Posted by: @lucia

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My question - brain knot - is am I better going 4KW with more /bigger rads or 6kw with more/bigger rads

We have a heat demand of 3.5kW at -9 and are using a 6kW heat pump, which outputs nearly at 7kW at 12 degs and just under 6kW at -10. It cycles, but importantly doesn't short cycle. It will run for 10 to 15 mins and then circulate water for the next hour or so, then run for another 10 to 15 mins. It is a normal capacity control.

I would go for a 6kW, remember you need to heat your hot water cylinder also. My 6kW heat our 210L twice a day and it takes around 40 mins at a time. The 4kW will take 50% longer.  So if you choose 4kW your heating will be off for 2 hrs minimum a day.

The colder it is outside and the longer the heating spends off the more the heat pump has to play catch-up, so running full load instead of part load. On the coldest day the 4kW may never catch up, so runs at full load all day.

I would choose the 6kW every time

@johnmo 

Thank you soooo much for this reply - it was exactly the kind of informed response I was hoping for. 

Efficiency/running costs are fundamental for me.

Also, I feel the cold via humidity too and not just temperature. In the winter, even if it's say 8-10º outside, if it then rains I have to turn the heating up or I get cold. 

I'm not sure how they'll respond to the rad upgrades but my fingers are crossed because it will give me more leeway with lower flow temps. 🤞🏻

@lucia My octopus survey has just been completed and he has put a note on my survey that I requested a 45C design temp so they are listening? Out of interest his heat loss calc was 5.25 kW as BG was 3.46 kW! and Heatpunk gave me 4.9 Kw.

@jancold that's interesting. I'll talk about my design temp in my next post. 😁

OK Houston we have lift off 🚀 🥳

So 🐙 just phoned here's the deal:

6kw Daikin 

My two extra rad upgrades (K2s in hall and bedroom) accepted. ✊🏻

One year's free service plan. 

Big NO to 45º design temp (with the implication that was pushing it re changes to original design) but I think it's no worries because I got the rads so I have more room for manoeuvre anyway. 

Contract on its way.

I think that's the best I can do? Feedback gratefully accepted. 🙂

@lucia Congratulations - you may now look forward to a comfortable warm winter and lower fuel bills! Toodles.

@toodles I hope so to both. 🤞🏻

Posted by: @lucia

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Posted by: @toodles

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@lucia Sounds to me as though you have made great progress (and I suspect, Octopus staff have learnt a few things about customer’s want and needs and heeding them!). Did you manage to get them to design out a LLH or buffer tank? Though I could not employ Octopus to install my system (as relayed ad nauseam elsewhere), I still think they will install a good system at a good price - and at least they are a company that listen and have the resources behind them to rectify any problems. Regards, Toodles.

I agree @toodles that for me with limited resources they are the best deal - plus I really liked the lead installer (who may not be my installer -dunno yet). 

But I don't know if I have made great progress ... that's the problem. 6kw is the result BG came up with if designing for no cavity wall insulation.

4kw was borderline for Octopus. 

My question - brain knot - is am I better going 4KW with more /bigger rads or 6kw with more/bigger rads (hall/bedroom as identified by me above)? 

I need to keep running costs as low as possible without getting cold. 🤔 

It hasn't got a buffer it has a volumiser and I'm not pushing back on that. 

I don't want the short cycling probs of a too big heat pump but I don't want a too small one working flat out when it's cold or wet. 

 

From your information you need 6kW. Bigger radiators DON’T give you the heat output you need, all you would have is lower temperature flow and a too cold house.

You seem to have won a lot from Octopus and you can always change the radiators in the study and hall in the future if leaving the doors open doesn’t get the rooms warm enough.

Obviously if you get 2 bigger radiators then that’s a big win for you but you have won enough with the 6kW.

From your information you need 6kW. Bigger radiators DON’T give you the heat output you need, all you would have is lower temperature flow and a too cold house.

You seem to have won a lot from Octopus and you can always change the radiators in the study and hall in the future if leaving the doors open doesn’t get the rooms warm enough.

Obviously if you get 2 bigger radiators then that’s a big win for you but you have won enough with the 6kW. 

Er?

Sure, I know what bigger radiators do - obviously they permit me to run at a lower flow temp which is precisely why I lobbied for them. Running costs matter to me.  I'm shoe string productions here. 

Thus the two radiators in the living/ dining room were already being upgraded to K2s and now I have two more in the next two most used areas - the bedroom/office and hallway. 

I'm miffed about the 50º design temp but hopefully the bigger rads facilitate dropping that. 

My aim is to run an open loop system at a flow temp of 40º - I think this gets me as close as possible to that for the cost and certainly leaves me much more room for manoeuvre. 

I have to supply the bathroom rad because I want a 'designer radiator' but otherwise I think/hope this will be ok. 

I'll try post the specs and floor plan eventually. 

Now I know the volumiser is not the greatest but Octopus will not fit a system without one. And at least it's not a buffer tank. 

It's a bit beggars can't be choosers but I can't afford those heat geek levels of money. 

@lucia My system was set out on the MCS forms indicating that the calculations had been based on a 50 degree C. flow temperature and that is where the installers aimed all the settings upon commissioning the system. It was a mild day in mid-February 2023 and not a lot of heat was required. The engineers did their bit and as fast as they tried to set up the LWT and then the radiators, I instructed them to ‘now leave that as you initially set it and I’ll do the tweaking’. Before they left, they knew I was lowering the LWT and within a day or two, I had the system down to 35 degrees C. Of course, I couldn’t be sure that this would suffice as it was still very mild. Come the autumn, I found a balance of LWT at 35 and WC set up, things were very stable and only needed a little tweaking when very cold or very mild. When last winter arrived I had the Homely smart controller in place and noted that 35 degrees C was rarely exceeded and we were very comfortable. When the OAT dropped to -7 or -8, Homely would adjust the LWT to about 43 - 45 but I have never seen it exceed this and we like a room temperature of 22.5 degrees C. 24/7. Regards, Toodles

@toodles Thank you that's really helpful.

I reckon I have the basis for a good set-up but installation is not likely to happen until October. 

I don't know why 🐙 are so obsessed with 50º (my installer said cheaper installations) when everyone else, including British Gas, design for 45º. 

When I've got some time I'll play with the very useful radiator output spreadsheet that @derek-m gave me. 

@lucia if your house needs 6kW then that is what you need. There’s nothing you can do with bigger radiators to make a 4kW heat pump give you enough heat power.

Yes you are completely right to consider operating costs and try to get a design and installation which will give you lower flow temperatures by using larger radiators. Good luck with it all. October isn’t too bad as a waiting period and it’s rarely that cold by then.