Heat pumps: how much efficiency is enough?

[Garald]

I have to make a decision between two indoor, high-temperature air-water heat pumps for a renovation project. See the link below for lots of possibly irrelevant detail.

 

In the end, the choice comes to: a heat pump that fits nicely in some space that I have available, but is somewhat less efficient (option 1) and a heat pump that occupies much more space (to the extent that what is now is a garage would become a small and somewhat inconvenient bike shed), besides being considerably more expensive (35% more) and involving waiting until February - but it is more energy-efficient (option 2). 

 

Let's simplify: space vs. energy-efficiency. COP = calorific power/absorbed power (I'm translating French terms - do correct me); higher is obviously better.

 

Option 1 (Amzair RENOV HT 15M):

 

when heating to 55 C                          

7 C outside: COP = 2.54                    

-7 C outside: COP = 1.93                   

Energy label A+

 

Option 2 (Stiebel Eltron WPL 19 IK):

when heating to 55 C                              

 7 C outside: COP =  3.3

-7 C outside: COP = 2.49

Energy label A++     

 

How shall I interpret these figures? Is it reasonable to go for option 1, or would that simply mean that I will be stuck with something that will be considered grossly obsolete and inefficient 5-10 years from now?

 

 

PS. Once everything is properly insulated, it may be realistic to set the system to 50C or even 45C (the old radiators aren't very small for the most part; the previous owners set their old gas heater to medium and that was quite enough, even though they had no insulation; I was able to test this during a brief cold wave at the end of May). At 45 C, option 1 would give me COP = 3.16 at +7 C and COP = 2.3 at -7 C, but Stiebel Eltron would also increase its efficiency, obviously.

[SteamyTea]

What is the temperature distribution for your weather.

Extremes are all well and good, but they are just extremes.

[ReedRichards]

Putting aside the space considerations, if you pay 35% more for the more efficient heat pump, how long will it take to get that money back through spending less on electricity?  

[Garald]

2 hours ago, SteamyTea said:

What is the temperature distribution for your weather.

Extremes are all well and good, but they are just extremes.

 

This is Ivry-sur-Seine, just outside Paris city limits.

 

Climate data for Parc Montsouris, in a Paris district close by:

 

hideClimate data for Paris (Jardin du Luxembourg, 1981-2010 normals, elevation: 46m, extremes 1978-present
Month	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec	Year
Record high °C (°F)	17.5 (63.5)	22.9 (73.2)	25.4 (77.7)	31.5 (88.7)	36.0 (96.8)	37.6 (99.7)	41.9 (107.4)	40.2 (104.4)	35.7 (96.3)	30.7 (87.3)	22.5 (72.5)	17.5 (63.5)	41.9 (107.4)
Average high °C (°F)	7.8 (46.0)	9.1 (48.4)	13.0 (55.4)	16.4 (61.5)	20.4 (68.7)	23.5 (74.3)	25.9 (78.6)	25.8 (78.4)	22 (72)	17.2 (63.0)	11.5 (52.7)	8.1 (46.6)	16.7 (62.1)
Daily mean °C (°F)	5.4 (41.7)	6.1 (43.0)	9.3 (48.7)	11.9 (53.4)	15.6 (60.1)	18.6 (65.5)	20.8 (69.4)	20.7 (69.3)	17.3 (63.1)	13.5 (56.3)	8.8 (47.8)	5.9 (42.6)	12.8 (55.1)
Average low °C (°F)	3.0 (37.4)	3.1 (37.6)	5.5 (41.9)	7.3 (45.1)	10.8 (51.4)	13.7 (56.7)	15.8 (60.4)	15.6 (60.1)	12.7 (54.9)	9.8 (49.6)	6.1 (43.0)	3.7 (38.7)	8.9 (48.1)
Record low °C (°F)	−13.8 (7.2)	−11.6 (11.1)	−6.2 (20.8)	−2.0 (28.4)	2.3 (36.1)	6.1 (43.0)	8.7 (47.7)	8.6 (47.5)	5.0 (41.0)	−1.0 (30.2)	−6.3 (20.7)	−8.0 (17.6)	−13.8 (7.2)
Average precipitation mm (inches)	53 (2.1)	43 (1.7)	50 (2.0)	55 (2.2)	68 (2.7)	55 (2.2)	61 (2.4)	57 (2.2)	54 (2.1)	64 (2.5)	53 (2.1)	62 (2.4)	674 (26.5)
Average precipitation days (≥ 1.0 mm)	10.5	9.4	10.6	9.3	9.9	8.4	7.9	7.8	8.0	10.1	10.6	11.1	113.6
Source: Meteociel[14]

 

(From Wikipedia)

 

Temperatures rarely reach -7C these days, to the point that I've considered growing a particularly hardy type of bougainvilleas on the southern wall (no success so far).

55 minutes ago, ReedRichards said:

Putting aside the space considerations, if you pay 35% more for the more efficient heat pump, how long will it take to get that money back through spending less on electricity?  

 

How do I do that calculation?

[joth]

27 minutes ago, Garald said:

How do I do that calculation?

This is pretty fundamental to the decision to get a heat pump at all. Heat pumps are expensive, and they're only value over cheap resistive heating is the efficiency gains they offer.

In other words it maybe a simple Willis heater is better value than either the heat pumps quoted. 

 

Did you get quotes from MCS installers? If so they should have room heat loss calcs and estimate of annual energy demand that you can use to get annual running costs of each system. 

If not, you can use your EPC to get a rough estimate. Or your SAP or architect.

 

For  demand of E kWh / year.

"(E / SCOP) x unit_price" Gives you annual running costs (ignoring DHW)

 

E.g.

2000 kWh / 2.5 x 0.3 = £240 per year.

 

Edited July 14, 2022 by joth

[SteamyTea]

17 minutes ago, joth said:

Did you get quotes from MCS installers

Gerald is in France.

They probably have more sense than to design an organisation like MCS, but will have more administration, some will still be available in 500 years.

[Garald]

Let me work back from the estimates given by the people who just did a somewhat disappointing job on the "audit énergétique".

 

They give two estimates for the energy we will spend on heating depending on how much work we do - one is 7483 kW/year and the other one is 6091 kW/year. (They actually have a little symbol h_{EP} after kW - no idea what that means.)

Let us work with 7200 kW/year. We are working with heating alone, neglecting differences in efficiency in heating water for consumption.

 

They are assuming a heat pump with SCOP =3.3. We are then talking about 3.3*7200 = 23760 kW/year.

 

I don't have SCOP data for Amzair or Stiebel Eltron at 55C (other than the labels A+ and A++, respectively). Since, at 35C, Stiebel Eltron has a COP of 4.99 when it is 7C and a COP of 3.32 when it is -7C, and a SCOP of 4.60 overall, I will just estimate that SCOP = (3/4)*(COP at 7C) + (1/4)*(COP at -7C). 

 

Then the SCOP of Stiebel Eltron WPL 19 IK at 55 C would be (3/4)*3.3+(1/4)*2.49 = 3.0975, and that of Amzair HT 15M would be (3/4)*2.54+(1/4)*1.93 = 2.3875.

 

So, the energy being used would be (3.3/3.0975)*7200 = about 7670 for Stiebel Eltron, and (3.3/2.3875)*7200 = about 9950 for Amzair;

 

At one-sixth of a euro per kW (current price at Engie, my current supplier),  we are talking about a difference of (9950-7670)/6 = 380 euros per year.

 

The difference in price is about 4500 euros. (There's also an additional difference in price, in the same direction, of 1500 euros; it goes into hot water for consumption - we are not taking that into account).

 

It will take then about 12 years for the investment to pay off. That seems about fair value; I imagine an indoors heat pump lasts roughly as much as an indoors cat.

 

----------------

Of course then there are environmental considerations, which are part of why we bother with all of this to begin with. But strictly in money terms, it seems that the extra efficiency of the Stiebel Eltron is paid for fairly by the extra price, grosso modo, and I should really make my choice based on other considerations.

 

[SteamyTea]

4 minutes ago, Garald said:

They actually have a little symbol h_{EP} after kW - no idea what that means

Guessing here, does it mean electrical power 

Edited July 14, 2022 by SteamyTea

[ReedRichards]

I think their units will be kWh per year.  The kiloWatt hour is a unit of energy consumption and you will be billed for your electricity consumption expressed in kWh (or you are in the UK).

 

Your current electricity per kWh cost is about half what we are paying in the UK.  I hope your electricity market is regulated or suppliers may work out that they can make more money selling electricity abroad than domestically and that could push your price up.    

[Garald]

France exports plenty of electricity, but sure, there have to be regulations in plave to make sure local demand is met first.

[ReedRichards]

Local demand at a price below the international market rate; you are fortunate indeed.

[joth]

UK higher prices may also be due to inefficiency in the supplier and distribution networks, higher margins, and higher taxes (including the so called Green Energy Surcharge that is applied to electricity but not gas here).

So exporting more to the UK does not necessarily mean higher revenue heading back to France for the producers. 

Edited July 15, 2022 by joth

[HughF]

What is your other heating energy option? That’s the deciding factor here. Do you need a certain scop to beat natural gas?

 

If your only option is electricity then compared to resistance heating, any heatpump will beat those on costs. Then it’s just payback inside the units lifetime to consider.

[kommando]

EDF have been strong armed to keep French Electricity consumer prices low, they are forced to sell at a loss to internal competitors. Next step is to renationalise EDF to ensure this continues without EDF going bust. So EDF will not be selling to the UK and leaving France short.

[SteamyTea]

5 minutes ago, kommando said:

Next step is to renationalise EDF

Thought I read they had been taken into public ownership already.

 

Still, they have 3 new reactors about to come on stream.  1 in France and two in other places.

Wouldn't it be great if a state backed company came to the UK, especially to a financially deprived area like the West Country, and build a new reactor.

All those cheap, low carbon, electrons flowing down the M5.  

What is not to like.

[Ferdinand]

Several factors here.

 

1 - I don't think you can rely on the long term Electricity price, because the French grid is in transition, Last winter at one time 70% of the French nuclear fleet was closed for maintenance, and it is still about half closed. The underlying issue is that no new reactors have opened since 2000, so whole shebang is like a clapped out Citroen - complex and unreliable, due to lack of consistent investment / problems with getting it done.

 

Mons. Macron made promises for umpteen billions of investment in nuclear and renewable, but then also in mini-nuclear. he has earmarked something like 100bn of investment in several directions. It is not clear what will happen, or who will pay for it via what mechanism. Difficult to see prices coming down, though.

 

2 - Not clear wrt EDF. Their share value was wrecked by the enforced reduced price, and the ~14% minority non-Govt shareholders badly hit. For - according to me - political election-related reasons.

 

Now it has been (or is being) nationalised. Share price has partially recovered (by about 30% after more than halving) in the last 7 days. 

 

3 - France has been importing up to 15% of its electricity in recent months, especially from Germany and the UK. This morning they are importing from UK, Germany, Spain and Belgium - the first 3 are countries that by policy invested in renewables whilst France was committed to nuclear.

 

I also see that the broken UK 1GW interconnector seems to have come back on stream as we have more capacity, but that the Govt has declined PP for one of the new proposals for another one for what seem to be confused reasons. 

 

4 - So what does that mean?

 

Personally, I would expect electricity prices to be rather less controlled, so I would prioritise being more independent from the network by reducing usage / more efficiency / self-generation - as the investments will not deliver until the mid-2030s. The current UK renewable surge is as a result of processes launched by Blair-Brown; it takes that long. 

 

Have you considered a place for solar pv to do your air-conditioning in the summer, spring and autumn?

 

5. As an aside - how cheap is it for you to replace radiators?

 

The last time I looked at having a system flush when gunge jammed up the workings of an old boiler, it was barely more expensive to replace all the radiators.

Ferdinand

 

 

[JamesPa]

"As an aside - how cheap is it for you to replace radiators?"

 

This resonates.  Radiators, in the UK at least, are remarkably cheap.  In planning my heat pump system I have gone from keeping as many as possible, to swapping out any I need to swap out to achieve my target flow of 45C, as I have come to realise how cheap they are, particularly of you can find a way to avoid less common sizes.  Plumbers don't charge much to fit them either, although it's a simple enough job if you have the time.

 

I definitely now have the impression that trying to avoid replacing radiators by increasing flow temp is a false economy.

 

Having said that my design still has a couple of problem rads where I will probably take a risk on a slightly smaller one than ideal, so that the space impact is not too great, and add a homemade fan unit to boost the output by ?%.

 

[Ferdinand]

14 hours ago, JamesPa said:

"As an aside - how cheap is it for you to replace radiators?"

 

This resonates.  Radiators, in the UK at least, are remarkably cheap.  In planning my heat pump system I have gone from keeping as many as possible, to swapping out any I need to swap out to achieve my target flow of 45C, as I have come to realise how cheap they are, particularly of you can find a way to avoid less common sizes.  Plumbers don't charge much to fit them either, although it's a simple enough job if you have the time.

 

I definitely now have the impression that trying to avoid replacing radiators by increasing flow temp is a false economy.

 

Having said that my design still has a couple of problem rads where I will probably take a risk on a slightly smaller one than ideal, so that the space impact is not too great, and add a homemade fan unit to boost the output by ?%.

 

I don't see any problem with this strategy, other than that you need sufficient leeway to allow Plan B if you calculate wrongly (in either direction).

 

One aspect of plan B for underestimating heat need could be awareness of double or triple rads for one or two of your locations as a future swap out if you need more hear. Or have some extra pipework installed.

 

If you need less heat them 45C is quite a high flow temperature, so if you Heat Pump can be reduced to say 35C that (and some TRVs) could give you that side of it.

 

Using common sizes is a universal tactic - for example the standard 18mm sheet materials will always normally be relatively less expensive as it is the standard thickness. The same goes for whichever thickness of floor insulation board *just* meets building regulation standards.

 

F  

[Garald]

On 15/07/2022 at 11:45, Ferdinand said:

 

Have you considered a place for solar pv to do your air-conditioning in the summer, spring and autumn?

 

I've considered PV, and will probably install them, though my architect says she doesn't know what I will do with all that electricity on sunny days (selling it back to the grid is not as profitable as it used to be). Come to think of it, powering a reversible Stiebel-Eltron would be a worthwhile use.

 

On 15/07/2022 at 11:45, Ferdinand said:

5. As an aside - how cheap is it for you to replace radiators?

 

It's mostly that low-temp radiators would require a lot of space (and a significant additional expenditure, once one considers the entire system,though we may have got that wrong). Let me live in the place once it's finished, and I'll see how low I can set the temperature with the old radiators (55? 50? 45?). Then I can see what decision to take on that matter in the middle run.

 

 

 

 

 

On 15/07/2022 at 11:45, Ferdinand said:

 

 

[JohnMo]

1 hour ago, Garald said:

 

I've considered PV, and will probably install them, though my architect says she doesn't know what I will do with all that electricity on sunny days (selling it back to the grid is not as profitable as it used to be). Come to think of it, powering a reversible Stiebel-Eltron would be a worthwhile use.

3kW, in summer, will run your heat pump in reverse, hot water, do the dish washer, washing machine, all your background electric usage.  You shouldn't be exporting any or very little.