Air Source Heat Pumps

Cardew

TiredGeek,

The question of humidity was discussed in another thread and this was posted by Graham2003(lest I be accused of plagiarism)

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.

I am no expert on Heat pumps(I am an electrical engineer) and the system I owned abroad operated in far warmer climate. I did carry out a long study on heat pumps, albeit for heating swimming pools - hardly relevant to your problem!!!!

As you surmised, it seems pretty certain that your system is correctly specified, but the inability to defrost correctly is at the heart of your problem.

I was amazed(and I mean amazed) that the EST field trial conducted with the co-operation of manufacturers produced such dreadful figures for a 12 month trial on some systems(as low as a COP of 1.2 with 1.6 being common) and they were unable to identify the cause of these results.

In some of the specialist plumbing forums on heat pumps, statements 'like defrost cycles are the killer for heat pumps' appear.

richardc1983

grahamc2003 wrote: »

Yes, and there's quite a lot of legroom for the implementaion of that strategy to cause the defrost cycling and instability reported.

For example (i can come up with thousands), the logic may assume there is no ice left when the coil temp risies to 1C, but there's a possibility of the water at 1c being completely encased in ice, yet the unit reports the deicing as complete (and furthermore that will, carry forward in the fuzzy logic to determine the timing of future defrost times).

It's impossible to come up with a solution to the problem on a BB, but from the info posted, it certainly seems the defrost strategy may not be working correctly, and the unit is reporting itelf clear of ice when, in fact, some - or even a great deal - may be remaining. This could of course lead to instability of the defrost cycle with the behaviour as described.

Hence I think it is a sensor issue. The defrost cycle is not running long enough.

richardc1983

TiredGeek wrote: »

Another thing, these units a brand new. Mitsi did a recall this year due to some faulty compressors or somesuch that could cause the units to literally blow up!
Both my EcoDans were commisioned in early November, they ran for two weeks unattended keeping the house at 10'c with ambients above freezing, all ok. Then we had that cold snap, they were left for 12 days before our next visit.

When we got there we found them partly covered in ice, but reading the electric meter we found that they had eaten 978 kwh (21/11/10 - 4/12/10) or £9 / 24hr average. To keep the house at 10'c!

So I checked the data from my weather station, cheap thing from Maplins but still accurate enough for this. It shows that for a while everything was fine, the system was keeping the house at a steady 8'c. But then the house temps start to drop off, eventually hitting -5'c indoors.

I assume that this is the time that they iced over outside and then just started chewing electric to try to heat the place. I reckon that that £9 a day isn't right, I think it was actually low until they iced up and then it soared as the COP hit 1:1 or worse.

My weather station then records a couple of days above freezing outdoors and some strong sun on the units. I think this has partly thawed them and allowed them to start working again.

Mitsubishi have essentially told my install company it's not their problem, I've seen the emails, and the installers are scratching their heads.
Looks like it may be GSHP for me in the future..

Do not give up,

I do not know why this system is using so much electricity. Either your electric meter is faulty or something seriously wrong with the unit.

Why has the system not been able to maintain 10C inside and temps dropped to -5C? Something is wrong here if it was struggling at 22C I would say yeh cos its cold outside maybe even at 18C but at 10C inside there is no excuse it should have floored it.

Has the refrigerant charge been checked in the outdoor unit, it could be short of refrigerant. But then again I am not sure why it is working properly other times? What was the outdoor ambient when indoors droped to -5C?

To quote the below in red:

[FONT=&quot]De-frost how does it work?[/FONT]

[FONT=&quot]When a unit is in heating mode a small amount of ice may form on the outdoor coil this is normal.[/FONT]
[FONT=&quot] This will give poor heat exchange which will cause the coil to drop in temperature.[/FONT]
[FONT=&quot]There is a defrost sensor that will measure the condensing temperature.[/FONT]
[FONT=&quot]NB if the liquid temperature is below 15 degrees this will cause the outdoor unit to go into defrost. [/FONT]
[FONT=&quot]NB If the unit is incorrectly charged of refrigerant ie undercharged this will cause unnecessary defrost causing little or no heat.[/FONT]
[FONT=&quot]After defrost the unit will resume normal action.[/FONT]

richardc1983

Cardew wrote: »

In some of the specialist plumbing forums on heat pumps, statements 'like defrost cycles are the killer for heat pumps' appear.

Only if the system is not correctly sized. If the system is running flat out when it gets cold outside then defrosts I agree do kill the operation and cop of the system.

If you specifically design your system for the worst case scenario i.e the lowest temp you can expect in winter you will end up with extra duty if the temp does drop very low outside.

You in affect over size the system so you have more to play with... this then means the larger capacity system is happily jogging away running at 70% output at -10C outside for example rather than running 115% and not able to keep up.

Yes it costs more to have a larger system but it works in the end. This is what I did with my air to air system I oversized by 4kws, so the duty that I lose when the outdoor air temp is at -10C means the system then is perfectly matched for the inside heat load.

zeupater

Hi

The other thing which needs to be remembered is that the if the SAP figure you mention is the heatloss calculation for the building it will be based on an outside temperature of -2C giving a differential of just over 20C ..... in your area at the temperatures quoted you'll probably be better to double the heatloss.

As others have mentioned, as the outside temperature fall so does the COP, but also consider that the overall heating capacity reduces as well, at -5C the 14kW systems will suffer a heating capacity reduction of somewhere around 30% (it's hidden in the brochure), so at -15C to -20C well below 7kW each is easily probable (can't find any data at such low levels) .... you then have a situation where the units are struggling to heat & defrost, working at a COP of about 1.5 at -5C, so close to a COP of 1 at your overnight temperatures, and only providing 25% to 50% of the building's heat replenishment requirement, the rest being drawn from the thermal mass.

The idea behind adding a large buffer tank is effectively to increase the thermal mass in the system whereby the ASHPs can produce more during the day when a higher COP can be achieved and utilising this stored heat overnight ..... hope it works for you and helps.

HTH
Z

richardc1983

Here are the pictures of the units that TiredGeek has sent to me: After seeing the pictures I can see that the ice build up on the coils is due to the snow as you also have ice on the surface on the centre of the fan inside which will be from snow blowing into the front and freezing on contact with sub zero temps.

The ice on the coils though I have witnessed on my own units, snow is sucked onto the unit and freezes on contact it then turns to ice when sun shines on it the next day etc. The water has also not drained from inside the unit probably because it has frozen underneath where it would exit from the holes. You need to perhaps put some salt or grit under neath the unit to try to stop this water freezing. My unit looked similar not as bad and only at the bottom inch of the coil due to snow fall behind the unit. The rest of the coil was clear though but you have had much worse temps up there. The blocked coil with ice has then caused air flow loss so the unit has had to work harder and draw more power. However ice should not form as thick as this and the defrost cycles dont look to be lasting long enough but it looks like the unit has been exposed face on into the snow as there is quite a lot on top of the unit as well. This depth of snow will have been sucked into the back of the unit at the same time and stuck straight away.

To me though as evidenced in the side view picture of the units you can clearly see that this is snow that has turned into ice. Snow fall and extremes of temps have been your issue here. The fact then that this hasnt been able to drain away out the bottom has caused these issues.

I was having to go outside and clear the build up of snow from the front and rear of the unit as mine isnt mounted off the ground like yours but my unit is sheltered at the front of the house so wasnt in the wind whereas at the rear we had big drifts of snow and at the front it wasnt too full on. If you was living at the property you perhaps could have been able to go out and assisted in removing some of the ice by pouring on warm water or something. I reckon the same will happen again if it snows bad again but I bet if it was -20C outside and dry you wouldnt have these issues. Snow is the culprit here.

Cardew

zeupater wrote: »

Hi


but also consider that the overall heating capacity reduces as well, at -5C the 14kW systems will suffer a heating capacity reduction of somewhere around 30% (it's hidden in the brochure), so at -15C to -20C well below 7kW each is easily probable (can't find any data at such low levels) .... you then have a situation where the units are struggling to heat & defrost, working at a COP of about 1.5 at -5C, so close to a COP of 1 at your overnight temperatures, and only providing 25% to 50% of the building's heat replenishment requirement, the rest being drawn from the thermal mass.

The idea behind adding a large buffer tank is effectively to increase the thermal mass in the system whereby the ASHPs can produce more during the day when a higher COP can be achieved and utilising this stored heat overnight ..... hope it works for you and helps.

HTH
Z

I certainly hadn't seen that the output dropped off by as much as 30% on the Ecodan at -5C .

http://www.celsiusair.co.uk/equipment/mitsubishi/ecodan/pdf%27s/Leaflets/Ecodan%20Brochure.pdf

It also shows the COP as much higher than you indicate at -10C it is approx COP = 2.3 and -15C = COP is 1.9C -(flow temp is 45C) see page 11 of brochure.

The BRE tests on a 8.5kW Ecodan showed that at -5C the output was 8.1kW @ 45C flow temp.(page 21)

Have you a more comprehensive dataset?

Anyway the output of the Ecodan is surely acedemic if they 'stall' because they don't defrost properly and a buffer tank will not help??

richardc1983

Cardew wrote: »

I certainly hadn't seen that the output dropped off by as much as 30% on the Ecodan at -5C .

http://www.celsiusair.co.uk/equipment/mitsubishi/ecodan/pdf%27s/Leaflets/Ecodan%20Brochure.pdf

It also shows the COP as much higher than you indicate at -10C it is approx COP = 2.3 and -15C = COP is 1.9C -(flow temp is 45C) see page 11 of brochure.

The BRE tests on a 8.5kW Ecodan showed that at -5C the output was 8.1kW @ 45C flow temp.(page 21)

Have you a more comprehensive dataset?

Anyway the output of the Ecodan is surely acedemic if they 'stall' because they don't defrost properly and a buffer tank will not help??

Cardew how come earlier you said that outdoor air temp didnt affect the output of the system?

richardc1983

http://www.celsiusair.co.uk/equipment/mitsubishi/ecodan/pdf%27s/Leaflets/Ecodan%20Brochure.pdf

THe article also shows that the ops particular model is guaranteed to operate down to -25C anything lower than that and it will cut out and not run to protect the compressor.

TiredGeek

Mine are the 14kw, they reckon they'll work fine down to -25'c. It has been known to get down that low once in a long while but not this year so far. Altnaharra is the record holder at -27.2'c a few years back. It's 11 miles from my place

This time it could be snow related, I wasn't there to watch it so I can't be sure. However, the last picture shows one fully iced up, that's last year when there was no snow. That's pure condensation.

If it's snow then the install company have a problem. One of my primary specs for the heating system was to be able to run unattended for 30 days at a time, all year. If the snow needs to be cleared regularly then they're not the right unit for me. :mad: