The subject of heat pumps is being brought up from time to time in various threads. As the subject is dispersed it isn’t so easy to view all relevant thoughts and facts.
Maybe we will be getting a category for heat pumps one day (that’s a prompt to you, @mattfranklin!) but meanwhile please use this thread. Doing so will help others, particularly newcomers, to find what they are looking for.
Joining @Russell_Selby @Christian_Unger in discussing the criticality of heat pump size I will stick my oar in.
The correct size for a heat pump is one that meets your heat requirements 90-99% of the time.
The cost of buying and running a bigger heat pump is too much. It is cheaper to have an additional heat source, such as an electric radiator or fan heater.
However, the government is not famous for understanding the nuances of energy usage and the BUS grant requires the heat pump installed to supply all heat to the building, thus contributing to anti-heat pump publicity about high running costs and discomfort.
Modern heat pumps modulate down to ⅓ to ¼ of the rated output. The times of the year when less heat is required the heat pump needs to cycle. Starting up the compressor requires a disproportional amount of energy. The heat pump is most efficient when not switched off and on. A lower powered heat pump would run for longer periods at a time, increasing efficiency.
This is before you even consider appropriate flow temperatures and Δt, on which the COP depends.
Cool,
So imagine this situation, lets take DHW from the discussion at this stage.
You have a heating load of 10kW on the coldest day of the year. And have a 12kW heat pump to satisfy that. You are suggesting that a heat pump minimum power threshold 1/4. 3kW roughly. As you suggested time, it may be best to have another heat source for those very cold days when efficieces get so low it becomes financially silly.
Then you upgrade your home and your heating load drops to 6kW, then the larger heat pump should still be able to manage at loads as little as 3kW. @Christian_Unger What you are suggesting is that that the issue could arise on the mildly cold days? where the heating demand is below 3kW and the system may be short cycling?
Poviding that the demands is not massivly below then maybe a solution would be …, with the upgraade of the home, increase radiator size, lower the supply temperature to the radiators and increase the volume of the heating system with a buffer tank? The increased volume will significantly reduce the short cycling?
As far as I am aware the only thing wrong with larger oversized HP is if they are short cycling. and as loing as that is not happening then your golden?
In addition i think some heat pumps can modulate down to 10%, but i will try and find some example. BUt if you know your going to refurb down the line, than a low modulating HP is a must.
Go for a maximum 9kW to satisfy most of that. As you retrofit your home you will quickly find it is adequate without supplementary heating. A 9 kW heat pump is cheaper and physically smaller than a 12kW one. The physical size of the unit is important for most people.
You should avoid buffer tanks if possible and increase volume with larger radiators and possibly a volumiser.
A buffer tank has 4 plumbed connections: flow and return from HP, flow and return to the house. Balancing the two different flows and returns is a nightmare that leads to wasted heat and decreased COP. In fact normally no effort is made to balance the flows.
A volumiser has 2 plumbed connections and serves to increase volume, to some extent hold heat and if plumbed in last thing before return to the HP assists defrosting of the HP.
Volumiser, I have never heard of this but i guess it has the same effect as a buffer tank, without the heat storage emphasis.
I will look into them cheers.
Adding a radiator for the purpose of adding more volume, I get. Given that a low temp radiator still runs hotter 45 degrees. Whilst the UFH runs say 35. Then would the radiator just feel obselte in regards to he heat that is emitted if it has 35 degree water through it? I think I like the idea of these volumisers, i guess hidden somewhere, instead of adding radiators, to what could be a radiatorfree house.
FYI-I am assuming for efficiency all emitters use the lowest and the same flow temp to opmtise efficiency. And remove the need of mixers which required wasteful heating cooling.
thanks Tim
Ideally for a heat pump all emitters should have the same flow temperature and there should be no zoning. Typically UFH is downstairs only, or maybe downstairs in an extension only. The radiators are for the rest of the house.
Given that heat rises only a relatively small surface area of radiator is needed upstairs but any downstairs rooms without UFH need large radiators to heat both the room they are in and to a lesser extent the one above.
Also, ideally there should be no TRVs to restrict flow. Instead of restricting flow reduce the flow temperature. In my own house I have eTRVs and radiators throughout. All thermostats are set to 30°C, i.e. always open, except bedrooms. This ensures a regular flow. There are no room thermostats to switch the heating on and off as this is extremely wasteful. Instead I use weather compensation to adjust flow temperature.
I just use the output from the eTRVs to monitor room temperatures.
See also my thread on starting off with a Daikin heat pump.
yeah I hear you. I guess I would recommend UFH throughout when gettin a heat pump if possible.
Just to clarify , thermostats at 30 your refering to the flow temp not the temperature in the room? that would be a hot house!
Good to hear that running 30 through your radiators works just fine. Yeah and of course , one temp, no zoning, as little flow restriction as possible. maximing the volume and minimise the temeprature difference between soruce and sink.
30°C is the maximum room temperature that can be set. I am using it to ensure the valves don’t shut and maximum flow rate is maintained. The eTRVs could equally be replaced by fully open lock shield valves other than their monitoring function.
Rooms never achieve such high temperatures. The flow temperature varies according to the weather but is generally in the 20s.
Bedroom valves do restrict flow to keep those rooms cooler.
AHHH ok ok, I was a little confused but that makes sense. Well now i am even more inpressed that your flow temperatures are in the 20s. And you seem to live in a cosy house. Bedrooms 21 to 23 degrees. downstairs hallway 20-22 degrees.
I guess thats the heat rising, as you mentioned previously.
Weather condensation is so important.
Thank you
That flow temperature is achieved for a 4 bedroom semi with “solid” masonry walls with no wall insulation and suspended timber floors with minimal insulation. 7ach @ 50Pa
On the plus side I have 200mm loft insulation, (poor) triple glazing and MVHR. My radiators are all oversized, multi finned aluminium.
By the time I’ve done some proper retrofit I should be able to reduce flow temperature further.
With regards to low temperature radiators, are there any good ones that can be recommended for the heat pump and may also be working for a boiler until I can change to a heat pump? I may need to exchange some of the radiator while I do put in new larger diameter pipes when I insulate floors and was wondering what to look out for.
There is no difference between a radiator for heat pump and boiler. It depends on room size and room heat loss together with temperature difference between circulating water and room temperature. The lower the temperature difference the larger the radiator will need to be to emit enough heat.
You want radiators with the largest surface area you can fit into the available space. Roughly speaking the larger the surface area the higher the COP once you get a heat pump.
I bought my oversized radiators in 2005-6, with an added one a year later. I was getting a condensing boiler for the first time and wanted to run it at minimal temperature to get as much condensation as possible, and thus gain as much latent heat of condensation as possible. I tuned the flow temperature to 40°C with manual adjustment to 45°C in really cold spells. I never really understood why anyone would run such a boiler at temperatures higher than 50-55°C.
I even asked for advice when I saw a vapour flume from the flue when there was frost on the ground. I didn’t see one at higher outdoor temperatures and thought I’d done something wrong.
I’ve been holding off from commenting but feel it’s only fair to people considering an ASHP to point out the noise can be very disturbing.
Part of why I’ve held back is because we are getting ours looked at by an engineer so ours may not be a good example, although the service engineer didn’t seem to think it was out of spec.
Watching YouTube you’d be under the impression these things are virtually silent in their enthusiasm to get people to move away from gas but ours is loud enough, often enough, to be noticeably annoying over the sound of the TV. This is through closed double glazing over 10m away. It’s a noticeable drone for a much higher percentage of the day than I imagined.
Don’t get me wrong, I’m hugely in favour of them, their reduction in carbon and the quality of heating supplied but the noise is causing some tension in our house and I can only imagine how that would feel to a neighbour, although this is not an issue for us.
NB I have long been convinced ours has been specified several Kw more powerful than required (16Kw) so most people will end up with something half the capacity and likely quieter but even months into ownership it’s not something we’ve got used to. My hope ours is an outlier to the majority experience.
@Jonathan_Keenan , your reply is concerning. It is good that you state the facts and let others decide. As you say, you have a huge heat pump and possibly it is either faulty or poorly installed. Please keep us updated on the engineer’s findings. Is this the engineer that fitted it or an independent engineer?
My heat pump is maximum 6kW output and totally silent when I’m standing next to it on a cold (ie hard working) day. Virtually the only ways I know it’s on are because the house is warm and there is a cold gale blowing out of the front.
Sorry to hear that.
16kW sounds like a beast. I hope the engineer can sort it out.
What model is it? Have you had a look at the manufacturer specs and what it says about noise generated?
I’ve been reluctant to post this as I don’t want people to be put off ASHP but think there is still enough potential for problems for people to take care.
We have Daikin EDLA16DA3V3
Although ours is 5 bedroom house it’s not a huge footprint as it’s all based on conversion of a 1930’s semi so a fair bit of the outer shell, walls and roof is now up to modern building standards. Seems to me people with older houses with bigger rooms are getting smaller ASHP than us.
When I get a chance I’m going to go back over my heat loss calculations and get Octopus (our installer and energy provider) to get the Daikin engineer to look over it. TBF Octopus did send someone out quickly and have already offered to get the Daikin engineer out so we will see.
Very glad others are getting a far quieter experience as I can’t imagine a row of houses all operating one like ours would be acceptable. I’ll keep you posted.
Hi everyone, and great to find this site!
We are downsizing to a bungalow and I am very keen to add green features but without too much disruption. What are our thoughts on adding a small ASHP to power the existing central heating / DHW system most of the time? I would like to retain the existing gas boiler for a boost on extra cold days.
Welcome to the forum @David_Brown.
A hybrid system is perfectly possible but will not qualify for a BUS grant.
If the boiler is your only gas appliance then remaining on gas “just in case” will work out very expensive in standing charges.
How practical this is depends on how low your boiler modulates, which you are currently not in a position to know. You only need to add the kW that the heat pump does not.
@Jonathan_Keenan yes please keep us updated. 16kW seems large, especially as you are working with a more modern fabric.
Is it possible to check the power consumption whilst it is generating on a cold day? Do you have a smart meter, or display?
Is it also loud when it is modulating down and delivering lower power on a warmer day?
@David_Brown with very little infomation: I would think that if your living in a bungalow (high surface area to volume ratio) that has not been upgraded for heat retention. Without any actual calcs, I would lean to think your heat pump COP will be hovering around the critical zone 3-3.5. Especially as the heating system is not optimised for HP
Meaning that it will cost a similar amount to run as your normal boiler, and as Tim said, coming of the standing charge may be the difference.
Welcome David.
I’d like to add to this comment that despite the problems we have had I would not wish to return to a gas boiler and the BUS grant meant that getting our ASHP was significantly cheaper than replacing the boiler. Add to this the financial savings due to the efficiency of an ASHP and, more importantly for us, the reduction in our carbon cost and I would still recommend going all-in on a transition to ASHP for heating and hot water.
In order of disruption I would rank;
The radiator changes had no real impact. Some space is regained where a boiler once sat and only you can decide how disruptive the rest would be for you but hopefully a little planning could mitigate most or all concerns.
