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Part 22 - Heating and DHW


Stones

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In this entry I'm going to discuss in more detail how I came to choose our heating and hot water system, and how it has performed to date.

 

As other forum members have found, deciding which fuel source and type of technology to use in a low energy house, is a challenge given the different requirements each of us has.  We had three stipulations – low running costs, hot water available on tap 24/7 and maintenance of the whole house at an even and constant temperature 24/7.

 

Having calculated our heating demand, taking the impact of solar gain, incidental household gain, human occupancy and wind speed into account, I was confident that I had a good indication of the amount of heating I would need.  I was also confident, based on historical use, of the amount of hot water we as a family use.

 

Living in an area without mains gas, my options were somewhat limited to using either oil or electricity as my fuel source. LPG was initially considered but discounted due to the lack of availability in my location. 

 

As part of the decision making process, I spent a fair amount of time carrying out a cost comparison of both oil and electricity based heating and hot water systems, using 500kWh increments from 2500kWh to 5000kWh.  I considered direct electric of various type, oil and air source heat pumps, both air to water and air to air.  Solar PV was also considered and costed in terms of each method of heat and hot water delivery.

 

In line with previous cost comparisons that I had carried out, I found direct electric to be the most cost effective in terms of capital outlay and running costs when both heating and hot water demand were less than 2500 kilowatt hours each year. As heating requirement and hot water requirement increases so the balance began to tip in favour of other technologies.

 

Oil was quickly dropped from the list as it became apparent that any rise in fuel prices over then then low point, would significantly increase running costs.

 

Having conducted significant investigation in respect of the viability of Sunamp units, although attractive in many ways, I found that the capital outlay and running cost was simply too high to be able to justify, given that the main benefit (low heat losses) were not as critical for me as they have been for others.  Part of that decision was also driven by the cost of fitting Solar PV, which in our remote location was extortionate.  I looked into a non MCS DIY install, but couldn’t make the figures stack up, the break-even point being around 17 years.  Much as I wanted to install PV, it didn't make any sense financially.  In time, I hope to revisit PV, if and when battery storage reduces the break-even point to a more realistic timescale. 

 

A wind turbine, given our location and the virtually constant presence of wind, would have been an ideal energy source and paired with Sunamp technology, probably unbeatable.  The proximity of nearby houses ruled out that option in terms of planning permission.

 

Air to Air heat pumps were ruled out based on my own experience of them and a road test at a friends house.  Neither myself or my good lady found them particularly pleasant as a heat source.

 

Having gone through the list of options, an air to water air source heat pump, paired with a large UVC and UFH for the distribution of heat, represented the best balance in terms of capital outlay, running costs and crucially, comfort and convenience.

 

We opted for a package from Mitsubishi Ecodan, an 8.5kW heat pump and 300 litre pre-plumbed cylinder fitted with the Mitsubishi FTC5 control panel. Given our location, we opted for the coastal model, which is treated with acrylic resin for enhanced corrosion resistance.  Whilst a pre-plumbed cylinder is more expensive than a bare cylinder and associated parts, after taking labour (plumber and electrician) into account, I found there was very little difference in cost.

 

I sourced the package from a trade supplier, Secon Solar. I found their price list while searching online and having phoned the company, and perhaps fortuitously speaking to the managing director of the firm, found they were quite happy to sell me package at trade / installer price, the bonus being that delivery to my location was free.

 

The package is configured for the UK market, the only difference to the system as sold in the rest of Europe (AFAIK) being that the cooling function of the heat pump is disabled so that the product complies with MCS approval for claiming RHI.  It is however a simple task to activate the cooling function, by flipping a dip switch in the control module on the cylinder.  Cooling can then be controlled from the master controller.

 

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As stated in an earlier blog entry, the heat pump and cylinder were fitted very quickly with simple connections on the plumbing side – flow and return from the ASHP, cold water, hot water and flow and return to the underfloor heating manifold.  Electrical connections consisted of power to the ASHP, a cable from the ASHP to the control module and a plug-in controller. 

 

I had initially planned to have the cylinder in the utility room close to the ASHP Monobloc, but changed the location to a service cupboard in the middle of the house, to reduce internal DHW pipe runs.  This does mean a 15 metre pipe run for flow and return to the ASHP, but as virtually all is within the insulated envelope, it doesn’t represent much of an issue, and does not appear to be having an adverse effect on performance.  

 

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The ASHP Monobloc itself is located beside our back door, open to the elements.  It seems happy enough where it is, despite the wind that traverses the space between house and garage walls.  Locating the ASHP within the garage itself was an option but one I decided against simply on the grounds that I didn’t want to give up floor space within the garage.  A timber housing for the ASHP is something we may look at in the future.

 

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We opted to fit individual room thermostats to all 3 bedrooms, to give us the option of being able to reduce the bedroom temperatures if we so wished.  We have not used these and keep the whole house at one temperature 24/7, treating the underfloor heating as a single zone.

 

At present I only have limited data as to how the heat pump has performed since moving in.  On board energy metering (energy consumed and energy produced) shows the CoP for heating has ranged between 3.5 and 4. DHW is maintained at 47C-50C in the cylinder, boosted every fortnight to 60 degrees by the immersion on an anti-legionella cycle.  To date the CoP for DHW is 2.4

 

As members know, heat pumps are best suited to the production of low temperature heat as opposed to the higher temperatures required for domestic hot water.  Whilst the CoP for DHW is lower than that for heating, the cost per kWh of our DHW, based on a CoP of 2.4, is 5p, which is significantly better than an E7 electricity tariff.  We may be taking a hit on efficiency, but in reality all of the other options would have cost us more.

 

The 300 litre capacity of the cylinder means that we have plenty of hot water on tap and can comfortably run a full bath and still have sufficient left over for another person to shower. 

 

The ASHP is currently operating on a 24/7 basis, providing heat input to the UFH and topping off the DHW as and when it determines it needs to, at whatever flow temperature it determines.  Whilst that does sound like a recipe for high bills and high flow temperatures, in practice, the heat pump delivers the lowest flow temp it can get away with to maintain our set temperature.

 

If I so choose, the controller lets me set various parameters such as heating curves or set flow temperatures, or indeed a timed schedule for heating and DHW. However,as the system is operating efficiently on its auto setting, and providing the level of comfort we want, I see very little reason to mess around and create my own settings.  If say electricity tariffs were to change from a single tariff to a dynamic tariff, then I would have the option of timing the heat pump operation to coincide with  lower rate tariffs. 

 

After much thought, and indeed discussion on this forum, I opted for an 8.5 kWh ASHP over a 5 kWh ASHP, as I felt happier running a larger unit more gently than pushing a smaller capacity unit harder.  A 5 kWh unit would probably have sufficed, and in time, may be what the current unit is replaced with when it reaches the end of its life.

 

We haven’t yet had to activate the cooling function as any overheating (defined as internal temperatures over 23C) caused by solar gain, can, as modeled, be managed by natural cross ventilation.  

 

Neither have we found it necessary to constantly circulate the UFH to even out the house temperature / redistribute solar gain from one part of the house to the other.  In the heating season, we found that there was sufficient circulation of the UFH during the heating cycle to maintain the house at an even temperature.  Outwith the heating season, when solar gain is at its peak, the house zones itself, the bedroom section remaining slightly cooler than the public areas, very useful on a warm summers day.   

 

Overall I’m very happy and impressed with our system.  It has, so far, delivered everything we have asked of it in terms of comfort and convenience, and the running costs are low.  I have the capability to cool the house (via slab cooling) if I so wish, and the option to bolt on a second zone pack onto the pre-plumb cylinder if I ever found it necessary to install a second heating / cooling function – i.e. fan coil or duct heater / cooler.  The one criticism that I have is about the controller thermostat function and its hysteresis - 1C increments only. A finer degree of control would have been preferable. 

 

Our installation was recently inspected by an MCS accreditor (our plumber is going through the accreditation process).  In due course that will give us the option to apply for RHI, although that will be very much dependant on whether the figures stack up.

 

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The heat pump looks to be much closer to the house than I have seen elsewhere. Do you think that in any way affects its performance?

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It's 250mm off the wall.  Doesn't seem to be affecting its performance.  Airflow certainly isn't a problem.

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250mm should be fine.  This is the diagram from the MI's for ours, showing that 200mm is the minimum spacing off the wall:

 

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Thanks @Stones, we're going through a phase of doubting our decision to go for a very similar setup to yours and this timely blog entry is not only very clear and informative but has gone a long way to settling any doubts we had. My only question would be, if mains gas had been available would your chosen solution have looked any different? 

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@JohnW, thanks for your kind comments. 

 

Our resident Welsh Wizard may be able to offer more insight but with gas you start needing buffers or maybe even a TS because of the short cycling issue. I'm only guessing but I believe fuel costs would be lower (for DHW at least), but you end up paying another standing charge for your gas supply. I haven't costed gas vs ASHP, but I suspect there wouldn't be that much in it once capital, running and servicing costs taken into account.

 

If you need cooling, gas isn't going to give you that capability.

 

I can't really give you a definitive answer as it simply wasn't something I had to consider, but my gut feeling is I probably would have still gone with an ASHP set up.

 

 

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4 hours ago, Stones said:

@JohnW, thanks for your kind comments. 

 

Our resident Welsh Wizard may be able to offer more insight but with gas you start needing buffers or maybe even a TS because of the short cycling issue. I'm only guessing but I believe fuel costs would be lower (for DHW at least), but you end up paying another standing charge for your gas supply. I haven't costed gas vs ASHP, but I suspect there wouldn't be that much in it once capital, running and servicing costs taken into account.

 

If you need cooling, gas isn't going to give you that capability.

 

I can't really give you a definitive answer as it simply wasn't something I had to consider, but my gut feeling is I probably would have still gone with an ASHP set up.

 

 

Thanks @Stones fair point about the lack of cooling.

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13 hours ago, Stones said:

@JohnW, thanks for your kind comments. 

 

Our resident Welsh Wizard may be able to offer more insight but with gas you start needing buffers or maybe even a TS because of the short cycling issue. I'm only guessing but I believe fuel costs would be lower (for DHW at least), but you end up paying another standing charge for your gas supply. I haven't costed gas vs ASHP, but I suspect there wouldn't be that much in it once capital, running and servicing costs taken into account.

 

If you need cooling, gas isn't going to give you that capability.

 

I can't really give you a definitive answer as it simply wasn't something I had to consider, but my gut feeling is I probably would have still gone with an ASHP set up.

 

 

It's very easy to get a house warm, but a bloody nightmare to keep it cool. 

Purely on the lesser amount of moving / complex components, I can't help but like Jason's setup. The CoP for DHW is an excellent bit of real life info, and equally impressive. Bigger house, bigger cylinder, ( if needs be of course ), and the figure should stay the same. 

A great blog entry Jason, many thanks. Nice to see all the neat copper work too B|

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Well done, real life feedback. I will be installing/fine tuning my heating/DHW system soon so this is good to read, i plan a very similar system but was thinking of utilising E7 to bring bills down further but yet to work out the logistics?.

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8 minutes ago, joe90 said:

Well done, real life feedback. I will be installing/fine tuning my heating/DHW system soon so this is good to read, i plan a very similar system but was thinking of utilising E7 to bring bills down further but yet to work out the logistics?.

 

Same here ... plan is that 300 litre UVC and a 95 litre buffer get  heated by ASHP overnight to 47c/ 38c respectively and then at 5am the immersion will kick in on the UVC to take it to 62c before the E7 cut off at 6:45am allowing the ASHP to start on the heating load. 

 

Plan is by 7am the floor should be up to temp along with a UVC at full capacity along with the buffer at 38c. PHE to warm the draw off leaving the UVC to just do the last 10c should mean ample hot water. Boost will be to the UVC by immersion as it's simpler, leaving the ASHP at max CoP on the floor/buffer. 

 

Or thats the plan ..!! 

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Well Peter, great minds think alike ( I hope?). I don't want to digress this blog so when I next get access to broadband ( trip to the pub) I will start a thread on exactly what I plan ( differs slightly from yours In theory). Will be good to get yours and others input.

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7 hours ago, Nickfromwales said:

It's very easy to get a house warm, but a bloody nightmare to keep it cool. 

Purely on the lesser amount of moving / complex components, I can't help but like Jason's setup. The CoP for DHW is an excellent bit of real life info, and equally impressive. Bigger house, bigger cylinder, ( if needs be of course ), and the figure should stay the same. 

A great blog entry Jason, many thanks. Nice to see all the neat copper work too B|

 

Yes, I'm pretty pleased with the overall CoP for DHW as well. We moved in in February and the CoP for that (part) month and March was 2.3     CoP for DHW the past couple of months is 2.5

 

December and January are pretty much the same temperature wise as February up here, so I anticipate a final annual CoP of 2.4

 

Obviously different parts of the country will fare better / worse depending on the local micro-climate, but it's certainly encouraging to have such reasonable performance.

 

We've lived with a heat pump before so are aware of their strengths and weaknesses.  Our decision to go with a larger 300 litre cylinder was informed by this experience as we didn't want to have to resort to topping off the cylinder with immersion (not that there is anything wrong with this method and it was one method I investigated as part of my cost analysis), and knew we would struggle with a smaller cylinder given the lower temperatures DHW is stored at in a purely ASHP set up.

 

2 hours ago, joe90 said:

Well done, real life feedback. I will be installing/fine tuning my heating/DHW system soon so this is good to read, i plan a very similar system but was thinking of utilising E7 to bring bills down further but yet to work out the logistics?.

 

E7 is only really viable or worth using if 50% or more of your usage is within the E7 tariff period.  Having lived with E7 before, I know how challenging this can be, and in some senses restrictive (we would have needed larger cylinders / buffers) if you size all your plant to operate solely with E7.  

 

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@Stones Great blog entry.  And some good detail on CoP

 

What sort of footprint is required for your setup in the cupboard?

 

I really like the pre-plumbed offering from Mitsubishi ECODAN and have decided to go this way.  However I'm a little unsure of cylinder size.

@Stones

@PeterW

As part of our PP conditions I have to install some solar PV and where possible would use this to run the immersion to 'top up' using something like http://www.intelligent-immersion.co.uk/ - would this make a smaller cylinder an option?

I'm hoping that will be an easy set up, and hope for some nice copper work too ;)

 

 

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I would max out the size - storing your excess in the tank (immersion at 75c and a TMV on the output) will more than pay for itself over trying to find an additional diversion load. 

 

Second option is a buffer tank as the divert load to also use for floor heating and DHW preheat. 

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2 hours ago, PeterW said:

I would max out the size - storing your excess in the tank (immersion at 75c and a TMV on the output) will more than pay for itself over trying to find an additional diversion load. 

 

Second option is a buffer tank as the divert load to also use for floor heating and DHW preheat. 

+1

 

@swisscheese the one Jason fitted is 300L and he seems to do well with that one. With solar pv I'd probably go a bit bigger as DHW will be a big energy consumer and I wouldn't want to waste any excess pv with a smaller cylinder maxing out too soon. 

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@swisscheese

 

Space wise, the cylinder and attached pre plumb gubbins takes up 850mm x 750mm, the expansion vessels and UFH manifold a little more. The cupboard is 1900mm x 750mm, so basically we have half left for storage space.

 

I went with a 300 litre cylinder to avoid the need for immersion top up. You could probably drop down to a 250 litre (IIRC that's what @jack has) or even a 200 litre but would need to boost the cylinder to get the same usable DHW volume as from the 300 litre.

 

Given you have to have PV this may well seem sensible, but given the pretty low additional cost of the larger cylinder, I think you would get more benefit going for a 300 litre, plus it means you wouldn't have to use immersion to top up when solar PV wasn't providing.

 

As I'd made the decision not to install PV I didn't look into the technicalities of using an immersun type device with the Mitsubishi pre-plumb cylinder / controller set up.

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Yes, I went for a 250L. No particular reason for the choice. We don't generally run out (I think we've done so twice in 18 months).

 

We also have shower waste water heat recovery, which (given shower usage patterns) probably makes the 250L tank act more like a 300-350L tank.  

 

 

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Stones, thank you for your comment on E7, rather than divert this thread I will start my own and pick the assembled brains on this forum?

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@Stones       Great blog post,

 

Do you go into detail around your calculations elsewhere, as I need to sit down and go through my own scenarios and make a final decision on what kit I actually need and how big? Up to now it’s all been like fantasy football, but as the TF kit has been ordered I need to finally decide on my actual lineup.

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@Triassic  when calculating what type of system to install, I compiled a table listing the various options, taking into account capital cost, servicing and energy costs (using 500kWh increments from 2500kWh to 5000kWh )then worked out the total cost over both 10 and 20 year timeframes so I had realistic annual figures to compare.  It was a pen and paper rather than PC job, so no longer have the actual figures.  Below 2500kWh demand for DHW and 2500 kWh for heating, direct electric worked out best overall, above that other types of system became cheaper.  Interestingly there wasn't a huge £ difference once all the cost factors were taken into account (maybe a £100 to £200 per annum difference IIRC), which for some may be a price worth paying for the simplicity of an immersion / willis heater vs an ASHP.  We went for an off the shelf plug and play system which has worked really well for us - no messing about, no trial and error, it just works and works well.

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@ProDave

 

Roughly speaking between 6 and 7 kWh per day for DHW (varies according to teenager use and time of year).  For heating, far more elastic - from 3kWh per day late autumn / spring rising to sometimes 10kWh day in winter depending on storms/ wind etc and all point in between!

 

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12 hours ago, Stones said:

@Triassic  when calculating what type of system to install, I compiled a table listing the various options, taking into account capital cost, servicing and energy costs (using 500kWh increments from 2500kWh to 5000kWh ).............

@Stones Thanks for an interesting and thought provoking post and response to my question. I’ve spent the best part of three days going back over around three years of posts covering every approach to providing heating and domestic hot water and I’ve come to the concluding your approach is the one for me.

 

Just a final question, what is your floor area of your UFH?

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