How (in)efficient are old non inverter "heat pumps"?

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Batman

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#128780 23-Aug-2013 08:41

I have a giant Mr Slim Mitsubishi heat pump rated 2.4kW by the internet but says 3.2kW on the outdoor unit and the model number is PUH-3KVA.

I'm trying to figure out whether our high power bill since moving in is due to this (we use it maybe 10 hrs a day on average); or the incredibly old giant hot water cylinder ....

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#883167 23-Aug-2013 08:47

A quick search finds this PDF

http://www.mitsubishielectric.com.au/assets/LEG/Mr_Slim_R22_Range_Brochure_rev_web.pdf

I can't find your exact model, but a COP (Coefficient of Power, ie efficiency) of between 2.7 and 3.1 is a little low by modern standards, mine is 4.1 or so from memory (Fujitsu Nocria). It's not awful though, my older inverter Daikin is around 3. That's what the figures say, though they may be more or less efficient in practice.

If your hot water cylinder feels warm to touch then put a cylinder wrap around it. New cylinders don't need them apparently, I put one on mine just because I had a spare laying around.

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#883171 23-Aug-2013 08:57

not sure what is a big bill ,but we have a Mitsubishi MSZ-ge71va model and run it for about 4 hours a day and it costs us about $70 a month and we have it set to 20 Celsius.

Common sense is not as common as you think.

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#883197 23-Aug-2013 09:17

Found the same PDF. :) I know very little about heat pump efficiency, but I'll still put my 2c in.

It appears the Model# is the outside unit only, which has a rated power consumption of 3.2kW. It also appears to say the outside unit is only suitable for ducted internal units, is that what you have?

I gather the model of the inside unit dictates the actual heating output and the actual power consumption of the outside unit.

If you're running it at full capacity (unlikely) for 10hrs/day that's going to be a BIG bill. Even at half capacity I'd hate the bill.
32 units/day
960 units/month
@ $0.30/unit
potential cost per month $288

Have you had a heat pump previously?
We're led to believe they're cheap as chips to run, and they are compared to other options, but still pretty hungry. I believe people get a bit of a shock when they get their first bill after installing them.

Your water heater could also be an issue, if it's old and doesn't have a newer insulating wrap around it, you would probably gain something by adding the wrap. Hard to say how much, but if the cupboard is warm now, it should be noticeable on your bill.

The 2 possible biggest gains are:
Turning down the temperature on the heat pump a degree or two. I understand this can have a pretty big impact.
Insulate the house better (if it needs it). No point in heating the whole street.

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#883199 23-Aug-2013 09:19

thanks I've read that document before but couldn't put A + B together ... now that I've taken a 2nd look it makes a little more sense ...

my ceiling module seems to be PKH-xFAK giving a COP of either 3.0 or 3.2 which isn't too bad for a large unit ... (smaller 6kW and under can achieve COP of 4s but the larger ones can't)

I guess it doesn't matter what the outdoor unit is (the one shown in the document has a Y instead of my V) ...

Well we've only just moved in and the power bill is all over the show ... one day i'll turn off the mains and see if the counter moves and then go from there

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#883201 23-Aug-2013 09:21

joker97: I have a giant Mr Slim Mitsubishi heat pump rated 2.4kW by the internet but says 3.2kW on the outdoor unit and the model number is PUH-3KVA.I'm trying to figure out whether our high power bill since moving in is due to this (we use it maybe 10 hrs a day on average); or the incredibly old giant hot water cylinder ....

You don't say what is high for a power bill? What area of NZ, type of house, size and number of rooms, how insulated, how many people, etc?

These issues have been discussed several times before so there's lots of info buried in the forum threads.

timmmay:If your hot water cylinder feels warm to touch then put a cylinder wrap around it. New cylinders don't need them apparently, I put one on mine just because I had a spare laying around.

Yes, it is well worth doing. Wrapping an old cylinder can save over $200 a year which is more than the cost of many wraps.

The real test for a wrap is not whether whether it feels warm to touch without the wrap but whether it is warm to touch under a wrap. This is because the losses under your hand can be small but the entire surface area is comparatively large. So use a blanket over the top of the tank to test this. A study in the USA - I've lost the reference - found that it was economic to insulate a hot water cylinder with up to 16 inches of insulation. Adding more insulation reduces the heat loss which reduces the convection currents in the cylinder which then reduces the heat loss, and so on.

Our cylinder is five years old and we can measure a substantial difference with one wrap so we are getting another one.

[Edit to fix typos]

timmmay

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#883205 23-Aug-2013 09:29

My power bill is around $350 this month for two people who shower twice a day, with the heat on 22 from 6pm - 10pm, 18 overnight, and on and off during the day, plus the other heat pump on for a half hour each morning and a few hours each weekend. My house is pretty old but I've spent a lot of time and money on insulation, including retrofit double glazing.

What's your bill?

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#883207 23-Aug-2013 09:29

joker97:
I'm trying to figure out whether our high power bill since moving in is due to this (we use it maybe 10 hrs a day on average); or the incredibly old giant hot water cylinder ....

Give the choice I would finger the Hot water Cylinder, the older it is the more uninsulated it is likely to be, If it likely to have at least a 3KW element in it and if it has a high temperature setting and an poor insulation it will not be cheap to run....

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#883290 23-Aug-2013 11:49

Two people + one infant, we both shower once a day, 6KW heatpump which never gets turned off (in winter), we had the house well insulated and treat it like we do a fridge or freezer...I think heatpumps use the most power when they are trying to get a room up to temperature...we've found by maintaining it at 21C our power bills are less than when we used to run a free standing heater for 4-5 hours each night. We also have a Econoheat Panel in babies room set to 20C between 6pm-9am.
We have on demand hotwater that is on gas...so that helps with the power bill. Ours is usually $150-170 at this time of the year.

Matt East

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#883329 23-Aug-2013 12:26

Hammerer:
joker97: I have a giant Mr Slim Mitsubishi heat pump rated 2.4kW by the internet but says 3.2kW on the outdoor unit and the model number is PUH-3KVA.I'm trying to figure out whether our high power bill since moving in is due to this (we use it maybe 10 hrs a day on average); or the incredibly old giant hot water cylinder ....

You don't say what is high for a power bill? What area of NZ, type of house, size and number of rooms, how insulated, how many people, etc?

These issues have been discussed several times before so there's lots of info buried in the forum threads.

the short answer is I don't know.

the long answer is
- prev house around 100 sqm insulated ceiling as much as possible, insulated water cylinder at around 65-70deg 2 6kw heat pumps on 18 hrs a day every day at low and the house is cold as fridge we are always shivering without jackets on
- bill $285 a mth in coldest winter

- new house around 300sqm but we only use 150sqm upstairs unfortunately staircase is not closed off from downstairs
- on most days we can walk around naked (not that we do) and do not shiver esp if there is sun
- roof is fully insulated but thin (not up to spec), probably has wall insulation
- we run one heat pump at most either the giant non inverter which only has high or low, or an inverter Mitsubishi 5kW on low
- over maybe the first 2 mths we are in here the bill is around $1000 but there has been an increase in power price of around 11%
- but we've had in laws staying over more often in our bigger house, they stay downstairs and there are 2 oil heaters which are used overnight when they are around

but we are in the process of heatpumping the entire downstairs to eliminate the oil heater use

I guess I can give you a better estimate next winter LOL ... hopefully when we don't use heating in the summer (remember in winter we can walk around naked no probs compared with the older house where we need jackets with heatpumps on low)

the question in the OP was - was it more likely to be old heat pump or old hot water or a combination of everything and I think I've just answered my own question

timmmay

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#883333 23-Aug-2013 12:33

I doubt a hot water cylinder could do that, unless you're having HEAPs of showers or it's boiling over/leaking.

You can get an energy assessment, and/or get a guy with an IR camera in to assess the cold spots.

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#883434 23-Aug-2013 15:08

joker97:
- new house around 300sqm but we only use 150sqm upstairs unfortunately staircase is not closed off from downstairs
- on most days we can walk around naked (not that we do) and do not shiver esp if there is sun
- roof is fully insulated but thin (not up to spec), probably has wall insulation
- we run one heat pump at most either the giant non inverter which only has high or low, or an inverter Mitsubishi 5kW on low
- over maybe the first 2 mths we are in here the bill is around $1000 but there has been an increase in power price of around 11%
- but we've had in laws staying over more often in our bigger house, they stay downstairs and there are 2 oil heaters which are used overnight when they are around

300sqm is a huge space to heat, even with a number of units, I would look at some form of doorway (or at the least a huge a$$ blanket curtain) on the stairs so that you can heat each level separately, and retain the heat.

The reason you are able to walk around naked is you are spending $500/month on power, or is it $1000 a month, if so you should be also be growing palm trees and pineapple inside to make good use of all the heating going on:)

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#883479 23-Aug-2013 16:05

the point is the combined heatpump on time is half in this house compared to the old house.

it's tropical without any heating. the heatpump is only on when there is no sun. and on low fan. (ie the heatpumps are not doing work the whole time they're on because the house retains heat a lot better)

in our old house we had 2 x 6kW heatpumps (COP4) going on most of the day on medium fan without making any headway into the house heating (ie the compressor is doing work the whole time it's on).

the differences are either
1) our current heat pumps are working harder despite my observations
2) (one of) our current heat pumps are wayyyy less efficient
3) we have used other heat somewhere like the water cylinder, oil heaters, ETC and our heatpump difference is insignificant
4) something else is using power that is not the heater (we have not changed our lifestyle habits ie haven't bought a giant swimming pool for example)

timmmay

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#883513 23-Aug-2013 16:52

Sounds like power monitoring would help you, probably a clamp type since nothing has plugs. Not sure about specific products, someone else may have some suggestions.

andrewNZ

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#883540 23-Aug-2013 18:11

The ones Jaycar sell are pretty good, Watts Clever is the brand. They are wireless, and I think some can monitor multiple points. You can also access the data to your PC.

A couple of thoughts:
-You don't have underfloor heating or something like that?
-Lighting can be a big factor. Are there significantly more lights than the previous house? are they incandescent rather than CFL?
-Have you checked the hot water cylinder isn't overflowing a little, and so constantly heating? It's happened to me, I could only tell because I heard a trickle water going down the downpipe in the middle of summer, and the pipe on the roof was just wet down one side.

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#883551 23-Aug-2013 18:51

all lights are the power saving type. no underfloor heating.

i'll try the other power spy-sniffing suggestions

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