Heat pumps - how to make the numbers work?

JKenH

michaels said:

So still trying to make the numbers work, I am trying to estimate how much of my 32kwh gas consumption is for hot water and how much for space heating.  In July when there was probably zero space heating we used about 1100kwh for hot water.  Should I just multiply this by 12 or will we use more in winter because the inflow temperature is lower, and if so about how much more?

Are you sure about that 1100kwh? For a large part of the year we get all our hot water from the solar IBoost and typically 6-8kwh is diverted to the 2 hot water tanks each day. There is some loss from cooling overnight so I would be surprised if we would use more than 6kwh each day if we were heating the water as we needed it.

michaels

JKenH said:

michaels said:

So still trying to make the numbers work, I am trying to estimate how much of my 32kwh gas consumption is for hot water and how much for space heating.  In July when there was probably zero space heating we used about 1100kwh for hot water.  Should I just multiply this by 12 or will we use more in winter because the inflow temperature is lower, and if so about how much more?

Are you sure about that 1100kwh? For a large part of the year we get all our hot water from the solar IBoost and typically 6-8kwh is diverted to the 2 hot water tanks each day. There is some loss from cooling overnight so I would be surprised if we would use more than 6kwh each day if we were heating the water as we needed it.

Sadly its true, one bath plus 11 long hot showers per day.  We may also have put a few thousand litres of hot water into the kids pool during them month so it may not be quite that bad.

ed110220

Pile_o_stone said:

ed110220 said:

How about spilt air conditioners (air to air heat pump)? The best have published SCOPs of 5 or more, but I'm not sure how achievable that is. With gas at 3.416p/kWh and electricity at 12.054 I'd only need a SCOP of 3.2 to break even.

I'm currently retro-fitting ducting for my MHRV system and it's a bit of a nightmare trying to accommodate relatively small ducts in ceiling/floor joists. I'd hate to think of how to install ducts large enough to carry significant heat without blowing a gale.

Rather than heating the air centrally and distributing the air with large ducts, in a split air conditioning system the heat is transported between the outside unit to the indoor unit by small diameter pipes. There can either be one outdoor unit for several indoor ones, or one each.

Martyn1981

michaels said:

So still trying to make the numbers work, I am trying to estimate how much of my 32kwh gas consumption is for hot water and how much for space heating.  In July when there was probably zero space heating we used about 1100kwh for hot water.  Should I just multiply this by 12 or will we use more in winter because the inflow temperature is lower, and if so about how much more?

Sorry to be a pain, but I have to go back to your OP again.
If swapping mains gas and an existing GCH for a heat pump, then you will not make the 'numbers work' today, that's been the case and will remain so for a long time*.

It was only your statement that you wanted to choose the green option that opened up the discussion to debate and analysis. If you go back to the numbers each time, then for now, you will be repeating the same experiment and hoping for a different outcome.

However, that said, I was pondering your DHW, and previously you will recall, I was shocked to find that it alone consumed around twice the gas pa as our households entire annual gas consumption for heating, DHW and gas oven. Given the very high efficiency of an ASHP during the warmer months (COP 4+), plus your PV, perhaps you'd be able to cover around 6 months of DHW from own generation, thus reducing the leccy import part of the ASHP equation. [Note - Would even the largest tanks that come with domestic ASHP be able to cope with the vast amount of DHW you use - if on checking, they can't, then this alone would end your inquiries.]

*Note that the GCH costs do not really account for all externality costs (neither would the gas part of the leccy import if you didn't switch to a green tariff), so that also helps with the numbers, though more in theory than on your bill. However, the recent article I posted on the G&E energy thread may be of interest .......

Wind and solar are 30-50% cheaper than thought, admits UK government

....... because there is an interesting element in the first graph showing gas generation with CCS at roughly the same cost as unabated gas generation. Now that's simply not possible given the costs of the CCS kit and operations, and the additional gas consumption needed to power the CCS part, so I assume it reflects a carbon cost applied to the unabated gas generation bringing the two estimates closer together.

That means that all gas prices will be subject to rising carbon tax (I suppose they already do) and that this could be factored into your calculations as it would bring both the 'green' and 'cost' elements together quite neatly by accounting for the externality costs.
If you look at the third graph (no timeline unfortunately) you will see the rising cost of gas generation due to carbon pricing, and the statement beforehand:

BEIS has also significantly reduced its levelised cost estimates for gas CCS, meaning the technology is seen as competitive with unabated gas in 2025 and cheaper thereafter, as the price of emitting CO2 rises (see chart below).

[I suppose we should also consider bio/synthetic methane if going down this rabbit hole, as they would not include a carbon tax, but since they cost more to produce I'd hope the all in numbers being considered would remain similar.]

michaels

Matt.  theory and practice.  One financial factor is whether the RHI / green home grant can cover the install cost.  
For ASHP the GHG will probably cover 100% of the cost whereas for GSHP the RHI would need to cover the cost.  The level of the RHI depends on the deemd household usage.  Our actual usage is sufficient to qualify for the full grant that might cover the full installation cost, however the split in our actual usage between HW and CH is very different form that in the RHI formula so if we end up with a lower deemed heat use than our actual because of this then this means that the capital costs are not covered for GSHP.
Why does this matter?  Because the higher COP for GSHP may make the running costs part of the equation better.


I agree re carbon pricing but sadly although the govt seem to pay lip service to this in reality I can't see them increasing something as politically sensitive as domestic gas prices to reflect the true carbon cost.


However I do think there may be scope for savings via time of use tariffs with a heat pump, obviously heating can not really be 'stored' but could to some extent be shifted to running as much as possible overnight and hot water can definitely be stored - as can electricity with our v2g set up.

Martyn1981

I think the RHI is capped at 20,000kWh, so you probably exceed that already, but when the house is assessed for its maximum allowance, this may be lower than 20k anyway as they probably won't be as generous as to meet your DHW demands.
From there you can claim about 10p/kWh 'generated', so if you consume 20,000kWh of heat, then the leccy input is probably about 1/3rd of that (based on an average UK COP of around 2.9), so you could get about £1,300 pa for 7years, which I assume, doesn't cover the cost estimates you've received?
But seriously, do you actually have 12 people bathing/ long showering each day, as you'll really want/need to raise this regarding a tank large enough to reheat on heat pump temps each day - I honestly have no idea, it may be fine, or impossible, it just sounds like one hell of a task for it to achieve to my naive brain.

BTW, I share your doubts about the Govt grasping the nettle and applying realistic carbon pricing, but that BEIS chart does seem pretty specific, with a gas generation price of £85/MWh in 2025, rising to £125/MWh in 2040.

From there (and I'm really just thinking out loud, so please anyone correct this) that suggests an additional £40/MWh on top of any carbon taxes up to 2025. If the leccy generation of 1MWh requires 2MWh of gas input, then that's an additional tax of £20/MWh on gas, or 2p/kWh. May be worth bearing in mind, or may be complete nonsense I've over-thought.

michaels

So I can get a green home grant of up to 10k which I hope would cover an ashp installation so no need to worry about RHI eligibility.

However I think running costs wise the gshp would have an appreciably better SPF so if I could get a gshp with the capital cost met by the RHI that would be ideal.  The RHI for GSHP is much higher and I think goes up to 30k kwh pa which gives a max payment of £33k if you can get an epc which says 30k kwh pa.

Whether you can get a borehole/vertical gshp for as little as 33k is another question, I got a quote yesterday of approx. 45k!!!!!  we would need at 12kw system (possibly 16kw for ashp due to the lower efficiencies) but I think it is more likely that an ashp could be got for 10k (the green home grant no worrying about rhi limit)

michaels

ON the assumed carbon price of gas for electricity, I don't fault your calcs but just suspect the govt would never dare add the carbon price levy extra to domestic gas prices.

Martyn1981

michaels said:

ON the assumed carbon price of gas for electricity, I don't fault your calcs but just suspect the govt would never dare add the carbon price levy extra to domestic gas prices.

I have strong doubts too, but remain hopefull. Maybe it would happen when there are 'better' alternatives, such as cheaper heat pumps, and low carbon gas - but Ecotricity's bio-gas plans from grass seem to have stalled, and even then the price would still be higher than 'cheap' FF gas.

Reed_Richards

I have just been quoted £17.5k to install a complete system with a 17 kW ASHP.  The price includes supplying and fitting a new hot water cylinder and 12 new radiators so apart from the pre-exisiting pipework it's a new heating system.