A_L Posted November 23, 2018 Share Posted November 23, 2018 26 minutes ago, PeterW said: I’d be looking at an ASHP that can provide 100% of the heating input at 50% capacity. This means it will have a larger condenser so should defrost less often. Actually isn't it the evaporator that freezes? ? 1 Link to comment Share on other sites More sharing options...
Jeremy Harris Posted November 23, 2018 Share Posted November 23, 2018 4 hours ago, Ballynoes said: I am under the impression that an ASHP is not that efficient in the winter when needed the most, just out of ignorance really, as I don’t see how it can get much heat from a temp outside of say -10. Please feel free to correct me as I am new to all this. There is about 7% less heat available in air at -10 deg C than there is available in air at +10 deg C, so the tiny reduction in heat available in the air to extract using a heat pump is negligible, and doesn't make a significant difference. What does make a difference is the design and sizing of the ASHP, as few can operate below about -18 to -20 deg C, and if badly sized for the heating demand then icing of the external evaporator coil is likely, which hits performance pretty hard. As above, oversize the unit, and choose one that's inverter controlled, so it can modulate right down in output when there isn't a high heat demand and the efficiency can stay high in cold weather. I've found the worst case isn't when it's well below freezing, but when it's between 0 deg C and about 4 deg C, when the air still has a lot of water vapour in in that can easily ice up the colder evaporator. Once the temperature drops well below freezing evaporator icing tends not to be such a problem, probably because in very cold weather the air tends to be a fair bit drier. Link to comment Share on other sites More sharing options...
Jeremy Harris Posted November 23, 2018 Share Posted November 23, 2018 16 minutes ago, A_L said: Actually isn't it the evaporator that freezes? ? Yes, it is, but [pedant_mode_on] the evaporator becomes the condenser during the defrost cycle [/pedant_mode_off]. 1 Link to comment Share on other sites More sharing options...
dpmiller Posted November 23, 2018 Share Posted November 23, 2018 @JSHarris what is the fan doing in that 0-4c zone- Is it at low speed? Is there a chance more airflow might reduce icing? Link to comment Share on other sites More sharing options...
Jeremy Harris Posted November 23, 2018 Share Posted November 23, 2018 46 minutes ago, dpmiller said: @JSHarris what is the fan doing in that 0-4c zone- Is it at low speed? Is there a chance more airflow might reduce icing? As far as I can tell, the fan speeds up and slows down independently of the compressor speed (but bear in mind this is far from a scientific test; all I've done is stand by the thing and listen to what it's doing). My gut feeling is that the fan speed is controlled by the evaporator temperature, so when that cools down the fan speeds up to chuck more air through it, which makes sense, as more air = more enthalpy (in rough and ready terms). The compressor speed seems to be determined by the difference in temperature between the water flow and return temperature. As the water flow rate is fixed for the installation (the circulation pump is a constant power unit, so more or less constant flow for a given set of operating conditions) the flow temperature determines the power that the heat pump needs to work at to deliver the target flow temperature, and this seems to be the primary parameter governing the compressor speed (there may be other factors at play too, like a variable expansion orifice - not sure exactly how that's controlled). There is a definite change in the fan behaviour at about 4 deg C, as it speeds up. There is a humidity and temperature sensor on the rear of the unit that measure the air flowing in towards the evaporator (in heating mode), and that seems to play a significant part in determining when a defrost cycle will start. Link to comment Share on other sites More sharing options...
Stones Posted November 23, 2018 Share Posted November 23, 2018 @Ballynoes I now live up in Orkney, where winter temps average in the 0 - 4C range (rarely colder) and there is high RH due to coastal position. I run an ASHP without issue - over the 12 months March 2017 – March 2018, heating COP ranged between a February low of 3.3 to an October high of 4.6 over the course of the heating season, with an overall SPF of 3.7 DHW COP ranged between a February low of 2 to a summer high of 2.5, with an overall SPF of 2.3 Colder and drier air temperature is in many ways better for an ASHP compared to the 0-4C range. If fitting to a new build and appropriate heat emitters and hot water storage, ASHP can work very well. If you want to speak to someone local(ish) you could try contacting Home Energy Scotland - they hold details of installations which you can visit and get real world feedback from owners (rather than sales pitch). Out of interest, how close to Dunkeld are you? Link to comment Share on other sites More sharing options...
joe90 Posted November 23, 2018 Share Posted November 23, 2018 @Stones, that’s very good figures, what water temp do you have for heating and what temp for DHW? Link to comment Share on other sites More sharing options...
Stones Posted November 23, 2018 Share Posted November 23, 2018 @joe90 Heats to 50C for DHW Flow temp varies (weather compensation) on heating side but I've not seen it more than 33C in winter, albeit generally under 30C. Link to comment Share on other sites More sharing options...
Declan52 Posted November 23, 2018 Share Posted November 23, 2018 3 minutes ago, Stones said: @joe90 Heats to 50C for DHW Flow temp varies (weather compensation) on heating side but I've not seen it more than 33C in winter, albeit generally under 30C. Have you worked out the rough cost in electric over the year??? Link to comment Share on other sites More sharing options...
joe90 Posted November 23, 2018 Share Posted November 23, 2018 19 minutes ago, Stones said: @joe90 Heats to 50C for DHW Flow temp varies (weather compensation) on heating side but I've not seen it more than 33C in winter, albeit generally under 30C. Impressive, mine should be good down here In Devon ?♂️. Link to comment Share on other sites More sharing options...
Stones Posted November 23, 2018 Share Posted November 23, 2018 @Declan52 Based on a kWh electricity unit price (inc standing charge) of 12.3p, I paid 3.32p per kWh of delivered heat, and 5.34p per kWh of DHW (inc losses). Just under £300 for DHW and just over £200 for heating last year (includes amortized cost of standing charge). Been a bit colder / higher wind speed recently so looking like this year will use a little more energy on heating. Link to comment Share on other sites More sharing options...
Declan52 Posted November 23, 2018 Share Posted November 23, 2018 1 hour ago, Stones said: @Declan52 Based on a kWh electricity unit price (inc standing charge) of 12.3p, I paid 3.32p per kWh of delivered heat, and 5.34p per kWh of DHW (inc losses). Just under £300 for DHW and just over £200 for heating last year (includes amortized cost of standing charge). Been a bit colder / higher wind speed recently so looking like this year will use a little more energy on heating. Pretty impressive indeed. When you where doing your cost comparisons how much did you reckon oil would have cost per year. Link to comment Share on other sites More sharing options...
reddal Posted November 24, 2018 Share Posted November 24, 2018 Hi, We have a GSHP connected to a couple of boreholes. It was installed a few years ago. It was expensive - the whole setup was a lot more than the £17k you estimated. However we also got an extra borehole for water at the same time which has worked well. We have had various issues with the system over the years. The biggest problem is that there are very few tradespeople around us who know anything about GSHPs and getting problems fixed has been difficult and expensive. I get the impression that ASHPs are more widely used and there should be more people with experience of them. Anyway an ASHP is a simpler system. The one thing I do like about our GSHP is its ability to provide passive cooling in the summer, where the heat in the house is effectively pumped deep underground for very little cost. An ASHP can provide a similar functionality using active cooling but costs a bit more to run. If I was doing it all again today I would probably go with an ASHP - or even just an old-school oil boiler plus AC for cooling. - reddal Link to comment Share on other sites More sharing options...
Onoff Posted November 24, 2018 Share Posted November 24, 2018 (edited) If I win the Lotto tonight I'm going to have a GSHP just because I've got the land to do so! Wasn't it @Barney12 did one? Or at least laid the pipes? Seem to remember a picture of a digger in/on the edge of a bfo trench... And someone else was in about sinking a loop weighted down with bricks into their lake...? Edited November 24, 2018 by Onoff Link to comment Share on other sites More sharing options...
newhome Posted November 24, 2018 Share Posted November 24, 2018 1 hour ago, Onoff said: And someone else was in about sinking a loop weighted down with bricks into their lake...? You're probably confusing this with someone plotting a murder 1 Link to comment Share on other sites More sharing options...
Nickfromwales Posted November 24, 2018 Share Posted November 24, 2018 3 hours ago, Onoff said: If I win the Lotto tonight I'm going to have a GSHP just because I've got the land to do so! Wasn't it @Barney12 did one? Or at least laid the pipes? Seem to remember a picture of a digger in/on the edge of a bfo trench... And someone else was in about sinking a loop weighted down with bricks into their lake...? “Geothermal” ( aka pipe in ground ) passive brine loop. Link to comment Share on other sites More sharing options...
Ballynoes Posted November 26, 2018 Share Posted November 26, 2018 All very interesting, I just assumed GSHP's were better than ASHP. Not going to be a huge house to heat, and we are planning a log burner for those cold winter nights to cook us. As for electricity, well, when your wife has 18 flood lights round her outdoor riding school, and a 9Kw shower...just for the horses.... you get the idea. About 6 miles from Dunkeld, but in the countryside so we get no shelter from anyone. Link to comment Share on other sites More sharing options...
PeterW Posted November 26, 2018 Share Posted November 26, 2018 Consider having a log or pellet boiler and creating your own district heating system. I know someone who has done this and gets commercial RHI for the boiler, supplies heat to the house via a heat metered underground pipe system and essentially the whole lot pays for itself. And the horses get a free shower ! Link to comment Share on other sites More sharing options...
jack Posted November 26, 2018 Share Posted November 26, 2018 On 24/11/2018 at 11:30, reddal said: An ASHP can provide a similar functionality using active cooling but costs a bit more to run. Not if you have PV and only run the cooling when the sun's out! 1 Link to comment Share on other sites More sharing options...
Nickfromwales Posted November 26, 2018 Share Posted November 26, 2018 9 minutes ago, PeterW said: and gets commercial RHI Formally known and referred to as 'non-domestic' RHI. 20 years of payments too so not to be sniffed at, albeit at a far lower rate than domestic RHI. Link to comment Share on other sites More sharing options...
Nickfromwales Posted November 26, 2018 Share Posted November 26, 2018 1 minute ago, jack said: Not if you have PV and only run the cooling when the sun's out! Tis a no-brainer if you've even 2% or greater overheat risk. Why not let the thing causing the problem, pay to cure it Link to comment Share on other sites More sharing options...
jack Posted November 26, 2018 Share Posted November 26, 2018 6 minutes ago, Nickfromwales said: Tis a no-brainer if you've even 2% or greater overheat risk. Why not let the thing causing the problem, pay to cure it It was a lifesaver during the extended heatwave we had this summer. Link to comment Share on other sites More sharing options...
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