UPDATED: Air Source Heat Pumps/Air Con - Full Info & Guide, is it cheaper to run than mains gas?

Swipe

My Scottish power online saver 10 tariff was 6.448p before it went up but is now 7.577p (Midlands area). And my heat pumps vary considerably at cold temps depending on humidity. When it's -1C and snowing or foggy they are awful (constant defrost cycling) but at -12C and dry as a bone they work great.

richardc1983

Cardew wrote: »

Agreed! with one exception!

A COP of more than 1 does not necessarily mean a saving over normal electrical heating for those out of the house. Bear in mind the example about 150kWh. For someone working, 150kWh output from an ASHP is less effective than 150kWh used when needed as the some of the ASHP output is being produced when not required.

However anyone reading this thread, and others, are very likely to believe that ASHPs are a 'no brainer' and plenty are recommending them even if gas is available.

Unfortunately bushyh's case doesn't seem to be that unusual. Bear in mind that the systems in the EST trial were not installed by cowboys, some major firms co-operated and the results were poor.

Given the large capital outlay for ASHP systems, they should make considerable savings before being viable.

AIr to Air systems are more efficient than gas central heating systems and cheaper to run - Most people would prefer heat delivered via a radiator/underfloor (air to water) but then this is when it gets complicated as the water has to be heated first via the refrigerant where the heating medium for air to air is plain and simple air which is easier to heat than water.

Cardew

richardc1983 wrote: »

AIr to Air systems are more efficient than gas central heating systems and cheaper to run - .

Back that up with some verifyable facts please(controlled trials etc) - not opinions!

richardc1983

We are going back to the days of Steve head who provided detailed usage statistics of his air to air system.

Cardew

richardc1983 wrote: »

We are going back to the days of Steve head who provided detailed usage statistics of his air to air system.

Hardly a scientific study was it? One man living in Devon(mild climate) with a 3 storey house posting some figures.

He also used gas CH for heating water.

I appreciate that you have huge enthusiasm for 'all things heat pump', but IMO you really should try to be more balanced and objective in your postings on the subject.

This forum is read by many who are considering spending £thousands on a system and they should be aware that as well as advantages, there are considerable disadvantages. If nothing else even the manufacturers don't seem able to get some installations working correctly, and according to the EST report more studies are required to understand the reasons.

I appreciate this is the pantomime season, but this thread shouldn't turn into a 'Oh yes they are - Oh no they're not' debate';)

Cardew

Swipe wrote: »

My Scottish power online saver 10 tariff was 6.448p before it went up but is now 7.577p (Midlands area). And my heat pumps vary considerably at cold temps depending on humidity. When it's -1C and snowing or foggy they are awful (constant defrost cycling) but at -12C and dry as a bone they work great.

Again this 'When it's -1C and snowing or foggy they are awful (constant defrost cycling) but at -12C and dry as a bone' is doubtful in UK.

I have looked at the BBC weather 5 day forecast for several areas in UK and the humidity is high in this sort of weather. This is typical - but check your own area:

http://news.bbc.co.uk/weather/forecast/2123?&search=brecon&itemsPerPage=10&region=uk

86% + humidity during this freezing weather( My area has even higher humidity at over 90%) Leave your car outside and see if any frost forms!

Swipe

It was a figure of speech, not meant to be taken literally. Ok, let me change, 'dry as a bone' to 'not foggy / snowy' then. In other words they are awful when snowing or freezing fog but ok at low temps when dry. FYI, just 10% humidity can make a huge difference. At anything lower than 90% Rhu they rarely defrost cycle even at -10C temps whereas at close to freezing temps at 100% Rhu, if running hard, they can defrost up to once every hour.

zeupater

Swipe wrote: »

It was a figure of speech, not meant to be taken literally. Ok, let me change, 'dry as a bone' to 'not foggy / snowy' then. In other words they are awful when snowing or freezing fog but ok at low temps when dry. FYI, just 10% humidity can make a huge difference. At anything lower than 90% Rhu they rarely defrost cycle even at -10C temps whereas at close to freezing temps at 100% Rhu, if running hard, they can defrost up to once every hour.

Hi Swipe

Guess that your system has been defrosting pretty regularly recently then .... your area of the country has been particularly cold for the past couple of weeks but has been misty or foggy for a good proportion of the time. The air is relatively dry, but the low temperatures are close to the dew point so any surface even colder than the air is causing condensation and which freezes immediately ....

It's -10C here at the moment and it's misty again with the sun trying to break through .... I'm probably not that far from you so, knowing what the weaher has been like, I'd be interested whether you have been recording daily energy usage from your ASHP and measuring the usage against the outdoor temperatures (RAF Shawbury if you haven't got a thermometer ??) .... If you know the house heatloss per degree difference indoor v outdoor you will be able to calculate a rough daily COP and compare this against the manufacturers specification claims ...

Z

grahamc2003

Swipe wrote: »

It was a figure of speech, not meant to be taken literally. Ok, let me change, 'dry as a bone' to 'not foggy / snowy' then. In other words they are awful when snowing or freezing fog but ok at low temps when dry. FYI, just 10% humidity can make a huge difference. At anything lower than 90% Rhu they rarely defrost cycle even at -10C temps whereas at close to freezing temps at 100% Rhu, if running hard, they can defrost up to once every hour.

I'd expect the defrost cycle to be much less once the temp drops below zero. There seems to be a bit of a mix up between relative and absolute humidity.

Below zero, the ability of air to hold water is very low indeed - just a very small absolute amount of water can saturate the air (something like 1% iirc), giving 100% humidity, yet only tiny amounts of water int he air. The ability to store water rises rapidly - so at 7 celcius, you can have someting like 3% water vapour in the air (this rises to 100% at 100 celcius) (at normal pressures).

So at 7 degrees, I'd expect much more condensation and freezing (or more correctly deposition) as the air on the outside unit is cooled from ambient (possibly removing 2% at 100% humidity of the air as water, which forms as ice on the coils).

At 0 ambient, there's so much less water possible in the air (even at 100% relative humidity), cooling it a few degrees can only produce minor deposition (i.e. ice formation). The situation is the same the lower the ambient temps - at mins 7 say, you'd probably get harldy any ice formation (because the air can only hold a small amount of water vapour at that temp).

So beware of relative humidity readings - at 25 degrees, 100% humidity can means lots of water in the air, yet at 0 degrees, 100% can means tiny amount, and at -15 100% means an even tinier amount.

The bummer is that there's less heat to extract anyhow from low temp air, so, ignoring the defrost cycles, the efficiency drops anyhow.

Swipe

zeupater wrote: »

Hi Swipe

Guess that your system has been defrosting pretty regularly recently then .... your area of the country has been particularly cold for the past couple of weeks but has been misty or foggy for a good proportion of the time. The air is relatively dry, but the low temperatures are close to the dew point so any surface even colder than the air is causing condensation and which freezes immediately ....

It's -10C here at the moment and it's misty again with the sun trying to break through .... I'm probably not that far from you so, knowing what the weaher has been like, I'd be interested whether you have been recording daily energy usage from your ASHP and measuring the usage against the outdoor temperatures (RAF Shawbury if you haven't got a thermometer ??) .... If you know the house heatloss per degree difference indoor v outdoor you will be able to calculate a rough daily COP and compare this against the manufacturers specification claims ...

Z

I have been recording daily usage but my living room ASHP is supplemented by a multifuel stove which I have been burning inconsistently depending on the weather. Some days I've been burning wood and switching off the pump and other days I've been burning coal with the heat pump topping the temps between 21/22C. So as you can see my usage data has been somewhat tainted. I've also had the flu for the last week so I've been keeping the house much warmer than I normally would so again this affected my stats. But, yes, I have to agree we are definitely situated in one of the colder parts of the country.

Defrost cycling occurs most when the pumps are working hardest. As I've been off work for the last week they have been switched on much longer and haven't had to work quite as hard as they normally would. They have mostly been maintaining temps rather than generating serious heat. That said, although I have seen quite a few defrost cycles during this cold spell, they been a lot less frequent than I was expecting.