EverHopefull Posted February 4, 2019 Share Posted February 4, 2019 This may be a completely stupid question as I do not currently have an ASHP but it is factored to be used in my build. Ok, be gentle with your replies. If the ASHP external unit is happy to operate within a range of temperature and it starts to struggle in the recent spells of -7 or below which is when you require it to work for comfort more than ever. Would it not be possible to have the unit itself one stage removed from being outdoors and take advantage of a warmer environment to boost performance. Maybe in a slightly insulated out building? Only a thought! Link to comment Share on other sites More sharing options...
A_L Posted February 4, 2019 Share Posted February 4, 2019 9 minutes ago, EverHopefull said: Would it not be possible to have the unit itself one stage removed from being outdoors and take advantage of a warmer environment to boost performance. Maybe in a slightly insulated out building? Unfortunately the amount of heat stored in any fixed volume of air is too small for this to work. ASHPs process several hundred cubic metres of air per hour to obtain their energy and one/two passes would almost certainly reduce the temperature to no higher than the outside. After this it would be colder than ambient air. Link to comment Share on other sites More sharing options...
Jeremy Harris Posted February 4, 2019 Share Posted February 4, 2019 2 hours ago, EverHopefull said: This may be a completely stupid question as I do not currently have an ASHP but it is factored to be used in my build. Ok, be gentle with your replies. If the ASHP external unit is happy to operate within a range of temperature and it starts to struggle in the recent spells of -7 or below which is when you require it to work for comfort more than ever. Would it not be possible to have the unit itself one stage removed from being outdoors and take advantage of a warmer environment to boost performance. Maybe in a slightly insulated out building? Only a thought! Sadly this doesn't work well, as an ASHP shifts a lot of air, many times the volume of an outbuilding per hour. Also worth looking at the variation in the amount of heat available in the air for a given outside air temperature. The heat energy in the air is zero at absolute zero, -273°C, so air at -10°C has about 3.5% less heat than air at 0°C, or about 5.3% less heat than air at +5°C. The effect of varying air temperature isn't that great, in terms of heat pump performance, at least until you get down to the lowest working temperature of the refrigerant used (typically around -20°C for R410A). 1 Link to comment Share on other sites More sharing options...
ProDave Posted February 4, 2019 Share Posted February 4, 2019 If you put it in an outbuilding, it would very soon cool that outbuilding to lower than ambient temperatire. I suspect the climate here in our pat of the Highlands is the most extreme of any on here. In the last 2 weeks we have had temperatures down to -14 one night, more typically -10 at night, and not above 0 in the daytime My ASHP has been keeping the house warm througout this.. I monitor the electricity consumption for heating and hot water independantly, and the HW usage goes up when it is really cold and I observe the heat pump defrosting from time to time when heating the hot water, but when working a lot less hard to heat the lower temperature for heating, it rarely if ever needs to defrost. Another factor is my house is very well insulated, so I only heat it in the daytime, so when we get those very cold night temperatures the ASHP is not actually working which will help a little. Link to comment Share on other sites More sharing options...
EverHopefull Posted February 4, 2019 Author Share Posted February 4, 2019 Ok that all makes sense. Maybe I am letting the posts regarding the inability of some installs to provide the required comfort levels worry me too much. I really could not face the prospect (or the other half to that matter) if the day ever came that once living in our completed modern marvel that it was deemed not able to keep us warm enough. Our current 15 year old house over three floors is a drafty place that is a complete pain to keep evenly heated as the middle floor (front door) is open plan, letting cld air downstairs to the bedrooms and all the generated heat to the top floor! We did toy with fitting aftermarket heat recovery to attempt to balance the temp between floors or at least circulate it evenly but thankfully the prospect of new build has saved me from that not working as well! Link to comment Share on other sites More sharing options...
ragg987 Posted February 4, 2019 Share Posted February 4, 2019 19 minutes ago, EverHopefull said: and it starts to struggle in the recent spells of -7 or below This will depend on sizing more than anything else. An over capacity hp will not struggle, a 'just-right' one will be running at, or over, capacity. Though do not way over size as it will cycle excessively in milder weather. A hp will typically modulate up to 30%. Having lived with one through 3 winters, I suggest you don't overthink this, just make sure the sizing takes into account the heat demand. Link to comment Share on other sites More sharing options...
EverHopefull Posted February 4, 2019 Author Share Posted February 4, 2019 It does highlight to me that there are possibly a lot of people that have been sold the heat pump solution by inexperienced people that either offer poor advice, incorrect spec devices for the property and or lack of knowledge by the installers when setting up and handing over the systems. Leaving a home owner high and dry for the winter is probably a very upsetting and sobering moment after such huge efforts and probably financial commitment. 1 Link to comment Share on other sites More sharing options...
Jeremy Harris Posted February 4, 2019 Share Posted February 4, 2019 FWIW, I've found that our re-badged Carrier ASHP seems to modulate over a wider range than 30%. Input power seems to vary from around 700 W minimum, to around 1,800 W maximum (although the manufacturers maximum input power rating is 2,320 W). I've no easy way to accurately measure output power, but the rated maximum output is 7 kW and a rough calculation based on ∆t and the UFH flow rate indicates that the UFH tends to use around 2.8 kW when running. The ASHP rarely ever runs at maximum power for more than a minute or two; when running the UFH it throttles back to around 700 W to 800 W input within a few minutes of starting up and will just sit running at that level for hours on end, without defrosting. When I initially installed it I did experiment a lot with the settings, primarily because, as supplied, the settings were hopeless for the typical conditions we have in the southern part of the UK. The biggest problem was defrosting, which has a pretty big impact on efficiency, as the heat pump reverse-cycles to defrost. By experiment I found that the worst case for ice build up was at an outside air temperature of around 4°C to 5°C, when the RH was high (typical cold, wet, winter weather). As soon as the outside air temperature dropped below zero the icing pretty much went away and the performance improved markedly as the temperature dropped below zero. By limiting the ASHP flow temperature to 40°C maximum I found that I could restrict the heat output power to a level where the ASHP stopped needing to ever run a defrost cycle. Our ASHP is significantly over-sized for our heating requirement though, and this definitely seems to make a big difference, as by modulating the compressor and fan speed down it seems to operate with a high COP (always better than 3.5) in cold weather. Link to comment Share on other sites More sharing options...
jack Posted February 4, 2019 Share Posted February 4, 2019 Absolute worst case, some form of resistance heater can always be employed to supplement the ASHP output on the very coldest days of the year. For example, judicious use of a fan heater would be a low capital way of topping up any shortfall. By way of example, we got by through all of last winter with a 1.5kW column heater in our kitchen after our ASHP packed up. I should add that this was the only proper heat source we used to bring the house up to nearly 20 deg C from the 14.5 deg C it reached while we were away for three weeks over Christmas 2017. Sure, I would have preferred parts of the house to be warmer at times, but it wasn't the end of the world. Link to comment Share on other sites More sharing options...
Jeremy Harris Posted February 4, 2019 Share Posted February 4, 2019 Just now, EverHopefull said: It does highlight to me that there are possibly a lot of people that have been sold the heat pump solution by inexperienced people that either offer poor advice, incorrect spec devices for the property and or lack of knowledge by the installers when setting up and handing over the systems. Leaving a home owner high and dry for the winter is probably a very upsetting and sobering moment after such huge efforts and probably financial commitment. I'm afraid you're probably spot on with this observation. When I was looking around at ASHP suppliers/installers initially I was given some really wild options to meet our requirement. It was getting some barking mad specifications for systems, together with very high installed prices, that made me investigate buying and installing a ASHP myself. That turned out to be both a big cost saving and also a useful exercise in learning about the way heat pumps work in a real installation. Link to comment Share on other sites More sharing options...
EverHopefull Posted February 4, 2019 Author Share Posted February 4, 2019 I can accept that there will be moments in the lifetime of any heating system that there will be "outages" due to breakdowns and possibly power cuts. I plan to have a Bioethanol type fireplace in the living room to give the backup in that instance. Only planning on about a 4kw version which I hope will provide a boost at the time of year that a flame gives a psychological uplift as well as an emergency to cover breakdowns and no power. I guess underspec does occur with conventional boilers and rads but I cannot ever remember hearing of any such situation in 30 years of home ownership. Could we one day get phone calls and emails bombarding us about being miss-sold heat pumps in the vain of PPI....... 1 Link to comment Share on other sites More sharing options...
jack Posted February 4, 2019 Share Posted February 4, 2019 Our plumber nearly choked when I told him we were considering a 5-6kW unit for our 290m2 house. He wanted to install a 15kW ASHP as a minimum. His main concern was how cold the house would get during the 1-2 hours that the ASHP would be heating the cylinder early in the morning. In reality, it takes something like 24 hours for the house temperature to drop by any significant amount! Link to comment Share on other sites More sharing options...
scottishjohn Posted February 4, 2019 Share Posted February 4, 2019 (edited) I know when Iooked at ashp and contacted plumbers etc the list of those i would consider got short very quickly after asking questions and I do not profess to be as switched on as alot of others here . but you could tell by responses that most had done very few and waffled alot . It was that that made me eventually go for eco dan unit with plumbed cylinder package --as it was pretty well a simple fit . even then the programming confused them an and alot of calls to mitsubishi technical fair enough though --when sorted it has run faultess i just leave it on 24/7 and let the room thermostats control temps . what i can say is it has cut heating bills by £1000 a year over the lpg --or 2/3rds of heating costs this house is no where close to even a good new build TF --as far as insulation Edited February 4, 2019 by scottishjohn Link to comment Share on other sites More sharing options...
joe90 Posted February 4, 2019 Share Posted February 4, 2019 (edited) My HP is only 5Kw and heats a 240sm house and DHW, the downstairs is a constant 21’ and upstairs 19’ (give or take) I have no consumption figures yet . We also have a wood stove and when that’s lit the lounge gets up to 30’ (er indoors thinks that’s a nice temp ?). We have no heating upstairs apart from electric UFH in our en suite and electric towel radiators in both bathrooms and my backup plan fir a very cold period is a couple of electric panel radiators for the bedrooms. I recon it’s not worth the cost of upstairs heating fir the couple of weeks a year it may be needed. i did specify an immersion heater in the UFH buffer tank so if the ASHP failed we could still heat the house if at additional cost. Edited February 4, 2019 by joe90 Link to comment Share on other sites More sharing options...
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