Badly performing heat pumps?

Are some heat pumps performing as badly as various recent stories have been reporting?

Let’s imagine if we procured cars in the same way we do home heating.

We would start with a body shell, then get someone to fit wheels to it, hopefully placed in the best position and sized for our purpose. We would then find an engine of a suitable power for our needs and couple it up to a gearbox with the best ratios.  We would need some way of controlling it all, and then learn how best to drive it.

In many ways, installing home heating can be little like this.  We have a ‘body shell’ that may be insulated or not. It may be lightweight or slow-to-heat heavy stone.  We then fit radiators or underfloor pipes.  Then we install a heat pump along with all its gubbins… we now have to set the controls and ‘drive’ it as efficiently as possible.

So, what is it about heat pumps that could make them prone to inefficient running?  I have likened a heat pump to a sailing boat.  You need to get a lot of things ‘just so’ in able to speedily slice through the water.  A heat pump is transferring heat from the environment, and it’s actually an amazing feat getting considerably more out than you put in (at the expense of cooling the outdoors more than it already is).  It does seem magical to get 3 kW of heat for 1kW of electricity, but it’s far from magic.  Let’s face it, there’s nothing magical about a fridge!

The concept does at first seem to defy the rules of science, so one might expect scepticism.  But it really doesn’t take long to understand the principle. However, if your heat pump has been pieced-together badly, you have high energy bills and are not warm enough, then you can see what feeds the notion of doubt.

If only heat pumps were as ‘visual’ as those cobbled together cars that we are imagining.  …..engines either too big or too small, either chugging or screaming at the wrong revs. How could you fail to spot the inefficiency of a car at full throttle in neutral while waiting at traffic lights.  This sort of thing can happen to a heat pump unnoticed.

I’m sure if anyone pointed their finger at such an inefficient car, the obvious answer would be – “make some modifications and get it working properly”!  The cheap-to-run cars would at-a-glance look and sound impressive.   Hopefully everyone would look to these best-performing cars and realise that they are the way forward.

It’s been odd for me seeing recent press reports that speak as if heat pumps are a new idea. In many ways, heat pumps are technologically advanced. They have been in common use in countries like Sweden for over 40 years.  A vast amount of product development has taken place in America and Japan, to name a few, so we have a great variety of tried and tested products available.

We also have, of recent years, got good training courses and knowledgeable installers able to fit them.  We seem however to sometimes be falling foul of the pitfalls with respect to how we integrate this technology into our motley (with respect to energy… no offence) range of dwellings.  I think we should acknowledge that many older buildings will be a challenge, but I feel it wont take long to learn what works and what doesn’t… we should already know!

So, how fussy are heat pumps?  What can go wrong?  In a nutshell, it’s a lot to do with the temperature of the water being heated. They love lukewarm water and whilst they can manage ‘hot’, they don’t particularly like it.  It’s simply a bit more tricky and expensive to install a heating system in older buildings if you are working with lukewarm water. In practice the water might need to be a bit warmer.

So, what actually goes wrong in these underperforming systems?  To name a few….  heating cycle duration too hot and too short, heating zones switching on/off unsynchronised, house allowed to cool off to much overnight, mixing valves ‘diluting’ the water temperature, circulation pumps running when not needed, flow rates too low.  Furthermore, given that we sometimes can only guess the actual heat-loss U values, systems might be either too big or too small.

We have learnt a heck of a lot over the recent decade, and given that a lot of heat pumps have either heat-meters and electric meters, or on-boards performance (COP) estimators, there is no reason why we should not be able to know with some certainty how well heat pumps are performing in practice, and how we can ensure that all installations achieve their potential.

I have on occasions come across owners who have fallen out with their heat pump so badly that they don’t give them a chance… they don’t even try to get them working properly.  Hopefully, as time passes we will learn to avoid the pitfalls, and correct those that have already happened.

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6 thoughts on “Badly performing heat pumps?”

  1. Is it more efficient to heat the whole house or turn off heating in rooms that are not in use.
    Many thanks for any comments

    1. Good question. I would expect there to always be a saving by turning un-used room off. I think the notion of leaving-your-whole-house-warm stems from the fact that you will probably need radiators hotter in the lived-in rooms since these have a higher heat-need if surrounded by cold rooms. The hotter rads generally mean a lower COP. However, if we consider the outer surfaces of the house, then heating that un-used spare room must add significantly to the net heat loss from the house. The question is… in practice, do you need rads hotter to combat heat losses to the cold rooms?, and will it reduce COP significantly?.is the COP significantly worse if you have a few rooms on.
      If you could magically move your radiators from the un-used rooms into the rooms you need heating, you you would do so. The problem can arise if your system is not very good at heating just a few radiators… if your rads are a bit small and your heat pump is a bit big.. e.g. if by turning radiators off, your heat pump goes into an unhealthy cycling on/off routine. It is however very hard to know if cycling is detrimental.. some monitoring we have done shows some cycling drops the COP only a small amount, other times it drops it notably. It’s worth test your system out by comparing a couple of days with all-on and a couple of days with some rooms off, and read your energy use difference. The COP may well get worse with more rads off, but total electricity consumed should be less. It is also a thing that will vary with the seasons, and unfortunately, it is spring and autumn where some heat pumps prefer a bigger load,.. preferring all rads on. Again , this is mostly a problem for oversized heat pumps and not-big-enough radiators. I would be interest in results from anyone experimenting.
      I think this whole topic could posed in a different way.. ‘how much more expensive is it to keep all my rooms warm’? I think the only way of knowing is to try it out.

      1. I may of left this late in season, but we will still have lots of cold days.
        My energy usage for the month ends on the 4th. So I live in 4 bed bungalow with open plan living area and air pump with under floor heating.
        At the moment I have 3 bedrooms , and main bathroom turned off, after 4 th I will turn on all rooms in house and compare bills for month. I will post.

        1. It can be a little bit complicated, and I’m wondering if I should write a blog about it in an attempt to explain it better. ??

          In a nutshell.. there are different control methods used with heat pump systems.. some have a ‘weather compensation’ function that is adjusted by the owner or installer. If you are expecting to heat more rooms and trying to save energy, you will usually need to have a different ‘heat curve’ setting so you get the lowest water temperature still gives you enough heat. Theoretically, the radiator water can be at a lower temperature if most rooms heated. If however you have a ‘room target-temperature’ control, then your system should automatically try to operate on the lowest water temperature. It will ‘auto adapt’ to the new conditions.

          Maybe experimenting is the only way of finding out. It would be worth taking some daily readings, and record approximate outside temperure. My fear is that for many systems, simply turning rooms on, will increase running costs considerably.

  2. With your knowledge on heat pumps I was hoping you might be able to give us some advice. Our heat pump is 16yrs old, and I’m hoping for a little advice. For nearly one year now our heat punp has been running on one compressor as it kept tripping out just after the second compressor started up. We were advised at the time that it was coming near to its lifespan and to run it on the single compressor until it failed.

    With our IDL Terra 20kw twin compressor heat pump now running on one compressor, before this one finally gives up on us, (as I would presume it’s not a simple case of topping up the refrigerant), I thought it would be prudent to find out what heat pumps are available on the market.

    With your experience are there any specific heat pumps you would recommend as a replacement? And any you would advise us to avoid?

    One person did say a while ago that he thought our heat pump was over sized and recommended something about 8kw. Personally, I don’t think that would be big enough.

    Having seen the cost of electricity increase and our heat pump taking longer than before to increase room temperatures I thought it might help if I could utilise the “Off Peak” electricity times for certain individual rooms by replacing some of the room stats for digital ones with the following in mind.

    With a heat pump than can utilise “Peak” and “Off-Peak” electricity times with 3 “Off-Peak” periods each day.

    I’m looking for a Digital room stat (preferably single socket) that can do the following, is there anything on the market that you can recommend.

    A Digital room stat that can be programmed to have a higher set point temperature during the 3 “Off-Peak” periods each day, i.e. program 6 separate time periods within the day. 3 time periods at 22 degC and 3 time periods at 18 degC . The current room stats are 240v, 3 wire + earth.

    I hope all this makes sense and hope to hear from you at your convenience.

    Kind regards

    Lindsay

    1. Hello Lindsay,
      It sounds good advice to run your system on one compressor. I would not assume the 2nd compressor may fail.. it could go on for years. It would be useful for you to assess if the 10kWs is big enough for your needs. Some sort of monitor might help you know how many hours/day the unit is running. You could try to extrapolate for the coldest day. I tend to favour smaller systems because they are more likely to operate at lower flow temperatures, hence be more efficient. Generally, controls seem more critical for bigger systems (bigger watts/sqm). BUT if you want to benefit from off-peak, a bigger size is an advantage in mid winter.
      Hard to recommend any specific model, but you may want something that will fit most easily with the rest of your existing system. I was sceptical about inverter models, but I have found that the ones I have monitored are giving very good COPs… the down-side would be possible failure and expensive repair.
      Finding good controls is a problem, but many (programmable thermostats) do time and temperature. You want something that doesnt pulse, as some boiler room stats do. Anything ‘intelligent’ may be unhelpful. Something with a simple cut-in/cut-out differential (say 0.5 degrees) will be best.
      Sorry, I have been rather vague, so not sure how helpful this is

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