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Air Source Heat Pumps
Comments
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Tired geek,
There aren't many gurus's unfortunately on MSE - although a couple of people who do install systems and hopefully they will read your post.
As detailed in the other thread on ASHP's there are several people who are less than satisfied with their systems and this post mentions the EST field trial:
People should read the EST report for field trials on Heat pumps - particularly on ASHPs. http://www.energysavingtrust.org.uk/...mp-field-trial
The study was a year long and had 29 ASHP systems from a number of manufacturers.(who installed the systems)
It is quite a long report but of the 29 ASHP systems the annual COP for 19 of the systems was 2.2 or less(the highest number had a COP of 1.6). So if that is a average spread across 12 months - little wonder systems are struggling in this current weather. In fact with the requirement for wasteful defrosting cycles, it is quite easy to envisage a situation with some systems where there will be a COP of less than 1.0 and people would be better to switch off the ASHP and rely on Granny's old 1/2/3 bar fire!!!!!!
It is also most important to note that they state because of the low water temperatures, the heating must be on much longer. This for comparison with gas/oil etc further weakens the case for ASHPs. For people who are out of the property or in bed, there is no need to have gas/oil heating on until, say 30 minutes before they get back in the house/get up; so little wasted heat. Not so with an ASHP as it has to be on for many hours while the house is unoccupied/people in bed.*
If our sensitive contributor thinks the above is Troll behaviour he should read the report and complain to EST!
* to explain if an ASHP output is, say, 150kWh in a day, that is far less effective than a gas CH output of 150kWh as the latter(with water at 80C+) is used when it is required - not when people are out or in bed.
** The COP figures quoted above are strictly speaking the overall system efficiency i.e. kWh in and kWh out.
I would contact EST and then start making a big fuss about the system being 'not fit for purpose' if Mitsubishi don't get it sorted.0 -
Thanks for the replies guys
richardc1983,
just to clarify a couple of things
We have two 14kw units so 28kw available to us, 15kw for the house according to SAP report so 28kw should be enough even in the very cold when the units drop off, say they drop 5kw each due to the cold and we should still be in the clear.
18'c is the temp of choice for us, it's nice and warm in the house at that so we're happy. The water outputs from the Ecodans are usually around 31'c and this works with our radiators fine. We have turned the stat up some times to 20'c when we're sitting watching telly (40'c output from them), by using the boost feature, this makes the rads hotter but uses more electricity, so we're happy with 18'c in general
Snow is not building up behind the units to any extent, we have them off the floor by 9 inches or so, the snow was only about 6 inches deep this year when they failed. Neither had snow fallen from the roof behind them to block up the breathing. There will obviously be some snow gets behind them and sitting on the outdoor pipe work, and snow will be sucked into them, but from what I have seen it's not snow that's causing this it's condensation on the heat exchanger that builds up.
grahamc2003,
nope, not foggy where we are, it was my thought initially as well. It's generally too windy for fog or mist to form, there is pretty much a constant steady breeze down the valley, though the humidity the weather station records is always very high. We have a large loch within the valley and the river draining it runs past my house to the sea 5 miles further up. We don't have salty air to contend with but this is northern Scotland, it only stops raining so it can snow!
The main problem as far as I can see is the failure to defrost. It's not the fact that the units defrost but fail to melt all the "frost" on them, when they do a succesful defrost they clear completely down to the fins.
It's just that sometimes they try to defrost but fail to complete the cycle, actually, they fail to start the cycle. The indoor unit says "defrosting" but the outdoor unit is completly shut down, doing absolutely nothing. It tries to start the defrost, makes a huffing noise and then totally shuts down. The indoor unit says defrosting for another two mins or so, then kicks back into heating mode. This then sucks more moisture onto the exchanger and it builds up a sheet of ice.
When the exchanger is completly covered, it keeps the ice well below freezing by trying to extract heat from it and any moisture that touches it then adds to the thickness.
Even if a succesfull defrost occurs it leaves a sheet of ice clinging to the frame over the exchanger but not actually touching it so it cannot be defrosted by the unit. This in turn leades to no airflow and extreme electric consumption. Add in ambient sub zero temps and the ice simply stays put. Mitsubishi initially said it's wind chill, there was no wind this time, and the ice was only on the heat exchanger end of the case, so I don't think that's the cause.
I have photos, though with such a small post count I can't add them or PM them to anyone. If you want to see them PM me an email addy and I'll be more than happy to send them
Cardew, after reading all the posts in this thread I think there's a good few on here know what they're talking about. I've read lots of your posts and they all seem to be knowlegable too
Thanks for that link, I hadn't seen that before, it'll be usefull in my case against the fitters / Mitsubishi.....A pair of 14kw Ecodans & 39 radiators in a big old farm house in the frozen north :cool:0 -
Tired geek just to point out again from my previous post:
I think you should also copy and paste this over at http://www.refrigeration-engineer.co...splay.php?f=78 many many skilled ASHP engineers on there who will be able to help.
I would copy and paste word for word you post was very detailed.If you found my post helpful, please remember to press the THANKS button! --->0 -
Thanks for the replies guys
richardc1983,
just to clarify a couple of things
We have two 14kw units so 28kw available to us, 15kw for the house according to SAP report so 28kw should be enough even in the very cold when the units drop off, say they drop 5kw each due to the cold and we should still be in the clear.
- Have you actually seen the performance charts so you know what output your getting at various outdoor temps... this would be useful for you to know I think, though on a Mitsi system it wont lose much.
18'c is the temp of choice for us, it's nice and warm in the house at that so we're happy. The water outputs from the Ecodans are usually around 31'c and this works with our radiators fine. We have turned the stat up some times to 20'c when we're sitting watching telly (40'c output from them), by using the boost feature, this makes the rads hotter but uses more electricity, so we're happy with 18'c in general
- So it heats the house upto 18C & maintains it at this in a short space of time you say... This confirms that the system has enough capacity... this rules out it being undersized. Your flow temps are fine if they are achieving the indoor temps you are comfortable with.
Snow is not building up behind the units to any extent, we have them off the floor by 9 inches or so, the snow was only about 6 inches deep this year when they failed. Neither had snow fallen from the roof behind them to block up the breathing. There will obviously be some snow gets behind them and sitting on the outdoor pipe work, and snow will be sucked into them, but from what I have seen it's not snow that's causing this it's condensation on the heat exchanger that builds up.
Correct... snow being sucked onto the back of the exhanger can freeze it over in a short space of time - I have seen this with my own unit. It freezes on contact and in a short space of time it would just be recognisable as white ice like in your freezer.
grahamc2003,
nope, not foggy where we are, it was my thought initially as well. It's generally too windy for fog or mist to form, there is pretty much a constant steady breeze down the valley, though the humidity the weather station records is always very high. We have a large loch within the valley and the river draining it runs past my house to the sea 5 miles further up. We don't have salty air to contend with but this is northern Scotland, it only stops raining so it can snow!
The main problem as far as I can see is the failure to defrost. It's not the fact that the units defrost but fail to melt all the "frost" on them, when they do a succesful defrost they clear completely down to the fins. - When is this most noticeable? WHen its snowing, when the weather is cold but dry?
It's just that sometimes they try to defrost but fail to complete the cycle, actually, they fail to start the cycle. The indoor unit says "defrosting" but the outdoor unit is completly shut down, doing absolutely nothing. It tries to start the defrost, makes a huffing noise and then totally shuts down. The indoor unit says defrosting for another two mins or so, then kicks back into heating mode. This then sucks more moisture onto the exchanger and it builds up a sheet of ice. - This indicates a fault with the unit... it should complete its defrost cycle and shouldnt start another defrost for at least half an hour if it did it properly the first time. This is something that needs raising with Mitsi. The sensors on the coil and pipework inside the outdoor unit will detect certain conditions which will cause it to go into defrost... it will then stay in defrost until certain conditions (temps etc) are met and then switch back into heating. If it is below 0C the outdoor fan should not be running & only the compressor will be whirring away when it is defrosting to enable the heat to build up in the coil. If it is running during initial stages of defrost this suggests a fault with the outdoor air temp sensor.
When the exchanger is completly covered, it keeps the ice well below freezing by trying to extract heat from it and any moisture that touches it then adds to the thickness. - Spot on
Even if a succesfull defrost occurs it leaves a sheet of ice clinging to the frame over the exchanger but not actually touching it so it cannot be defrosted by the unit. - Ice should never build upto this level anyway... if defrost cycle worked properly it would never be an issue.
This in turn leades to no airflow and extreme electric consumption. Add in ambient sub zero temps and the ice simply stays put. Mitsubishi initially said it's wind chill, there was no wind this time, and the ice was only on the heat exchanger end of the case, so I don't think that's the cause. If the unit is covered in ice electric consumption goes up because it has to work harder we need to solve the issue of the defrost cycle. I would have your installer replace the coil and pipe sensors on the outdoor unit, if this is still an issue then it seems to me like a fault with the outdoor PCB. This is a premium product and should not be behaving like this. THese units are tested in extreme controlled environments indoor freezers I have seen them covered in ice and the walls of the test rooms covered in thick ice yet the units still perform without issue and coils are clear.
I have photos, though with such a small post count I can't add them or PM them to anyone. If you want to see them PM me an email addy and I'll be more than happy to send them - Email addy sent I will upload them on your behalf.
I have responded above in RED hope this helps. MItsubishi have a range over in new zealand, much harsher climate than over here.
If you found my post helpful, please remember to press the THANKS button! --->0 -
Tired geek,
There aren't many gurus's unfortunately on MSE - although a couple of people who do install systems and hopefully they will read your post.
As detailed in the other thread on ASHP's there are several people who are less than satisfied with their systems and this post mentions the EST field trial:
I would contact EST and then start making a big fuss about the system being 'not fit for purpose' if Mitsubishi don't get it sorted.
Mitsubishi are market leaders in heat pumps, they have a lot of experience. There is an issue with the unit as I have suggested and it just needs the installer/mitsi to take the bull by the horns and sort it out.
There is absolutly no way one of their systems is designed to be behaving like this.If you found my post helpful, please remember to press the THANKS button! --->0 -
richardc1983 wrote: »Hi TiredGeek.
I understand the very low temps you have up there but are you aware that at -20C you are not going to get anywhere near the 14KW per unit that you need to heat the house.
The output of an ASHP does not vary with outside ambient temperature!
The unit will(should) still output 14kW regardless of the temperature.
As the temperature drops the COP will reduce and so the input will increase to achieve that 14kW.
I think you have misread tired geek's post. The house is unoccupied and he has set the house temperature to 10C without any DHW. This is a very low heating load and the system should easily cope.
He also doesn't state ' he is only able to get 18C'.
Given the system is professionally installed and Mitsubishi have not stated that it is under spec, there is little point in you making an assumption that bigger radiators and higher outputs are needed.
It seems pretty clear that the inability to defrost properly is the root cause, and this is often quoted as the achiles heel of some ASAHP systems.
EDIT
written before posts above appeared:0 -
I have just managed to find Mitsubishi's defrost logic:
Defrost Strategy
When the outside temperature drops below zero all heat pumps must perform a “defrost cycle” to remove ice build up on their outdoor coils.
Defrost strategy is determined by individual Heat Pump manufacturers. These strategies vary greatly between brands. Older style Heat Pumps initiated defrost by a fixed time or coil temperature. This system was not efficient as it often caused Heat Pumps to defrost too often or effected performance by not defrosting often enough. Defrost cycle is required when the outdoor coil is too cold or covered in ice preventing heat transfer and unit performance.
All Heat Pumps must defrost. Mitsubishi heats pumps utilise a Fuzzy Logic software program, a form of Artificial Intelligence contained in the chip of the outdoor unit and typically lasts between 3 to 5 minutes.
The program measures and records:
- Ambient Temperature
- Outdoor Coil Temperature
- Accumulated Continuous Heating Running Times
- Defrost Initiation Time and Termination Times
The program optimises this data based on history to produce defrost initiation only when absolutely required.
This is important as Heat Pumps are unable to produce heat when they are in defrost mode. This is extremely important to real performance in low ambient conditions.
Defrost Cycle Management
Heat Pumps optimise its defrost cycle once selected in three ways:
When the outside temperature drops below zero all heat pumps must perform a “defrost cycle” to remove ice build up on their outdoor coils.
Defrost strategy is determined by individual Heat Pump manufacturers. These strategies vary greatly between brands. Older style Heat Pumps initiated defrost by a fixed time or coil temperature. This system was not efficient as it often caused Heat Pumps to defrost too often or effected performance by not defrosting often enough. Defrost cycle is required when the outdoor coil is too cold or covered in ice preventing heat transfer and unit performance.
The program measures and records:
- Ambient Temperature
- Outdoor Coil Temperature
- Accumulated Continuous Heating Running Times
- Defrost Initiation Time and Termination Times
The program optimises this data based on history to produce defrost initiation only when absolutely required.
This is important as Heat Pumps are unable to produce heat when they are in defrost mode. This is extremely important to real performance in low ambient conditions.
1. Compressor Control
When a Heat Pump is defrosting it is not providing heat to the controlled space. It runs the compressor(inverter drive) at maximum speed during defrost to bring the outdoor coil up to temperature as quickly as possible. This melts any ice formed on the coil fins quickly and minimises defrost time. Minimising defrost time maximises heat output per hour real time.
2. Dry Coil Defrost Cycle
Once the outdoor coil is up to temperature and the compressor cycle has completed there is generally water between the outside coil fins. If the outdoor unit were to immediately resume heating the outdoor coil would freeze and prevent heat exchange. To prevent this the outdoor fan is run at maximum speed prior to resumption of the heating cycle. This is often characterized by steam blowing from the outdoor unit. This ensures the coil is completely dry before the heating cycle resumes.
3. Time optimization through Fuzzy Logic
Time between defrost cycles is continually being reviewed and optimized by the Heat Pump microprocessor software. Algorithmic calculations based on previous history is used to calculate the next defrost period.
Fuzzy Logic or learning logic is a form of artificial intelligence. Defrost cycle termination is based on a combination of time and temperature. These parameters are used to calculate the next defrost period.If you found my post helpful, please remember to press the THANKS button! --->0 -
The output of an ASHP does not vary with outside ambient temperature!
The unit will(should) still output 14kW regardless of the temperature.
Not sure where you have got this from but the outside ambient temperature is directly related to the output that you will get from the unit. Are you saying that at -20C outside temp you will get 14kw of heat out of a 14kw system that is designed and rated at 5C. An example of an LG system below shows the capacity tables. As the outside air temp drops the output also drops. It does not remain the same. THe column on the left is the indoor units connected and their capacity. You dont really need to take note of this column. The data that is highlighted in bold is the rated output.
Most manfacturers in the UK design their systems so that at 6C outside temp you get the rated output. WHen temps start dropping away the system has to work harder to achieve its output. When temps drop even further and the system is already working flat out it simply cannot meet its rated output so then you start losing KW per hour output which is what COP is all about. The output for every KW consumed which drops the lower the ambient.If you found my post helpful, please remember to press the THANKS button! --->0 -
richardc1983 wrote: »I have just managed to find Mitsubishi's defrost logic:
Defrost Strategy
When the outside temperature drops below zero all heat pumps must perform a “defrost cycle” to
remove ice build up on their outdoor coils.
The program measures and records:
- Ambient Temperature
- Outdoor Coil Temperature
- Accumulated Continuous Heating Running Times
- Defrost Initiation Time and Termination Times
.
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.0 -
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..
A pair of 14kw Ecodans & 39 radiators in a big old farm house in the frozen north :cool:0
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