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

Swipe

grahamc2003 wrote: »

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.

Graham, thanks for the info on relative humidity, seems to make sense and kind of backs up my experience of seeing more defrost cycling at higher temps. I don't think I've even see either of my units go into defrost cycle at temps higher than 3C though, even in misty rain. The worst case scenario, in my experience, is when there is wet snow at around 1C temps.

zeupater

grahamc2003 wrote: »

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.

Hi

Agree with the above .... air can carry less humidity as the temperature falls, but it has been foggy and misty in the area where Swipe has an ASHP, therefore there is relatively high humidity to condense and freeze, it has also been the area of the country with the lowest average temperatures over the last couple of weeks which creates an additional problem with the defrost cycle, that being the energy required for the defrost cycle has to battle against the ambient temperature as well as the temperature of the ice on the evaporator surface which pulls the COP down even further .... this is why I'm particularly interested.

Z

grahamc2003

zeupater wrote: »

Hi

Agree with the above .... air can carry less humidity as the temperature falls, but it has been foggy and misty in the area where Swipe has an ASHP, therefore there is relatively high humidity to condense and freeze, it has also been the area of the country with the lowest average temperatures over the last couple of weeks which creates an additional problem with the defrost cycle, that being the energy required for the defrost cycle has to battle against the ambient temperature as well as the temperature of the ice on the evaporator surface which pulls the COP down even further .... this is why I'm particularly interested.

Z

Yeah, I see the point. If the unit is sucking in snowflakes or mist, then there'll be lots of Ice formed from those, rather than from deposition (i.e. ice forced out of the air due to cooling). Not sure if mist can easily be filtered out, but I'd take steps to ensure no snow was ever sucked inside.

It's becomming clear to me that totally relying on ashps for heating may not be the most cost effective way to use them, especially if this years winter is going to be typical going forward. I think 'bivalent' is the word. I'm likely to get some air to air ones installed, but I think I'd simply turn them off when de-icing cycles become excessive, or the temperature drops really low (say where the figures suggest the cop drops below 2, at which point my storage heaters make more sense). But that to my mind leaves loads of time (probably 97% of days) when you could get very cheap heating, operating with a cop of 4 or 5 with the best units (according to manufacturers numbers ... hmmmm.) installed correctly.

I've had a couple of quotes, reflecting others experiences. Non-mcs have quoted £1.5k for one 4kw inside/one outside unit, and an mcs-qualified has quoted £8k for three 2.5kw inside/one outside unit (they had different briefs!). I may wait until the rhi grants are finalised before proceeding.

djpudsey

Hi all,

I wonder if anyone can share their experience of defrosts for inverter heat pump systems in the current UK weather conditions.

We have a Daikin 4MX86 multisplit inverter system which is behaving very erratically. Some of the time it is working fine, running for 30-40 minutes before defrosting, getting temperatures up to set point and only defrosting when the heat exchanger on the outdoor unit is well frosted up. Then it will switch to running for 5 minutes then defrosting (which takes about 6 minutes) running for 5 minutes then defrost etc for an hour or more - with little visible frosting/icing of the heat exchanger. Then it will run normally again for a while.

When running with the frequent defrosts, at the end of the 5 minute run the compressor always seems to have just stepped up in speed, then quickly ramps up to max speed in a few seconds before stopping instantly at which point the indoor unit(s) also stop. It then goes into defrost.

With the current temperatures, the room temps cannot be maintained and drop quickly when the system is only running 5mins in every 11 or 12 minutes.

It doesn't seem to be outdoor temperature or humidity related. It can switch from one state to the other at any time of day and has been the same on the cold/dry days and slightly warmer/foggy days we've had recently.

It is also occasionally failing to start correctly following defrost and is showing U2 fault code (Power supply under/over voltage). The engineers are saying that it is a problem with the mains supply and passing the buck, but YEDL have done some basic checks and all appears okay and there are no signs of supply dips showing with the lighting or other equipment. Supply meters at full 240V. We are in an suburban residential area only 50m from the local substation.

Is this normal? Anyone else have a similar system that is behaving the same way, or is working normally in the current conditions?

Thanks,

David J (Yorkshire)

richardc1983

I only notice defrost cycles when outside temp is below 2c. Even at 2c there are hardly any defrost cycles.

Even at temps below zero a decent and well specs system is more efficient than storage heaters and electric heaters.

You get more heat out than u put in so until your getting less out than your putting it is still more efficient unless u go by the cost per unit etc.

To the poster above who has problems with his air to air multi split it sounds like a dodgy defrost/coil sensor that keeps putting it into defrost.

Try copying your post over at https://www.refrigerationengineer.com under the air conditioning section.

Many engineers on there who will be able to give advise.

bushyh

OK all you helpful people here is the technical report in respect of my system. Builder is opting for option 1 followed by 3 and if that fails 2 - presumably because they are the cheapest for him. So my question now is will either of those 2 options a) reduce the cost of running the unit and b) reduce the noise of running it! I suspect not, but what do I know. I'm assuming they will keep me warmer!



Initial inspection

The ASHP unit was operating correctly. Temperature difference between flow and return was 6oC, as expected
There were no apparent issues with the installation or operation of the heating system.
The immersion heater in buffer tank was wired in and switched on via a relay. There did not appear to be any additional control of the relay.
The customer is reporting a problem with the level of heating within the property, with the unit being able to satisfy the hot water demands.
The programmer is set up as advised with separate time periods for hot water outside the required periods of heating. It was also stated to the customer that during periods of cold weather the ASHP will need to be run continuously with the temperature on the room stat set back to lower temperatures during the night and during the day when occupants are out.

Review of ASHP sizing and system design

The design draft SAP calculation provided states a heat loss coefficient of 270.71 W/K for the 208m2 4-bed property.
Using the standard UK design condition of an external temperature of -3oC and an internal temperature of 21oC, the heat loss from the property should be 6497W.

This supports the selection of the Calorex AW12002AHY unit which has a heat output of 7280W with air on at -3oC and water flow off at 50oC (A-3/W50). At this condition there should in theory be at least a 10% safety margin on the selection.

However we have been experiencing weather conditions considerably below -3oC in recent weeks. In extreme cold weather the heat output of the unit would fall below the requirements of the building. This has been a common feature

It is noted from the design information generated by Evans Design, that was supplied, that the radiator selection was based on a heat loss estimate of 9816W. The radiators selected have a combined heat output of 13,227W based on a mean water temp of 50oC.

This estimate is considerably higher than the SAP estimate. This is primarily due to the higher air change rates assumed in the Evans estimate compared to the 0.6ac/h assumed in the SAP.
If the heat loss from the property is nearer the Evans estimate than the SAP calc then the shortfall on the heat supplied by the ASHP will be higher. This corresponds with the occupants experience, where the heat pump can only provide sufficient heat during milder temperatures.

The radiator outputs are considerably higher than the heat output of the ASHP. This will result in a lower system temperature being achieved. A balance point will be achieved where the output from the radiators matches the input from the heat pump, this will be significantly lower than the 50oC design temp. The occupant will then see radiators at a lower temperature but does not in itself mean that the property will not be warm.

Conclusion
The system has been installed correctly and the ASHP is operating correctly and providing as much heat as it can. However the heat output from the ASHP is is proving not to be sufficient for the property. Indicating that the heat losses are higher than those indicated in the SAP.

This is confirmed by the fact that the 3kW immersion heater in the buffer tank has been utilised since the visit to site but with little improvement reported.

Hence more heat input is required into the property, particularly to cope with periods of extreme cold weather.

Recommendations

There are a number of options that could be considered to increase the heat input into the property.

1. The existing ASHP could be modified to use R407c refrigerant in place of the current R134a. This would give the advantage of an additional 25% heat output from the unit. However it would have the disadvantage of lower the flow temperature that the unit can achieve to 55oC (currently 65oC), this would mean that the DHW cylinder immersion would need to be used on a timed basis to boost the temperature of the stored hot water.
This in conjunction with the use of the 3kW immersion in the buffer tank should be sufficient to cope providing the heat losses are no higher than those assumed in the Evans estimate. The buffer immersion would need to be controlled through a frost and buffer tank thermostat so that it is operated automatically only when required.
The cost for the modification to the unit would be approx £300.

2. Install a supplementary gas boiler in a bi-valent installation to support the ASHP in heating mode during periods of colder weather. We can provide support with plumbing and electrical schematics to show how this can be achieved.

3. Install a second heat pump. An additional AW9002AHY unit with a heat output of 5.1kW at A-3/W50 should be sufficient providing the heat losses are no higher than those assumed in the Evans estimate. An additional 32A power supply would be required and the heating circulating pump would have to be increased to a Grundfos 25-80 from the current 15-60 being used.

richardc1983

bushyh wrote: »

OK all you helpful people here is the technical report in respect of my system. Builder is opting for option 1 followed by 3 and if that fails 2 - presumably because they are the cheapest for him. So my question now is will either of those 2 options a) reduce the cost of running the unit and b) reduce the noise of running it! I suspect not, but what do I know. I'm assuming they will keep me warmer!

3. Install a second heat pump. An additional AW9002AHY unit with a heat output of 5.1kW at A-3/W50 should be sufficient providing the heat losses are no higher than those assumed in the Evans estimate. An additional 32A power supply would be required and the heating circulating pump would have to be increased to a Grundfos 25-80 from the current 15-60 being used.

I say option 3 is part way there but I would not bother having a 2nd unit installed I would just get them to take the existing unit out and fit a higher powered unit that can meet the needs.

How is your system performing with day time temps of 3C? I dont think an extra 5kws is going to help when we end up with low outside temps as appears to be the trend.

They should either remove the heat pump all together and install gas boiler or they remove the existing one and fit a decent system that is rated down to -15C as most of the decent brands are.

You need to speak to them about this, fitting another cheapy heat pump is just going to cost you money to run. I have had a look at that model number and it is mainly for a swimming pool heat pump not for your home central heating.

IF they are looking to budget perhaps look at the LG therma V which is upto 16kws output and maintains this pretty much down to -15C without much loss. Certainly at -3C it will still nearlly be outputting 16kws of heat.

How do they propose you control this other heat pump are you going to have to switch it on when the exisiting system doesnt cope. Seems like a lot of faffin about.

You are also paying the cost of running 2 compressors and units by 2 different manufacturers the system isnt going to be matched. Its like putting a part from a ford car into a Mazda and expecting it to run with out compatibility issues.

You should not be having to run those backup elements apart from having to raise the water temperature once a week which is what all the decent systems do anyway. The unit you have in now are using very old style refrigerant and even if they change it to the other stuff its still old style! The new modern units use R410a.

Your unit is to sum it up a rubbish cheap nasty unit and not upto the job along with the one they are planning to put in. They need to remove it completly and put a unit in that will cope with the heat requirement to that house not start having pick and mix outdoor units as this is going to make running the system complicated and there will be issues in how you get them to control each other.

IF they are worried about cost then they should have a look at the LG Therma V, it does a 14kw and a 16kw outdoor unit that would be suitable for your needs.

Heres a brochure for you to look at http://www.airconwarehouse.com/acatalog/LG_Therma_V.pdf its a very good system and not as expensive as the other systems out there. LG are 100 times better than what they are planning on puttinig in and what you have now but they are not "top dollar" systems.

At the end of the day its their problem and responsibility to fix it so putting in another cheapy heat pump just so you have the required heat load into the house is not acceptable as below freezing the output will drop off too quickly. Not to mention again the compatibility and noise from these cheaper units.

If they do as they are suggesting you will end up with 2 cheapy compressor units outside and more noise. Running 2 cheapy units when the outdoor temp drops off they will be running flat out but not generating much heat but still using the same power whereas the lg system or the top end systems will run flat out but you are still getting the required heat output into the house so nice and warm and the unit will then cycle off and slow down once its got the house warm enough etc.

FItting one single unit that has the required duty speaks many words really. Please dont let them do this, make it clear what you want doing. Either completly take the heat pump out and have GCH or get them to fit the LG Therma V system.

A gas boiler will work below -3C a cheap nasty heat pump will not. SO what are they suggesting when we get colder winters that as well as running 2 heat pumps you will have to use the booster heaters as well... UNBELIEVABLE and he is clearly trying to cut his losses. Tough... they should have done it properly in the first place.

Lots of information for you above so I hope that helps.

Cardew

Interesting. - 'Not fit for purpose' comes to mind.

I would be wary of any 'routine' reliance of a 3kW immersion heater in the buffer tank.

If the system couldn't cope even with the 3kW immersion heater, fitting of a additional heat pump of only 5.1kW would not fill me with much confidence.

The problem is that if option 1 or 3 are chosen, and are not satisfactory, what then? Also the conditions of the last few weeks might not be replicated for years.

This statement is something I have been 'banging on' about!

It was also stated to the customer that during periods of cold weather the ASHP will need to be run continuously with the temperature on the room stat set back to lower temperatures during the night and during the day when occupants are out.

When considering the COP of ASHP firms never take into account that the heat produced when occupants are out, or in bed, is wasted. For instance it might be necessary to keep the ASHP running 24/7 because of the inability of the system to quickly bring the house up to temperature. It might be that, say, 30kWh will be produced when occupants are out in bed.

With gas/oil CH with water at 80+C there is no need to heat the house when occupants are out/in bed, as 20-30 minutes will bring a house up to heat. Yet in the inevitable comparisons of running costs, the 'unwanted' 30kWh is always used to swing the comparison in favour of heat pumps.


Option 2 or full gas CH would be the only solutions I would consider - preferably the latter

richardc1983

Good post Cardew. The reliance of immersion heaters for anything other than raising the hot water tank temp once a week is unacceptable. THey should not even be coming on unless its below -10 or -15C which is when performance starts to drop off on the decent systems. Even then the decent systems will only use backup heaters for exactly that, backup if there is a fault and you have no heating as stated in the LG Therma V brochure.

I feel sorry for the OP as this could go on for ages and if they went for the options that are being proposed which I guarantee will not resolve anything then they are back to where we are now.

Having 2 cheapy units running flat out using for example 4kws of electrictiy each you are using 8kws there. IF they are not able to get the heat because its 0C outside they will still be using 8kws and wasting it.

If you go for one bigger outdoor unit by LG or some other decent brand (doesnt have to be top end just something better than a pool heat pump) the max that unit will ramp upto is 3.8kws and generate 16kws of heat.

Easier way to look at it is the calorex unit has 2.93kw of input and outputs 7.86kw of heat this is at 0C. When it gets to -5C outside you have dropped down to 4kws of heat output but its still using 2.93kw of electric.

The LG THerma V 16kw unit has a max input of 3.80kws of electric but you get 16kws of heat out of that at a flow temp of 35C. At 45C flow temp you get 13.3kw out of it which is the design flow temp. Its a bigger compressor and uses a little bit more electric you are getting double out of that compared to the calorex at -6C the output at 45C flow temp is 12.7kw. JUst to show you that these units are decent at 45C flow temp the output drops to 9.86kw.

ITs only at -15C would you then need to start using the backup elements etc but when is it ever -15C during the day and at night you would have your set points turned right down anyway so it would only be ticking over.

bushyh

oh god this is really not good is it, I'm sorry to keep pestering you but would you recommend I basically use your comments in a letter to builder or do I now do this through a solicitor I am starting to feel a bit out of my depth albeit I fully understand and take on board everything everyone has very helpfully said.