ASHP Running Costs / Efficiency

[Sailchick]

Hey, which is the most efficient way to run a 14kW ASHP. Times intervals or constantly on? 
 

1. We have one specific for DHW the water remains pretty warm for 24hrs (although my husband tends to drain out all the water when he has a shower 🤦🏽‍♀️) 

 

2. We have another one for UFH, no buffer tank. New build, well insulated. 

[ReedRichards]

1. Keeping a hot water tank at temperature is the least favourite task of a heat pump because it's the least efficient thing it has to do.  If the water in the tank goes a bit tepid then at least it can be more efficient whilst heating the the water from tepid to warm.  So there might be an efficiency saving turning off the tank heat pump when you don't need hot water (e.g. overnight).  Or you could offset the tank temperature at night if you are able to do that.

 

2. UFH tends to be very slow to respond, particularly when it's buried in a big slab of concrete, as is likely in a new build.  You keep the slab at temperature, that warms the rooms above it.  You really have to keep it on all the time.  I'm surprised you can get away without a buffer tank.     

[ProDave]

1 hour ago, Sailchick said:

2. We have another one for UFH, no buffer tank. New build, well insulated. 

Do you mean you have two ASHP's?

 

 

[SteamyTea]

1 hour ago, Sailchick said:

Hey, which is the most efficient way to run a 14kW ASHP. Times intervals or constantly on

There is no one answer to that question.

What are you trying to achieve, and what do you mean by efficiency?

[Sailchick]

1 hour ago, SteamyTea said:What are you trying to achieve, and what do you mean by efficiency?

Aiming to minimise electricity bills. 

[Sailchick]

1 hour ago, ProDave said:

Do you mean you have two ASHP's?

 

 

Correct, i apologies for my poorly written questions 

[Sailchick]

2 hours ago, ReedRichards said:

2. I'm surprised you can get away without a buffer tank.     

Well it was a debate whether to install one or not. The original design didn’t have it but that was designed almost 3 years ago.  We only just commissioned the whole thing. We’re trying to see what works best. And yes we have very thick concrete slabs. 

[Iceverge]

Do you have a time of use tariff? E7 or similar? 

 

What flow temp do you use? 

[SteamyTea]

38 minutes ago, Sailchick said:

Aiming to minimise electricity bills. 

Basically, the efficiency (CoP) of a heat pump increases the smaller the temperature difference between the outside air (OAT) and the delivered temperature. This is the power (W or kW).

Depending on how long the heat pump is working, in hours, multiplied by the power it takes to run it, is the energy, the kWh.

The problem is finding the right position under the curve (integral) that is most cost effective. This is especially hard if the outside temperature is variable i.e. the day starts cold  then warms up, then gets cold again.

The 'on' times need to coincide with this. 

This can be done manually i.e. switch the HP off for 6 hours during the warmer period of the day.

The trouble with this is the efficiency of the HP may well be quite low as it tries to reheat everything up to operating temperature. This can cause icing up of the outside unit that takes energy out of the system to clear the frost off the fins.

 

My view, and this is purely academic as I don't have a heat pump, is that, if you can  run the unit for as long as possible, at the lowest output temperature possible, your increased CoP will counteract the longer running times.

This may seem counter intuitive as the HP is using electric power for many hours. But ten hours at 1 kW is less than 4 hours at 3 kW, even if that 4 hours is spread across the day.

The problems (calculations) gets compounded if, for tariff reasons, the amount of time the HP can be run on a cheaper tarif, especially if that tarif time is in the middle of the night.

 

So summing up.

Run the HP at the lowest output temperature you can get away with.

Accept that the house temperature may vary during the day by 2 or 3⁰C.

Learn how the weather compensation curves are set up, these are designed to deliver more power, but at a lower CoP when it is really needed then back off as the temperature increases. It is like driving up a hill, you have to put more pressure on the gas peddle, and sometimes change down a gear, then you can coast down the road for the next bit, as long as you did not loose too much speed on the up hill (or you just get in the way of everyone else).

Edited April 5, 2023 by SteamyTea

[Sailchick]

It’s a balance, and trying to figure out that balance is the hardest. At the moment we’re on a flat electricity rate. I’m hoping to get PVs in soon so this could benefit us. I guess I need to get my excel sheet out and do the calculations. 

[JohnMo]

I would set up WC (Weather Compensation - Ed) curve and have a thermostat that can switch off the heat pump.

 

Test 1 optimise WC curve, so HP runs 24/7, without tripping thermostat (set thermostat a couple of deg hotter than target house temp) record energy usage daily.

 

Test 2, increase WC curve up a couple of degrees, set thermostat 0.5 below target house temp.  The HP will now batch charge the floor over an 8 to 10 hour period, the temp in the house is likely to overshoot the thermostat 0.5 to 1 degree. Record your finding

 

compare with test 1. Publish results.

 

As a note, although on a gas boiler, I am finding test 2 results in about 10 to 30% reduction in gas consumption - it changes with outside temp.

 

With a thick floor, on off scheduling doesn't work that well, due to the huge heating time.

 

 

[mattman]

Hi we have a small 8.5kW air source heat pump and find it works best at a very low flow temp, which is controlled on the heat pump controller, and on all day, in all zones. During very cold -6c outside weather we found we were using about 25 to 30kWh a day for all our domestic hot water and heating (this is just ASHP usage). We schedule our DHW to heat twice a day if it is below temp. We only have heating on the groundfloor via wet UFH, are well insulated with a reasonable air tightness score of 2.5, with MVHR which distributes warmth upstairs. With the milder weather we've been able to turn off our space heating which further reduces cost. We are on the Octopus Tracker tariff which we have found works really well as it is a low cost all through the day. which suits the heat pump as its on all the time. It's currently at 17p per kWh. It was 16p yesterday. 

Edited April 11, 2023 by mattman