101 Intro to Heat Pumps & running costs - help required

5lugger

Hi,

Had a survey for a heat pump via Octopus last week and have just received the response.  I'm scheduled to have a call with the sales advisor a little later.

In anticipation I'm looking for a little help in trying to work out the likely running costs, so I've got a better idea if this is all worthwhile or not, so any practical first hand experience would be gratefully received.

The recommended system is a Daikin Altherma Monobloc - 4kW with a Stelflow Slimline Hot water cylinder

For background info my property is a small 2 bedroom, currently heated via a condensing gas boiler, approx 15 years old.  The property is in the south east, and I generally work from home most days.  The room I work for is south facing, so even in Winter I don't generally have the heating on for more than a couple of hours a day, at 19 degrees.  Hot water is generally on for 45 minutes per day.

From what I can work out my annual gas KWH usage for the past couple of years was 4000 Kwh for 22/23 and 3500 Kwh in 23/24 (which I think has been a milder winter)

I also have solar panels (but currently no battery storage) This is a 3KW system and on average I generate 2500 Kwh per year.  Appreciate the majority of this will be in the summer when I'll not requiring heating.

Can anyone give me a steer on running costs, or how these can be best calculated?  I will wade through other posts over the weekend, but grateful of any hints ahead of todays phone call.

Cheers!

tacpot12

Even though you won't have your heating on, your solar production should be helpful to heat hot water during the summer, and should reduce the costs further. 

Your existing boiler will be about 90% efficient, so you would have needed about 3,6000kWh of heat energy to heat your home and hot water (90% x 4000kWh). (You are burning 4,000kWh of gas to get 3,600khW of heat)

To figure out how much this heat should cost you if it comes from a heat pump will depend on the seasonal Coefficeint of Performance  (SCOP) of the heat pump. 

If the SCOP is expected to be 4 (for the sake of arguement), then 3,600kWh of heat energy will take 900kWh (3,600 divided by 4). Multiply the cost of 900kWh of electricity to find out the annual cost. I'm paying about 50p per kWh, so the cost or me would be about £450 a year or £37.50 a month. This is a very low amount, but you are being very frugal with your heating and hot water usage, so if your figure of 4,000kWh is correct for a full year, then the figures above will also be correct. 

Reed_Richards

Octopus ought to tell you what they estimate your annual heat and hot water requirement to be and how much electricity that should require.  From what you write, you may find that they estimate you use more energy than the 3500 to 4000 kWh p.a. that you actually use.  If they overestimate by, for example, 50% then they will overestimate your electricity usage by 50% too, so you can get a more accurate estimate by allowing for that.  Plus you'll get some of that electricity from your solar panels, although probably not a high proportion.  

5lugger

Reed_Richards said:

Octopus ought to tell you what they estimate your annual heat and hot water requirement to be and how much electricity that should require.  From what you write, you may find that they estimate you use more energy than the 3500 to 4000 kWh p.a. that you actually use.  If they overestimate by, for example, 50% then they will overestimate your electricity usage by 50% too, so you can get a more accurate estimate by allowing for that.  Plus you'll get some of that electricity from your solar panels, although probably not a high proportion.  

Cheers, 

This was the bit I couldn't quite work out.  Info they provided is:

Estimated System Performance / Comparison Energy Requirement for the building

Net Energy required to heat property

Heating   Hot Water     Total

7070       2453              9523 kWh

Existing System Consumption

Heating   Hot Water     Total

7684       2666             10351 kWh

New HP System Estimated Consumption Full Heat Pump System (if selected above)

HP System Electricity Consumption

Heating   Hot Water     Total

2062       1019              3081 kW

On this basis then if I'm actually using say 4000 kWh of gas, and some electricity for pump etc, I'm using under half of what they have suggested in the 9523 kWh.  If that is the case then my overall electrical usage would be half? So say 1500 kWh rather than 3081 kWh?

Or should I expect slightly more electricity usage with the need of the heat pump system to be on longer?  Again, another bit I can't work out

5lugger

tacpot12 said:

Even though you won't have your heating on, your solar production should be helpful to heat hot water during the summer, and should reduce the costs further. 

Your existing boiler will be about 90% efficient, so you would have needed about 3,6000kWh of heat energy to heat your home and hot water (90% x 4000kWh). (You are burning 4,000kWh of gas to get 3,600khW of heat)

To figure out how much this heat should cost you if it comes from a heat pump will depend on the seasonal Coefficeint of Performance  (SCOP) of the heat pump. 

If the SCOP is expected to be 4 (for the sake of argument), then 3,600kWh of heat energy will take 900kWh (3,600 divided by 4). Multiply the cost of 900kWh of electricity to find out the annual cost. I'm paying about 50p per kWh, so the cost or me would be about £450 a year or £37.50 a month. This is a very low amount, but you are being very frugal with your heating and hot water usage, so if your figure of 4,000kWh is correct for a full year, then the figures above will also be correct. 

Thanks, and yep frugal a good description    I'd not sit in the cold, but I have found the room I work from to be generally warm enough to work from without the need to have so many layers on that I look like the Michelin man!

The SCOP is estimated to be 3.43 for heating, based on a flow of 50c.  So, if I were to be conservative and take the 4000 kWh figure I'd be looking at say around 1200 kWh (4000 / 3.43) with a small reduction for the solar power.  If that's correct then based on the standard tariff at present being 29.73 per kWh I'd be paying say £350 a year, or do I need to allow some extra given the system needs to run more often than a boiler firing up for an hour or two?

Strummer22

5lugger said:

On this basis then if I'm actually using say 4000 kWh of gas, and some electricity for pump etc, I'm using under half of what they have suggested in the 9523 kWh.  If that is the case then my overall electrical usage would be half? So say 1500 kWh rather than 3081 kWh?

Or should I expect slightly more electricity usage with the need of the heat pump system to be on longer?  Again, another bit I can't work out

Yes if you currently use 4,000 kWh of gas a year (producing 3,600 kWh of heat assuming 90% boiler efficiency as noted above) but Octopus estimate your existing consumption is 10,351 kWh per year, then you can factor their estimate of electricity use by the heat pump by 4,000 / 10,351 = 38.6% (well done you for being frugal... I also use much less energy than my EPC indicates, and I expect this is true of a lot of forumites; after all, we are on the money saving expert forum!).

You can expect to use 38.6% of Octopus's estimated 3,081 kWh of electricity per year, i.e. 1,191 kWh. Your calculation based on the SCOP should use the heat demand of 3,600 kWh as boiler efficiency isn't relevant to the calc. Either way it's very close to your 1,200 kWh estimate based on usage and SCOP. £350 a year to heat your house is dirt cheap... but is just slightly more than what you've been paying (gas: 7.24 p/kWh, 4,000 kWh per annum = £290). 

Your heat pump will run longer than the boiler - possibly all day in cold weather - but it will use energy and produce heat at a slower rate than the boiler does. No adjustment to the figures is necessary because of the longer running hours. 

Is it proposed to replace the radiators with bigger ones, or even install underfloor heating? Getting larger heat emitters means your heating can function at a lower flow temp. Heat pumps are more efficient at lower flow temp. 50°C is fairly high for a heat pump, you'll get a better real-life SCOP if you reduce the flow temp. The systems are designed to a flow temp of 50°C to appease people who want a 'hot' radiator - it just needs to be warm (not hot) to warm up the house though. 

Note the £350/year is based on the system SCOP of 3.43. You can almost certainly beat that if you run your heat pump correctly, lower flow temps as much as you can, use weather compensation, run it to heat your hot water in the afternoon (when the air is warmer) and not 6am when it's coldest outside. Others will be able to give further pointers. 

Reed_Richards

There are two ways you can run a heat pump.  You can used a fixed output water temperature (at the high end of the possible range).  If you do that and the surface area of your radiators is large enough then you should be able to warm up a room/house about as quickly as with a gas boiler.

Or you can use Weather Compensation in which case the output water flow temperature is reduced as it gets warmer outside.  This will tend to make your room/house slower to warm up but for the same total amount of heat it should work out cheaper.

I think that the calculations Octopus gave you are for a fixed output water temperature (which should be specified).  But I could be wrong about that; ask them if you like.  

5lugger

Strummer22 said:

5lugger said:

On this basis then if I'm actually using say 4000 kWh of gas, and some electricity for pump etc, I'm using under half of what they have suggested in the 9523 kWh.  If that is the case then my overall electrical usage would be half? So say 1500 kWh rather than 3081 kWh?

Or should I expect slightly more electricity usage with the need of the heat pump system to be on longer?  Again, another bit I can't work out

Yes if you currently use 4,000 kWh of gas a year (producing 3,600 kWh of heat assuming 90% boiler efficiency as noted above) but Octopus estimate your existing consumption is 10,351 kWh per year, then you can factor their estimate of electricity use by the heat pump by 4,000 / 10,351 = 38.6% (well done you for being frugal... I also use much less energy than my EPC indicates, and I expect this is true of a lot of forumites; after all, we are on the money saving expert forum!).

You can expect to use 38.6% of Octopus's estimated 3,081 kWh of electricity per year, i.e. 1,191 kWh. Your calculation based on the SCOP should use the heat demand of 3,600 kWh as boiler efficiency isn't relevant to the calc. Either way it's very close to your 1,200 kWh estimate based on usage and SCOP. £350 a year to heat your house is dirt cheap... but is just slightly more than what you've been paying (gas: 7.24 p/kWh, 4,000 kWh per annum = £290). 

Your heat pump will run longer than the boiler - possibly all day in cold weather - but it will use energy and produce heat at a slower rate than the boiler does. No adjustment to the figures is necessary because of the longer running hours. 

Is it proposed to replace the radiators with bigger ones, or even install underfloor heating? Getting larger heat emitters means your heating can function at a lower flow temp. Heat pumps are more efficient at lower flow temp. 50°C is fairly high for a heat pump, you'll get a better real-life SCOP if you reduce the flow temp. The systems are designed to a flow temp of 50°C to appease people who want a 'hot' radiator - it just needs to be warm (not hot) to warm up the house though. 

Note the £350/year is based on the system SCOP of 3.43. You can almost certainly beat that if you run your heat pump correctly, lower flow temps as much as you can, use weather compensation, run it to heat your hot water in the afternoon (when the air is warmer) and not 6am when it's coldest outside. Others will be able to give further pointers. 

Thanks for the reply.

Pre pandemic & working from home (and higher energy prices) I'd guess I heated the house more using the Hive scheduler, so warm when I got home etc.  I now rarely use the schedule and mainly get by using the boost for an hour or so.  This started off mainly as an experiment, the room I work from is south facing, so even on a cold day, it seems to get decent warmth from sun.  Currently 19c in here and not had heating on all week (thermostat is in hallway downstairs and is 14.5, again no heating)  Anyway less of my frugality!!

The current rads are well spec'd all K2 / T22 so the only suggested replacement was the bathroom which is a 600x600, and they simply suggested a K3 was fitted here, again 600x600, otherwise the survey said the current rads would be ok, and no suggestion of underfloor heating.  I was slightly surprised the rad in the lounge doesn't need replacing, as I'd thought that might need swapping. 

 

5lugger

Ok, so have now spoken to the helpful chap at Octopus.

Again, when running through the calculations I mentioned my actual usage and that moved the figures towards those kindly provided in the replies, though there is a bit of a small 'unknown' cost of running an additional immersion heater once a week.

As I understood from the the run through if we took the 4000 KWh of gas, the heat pump figures would be:

3600 kWh (to allow for boiler efficiency) and then tariff dependent

1200 kWh would be on a cheaper off peak rate & 2400 kWh would be on a peak rate.  The working figures here were 12p & 35p, though I note the standard rate from 01 April will be 24.5p in my region, which would overall be less expensive than a split rate, so would be interested to hear of others experiences with split rates.

There was an offer to not upgrade the radiator in the bathroom, and receive a discount on the overall cost, and given my slight concern if the lounge rad will be warm enough there was an offer to potentially split the cost of upgrading that rad for piece of mind.

The other points of note were firstly the removal of flooring to allow for a D2 pipe to be fed under the floors and out through the wall, I've asked why this can't leave in the same route the pipework comes in.

Finally as it stands planning permission is needed as the noise level is one decibel too high!!!  As the neighbours window is within range.  I've asked how this might be mitigated without the need for planning permission, and it seemed increased fencing could be an option, so a discussion needed with the neighbours!

Again any views or experiences over any of this, particularly those last couple of points would be appreciated.  

Reed_Richards

If you ever find yourself naked and wet in your bathroom then I wouldn't advise you to skimp on radiator size if that causes that bathroom to be on the chilly side.  And if you have a room that you want to warm up quickly then oversize the radiators in that room.

Split rates work for me because I have solar panels with a battery.  I charge the battery cheaply overnight, it powers my heat pump in the morning then at this time of year it gets recharged from my solar panels; today (sunny) I didn't have to start using mains electricity until 9 pm.  But without a battery the best you can do is heat your water at the cheap rate  

FreeBear

Strummer22 said: Note the £350/year is based on the system SCOP of 3.43. You can almost certainly beat that if you run your heat pump correctly, lower flow temps as much as you can, use weather compensation, run it to heat your hot water in the afternoon (when the air is warmer) and not 6am when it's coldest outside. Others will be able to give further pointers. 

Just had a quick look at the spec sheet for the Daikin Altherma. SCOP at 55°C flow temp is given as 3.26, and at 35°C, 4.48 (for space heating). So if you are running with weather compensation along with smarter controls, that should push the running costs down even further.