I would like to ask the community on their thoughts around the economic benefits of upgrading radiator sizes to reduce the COP. In particular for the sub zero ambient operation.
I already have a Samsung HT (8kW self installed) and operating very satisfactorily with the higher flow temperatures keeping the whole house at 19 degC in -2 deg C ambient conditions. I have kept the pre-existing radiator system which was already designed for an Oil burner working at 50 degC FT for best efficiency so my radiator sizing may have already been generous but is not unusual for more recent radiator installations.
The water law settings are currently -2C+15C ; 50C 35C and with the heat pump on 20 hours a day it maintains target temperature nicely. The SCOP this season is projected to be 3.2
I calculate (using standard COP comparators below) that reducing the FT by 5degC would only reduce the COP by 7.5% at -2C (50 reduced to 45 is 2.57 and 2.76 resp). The calculation at higher ambients of 5 C is better at (45 reduced to 40 is 3.14 and 3.52) 12% reduction.
However, most power is used at the lower ambients so this means I can only really save around 10% or less on my energy bill, say £100 per annum. To make this kind of change to the radiator emitters is likely to cost around £1500 to £2000.
Is this a reasonable investment (that is also good for the environment) or is it just a waste of perfectly good radiators!
It surprised me that flow temperature has such a diminishing effect on COP at typical subzero ambient conditions. It makes me think the calculation is wrong but perhaps someone could check it.
Hi Ringi, thanks for the reply I have 7.2kW of battery which as you say saves on ££ Octopus Cosy tariff. The radiators are in very good condition and I have experimented with fitting two of the double radiators with computer fans as a DIY job and that works well by increasing output but not everyone particularly likes the noise they make.
My question is more of a general one for installers on the economics of scrapping existing radiators and replacing them with doubles or triples to enable a lower flow temperature when it is still economic say over a ten year period to run at the higher flow temperature of the Samsung HT and avoiding yet more capital investment - if my calculations are right?
Unless you can add enough battery to cover most days’ usage on a single charge, it will be hard to improve on your existing battery. With radically more batteries you can get on to a overnight cheap rate of about 7p, but unlikely to justify changing what you have.
I expect this is the issue, the UK design standards require a higher room temp and the grant is based on installing a heating system that will work for everyone not just someone who is happy with 19c. Your system is unlikely to give the British Standard room temps at a 50c flow when -3c outside.
But economically for you, I can’t see any justification for upgrading the radiators, even if you sold your home, it will not be worth more with larger radators.
I have Octopus Cosy which has three cheap rates and one peak rate so I only need 4 hour storage. Under moderate heating conditions with temperature set backs and 7.2kWh battery I only use 25% on normal rate, nothing on peak rate everything else on the cheap rate 12p and that results in just 15p per kWh average. Quite a saving and is more than any small changes I can make to COP I think.
Proper PWM controlled fans at sufficiently low speed are inaudible. Apart from the ones in my bathroom which run on full blast to give 23-24°C, I can hear none of my 120 or 140 mm fans. Have a look at my project here:
I guess that is the point I am making - no MCS grant needed for my simple replacement of an existing oil boiler so I can do what I like and save on the capital cost. If there are no grants the owner can compromise on comfort if they want just like the fossil fuel systems they replace. I can always turn the FT up in very cold weather to get 20C or more this would not need to happen very often so would not add too much to the heating cost, my choice.
I expect in time the grant will be greatly scaled back anyway as the take up increases and the additional regulations, conservative choices of additional equipment and the training costs start to drop out, similar to what happened with solar.
MCS SCOP ratings should give a better idea of the efficiency over the whole heating seson. For the Samsung 8kW HT they are are:
3.66 @ 50C
3.98 @ 45C
So dropping by 5C would give an 8% saving.
Dropping to 40C would give SCOP of 4.3, so a 15% saving
If you’re expecting a SCOP of around 3 running at 50C it seems like something’s not working as well as it should. Are you using weather compensation?
I used a buck converter to reduce the voltage of 12 volt computer fans and turn them down until you can barely hear them but there is still sufficient flow to make them effective. I do recommend fans they are cheap and effective. I bought a size that will clip between and to the bottom of a double radiator with a strip of foam rubber packing. I put them 200mm apart.
Hi thanks for that information. It confirms my calculated saving for a 5C flow temperature drop which is very helpful. I do have weather compensation operating at all times and the system is nicely in balance on both mild and cold days running continuously. My estimate of a SCOP of just over 3 includes a lot of DHW so my space heating performance is probably not wildly out. I will know better by the end of the season.
No, but I am planning to get them, based on larger batteries not geting a full cycle most days, compared to a small battery getting nearly 3 complete cycles a day, so not needing as high quality cells if 30kwh.
The other side of this calculation to work on is the cost of the radiators. That is, upgrading a single panel to a triple panel of similar width might be doable as a DIY job without draining down the system, if you can arrange to use (mostly) the same pipework, depending on your competence level and ambition. Then, it’s just the cost of the rads and fittings, which presumably makes it considerably cheaper.
The other thing people often do is work on the least well heated rooms first - i.e. that bathroom on the side return with 3+ external surfaces and just a little towel rad. Increasing thermal comfort in those rooms and reducing flow temps at the same time makes addressing them a no-regrets early measure. So: go one by one, knocking off the low hanging fruit first.
Thanks . That is good advice and will take that up. Changing the radiators where they can easily be swapped out by the DIYer and are most likely to increase comfort will be most cost effective. This approach is likely to pay for itself over a reasonable time frame of say 10 years.
As footnote I should say that after improving basic insulation (DIY again) installing a heat pump has been the best upgrade to my victorian property I have ever done. I would never want to go back to an oil burner. (the end user just needs to understand the HP must be on all day to be able to balance heat losses which some home owners seem unable to accept)
Can I build on Davids suggestion but with a twist. If you fit a larger radiator in the room you want warmest then as well as having that room toasty you can often daisy chain the radiators down to increasingly smaller/ less warm rooms. I only bought a couple of new radiators for my house and just refitted the others into different rooms. It worked well and was cheap to do.
I had a 12 KWh Samsung ASHP fitted in mid November and by 31st December had balanced the flows with lounge and hall room temperatures at about 19 C. I have 13 radiators of which 8 were upgraded. Your COP sounds a little low to me. During January my SCOP for the month was 3.79 based upon an average outside temperature of 3.27 C. February to date is SCOP 3.81 based upon an average outside temperature of 4.5 C. I have uploaded a correlation graph for my data of temperature v COP.
Thanks all. I like the idea of reusing the double radiators that are replaced with triples to reduce the waste and cost. I will upgrade three radiators and add one more in an extension and that should improve heating SCOP which currently looks like 3.2 and I would be happy with 3.5+ with a retrofitted radiator system. (I do not have a way of separating DHW yet so not an accurate measurement).
This change should then save up to £120 on space heating cost per year so worth doing.
I have also heard that the resulting increase in volume along with the lowering of return temperatures should reduce compressor cycling on my Samsung HT 8 kW.
