@jeff, welcome to the forum and it's good that you are considering an ASHP.  Being in a conservation areas presents a couple of issues (we are too) with siting of the ASHP but nothing too onerous. 

Although it's very early days for us and we haven't lived through a winter with an ASHP, we don't heat the house all day and it works fine.  The are a few myths about ASHPs and one of them is that the low water temperature in itself means the house heats up slower.  If the radiators are sized correctly for the lower temperatures this isn't the case.  As long as the power output from the radiators is the same, the house will heat up just as quickly with small very hot radiators as with large not so hot ones.  I think (although I don't know) a gas boiler may be able to get the water up to its operating temperature quicker than an ASHP and if so, that will mean the house will heat up a bit faster.  I also don't know if keeping the house at a constant temperature with an ASHP uses less power than switching it on and off; I've heard that claimed but have never seen any proof. A bit like keeping your HW on all the time vs. for a few hours a day. 

The above only applies to radiators; if you have underfloor heating then it's different because that involves keeping the whole floor 'slab' warm and that does take a long time to achieve.  

Please feel free to ask more questions; there is a lot of knowledge and experience on the forum.

No underfloor heating, just radiators and a combi boiler. Not surprising, even with mains gas boilers some radiators are definitely oversized in our house and some undersized even now. This may be a good real world  input when the time comes to switch. I have found it interesting that some of the issues with air source heat pumps appear to equally apply to gas boilers in terms of sizing, radiators, insulation etc.

There was a bit about running heat pumps intermittently and also about high heat air source heat pumps in an article from octopus energy a few days ago. 

https://octopus.energy/blog/heat-pumps/

So it will be interesting to see how things evolve as we get a much wider installation base and a variety of real world examples/solutions in different UK houses.

Our house started with coal fires, moved onto oil fired heating and then mains gas. Am sure it will cope with another type of heating in the future.

@Jeff, welcome to the forums. We've upsized our rads, and I think that this coming winter will be first time that our heat pump may not run 24/7 because I think rooms will come to temperature and the pump will cycle. It will be interesting to see how that pans out for us.

On some level, I feel that oversized/big K3 rads may actually be more effective than UFH running in conjunction with an ASHP. In our house, I think we would see more benefit from this.

When are you considering making the potential switch to a heat pump?

Hi Jeff,

Welcome to the forum.

Many years experience has taught me that there is no 'one solution fits all'. Even supposedly identical homes, on the same street, will have slightly different heat loads, due to shading by trees or other buildings or the cooling effect of the wind etc.

If you can adjust the central heating water temperature from your gas boiler, you could try lowering the setting to see how well your present radiators perform at lower temperatures.

Having read the information from Octopus, without wearing rose coloured spectacles, I agree with some of their statements, but find others quite misleading. Obviously they don't point out that ASHP's are at their least efficient during the Winter months when they are working hardest and being used the most. For most homes with mains gas an ASHP would probably be more expensive to run.

Over the past few months I have periodically checked the 'Agile' tariff, and unless I am mistaken it appears to be creeping upwards. Maybe someone who is on this tariff could confirm.

My advice for anyone wishing to reduce their carbon footprint would be to start with insulation and draftproofing. It is much better to not use the energy in the first place, rather than use some more efficiently.

Posted by: @editor

@Jeff, welcome to the forums. We've upsized our rads, and I think that this coming winter will be first time that our heat pump may not run 24/7 because I think rooms will come to temperature and the pump will cycle. It will be interesting to see how that pans out for us.

On some level, I feel that oversized/big K3 rads may actually be more effective than UFH running in conjunction with an ASHP. In our house, I think we would see more benefit from this.

When are you considering making the potential switch to a heat pump?

 

That is a good question. The reason I thought it was worth thinking about switching is that the RHI scheme is due to finish next year, to be replaced by a single upfront payment. I thought it was worth exploring whether it was worth making the jump now. Also all the climate news made me think about it. I very much doubt it makes economic sense to switch at the moment for us but worth exploring especially as the rules generally around heating and insulation may change quite quickly.

The payback time may be simply too long currently given how long we may stay in the house and I can't image running costs will be lower until the government make more radical long term changes rather than simply RHI payments. 

We have no burning (excuse the pun) need to switch. The current gas condensing boiler is not expensive to run or any trouble and the solution suits the house and our current lifestyle.

The last occupants of the house were independent 89 and 91 year olds and moved to a bungalow to be nearer family so clearly the house can accommodate different lifestyles currently but I suspect with relatively high bills to maintain the expectations of a family. 

Of course there is the green angle and all gas boilers will go over time especially as electricity generation switches consistently to more non fossil fuel energy.

Good luck with the radiators! We have 3 large double panel double fin radiators downstairs with our mains gas heating. From memory most of the rest are double panel single fin. One of the double panel double fin is never turned on. Simply from experience rather than any calculations the larger radiators at least appear disproportionately better in our situation. There definitely at least feels disproportionately more of a convection effect which I have read others talk about as well. So I can see how getting the size, type and from my experience the position of radiators correct is important in any system. We have one radiator that is poorly positioned down one end of a room.

Am hoping to learn more about our house with its current heating, drafts and insulation requirements which I hope will help when the switch comes and will help inform the calculations needed to make sure an ASHP system works. 

We may simply downsize before we decide/have to switch. In some ways this makes more sense as then the next occupants could make changes as part of a full refurb/house extension which we would not do. 

Another very general question as I start to think about ASHP. Just looking for thoughts and experiences rather than definitive calculations. 

We don't have a bath in our house. Just a shower in the bathroom and a shower in the ensuite. Both power showers running off the ventilated tank (Glenhill envirofoam) connected to our gas condensing boiler. Tank timed for 45min once a day each morning. Never adjusted timing since it was installed. Tank thermostat set to 60 degrees. Works fine with no issues.

Am curious about people's general thoughts about water heating with ASHP especially as we don't have a bath. Any advantages to having electric showers when an ASHP is installed so there is less issues with heating water generally. Clearly electric showers are not that efficient but at least you only heat the water you actually need. I can't say we ever wash up. Dishwasher and washing machine are obviously cold fill. Or is water heating generally not an issue with ASHP? Obviously we would need to heat some water for the tank. 

We still have electric showers with our ASHP.  We have the pipework ready to and will plumb in direct (from the tank) showers later. Currently we have the HW on constantly but use almost none; just the handwashing and a few pots and pans that won't go in the dishwasher. With a modern tank, once it heated up, it doesn't take much power to keep it hot.  We will be ditching the electric showers as soon as we can. I doubt it will be an more expensive and the showers will be a lot better.  Your HW won't be as hot with an ASHP, maybe 50 deg, but hot enough for a good shower.  If you have a big family who all like baths you might have a problem.

@kev-m. I’ve just read that an ASHP is incapable of heating water and radiators simultaneously.  How does this fit in with your regime of heating the water 24/7?

I think that is right. I remember our installer saying HW took preference.  What I meant is that I'm not setting a time to limit HW it but letting it heat when it has to.  The heating won't come on at night at the moment so the HW will be up to temperature when it does come on in the morning.  I suppose there might be a conflict when it's really cold and you have a bath.  But that would happen with a ff boiler; there's only so much power it can put out at once.  The boiler can do heating and HW at the same time but each will be sharing the output. The ASHP does it one at a time.  I think ... 

edit - how are you getting on with your ASHP - have you had any firm quotes?

@kev-m

only the one quote from JL Phillips.  Given the impression I got from him and how pleased someone on here (you?) is with their work I will go with them.  Honestly trying to get any tradesman to even respond to a request at the moment is a nightmare.  All busy apparently with folk having loads of money due to not being able to spend on holidays and eating out etc.  Literally just managed to get cavity wall insulation installed.  Now awaiting a guy to remove the back boiler from the Aga, a shower reinstall so that a suitable wall is vacated for a radiator then Phillips will come back and do a full design survey.  I’m confident all will be sweet by onset of winter.

Hi Everyone,

In theory it should be possible to heat your home and hot water, using an ASHP, at the same time, during most of the year, but it is probably discouraged for the following reasons.

1) For the most efficient operation of an ASHP, it is necessary for the water flow temperature to be kept as low as possible, in the region of 35C to 40C. Hot water on the other hand needs to be periodically heated to 60C to prevent Legionella.

2) The consumption of HW will vary from household to household and even from day to day within the same household, so running the ASHP at higher temperatures to provide HW and heating at the same time would not be the most efficient.

In a well designed system, it should be possible for the ASHP to provide water for heating, at a lower temperature for best efficiency, to heat the radiators or UFH, and whilst this heat energy is being transferred into the building, to then produce water at a higher temperature, should it be required, to heat the water in the HW tank. Obviously there could be periods of cold weather and high HW demand, when the system may struggle to satisfy both requirements.

It is possible to calculate the approximate energy requirement for producing HW from the formula below.

It takes approximately 1.2Wh of energy to heat 1kg of water by 1C.

So to heat water from cold (say 15C) up to 60C in a 300 litre tank would require:-

1.2/1000 * 300 * 45 = 16.2kWh

It would therefore take an 8kW ASHP approximately 2 hours to heat the water from cold, whereas a 16kW ASHP would only take approximately 1 hour of continuous operation at maximum output.

Providing heating and HW is therefore a balancing act, that will probably require some time and effort spent optimising your system, to achieve the most efficient method. Do you heat your HW during daytime when the outside air temperature is higher, and hence your ASHP is probably more efficient, or do you invest in solar PV or solar thermal to help provide HW and hence reduce the load on your ASHP?