadsheet (and a load of other stuff). Its very crude, basically it uses the figures in the A7COP column to calculate consumption and cost. Really you want SCOP but since this is quoted only at 35 and 55 I used COP at 7C and the design flow temperature as a proxy. If you want to be precise you could calculate a SCOP from the COP tables for your heat piump, but a crude indication may be good enough. My guess is that you will have a sound business case to upgrade, the only question being to what FT. Realistically the cost of the rads doesnt vary that much with size so probably its the lowest FT you can get to without degrading appearance or doing something extreme. That will require some playing with figures. You can find a table of output per metre length for various heights in the Myson or Stelrad catalogue, which makes it fairly easy to play around with options. Here is a lookup table I use. Hope that helps. Output of most radiators is proportional to DT^1.3, so given outputs at DT50 its easy to calculate output at any other DT.
I totally agree that K2 are worth it. They don’t look very different to K1 or P+ and are SO much more effective. Moving to K3 is a different step in both esthetics and price.
2kW + Growatt & 4kW +Sunnyboy PV on south-facing roof 9.5kWh Givenergy battery with AC3. MVHR. Vaillant 7kW ASHP (new & still learning it)
Moving to K3 is a different step in both esthetics and price.
But equally you need to have radiators that will put out enough heat at your heat pump's flow temperature, otherwise on cold days your house will be cold.
We've only touched on this very lightly so far, but if the spreadsheet models used to determine total heat loss do that by summing each room's heat loss, and those room losses are used to size the radiators, as they usually are, then in cases where the spreadsheet model over-estimates the total heat loss, then the specified radiators (based on the spreadsheet model) will be larger than needed. The answer is presumably to scale the total empirical heat loss, allocating to each room the proportion it contributes to the spreadsheet model total heat loss, for example if my kitchen accounts for 15% of my total spreadsheet heat loss, then to size the radiator for the kitchen, I take 15% of my total empirical heat loss. But I didn't do that. At installation time, i was locked into the spreadsheet model, and my rads are sized to the spreadsheet model room heat losses.
Midea 14kW (for now...) ASHP heating both building and DHW
Moving to K3 is a different step in both esthetics and price.
But equally you need to have radiators that will put out enough heat at your heat pump's flow temperature, otherwise on cold days your house will be cold.
Indeed. Based on my own experience and sense of aesthetics I would, if I were doing this all again, avoid K3s unless
the depth is hidden
they can be built in as an architectural feature
there is absolutely no other option
I upgraded our upstairs radiators myself early on and ended up with one K3 which I can tolerate but my wife hates. We could probably just have got away with a K2 (type 22). Its in a bedroom so a fan isnt really an attractive option (although it strikes me that, with a bit of control logic, one could probably boost just before going to bed then turn the fan down to a minimum - there is some thinking to be done about how to use fancoils intelligently with ASHPs, they have interesting and useful properties!).
I wish, in retrospect, Id gone for the 22 and if necessary bumped the FT up a bit. The downstairs is being done as a second tranche of rad upgrades along with the ASHP installation. There are a couple of locations where K3 or fans are the only viable options. I've gone for fans, one home made, the other purchased. Of course fan noise is less of an issue in living rooms than in bedrooms.
We've only touched on this very lightly so far, but if the spreadsheet models used to determine total heat loss do that by summing each room's heat loss, and those room losses are used to size the radiators, as they usually are, then in cases where the spreadsheet model over-estimates the total heat loss, then the specified radiators (based on the spreadsheet model) will be larger than needed. The answer is presumably to scale the total empirical heat loss, allocating to each room the proportion it contributes to the spreadsheet model total heat loss,
Agreed. My upstairs rad upgrades were based on (my own) fabric spreadsheet (loss=10.5kW) and so I now know to be oversized (measured loss 7.5kW). Im hoping that with a bit of judicious playing with the downstairs ones I might be able to turn the FT down a degree or two from the intended. However this is limited by not wanting any more K3s (see above). A bit of exploration will be necessary once the job is done.
In total I spent a lot of time fretting about radiators and even now am not sure I got the optimum combination taking visuals and performance into account. It will be interesting to see, in a few years time, whether the Adia technology takes off as a way both to optimise FT and radiator adjustment and to help with selecting upgrades. The concept certainly has potential IMHO if they get pricing and market positioning right.
Living in a listed building, I attach a lot of importance to aesthetics, but I also have another constraint, limited wall space. Not only are the rooms small, but often the vertical height from floor to window sill is limited, especially upstairs. The house was built long before building regulations were even a gimlet in the local authority surveyor's eye. This meant (at design time, using spreadsheet model heat losses) that, to be of adequate size, some K2s had to be very long, almost absurdly long, and that of itself becomes unsightly, as well as taking up wall space, which pushes out the furniture, making parts of the room look bare (why is all the furniture on that wall squished into the corners?). Having visually imagined the various options, I decided K3s were the least bad, and went with them, and the end result hasn't in fact been too bad, because you get used to them. Initially I did think they were a bit of an eyesore, but now I hardly notice them.
I now know that some/all of them are very likely over-sized, but hey ho, there you go... And better that way than too small. It may in fact mean I can reduce flow temp a bit, and still get adequate heating. Still mild hereabouts and the heat pump is barely ticking over, but when it gets colder I shall do some experiments.
Midea 14kW (for now...) ASHP heating both building and DHW
Aesthetics are important. A K2 radiator just about manages to look in proportion most of the time, while K3s definitely look boxy, especially in smaller sizes, but for a given output, they take up far less wall space. For this reason, I accepted mostly K3s because had I gone for K2s, some rooms would have been dominated by a K2, making my house look like a radiator showroom
If you paint radiators the same colour as the wall they sit on they 'disappear' quite nicely. I use a Finnish paint called Tikkurila Optiva 3 on walls because I like really matt finishes and you can tint Tikkurila any colour you like (and it works out cheaper than Little Green etc.,) . I then colour match their Helmi 10 to the same colour which gives me a super tough but matt paint for woodwork, radiators or whatever. They offer numerous sheen levels with the number indicating sheen level.
It works. Even quite chunky radiators merge down and don't stand out so much. You can obviously use any brand of paint you like but Tikkurila can handle heat and multiple surfaces.
@lucia In my last house, I painted the radiators in my children's bedrooms. They were in their early teens at the time and they were allowed to choose their own decorations. One had a black and white scheme with a matt black radiator !
My fabric analysis came out at 11.7kW but I recall erring on the side of pessimism with measurements, U values, etc.
Since we've had several nights with sub-zero temperatures recently (with the average temperature yesterday of 0.4°C), I though I'd quickly revisit this topic. With the reasonably stable temperatures over the period of a day or two, I have chosen a wide sample window to avoid any time correlation issues between my boiler gas use and external temperature. Since my gas use yesterday was 205kWh (according to my Octopus app) this somewhat confirms the heat loss figure @cathoderay mentioned of 10kW at -3°C. Because I always like to consider 'what does this look like', (and to show my age!) I visualise this as the heat from three 2-bar electric fires spread-out across whole surface area of the house.
I have downrated my boiler which now gives a flow temperature of 68°C - fairly constantly overnight so I think it's running at its limit. Despite a return temperature of 58°C there are still plumes of water vapour so I assume efficiency is poor. I have no way of knowing the flow rate, unfortunately. The heating thermostat is in the kitchen and the two 1.8m column radiators I mentioned in another post are not quite keeping up with demand as the room temperature dropped by 1°C overnight last night.
