You've all heard my grand (ok, not really) plans to use A2A heat pumps to reduce, hopefully, the carbon (by 40%) and cost (by 25%) of my space heating compared to my current, high efficiency combi gas boiler at reasonable installation cost (circa £2k).

I can't for the life of me decide what I, or someone in my position, would do about DHW, though. The economist in me says that, given the current cost of my hot water, any solution would have to cost about £1k installed maximum, and preferably £750. Can anyone think of a system that provides hot water at significantly lower cost and carbon than a natural gas boiler for this amount? Or are usage reduction measures the only sensible way to go?

Just let me know if I've missed any recent threads on this question.

Defra says that natural gas emits 0.184 kgCO2e/kWh. Given 85% boiler efficiency, that's 0.217 kg/kWh delivered to space heating. They say that electricity produces 0.462 kg/kWh. (see: http://tinyurl.com/nwc4rlq).

(As an aside, this site: http://tinyurl.com/kmcyvo suggest an average grid intensity of much less than 400 - don't know where the differences come from).

Anyway, using Defra's figures, you need a SPF of 2.13 in order for an electric heat pump to emit less carbon than natural gas. To achieve a 40% reduction in carbon relative to natural gas, you only need an SPF of 3.55.

As discussed in other threads, empirical evidence shows that A2A heat pumps manage SPFs of around 4 in Sweden. I'd expect them to manage at least this in the UK, and even better if I wait for R32 based systems (but there's too much of a premium).

Long story short, for these reasons I expect to achieve -at least- 40% carbon savings, and I'd hope for near 50%.

The -claimed- SCOPs of A2As are much higher than A2W devices. I think the difference is because a) output temps are lower than A2W, and b) they're far simpler machines. This unit claims 5.2: http://tinyurl.com/omsseja. The same unit, actually tested over a year in Malmo, managed 4.1 in practise.

Now, Malmo's colder on average than the UK (colder even than Edinburgh) AND suffers greater extremes of cold temperature. Both these things would tend to argue that results in the UK should be better. On the other hand, as you say, our higher humidity would mean that defrost cycles would be more frequent. All told, I'd expect the former issue to trump the latter, BUT, to be conservative, I still tend to assume that I'd get 4 (which several units tested in Malmo achieved). Once R32 arrives, I might have the courage to bump that up to 4.4 or something.

DECC or DEFRA recently published a study that found that even the best, most modern system boilers achieved 80%, and similar combi boilers 85% (might even have been the other way around!). But I can't find the link to it at the moment. Results for DHW were really poor. I think something like 65%.

Doesn't that mean that your DHW system is only 25% efficient, max?? Did you mean cut your energy usage BY 25%?

You use E7, don't you, so despite the decrease in energy usage, presume the actual cost would rise? Is there a thread you can link to where you discuss this?

IIRC, I don't think I managed to find comparative RH stats. I remember that precipitation is a lot lower in Malmo than the UK, which would result in lower RH, as would the lower average temperature I think?

Posted By: mike7You'd expect the SCOP of a DHW heat pump to be higher than for one doing space heating as it would spend a bigger proportion of its time in mild COP-friendly temperatures.

Against that, the DHW heat pump will be reqd to produce high output temp all the time, while a space heating one can output at low temp much of the time.

Posted By: fostertom

Posted By: mike7You'd expect the SCOP of a DHW heat pump to be higher than for one doing space heating as it would spend a bigger proportion of its time in mild COP-friendly temperatures.

Against that, the DHW heat pump will be reqd to produce high output temp all the time, while a space heating one can output at low temp much of the time.

Aircon style HPs modulate their output - I anticipate it would be possible to arrange for the output temp to ramp up as the tank temp rises, so avoiding the supposed need for permanent high temp.

Mike7's right. So long as you're happy with 40 degree hot water, a heat pump -should- be great for DHW. I presume that it's poor space heating performance, due to using water to transfer heat via big radiators and other complexities, that screws up the SCOP performance of conventional Air to Water systems, not DHW production.

So what I'd like to buy is a typical multi-split air con type system, which can manage an SCOP of 4+ for space heating and which, if installed on a standalone basis, would cost around my £2k.

But what we need is a system where, over night, when not being used for heating, refrigerant from the same outside unit is instead diverted to heat a (preferably existing) hot water tank. That feels like something that shouldn't be too difficult or expensive for a manufacturer to produce for my marginal £1k.

A caveat is that, if I recall correctly, Japanse EcoCute systems, which do DHW only, don't have spectacular SCOPs. But that might reflect tank losses, which is the same thing that causes gas boilers to manage relatively poor performances for DHW.

If we assume that's the case, then, a natural gas boiler emits 0.263kg/kWh of hot water if it does so at 70% efficiency (I'll dig out the actual figures when I have time, but it's something like this). Then you'd only need the DHW part of the heat pump to deliver an SCOP of 1.8 to break even in carbon terms, whereas I think it should deliver about 3.

So, now I just need to give the nice people at Mitsubishi a bell and ask them to produce a new type of system just for me...

Posted By: GarethCI remember that precipitation is a lot lower in Malmo than the UK, which would result in lower RH

It's right on the coast, so RH will be dominated by the sea, not the rain.

Posted By: mike7You'd expect the SCOP of a DHW heat pump to be higher than for one doing space heating as it would spend a bigger proportion of its time in mild COP-friendly temperatures.

My brain hurts. What does that mean. The delta T for DHW is a lot greater than that for space heating, so how come a greater COP?

My solution to Gareth's question would to try putting a refrigerant-to-water heat exchanger in the hot line of a small aircon unit - I'll try this when I get a round tuit.

Wasn't that what the Japanese tried before they came up with ecocute HPs?

Posted By: GarethCMike7's right. So long as you're happy with 40 degree hot water, a heat pump -should- be great for DHW. I presume that it's poor space heating performance, due to using water to transfer heat via big radiators and other complexities, that screws up the SCOP performance of conventional Air to Water systems, not DHW production.

40 °C is too low for a stored hot water supply (leg***ella), or for filling a bath, and is marginal for a shower. Passivhaus compact units presumably represent something like the state of the art?

Posted By: GarethCGiven the very modest capital cost, it might be worth going for direct electric for hot water. You won't do your green credentials that much damage if you're also frugal with water.

Will cost about twice as much (very roughly) in water heating costs, but the capital cost savings will make up for many years of that, and no expensive servicing of equipment needed.

Ooh, did I hear that properly? I have a direct electric thermal store which has been powered solely by PV up until two days ago when I had to give it a boost. I haven't yet bought any space heating gear (we have an old fan heater or two for any emergencies) but at the moment it still looks like direct electric for me.

Keep up the good work, Gareth. You might yet convert me!

<blockquote><cite>Posted By: GarethC</cite>Doesn't that mean that your DHW system is only 25% efficient, max?? Did you mean cut your energy usage BY 25%?</blockquote>
I meant that my cylinder looses a lot of heat (but have since insulated it more).

Part of the problem is that I use little hot water, so in effect have a very oversized cylinder.

I did post up my findings somewhere on here, and over at the other place, but did not book mark it, so struggling to find it right now.

Just trying to get some weather data from Sweden.

Assume you've read this: http://www.ebuild.co.uk/blog/12/entry-232-part-thirty-five-hot-water-and-leds/

Aberdeen has a strange microclimate off the sea and spends all summer under a seafog bank called "the Haar". No worries though, your data is probably from Dyce airport which is 5miles inland and only gets the fog in the evenings. Surprised to see any data points above 25degC ! Aberdeen has 30% less rainfall than Glasgow.

Back to the original topic, any scheme that means installing a cylinder is going to be tough on the OP's £1k budget, especially if you need to build a cupboard to put it in. Maybe best stick with the combi, at least till it needs replacement?

Or look at gas-fired instantaneous dhw heater, more power and less CO2 than electric instantaneous heater. Anyone tried these?

We have used 'Rinnai' dhw heaters. They work straight out of the box, and modulate well.
Adjustable output temp, and can accept inputs up to 65 IIRC?

Cheers

Thanks all for the input. Sorry been AWOL.

UK vs. Sweden performance: Malmo’s avg. temp is 8.1 degrees. Edinburgh’s is 9.3. Malmo has 14% more heating degree days at 15.5 degrees base temp and 41% more HDDs at ten degrees base. i.e. Malmo gets much colder than even Edinburgh, and that kills COP. With ST's data suggesting similar humidity, I'm fairly sure 4 is a conservative target COP (in England, you should get significantly better). WHY can't someone do a UK study!? I've contacted DECC in the past directly to suggest one.

Rikon Waterbox: Would be great to see a schematic, but I can't find one. Did it use a multi-split? You could use a four indoor unit multisplit, diverting one line instead to the DHW cylinder, then have that one come on overnight when others aren't in use, thereby being able to use all the outdoor unit's output.

Will's right that tank cost buggers things up if you haven't one already. If so, at £400 (+ installation) Rinnai units might be best for combi owners. Should be able to swap into where the combi is easily, and probably at least a bit more efficient. I've found the report I referred to (http://tinyurl.com/ocun5d6) which found that combis are 73% efficient for DHW (and regular boilers 38% on average! But with large variation).

http://www.rinnaiuk.com/hotechnology/technology/condensing-water-heaters/

This vendor claims 107% efficiency vs LHV due to very low condensing temp of incoming cold water feed.

Maybe not worthwhile investing to preheat the water with ashp as would reduce condensing flue gas effect?

They come in LPG variant so might be good alternative for off gas grid, in parallel with ashp for space heating. My dhw usage would use about 4 of those orange propane cylinders a year so no LPG tank required.

I'm suddenly interested - anyone see a drawback?

The really cheap ones (different brands) seem to be aimed at caravans, so flow a bit low.