ASHP - How to know the actual COP?

cretanrunner
cretanrunner Posts: 23 Forumite
edited 23 January at 3:24PM in Heat pumps
Thinking of installing an ASHP and have been following some of the threads on here. I may have missed this, but how does one actually calculate the real COP that is being achieved over a chosen time period? Obviously I can meter the electrical input, but how do I measure the actual heat output? Sorry if this is dumb....
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  • jeepjunkie wrote: »
    One of the building engineers at work explained how to measure COP yourself and can ask again if you wish but basically you need to measure power consumed, the ASHP will tell you the temp its heating to then you need sensors on the pipes at the rads. Stick it all into a formula...

    Oh boy, that sounds complicated. So when people rabbit on about they are getting this or that COP, they are just guessing? All most people can do in practice is compare with last year's costs?
  • Oh boy, that sounds complicated. So when people rabbit on about they are getting this or that COP, they are just guessing? All most people can do in practice is compare with last year's costs?

    It's not an easy one to work out. Alot of guesswork is involved.

    One way to estimate the COP though is to get an idea of the volume of water in your central heating system. Each rad. can be fairly accurately measured, then add a bit for the pipework.

    Start your heat pump from cold, noting that the temp. in your rads is say 18'C. Then when your rads have all stabilised at say 35'C, you know you have raised x litres of water by 17'C. The energy required to do this is easily calculated (use google). Try to keep the time short, so as to minimise heat loss to the room, but you will need to add some energy for this.

    If you have also measured the energy in kW used by the heat pump during this time (I have a counter fitted to mine), then you have your two figures. The former divided by the latter is the COP.

    Not terribly accurate or scientific, but should give a rough idea.
  • cretanrunner
    cretanrunner Posts: 23 Forumite
    edited 17 October 2011 at 3:36PM
    OK, that's starting to make sense. I have UFH not rads though, which may actually make it easier. I know exactly how much pipe is laid under the floors, and I can measure the main feeds to the manifolds too, so that will give me total volume of water. What would be the best place to measure the water temp change though? To be "fair" to the heat pump should I not measure at the HP flow pipe thus eliminating heat losses in the house?

    Actually, that won't eliminate heat losses will it? Doesn't matter where you measure does it?
  • Cardew
    Cardew Posts: 29,034
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    You cannot work it out! Requires sophisticated measuring equipment. Anyway there is no point in measuring a snap shot at a specific ambient temperature and attempting to extrapolate that result to give an annual overall figure.

    Read the EST report on their year long trial of 29 systems to see the very poor results acheived.

    Also beware of glowing reports from people who haven't had their system over a winter and want to justify a massive expentiture.

    The bottom line is at the current state of knowledge of those installing(often in unsuitable houses) you are taking pot luck if you get a good system for you £10,000+
  • Well, I realise we're only talking an approximation, but surely you're not suggesting I rely on the manufacturers' blurb......

    I detect that you may be a bit of a sceptic re ASHP, Cardew, and I agree the technology may not perform in the UK, but I actually live in Crete where the avge night-time minimum in deepest winter is around 6 or 7 degrees. Gas is not available. It's a new build, reasonably well insulated, and the heat pump will be driving UFH. In theory it should be able to deliver. My worry is finding a company that knows how to specify, install, and set it up properly.

    That was partly why I wanted a rough guide to COP performance so I had some idea of whether it is working properly before the electric bill comes in.

    BTW, do they not sell loads of these in Scandinavia?
  • Cardew
    Cardew Posts: 29,034
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    Well, I realise we're only talking an approximation, but surely you're not suggesting I rely on the manufacturers' blurb......

    I detect that you may be a bit of a sceptic re ASHP, Cardew, and I agree the technology may not perform in the UK, but I actually live in Crete where the avge night-time minimum in deepest winter is around 6 or 7 degrees. Gas is not available. It's a new build, reasonably well insulated, and the heat pump will be driving UFH. In theory it should be able to deliver. My worry is finding a company that knows how to specify, install, and set it up properly.

    That was partly why I wanted a rough guide to COP performance so I had some idea of whether it is working properly before the electric bill comes in.

    BTW, do they not sell loads of these in Scandinavia?

    Yes a self-confessed sceptic at the current situation with ASHPs in UK.
    My worry is finding a company that knows how to specify, install, and set it up properly.

    That is exactly the problem in UK.

    For a start in UK you need no formal qualifications as a heating engineer to install a system - Even the big firms like Mitsubishi etc allow anyone to install a system and take absolutely no responsibility if it is an unsuitable installation.

    Read through MSE forums to see the number of people who have ripped out ASHPs because they just don't work. That is replicated all over the web.

    Of course the technology should work - but all to often it doesn't work. People are paying up to £20,000 for systems that are useless.

    Read this thread that I started:

    http://forums.moneysavingexpert.com/showthread.php?t=2968958
  • grahamc2003
    grahamc2003 Posts: 1,771 Forumite
    edited 17 October 2011 at 7:01PM
    Thinking of installing an ASHP and have been following some of the threads on here. I may have missed this, but how does one actually calculate the real COP that is being achieved over a chosen time period? Obviously I can meter the electrical input, but how do I measure the actual heat output? Sorry if this is dumb....

    Difficult to get an estimate after installation, but would be pretty easy to calculate the dynamic cop if a couple of things were designed into the system.

    The cop is constantly changing of course, not a fixed value - sort of analogous to a car's mpg, and probably just as simple to calculate.

    The easy bit is the instantaneous power used, and no probs getting that. To get the rate of heating of the water (assuming a air to water hp), all you need is the instantaneous water flow rate, the temp of the water entering the hp, and the temp of the water leaving the hp. A simple bit of logic (say on a laptop reading the above parameters in real time) could easily calculate and display the instantaneous cop (it's the rate of heat input to the water divided by the power consumed).

    Sometimes of course, the cop would be zero, during defrost cycles, which is what seems to me to be the achilles' heel of hps.
  • albyota
    albyota Posts: 1,106 Forumite
    On the other hand, there are over 20,000, yes, twenty thousand....!!! systems (Mitsubishi, Daikin, Sanyo, LG, Panasonic, Hitachi, Vaillant, Worcester Bosch, Grant and 43 other manufacturers) installed in the UK (46,000 Ecodans in Europe) that do work...and work very very well, and have reduced their owners running costs for an average cost of £8.5K. The problem is...... you will never get to hear from these people, unfortunately, searching these forums will only provide you with negative comments from unfortunately, badly installed, incorrectly designed / specified systems.

    There is still a lot of unqualified installers as cardew says, that do not understand the complexities of advising, sizing, designing, specifying, installing, etc... and until these issues are sorted out, I'm afraid, I have to agree with cardew, people are taking a chance. Research the technology, get a SAP report, talk to people who have had systems installed...and have had them through the last couple of winters, ask for reference sites, check with the maufacturers that the specified equipment is sized correctly for your property, and what temperatures (external / internal) they have used in their calculations.

    Anyway, most heat pumps work on R410A refrigerant gas, and pressure and temperature is relative, so as the ambient temperature drops, the performance of most systems drops, this will affect the maximum output, and the temperature of the water it can produce, at the same time the COP reduces, therfore the electrical consumpsion rises.

    To put this into perspective, for example,

    at minus 7 degrees, Mitsubishi's 14kW Ecodan will drop to 13kW (energy out), but still heating water at 45 degrees, power input of 6.67 kW (energy in), and a COP of 1.95. (using standard UK tariff 13.2p/kWh) means 6.77p/kW of heat. (13.2p/1.95)

    at higher ambient, at minus 2 degrees, Mitsubishi's 14kW Ecodan will maintain 14kW (energy out), still heating water at 45 degrees, power input of 6.17 kW (energy in), and a COP of 2.27.

    at +7 degrees, Mitsubishi's 14kW Ecodan will maintain 14kW (energy out), still heating water at 45 degrees, power input of 4.38 kW (energy in), and a COP of 3.20.

    at the extreme UK winter (3 weeks)
    at minus 15, Mitsubishi's 14kW Ecodan will drop to 11kW (energy out), still heating water at 45 degrees, power input of 6.83 kW (energy in), and a COP of 1.61.

    Crikey...got carried away there.....any way to answer the OP, in Crete with ambient temps of 6 or 7 running UFH at 35 degrees, with the 14 kW Ecodan, power in 3.32kW the COP is 4.22, however, if your house only needs 6, 7 or 8 kW to maintain the required temps, then that is all the system will provide due to it being inverter driven compressor, so adjust the above figures accordingly.

    hope this helps

    good luck cretanrunner.

    Edit, as graham2003 says, the defrost strategy for some units in the UK can be a serious problem, but if this (6 min loss per hour) is factored in to the calculations, it should not be a reason to put anyone off.
    There are three types of people in this world...those that can count ...and those that can't! ;)

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  • Thanks to everyone for their contributions. I'm starting to see a way forward. A couple of supplementary questions: I already have a 1500L thermal store. (It's a long story...). Should I send the heat pump output to the store or link direct to the UFH? I've seen arguments for and against. Some people claim the heat pump will run more continuously when linked to a heat store, others claim it will work more continuously when running straight to the UFH!! Any views? Also are the COP figures for the Ecodan from Mitsubishi, and if so, why should I trust them? Call me cynical...
  • Cardew
    Cardew Posts: 29,034
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    Cynic as I may be, I don't think there is much doubt that the manufacurer's data is accurate for their heat pumps on a test bed. In the same way as my car will do XX mpg at YY mph according to the manufacturer.

    As a matter of interest, why in a climate like Crete(hot Summers and mild winters) have you gone for an Air to water ASHP in a new build? I thought Air to Air ASHPs, with ducting to each room, was normal in new builds in such climates?? - the obvious advantage being air conditioning. Certainly that is the preferred solution in the USA in similar climates and was in some properties in Malta that I looked at.
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