@editor ah that’s a good idea thanks.

Posted by: @marvinator80

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Changing from oil to ASHP we are only having to change some radiators from K1 to K2. Slightly thicker radiators. Same existing fittings.

No doubt that was OK for you, but a comment like this needs an added caveat, that in many cases moving from a fossil fuel based system with rads sized for that system to an ASHP based system will absolutely need new rads. This will become even more the case as more retrofits into older houses (less insulation, more heat loss) happen. Very roughly the change in delta t (temperature difference between rads and room temp) from a fossil fuel system, say 50 degrees, to a relatively high flow temp ASHP system of say 30 degrees, will need rads that have twice the output of the existing ones, either by doubling the area (not the linear dimensions) or moving to a higher output type (K1 to K2 to K3 etc). Unless the fossil fuel rads were hideously over-sized, that means for most fossil fuel to ASHP conversions, the rads will need to be upgraded, sometimes by a considerable amount.

I would even go so far as to say that homeowners considering replacing a fossil fuel boiler with an ASHP should assume they will need to upgrade the rads, with attendant costs and upheaval, unless and until detailed heat loss and rad output calculations show they don't need to do this.

Pipework is more variable, and sometimes the old pipes with minor adaptions will work. My new larger rads went onto the existing tails with 90 degree elbow connections to get the old tail to mate up with the new rad connections.

The key point is that ASHPs are not a cut and paste replacement for fossil fuel boilers. They work in a very different way, then have to be run in a very different way, and in many/most cases there will be major upheaval all round the house, not just in the boiler space.      

Hi all,

I have had A ASHP for just over three years. Was so glad I stubbled upon this forum as seems to be a wealth of knowledge!

Daniel

@cathoderay Wouldn't the extent of replacement depend on whether they are moving onto the original ASHPs or the new higher temperature ASHPs being released now?  As I intend to make the switch in the next year, I have been experimenting with reducing boiler outlet temperatures to see how well my heat emitters cope.  I've dropped it from 70C (that's what the annual maintenance guy sets it at) to 60C and it appears to easily keeping the house at 18C even in the current very cold conditions.  One difference is the radiators no longer feel hot to touch.  I can't really go lower and still be able to heat hot water hot enough for Legionella suppression.

I also looked up and did the heat emitter calculations according to their guidance.  I may need to replace two radiators in the small rooms if using high temperature ASHP.  I had expected worse but perhaps housing regs had improved enough by 1980s to forestall the worst case.  It will probably be more annoying that my rads are on imperial standards and will need pipework mods to fit larger current metric rads.

@ronin92 We are having 8 of the 10 radiators / emitters replaced; our gas boiler when set to 60 degrees C can’t cope in this very cold weather, but, then we are a retired couple with slight medical impairments such that we need 21 - 22 degrees C to be comfortable. Direct swaps to K2’s in most cases but one is in a tight corner with no wriggle room so has to go upwards instead (160 cms.!) The new hot water system is totally independent of the heat pump kit and as the Sunamp Thermino holds very little water and heats the water as required, those legionnaires will have to look for a harbour elsewhere. 

@Daniel, welcome to the forums – we look forward to hearing about your experiences and answering any questions.

@cathoderay In my opinion, the fact that they are not a cut and paste for a boiler could be a problem for many members of the public. 

We have UFH downstairs and cast iron rads upstairs. We tried to replace one of the cast iron rads in our bedroom but our engineer discovered the pipe work had been installed in a strange way by the previous owner (who happened to be a builder who renovated the property) and to replace the rad would have involved taking up a very large part of a very expensive wooden floor (also put in by the previous owner) and probably needing a replacement floor as a result. We decided to take the risk with the existing cast iron rad and thankfully it is emitting enough to get us to around 18 degrees, which is fine in our bedroom. But we can’t get up to more. My point is that installing larger rads isn’t always easy for everybody. 

Plus, people on here tend to be the type of people who want to find out how best to operate their heat pump. Most will want it to be plug and play, and there is a huge risk that people just trust their installers, suddenly find it hasn’t been installed properly and have huge bills and/or cold houses. That will damage the reputation of heat pumps. 

They need to be an easy switch for people that are cheaper to run and cause no issues for people in my opinion if there is to be mass take up. 

Posted by: @ronin92

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@cathoderay Wouldn't the extent of replacement depend on whether they are moving onto the original ASHPs or the new higher temperature ASHPs being released now?  As I intend to make the switch in the next year, I have been experimenting with reducing boiler outlet temperatures to see how well my heat emitters cope.  I've dropped it from 70C (that's what the annual maintenance guy sets it at) to 60C and it appears to easily keeping the house at 18C even in the current very cold conditions.  One difference is the radiators no longer feel hot to touch.  I can't really go lower and still be able to heat hot water hot enough for Legionella suppression.

I also looked up and did the heat emitter calculations according to their guidance.  I may need to replace two radiators in the small rooms if using high temperature ASHP.  I had expected worse but perhaps housing regs had improved enough by 1980s to forestall the worst case.  It will probably be more annoying that my rads are on imperial standards and will need pipework mods to fit larger current metric rads.

The rad/room delta t will be bigger with a higher flow temp ASHP, maybe 30 (rads 50, rooms 20) but almost certainly not as big as with a normal fossil fuel, typically 50 (rads 70, rooms 20). If you look at most rad catalogues, you will see the outputs are given for delta t 50, ie the fossil fuel 'standard'. At lower flow temps, significantly less heat is delivered, and the only way to get enough heat into the room is to enlarge the rads.

Experimenting with lower boiler temps is an excellent empirical way of establishing what your rads can achieve at lower temps. You need to go even lower though, you are only about half way between fossil fuel flow temps and ASHP flow temps. It is surprising how much larger the rads need to be, as I said earlier, a good rule of thumb is a rad will halve its output when going from a delta t of 50 to 30, or put the other way round, it needs to be about twice as large (in area) to put out the same amount of heat at delta t 30 compared to delta t 50.

Good that you are doing the heat emitter calculations, it will help you understand how it all works (or doesn't). I personally prefer the downrated output approach, ie correct the delta t 50 output to a delta 30 output using Stelrad's (or anybody else's) conversion factor, and see if it is enough for the room, if not go for a bigger rad until it is enough. The other approach, it is actually the reciprocal mathematically, is to determine how much bigger the rad needs to be (in area) and then find one that suits is to me slightly less explicit, and has less focus on the actual output in watts.    

You will read this time and time again here and elsewhere, but especially here, you want to run your ASHP at the lowest possible flow temperature that will deliver enough heat, the reason being they become progressively more inefficient at higher flow temps, and furthermore that gets compounded when the outside temps is low.

I personally don't bother with legionella suppresion, in my opinion the risk is very low in sealed domestic systems. One way of getting a legionella cycle if you do want one, and don't want to run the ASHP at very high flow temps, is to have a separate immersion heater that you run for an hour once a week (or not as the case may be). The immersion heater will also I believe satisfy relevant building regs. 

You also need to look very closely at the ASHP power output at high flow temps and low ambients. Some are a lot worse than others, but generally all of them put out far less than the nominal output on the box. My 14 kW Midea unit for example falls to 11 and a bit kW with flow temps at 50 and current ambient conditions. At the same time, the COP also falls through the floor. A double whammy.      

Posted by: @knukes

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In my opinion, the fact that they are not a cut and paste for a boiler could be a problem for many members of the public. 

We have UFH downstairs and cast iron rads upstairs. We tried to replace one of the cast iron rads in our bedroom but our engineer discovered the pipe work had been installed in a strange way by the previous owner (who happened to be a builder who renovated the property) and to replace the rad would have involved taking up a very large part of a very expensive wooden floor (also put in by the previous owner) and probably needing a replacement floor as a result. We decided to take the risk with the existing cast iron rad and thankfully it is emitting enough to get us to around 18 degrees, which is fine in our bedroom. But we can’t get up to more. My point is that installing larger rads isn’t always easy for everybody. 

Plus, people on here tend to be the type of people who want to find out how best to operate their heat pump. Most will want it to be plug and play, and there is a huge risk that people just trust their installers, suddenly find it hasn’t been installed properly and have huge bills and/or cold houses. That will damage the reputation of heat pumps. 

They need to be an easy switch for people that are cheaper to run and cause no issues for people in my opinion if there is to be mass take up. 

"Installing larger rads..." Absolutely, that is why I have tried to highlight the pitfalls. There are many constraints. In my house (an old small cottage with low window sills in the upstairs rooms), it is available wall space, and no sensible practical way of changing the pipework, eg it is embedded in a solid floor downstairs. Folks need to think through what is really needed beforehand, if they are not to find, as this forum shows only too well, they have been sold a pig in a poke. 

You are also right about the people here, we are a self-selected group. Most of the population will want, even expect, plug and play. Some wont have the numeracy to be able to do or check the calculations for themselves. Given that many installers are less than perfect, and heat pump manufacturers and sellers are dishonest in their headline heat pump outputs, its a minefield. Some installers for example (I had a couple in my quotes) simply say the existing rads will do, so they can give a lower quote, when even a cursory examination of the outputs shows they wont.      

Another general observation while I am at it: you need to think of a heat pump as a Steady Eddy that runs low and slow. It is a tortoise, not a hare. It can get there, but it is much better as keeping your house at a steady temp once it has got there, rather than heating the house up from cold. My house, for example, can take days to warm up from even a small drop in inside temperature. Again, this has a bearing on rad size, if they are too small, or just marginal, the system, not just the heat pump, will struggle to put enough heat into the building to warm it up. 

Posted by: @knukes

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people on here tend to be the type of people who want to find out how best to operate their heat pump. Most will want it to be plug and play, and there is a huge risk that people just trust their installers, suddenly find it hasn’t been installed properly and have huge bills and/or cold houses. That will damage the reputation of heat pumps. 

The lack of basic knowledge on the overall subject of home energy is frightening, let alone the introduction of heat-pumps into the mix. This leaves the bulk of UK population at the mercy of cowboys and scammers. Just an hour ago I received a cold-call from a gentleman with a SW Nigerian accent informing me that he was from "the Government grants department". You can imagine how the conversation went from thereon in. 😉 

Whilst most home-owners won't ever know enough to specify, maintain and optimise a heat-pump system, I still think we should be aiming for:

• separating production of the home energy survey (PAS2035) from the tendering process and work of prospective installers
• a HP installation with remote monitoring. The installer should then view the data remotely and see if a site-visit is required to increase system efficiency. Optionally, the home-owner could even allow access to one of the specialists here on this forum, which would save us lengthy posts requesting details of what's happening
• promoting community energy groups who become a knowledge-base of what needs to be considered and who are the best local installers. If there isn't one - start it... preferably in a neutral pub-setting to encourage discussion
• consider the storage aspects of any HP installation. This can be thermal-storage or electricity. This increases the initial cost but adds flexibility and resilience, especially if the design allows operation during a power-outage. These aspects are becoming more important as we migrate towards renewable-energy sources.

@knukes i posted on the forum a few weeks ago basically saying that the whole subject of heat pumps and the calcs/heat curves/flows etc is not understandable to the vast majority myself included (and as the owner of a specialist letting agency and sitting of committees of professional letting bodies with CIH qualifications and retired Royal Engineer)

90%+ of potential ASHP users are going to be swamped then sunk by the vast swathes of technical jargon and calculations they come across while trying to investigate heat pumps. Once we have spent thousands there is the incredible anxiety that comes with the massive usage when we hit the cold spells as we just have. I have spent a lot of time looking at usage etc and over the last 10 months my SCOP is 3.3 on 2x 7KWH another so from valiant. We have the main house between 19.5 and 20 the back end about 14 (mainly because the new rads were put in after I had had a built in library and the rad has to sit under the shelves so we lose a lot of heat and the is nowhere else) 

The system works to my spec and I am saving money on my old LPG/wood burner as well as being greener. I have also just installed a ground based 8KWH solar system + battery to augment the old 4kwh roof system. Over the last 6 months this has pumped in about 4700 kwh but of course most in the summer. BUT

The market seems full of contractors who will pop out (or in one case worked out what i needed by looking on line at my old EPC!) wonder round and cobble together a report and suggestion cost 15K plus 10k to replace all rads. There is no quality control available MCS seem to be a sales sticker only needed for grants etc and reviews and blurb are worse than useless even misleading. One firm I approached which looked good on paper admitted when pushed they had only fitted 1 pump to date. I appreciate everyone needs experience but at over 22k I'm not will to be a Guinea pig.

When we looked into all of this we calculated that a monthly electricity ddm of 220 would cover all our household use plus heating and hot water (much tumble dryer dishwasher and EV) my estimated forward projections on say .50p pkwh show 625pm ddm and this allows for our own solar as well. Admittedly this is hopefully worst case and push comes to shove i will just use my wood burner and run a cold house as we did for 12 years - but as bones get old the warm house is attractive.

One additional worry is will the heat pump set up put off potential buyers if we ever come to sell

so we need

a full professional body controlling the installation of installers

we need acknowledgement by Govt that those complying with their drive to install heat pumps are rewarded by some form of rebate on electricity (say remove the green tax on those using heat pumps - one use for the MCS cert!)

a rapid reduction or simplification of the jargon and large expanse of options to play with pumps 

and for those with the knowledge continuation of this website