Heat pump / inverter DIY

Cardew

anthonykeates wrote: »

Blimey, I dunno what to believe any more!

Max output of 4600w using 1237w to produce it would be a COP of 3.72 wouldn't it (or is the equation more complex than that?).

So if SteveHead's electric usage was 300kwh for Dec, multiply by say 3.72 = 1116kwh, compared to 3-4000kwh the previous year? If his boiler was only 50% efficient (think before it's been said before somewhere that it is old and inefficient) that would be 1500-2000kwh of actual heat. If the house has since been properly insulated it wouldn't be unfeasible to get a 500kwh reduction in the heating requirements would it? Plus his Gas usage included DHW and cooking, so that would further reduce the differential.

I'm no expert, but I can kind of see how the figures might stack up assuming my back-of-a-fag-packet calcs are roughly along the right lines. Also different weather conditions would have to be taken into account between Dec 2007 and 2008.

Any thoughts??

Unfortunately it is a lot more complicated. although the 4600/1237/COP of 3.72 would appear to be correct. It would have a COP of 5.45 at 440 watts input.

For a start the COPs quoted are at an outside temperature of 7C and 20C inside. Below that the COP drops.

Steve's 3000 to 4000 kWh per year was for heating alone(HW and cooking had been deducted)

What is not clear from the Mitsubish website is the 'running current' of 2.5/2.4/2.3 amps(for 220/230/240v) this is 550watts??????

The 2 fans in the indoor unit are 51watts that run all the time?, this will presumably not be included in the heater input?

mech_2

anthonykeates wrote: »

Any thoughts??

I agree. I don't think there's any need to assume the 304kWh isn't the consumption of the whole system. The figures for December are believable (I'm just not sure about the COP of 5.45 part).

It's still not clear to me that it's a cheaper option than a modern combi boiler on mains gas, though it's probably a pretty close-run thing. I suppose it depends mainly on what price-per-unit you can get. I suppose also that hot water is a sticking point.

thechippy

Cardew wrote: »

That is what I suggested in post # 146 could be the only way that the monitor could work - but Steve's post contradicted that - especially when he said Richard had explained it well.

The wiring Diagram on page 11 does not show it wired as you suggest.

If it is wired as you suggest how is the maximum consumption monitored only 360watts - less than 30% of the maximum input?

I just checked the wiring diagram - seems fine to me, but they are just using a single pulsed cable for the control.

The 360 watts that Steve gave was measured "at the time". The maximum, is exactly that - when the system is on full load. If the setpoint in the room is near to being reached, the compressor will slow down and simply attempt to put the amount of heat into the room which is being actually lost at that time, so it "ticks over".

Cardew

thechippy wrote: »

The 360 watts that Steve gave was measured "at the time". The maximum, is exactly that - when the system is on full load. If the setpoint in the room is near to being reached, the compressor will slow down and simply attempt to put the amount of heat into the room which is being actually lost at that time, so it "ticks over".

No that simply is not correct!

What he said on 06 Dec was:

I now know that I can heat my home to just-a-shirt comfort for peanuts by using electricity.
More specifically... I have a wallmounted unit in the (open plan) living room and one at the top of the stairs. They are set to 25/24 degrees (low power). Whilst on, they pull between 8 and 360 watts each. Thats 720 watts heating my whole house!

So on 2 units the maximum on either was 360 watts.

this is for 'just a shirt' comfort set to 25/24 degrees.

An average of 5kWh a day on each in Dec - apparently including all fans in both units, electronics, compressor, - everything.

With Gas CH he used between 100kWh and 130+kWh a day for a smaller(not extended) house, in Dec 07 which was not as cold.

I don't know how many times I have say it, but ASHPs are good, but to quote a COP of 5.45 as the figure you will get, and back this up with his statistics which are even more unrealistic, is just not supportable.

Nobody, but nobody, claims that COPs of 5+ are obtainable for systems. Not even Mitsubishi.

stevehead

anthonykeates wrote: »

Blimey, I dunno what to believe any more!

Max output of 4600w using 1237w to produce it would be a COP of 3.72 wouldn't it (or is the equation more complex than that?).

3.72? That's on MAXIMUM and being a bit harsh on the way the system actually operates. It spends less than 2 mins during startup at >1Kw input. It rapidly drops and stays dropped to 320W - 450W for the rest of its duty cycle. Usage bobs up'n'down all the time within this range, except when it's reached it's SetPoint drops to ~8W. (just running the variable indoor fan).
You wouldn't buy this unit if you wanted a 4600W heat output, you'd buy a bigger one.

Ah yes.. there's one more way to force >1Kw input by using Turbo Mode. This is only sustainable for a brief and expensive 10 mins, you can't keep it there and it's meant for occasional use.

anthonykeates

Cardew wrote: »

Nobody, but nobody, claims that COPs of 5+ are obtainable for systems. Not even Mitsubishi.

Erm, don't they quote a figure of 5.45 for these units (I know this is an absolute maximum and therefore unlikely to be obtained except under perfect, test conditions) but supposedly that's what it'll do!

If Steve's used 304kwh's to heat his house for a month then surely, that's what he's used!

thechippy

Cardew wrote: »

No that simply is not correct!

What he said on 06 Dec was:



So on 2 units the maximum on either was 360 watts.

this is for 'just a shirt' comfort set to 25/24 degrees.

An average of 5kWh a day on each in Dec - apparently including all fans in both units, electronics, compressor, - everything.

With Gas CH he used between 100kWh and 130+kWh a day for a smaller(not extended) house, in Dec 07 which was not as cold.

I don't know how many times I have say it, but ASHPs are good, but to quote a COP of 5.45 as the figure you will get, and back this up with his statistics which are even more unrealistic, is just not supportable.

Nobody, but nobody, claims that COPs of 5+ are obtainable for systems. Not even Mitsubishi.

COP's of over 5 are achievable, it just depends on conditions - it's just not the average by ANY means.
I see what you're saying now, no the systems are not going to run at 320w all the time, they may only run at this as the setpoint is being reached, until then, they will be consuming more power.
The manufacturers, unfortunately, leave the given data open to interpretation and I agree with you on this.
If you look at Mitsi's data on Steves - 440w to give 2.4kw heating is around COP 5.4, but if you use thier figure of 2.4a run current to give 2.4kw heating, then you get a COP of around 4.2, it can all be very misleading. If you used an average COP of 3.5 @7c, you will never be far out. Some will give a bit more and some less, but that's a good overall average.

anthonykeates

stevehead wrote: »

3.72? That's on MAXIMUM and being a bit harsh on the way the system actually operates. It spends less than 2 mins during startup at >1Kw input. It rapidly drops and stays dropped to 320W - 450W for the rest of its duty cycle. Usage bobs up'n'down all the time within this range, except when it's reached it's SetPoint drops to ~8W. (just running the variable indoor fan).

Was just using those input figures Cardew quoted as an arbitrary method of doing some kind of calculation to try and get it clear in my mind, because like you've said earlier there's no way for you to work out the actual COP your units are achieving.

I guess all this debate/questioning is because people are amazed at how little it's costing for you to heat your house - lucky devil!

Cardew

thechippy wrote: »

If you used an average COP of 3.5 @7c, you will never be far out. Some will give a bit more and some less, but that's a good overall average.

QED

That is exactly the point I have been making in post after post.

I have 'objected' to figures of a 5.45 COP being bandied about so the uninformed will think that they will get 5.45 units of heat for 1 unit cost.

Steve in his enthusiasm compounds that by using figures of a £120 gas bill only costing £30 with electric.

I think that if you can get an overall COP of 3 in a UK heating season it is excellent - particularly for electric only homes.

thechippy

Cardew wrote: »

QED

That is exactly the point I have been making in post after post.

I have 'objected' to figures of a 5.45 COP being bandied about so the uninformed will think that they will get 5.45 units of heat for 1 unit cost.

Steve in his enthusiasm compounds that by using figures of a £120 gas bill only costing £30 with electric.

I think that if you can get an overall COP of 3 in a UK heating season it is excellent - particularly for electric only homes.

As stated, the manufacturers figures are misleading and can be interpreted any way you like and I agree with you in this respect. Steve's system has a quoted COP of 5.41, but this would only be under ideal conditions. I looked at some of Mitsi's figures again - they state 2.4a run current for 2.4kw heating. 2.4kw of electric resistive heating would require about 10.5a, so I'm coming back to a COP of around 4.3 on Steve's system again, but certainly not the 5.41 they list for it. If it is indeed 4.3, then that's a bit above average, but then the Mitsi's are known to be pretty good in heating mode.
I'm not disputing Steve's figures though - it may be possible. Insulation is the key factor.
Let me try to give a simple explanation as to how these work.......(I know.....yawn!!)
You get home and the living room is very nippy.
You switch on the ashp and set it to 20c
It kicks in and because it has sensed that the room is well below 20c, within a few seconds the compressor will ramp up to full speed (full load, max heat)
As the room temp rises, the system will sense this and gradually start to slow down (less heat, less power consumption)
As the setpoint is neared, the system will sense this and will slow down enough to only input the amount of heat that is being lost.
Basically, all rooms will lose xx amount of heat per hour.
Lets say Steve's living room is losing 1kw of heat per hour.
The electronics would have already calculated this, and so the compressor will slow down to the point that it only tries to supply the room with 1kw of heat. It will attempt to tick over at this point.
Should the room temp continue to rise, but the compressor is already running as slow as the inverter will allow, then it will shut down.
It will restart when the room temp drops below a certain % below setpoint.
The "Inverter" is just what controls the compressor speed. Basically, it converts 240v AC single phase into DC 3 phase. The compressor is 3 phase DC. The inverter will adjust the frequency to control compressor speed.

Right,

I've bored you enough and I'm grabbing a glass of wine!