Air Source Heat Pumps

richardc1983

TiredGeek wrote: »

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:

You dont need to clear snow from them they are mounted off the floor but you need to ensure that the drainage holes in the bottom does not freeze up like it has done.

The last picture shows it iced up was the rear of the coil the same or just the side?

Not sure what the causes of last year were, I still think the system isnt defrosting properly and you need to raise it with them, if your saying it goes into defrost then the outdoor unit wont go into defrost this needs investigating.

paceinternet

If both units are misbehaving exactly the same, you may have to rule out faults with the units.
Have you been able to observe that the water circulation pumps are always running when it tries to defrost?
It needs water flowing and at a minimum temperature to provide the heat source for the reverse cycle.
If the defrost works some of the time, check if the water temperature / flow is different to when it doesn't work.
However, you would expect the installer to have done these checks already.

Cardew

richardc1983 wrote: »

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

I had assumed that the output would remain more or less constant within the specified ambient temperature range.

Indeed the BRE tests show that the 8.5 Ecodan output only drops to 8.1kW at -5C. less than 5%.

I can find no data for the 14kW Ecodan.

richardc1983

Cardew wrote: »

I had assumed that the output would remain more or less constant within the specified ambient temperature range.

Indeed the BRE tests show that the 8.5 Ecodan output only drops to 8.1kW at -5C. less than 5%.

I can find no data for the 14kW Ecodan.

Yes but this particular model is rated down to -15C. The ops system rated down to -25C also.

Cardew

richardc1983 wrote: »

Yes but this particular model is rated down to -15C. The ops system rated down to -25C also.

You are making my point for me!

I cannot find data for -15C on any model, or the output at -5C for either of the 14kW models.

The output has dropped less than 5% on a model that operates down to 'only' -15C.

Therefor it might be reasonable to assume that on a model designed to operate down to -25C that the percentage drop off in output will be less at -5C as it is designed to work in colder conditions..

richardc1983

Cardew wrote: »

You are making my point for me!

I cannot find data for -15C on any model, or the output at -5C for either of the 14kW models.

The output has dropped less than 5% on a model that operates down to 'only' -15C.

Therefor it might be reasonable to assume that on a model designed to operate down to -25C that the percentage drop off in output will be less at -5C as it is designed to work in colder conditions..

I would say maybe it loses a 2 kw's at -25C Higher humidity and more defrost cycles it will be more than that.

Either way Cardew it loses capacity based on the outdoor air temp earlier you said that outdoor air temp didnt affect it but it does lose duty. HOw much depends on the manufacturer... we are talking specifically mitsubishi now but on my system which is LG at 6C the output is 8.5kw's problem however output is only 4.84kws at -15C.

zeupater

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??

Hi

BRE efficiency test graphs on page 21 of the brochure is the source of the COP reduction .... also BRE output dataset shows the output reduction v temperature at speed 7, I took the 55C flow dataset for the example, my reasoning ? .... the manufacturer has obviously selected the speed dataset which gives the best figures for a sales brochure, so why not take the worst performance ratio within their best dataset as probably being more typical ..... I know it's not a highly scientific approach but in the absence of a full dataset, why take the dataset specifically selected by the marketing department without selecting their own published worse case ?

Maybe if the marketing departments for ASHP manufacturers were a little more open with their test data outside 'normal' operating conditions there would be a little more trust in this sector and systems would be correctly sized or only specified if supplimented with 'extreme' conditions backup .....

I would expect that an ASHP system which has been so oversized was designed to cope with the low heat output for colder than average conditions, but considering that two large systems have been installed at a total cost matching a complete GSHP, including the groundwork, the supplier provided a system which suited the installer & not the customer .... especially so considering the location.

(My thoughts only)

HTH
Z

grahamc2003

zeupater wrote: »

Hi

BRE efficiency test graphs on page 21 of the brochure is the source of the COP reduction .... also BRE output dataset shows the output reduction v temperature at speed 7,
HTH
Z

I'm a bit confused about the cop. I've read that the cop measure is just a reflection of the thermodynamic heat transfer, and doesn't take account of defrosting. (i.e. it's a performance measure when in steady state heating). The efficiency measure which takes into account the defrosting is 'system efficiency' - according to what I read (which was some valid technical literature somewhere or other). That in itself is confusing because heat pumps would generally have system efficiencies of >100% (which grates a little).

Is that the agreed standard? or have others read the cop takes account of typical defrosting?

lovesgshp

Having read the comments, regarding low outside temperatures. Does the Ecodan have a built in booster heater to help if the pump cannot cope? I have a GSHP, with either a 3/6/9 KW heater which switches on when required. I cannot see any reference to it on the info sheet in the Ecodan brochure.
With regard to output COP these should be based on input 7C/35c output or 7C/55C output or radiators.
The Ecodan is designed solely for the UK market, so possibly although the specs say it will run to -35, these temps may be optimistic. Even the IVT OPtima is based on -25 and that is for Swedish weather conditions.
On another point, a heat pump should never intentionally be oversized, 10% under is a good rule of thumb, as it then works at peak performance.
Settings for radiators should normally be in the 50C range as otherwise the pump will be running almost continuously trying to keep the house temp even. Check the flow rate return and pump running hours to get the details.
Hope this helps

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..

Doesnt seem like your the only one with defrosting problems on air/water system!

This seems to be worse than yours and is over in Netherlands: Its a Daikin Altherma!

http://www.refrigeration-engineer.com/forums/showthread.php?t=29513