Air Source Heat Pump running it efficiently 20 April at 2:09PM edited 21 April at 10:39AM in LPG, Heating Oil, Solid & Other Fuels 16 replies 427 views

Mstty

So please talk to me like I know nothing, moved into our house in June last year and the house already had a Mitsubishi Ecodan heat pump and it has been reasonably efficient.

What I want to do is maximise efficiency for this winter with the higher fuel bills.

Some do's and don'ts on how to run it and general advice please

shinytop

Mstty said:

So please talk to me like I know nothing, moved into our house in June last year and the house already had a Mitsubishi Ecodan heat pump and it has been reasonably efficient.

What I want to do is maximise efficiency for this winter with the higher fuel bills.

Some do's and don'ts on how to run it and general advice please

Can you re post in the Energy forum? You won't get many replies here as most people who have ASHPs don't hang around here. I have an Ecodan and willgive you some pointers. 

Mstty

Strangely someone said to post here, happy for a mod to move it👍

gefnew

I bought a heat pump.
You could Read this post.
https://forums.moneysavingexpert.com/discussion/6240076/i-bought-a-heat-pump#latest

Reed_Richards

I'm not sure where this was originally posted but it seems to be in as good a place as any now.  I would say:

Do use weather compensation.
Do check that the parameters used for weather compensation are at least roughly correct.  If the weather compensation algorithm sets the flow temperature too high you will waste money.  If the flow temperature is too low your house will not get warm enough.  I'm afraid you have to do this by trial and error, although I found the parameters specified in my MCS certificate were a good starting point.
Don't believe all the myth and mystique surrounding heat pumps.  They are not that different from boilers; it's just that getting the weather compensation right will achieve bigger cost savings.  
  

matelodave

As above, get the weather compensation slope set to the lowest flow temp that you can achieve commensurate with heating the place comfortably and within a reasonable amount of time based on your lifestyle. It's now probably the wrong time of year to try an evaluate it's performance as you probably don't need much if any heating

For instance, as we are at home all day ours can idle away at quite a low temp just to balance the heat loss of the house with a night set back of around 3 degrees but it takes hours or even days to reheat if we let the place get stone cold but that's due to the very low flow temps and sluggish response of the underfloor heating. 

Our flow temp varies between 30-40 degrees depending on how cold it is outside (-5 then it's about 40, +5 then its about 35 and +10 its around 30, but that suits our bungalow and keeps it at around 19-20 degrees inside)

However there are occasions when its wet and windy but not ever so cold and the weather compensation lowers the flow but cant anticipate the additional heat loss due to evaporative cooling of the house, so the the flow temp might need a bit of a boost so I might tweak it it to +3 or 4 for a few hours to compensate. In the end its down to trial and error but keep notes of what you've done, and what the effect was. Only tweak one thing at a time and give it a day or two to settle between tweaks.

If you've got radiators and want to let the place cool a bit overnight and during the day when you are out at work then you need either larger radiators or higher flow temps and it's high flow temps that reduce the efficiency of the system and often its best just to set it back 2-3 degrees rather than right off.

Likewise keeping your hot water tank temp lower will also save you money - we can get away with 45 degrees and just heat for an hour or two a day and that gives us all the hot water we need (dishes get washed in the dishwasher). We have a low flow shower (6lpm) and we dont spend ages in there.

Reed_Richards

Good advice from @matelodave.  I have only radiators so my maximum target flow temperature is 50 C.  But I checked my MCS certificate and that says that this is based on an external temperature of -3.7 C so I set my weather compensation settings to make the flow temperature less than 50 C if the external temperature is warmer than -4 C.  So far this has worked well.  

markin

Reed_Richards said:

Good advice from @matelodave.  I have only radiators so my maximum target flow temperature is 50 C.  But I checked my MCS certificate and that says that this is based on an external temperature of -3.7 C so I set my weather compensation settings to make the flow temperature less than 50 C if the external temperature is warmer than -4 C.  So far this has worked well.  

Where you rads upgraded? Or maybe you need more, A 50c target seems uncommon on these boards and must be expensive to use.

Reed_Richards

Yes, my radiators were upgraded and I got two additional ones.  50 C is the target for the Leaving Water Temperature. the target drop across the radiators is 5 C so the target average radiator temperature is 47.5 C.  I don't think that is very unusual for a radiator-based system installed as an upgrade to an existing system where some radiators needed to fit into the same space as their predecessors.  Now with hindsight I regret not running the numbers on a slightly lower temperature, say 45 C.  If that could have worked it would have got me a reduced running cost and a bigger RHI grant.  On the other hand, I am only actually using 50 C when it is -4 C or less outside.  At the moment it is 8.5 C outside and the target LWT should be about 38 C.      

thrope

matelodave said:

we can get away with 45 degrees and just heat for an hour or two a day and that gives us all the hot water we need (dishes get washed in the dishwasher).

Watch out for Legionnella. I think you have to heat above 60 for safety (at least once a week). Otherwise there is a real risk. https://www.hse.gov.uk/healthservices/legionella.htm

One thing I don't understand about the recommendation to leave heat pumps on all the time. With traditionally heating I thought "leave the heating on low" was a debunked myth, ie it is most efficient to heat the house just when you need it warm, so off during the day when out and overnight. In the winter with the heating off overnight it would definitely drop below 10, but with a heat pump we would put a set back from 19 to say 16. But we are still heating the house those 6 degrees every evening when not needing too. I know this is also because the heatpump has lower temperature output and takes longer to warm the house, but is this factored in when comparing the efficiency?

Reed_Richards

There are conflicting opinions about Legionella.  Some say you need to go above 60 C for about an hour once a week.  Some say that a longer time at a lower temperature can also be effective.

I would not recommend leaving a heat pump on all the time unless you have underfloor heating.  This can be very slow to respond and can take a long time to bring a room back to temperature if allowed to cool.  The same applies if you have underfloor heating heated by other means.  However, if a heat pump is trying to heat up a room/house quickly it may increase the water output temperature causing it to run less efficiently.  Not all heat pumps will do this but if you have one that does it might be more efficient overall to keep it on for longer running at a higher efficiency. 

I'm horrified that you house drops by so much in temperature overnight; it must be very badly insulated.  My heating is set back by 3 C for six hours at night.  Only on the coldest nights does it drop by 3 C and cause the heating to come on early.  The low temperature output of a heat pump has little or nothing to do with the time it takes to warm up the house.  Underfloor heating is slow because the is a large thermal mass involved.  Radiators can be slow if their output is closely matched to the room requirement; oversize the radiator and the room will heat up faster.