ASHP vs Oil in 2022

[ReedRichards]

6 hours ago, JohnMo said:

Short cycling is where the boiler/ heat pump, starts runs for short period and trips on high return temps. It is then locked out 10 mins and then repeats.

 

Lots of energy goes into heating all the metal in the system as this sucks the heat from the water.  Not energy efficient.

 

You always have to heat all the metal in the system, this always metal loses some heat and that wastes energy if it is located outside the heated fabric of the building but not if it is inside (which is why the pipes feeding radiators are not insulated). 

 

Actually the main reason short cycling is inefficient is because there is the potential to supply the same amount of heat using a continuous supply of lower temperature water and running a heat pump to supply lower temperature water is a lot more efficient.  At the moment my heat pump spends the day cycling but the peak power in the cycle is the minimum power the heat pump can provide (which is about 20% of its maximum output).  If my heat pump could modulate down more it has the potential to be more efficient when it's not too cold outside.  But dems da breaks, heat pumps typically don't have the modulation range of a gas boiler.     

[JohnMo]

But most gas boilers are way to big in the first place.  So although the modulation can be good, it's not good if too big in the first place.

 

Why put 10kW gas boiler when you can put in 20-30kW.  Which is what a lot of people have. 

 

The same mentally with heat pump can lead to a expensive to run water heater.

[PhilT]

17 hours ago, JohnMo said:

What 6kW of head room on a max 6kW heat demand? On a house with heat/time constant of 6 days?

 

Not sure I could agree.  But I will bow out, and leave you to it.

I do agree with you in principle but my supplier quoted the same price for Ecodan 8.5 and 11.2 models, and the databook suggests the older tech twin rotary compressor in the 8.5 is significantly less efficient at temperatures other than the 2degC midpoint. So If I was going for a smaller capacity model I would get one with a scroll compressor - either a smaller Samsung or Vaillant Arotherm Plus, costs permitting.

[MortarThePoint]

With an Heat Transfer Coefficient of HTC=372W/K, ignoring any gains, means 10% duty cycle per degree of difference between indoor and outdoor with 11.2kW Ecodan 11.2kW on min setting (4kW). E.g. duty=60% when 12C outside (2.4kW average), 100% when 8C outside (4kW). Gains work out as something like 4C equivalent, shifting those temperatures down to 8C and 4C respectively. That could allow lower flow temp and better COP. Perhaps FT=30C giving a COP of 5 when 7C outside.

 

lots of scope for optimising.

 

 

[Kevm]

If you want to see what cycling looks like here are some graphs from my 14kW Ecodan.  The first pic is from last year and you can see as the outside temp reaches about 8 degrees, it starts cycling at about 3 x per hour.  The Y axis scale for energy is joules per minute.  The minimum the ASHP will use steady state (no cycling) is about 1.1kW so maybe 5kW delivered.  

 

The second picture is from yesterday.  I now have my MMSP monitoring working so can show energy delivered.  It was more like 14 degrees and the heating was barely needed (it's running on weather comp only and keeping my whole house at about 20 deg).  The cycling is a bit less frequent, maybe 2x per hour.  The COP for the 3 hours or so is 3.9 and it's delivering an average of 2.2kW.  

 

 

 

Edited October 15, 2022 by Kevm

[MortarThePoint]

I've expanded my model a bit to consider each ambient temperature and its probability of occurrence. Factoring in Our intended ASHP (Ecodan 11.2kW), but not modelling capacity variation with temperature, I calculated an average COP of 3.9 and an average of 87% coming from E7 electricity.

 

Inputs:

• SAP numbers for heat demand to work out a fit line of heat demand vs ambient temperature (14 - T_ambient)*250 kWh/month in my case
• Cambridge weather numbers to get normal distribution mean and standard deviation (10.57C and 6.57C)
• ASHP capacity (11.2kW) and cubic fit to their COP curves based on range of flow temperatures
• Flow temperature of 35C which should be possible as I have UFH on both floors.

That 87% figure ignores:

1. Any heating requirement outside of optimised hours (e.g. for defrost or comfort)
2. Domestic hot water heating

I expect 80% may be more realistic for space heating and the DHW may lower that further(?).

 

Our supplier (not sure if will be for new house) has a ration of 3:1 between their normal rate and E7 rate:

 

That makes the cost of electricity for the ASHP vary as follows depending on what proportion is E7:

 

 

So I'm probably looking at electricity costing 22p/kWh for the ASHP and with the average COP of 3.9 that makes heat cost 5.6p/kWh.

 

Oil at the moment is expensive at around 80p/litre, so 7.7p/kWh (80p/litre / 10.35kWh/litre). [saw Black Friday price of 74p, seeing 81p-89p now]

 

If I were to use the 'without government support numbers', the ASHP would average 47p/kWh electricity and 12p/kWh heat.

 

Historically, a more normal price for oil is around £55/litre I think. That leads to near parity with oil costing 5.3p/litre (5% cheaper). I haven't heard that the government is subsidising heating oil at the moment, so I wouldn't be surprised if the future looks a bit like Electricity consolidating around the subsidised price and oil coming back down to 55p/litre. Still no clear winner on price. That's all crystal ball stuff though.

 

If you have a leakier house which needs a higher flow temp, giving lower COP, and stretching the ASHP more, lowering the E7 proportion, an ASHP is clearly a disaster. 

Edited December 17, 2022 by MortarThePoint

[ReedRichards]

2 hours ago, MortarThePoint said:

Oil at the moment is expensive at around 80p/litre, so 7.7p/kWh (80p/litre / 10.35kWh/litre).

 I would say it was cheap, the cheapest readily available fuel for heating.  Apparently the UK currently has 10 oil-fired power stations; time for some more.

[MortarThePoint]

2 hours ago, ReedRichards said:

 I would say it was cheap, the cheapest readily available fuel for heating.  Apparently the UK currently has 10 oil-fired power stations; time for some more.

 

Yes cheap by comparison

[Roger440]

2 hours ago, MortarThePoint said:

 

Yes cheap by comparison

 

Indeed. As i observed on another thread, outside burning your own wood, the cheapest heating option.

 

Its a crazy world. Traditionally it was an expensive option, for those with little choice.

 

Just need to go find another oil boiler for my barn/workshop. 

[MortarThePoint]

Simple calculation to work out the SCOP required for ASHP to be cheaper than oil:

 

SCOP > (elec_cost_kWh) / (oil_cost_litre / (10.35 * oil_boiler_efficiency))

 

Where 10.35 is the kWh per litre of oil. A modern condensing oil boiler has an efficiency of 92%, so that becomes:

 

SCOP > 9.5 * (elec_cost_kWh) / (oil_cost_litre)

 

Today, for me that works out as:

 

SCOP > 9.5 * (32.09p) / (54.43p)

SCOP > 5.6

 

The 32.09p/kWh figure is a weighted average of peak=38.63 and Economy7=16.21.

 

If I had battery storage (guess 95% efficiency) the elec_cost_kWh=16.21/95%=17.06.

 

SCOP > 9.5 * (17.06p) / (54.43p)

SCOP > 3.0

 

Remember, SCOP is the average across the season, so not the worst performance when -5C outside or the best performance when +15C outside.

Edited July 15, 2023 by MortarThePoint

[ReedRichards]

3 hours ago, MortarThePoint said:

The 32.09p/kWh figure is a weighted average of peak=38.63 and Economy7=16.21.

 

That weighted average would seem to be very similar to the price of "regular" (not economy 7) electricity.  Maybe you have an electric car so need the Economy 7 to charge that?

Edited July 15, 2023 by ReedRichards

[JohnMo]

If the weighted average is above 29p you may as well be on standard rate.

[MortarThePoint]

26 minutes ago, ReedRichards said:

 

That weighted average would seem to be very similar to the price of "regular" (not economy 7) electricity.  Maybe you have an electric car so need the Economy 7 to charge that?

 

(17*38.63 + 7*16.21) / 24 = 32.09

 

Assumes uniform use.

 

Hard to adjust to the new prices. Economy7 is about what peak used to cost.

Edited July 15, 2023 by MortarThePoint

[SteamyTea]

I had a letter from EDF telling me they are turning off the radio signal for E7 (was meant to be March).

So I need to arrange for a smart meter to be installed.

Prices are now dropping so shall stick with E7 as last year was not to bad.

[sharpener]

On 15/07/2023 at 12:37, SteamyTea said:

I had a letter from EDF telling me they are turning off the radio signal for E7 (was meant to be March).

So I need to arrange for a smart meter to be installed.

Prices are now dropping so shall stick with E7 as last year was not to bad.

i have a radio teleswitch too. There was a discussion on another thread and it was pointed out that it would now be easy to fit a meter with a clock that would be sufficienty accurate over its whole life (?20 years between renewals) for this to be a non-issue. I don't know why they are not going down that route except maybe it's to move ppl to smart meters by stealth.

 

Can't get good enough reception here for a smart meter so I don't know what their fallback plan is.