2xASHPs and WBS

[andyrob]

In my renovation project, with no floors, internal walls or heating - or wiring or plumbing, I effectively have a blank sheet of paper. (I do have good external walls and a lovely roof!)

With a chimney at each end, a log-splitter and plenty of wood, I'll start by having two woodburning stoves.

The woodburners will be used as "fall asleep in front of the TV" heating, and one will be used to feed a coil in the DHW tank, and a heat-sink radiator or two in the bathroom and loft conversion.

I have a 14kw Worcester/IVT GSHP and UFH in my present place but there is hardly room for a bore hole, let alone a ground loop in my "project" - a house for my retirement. So, surely (Martin), my options must include an ASHP? In fact the decision was made when I bought a job lot of 5 Calorex ASHPs. These were used for only six months last winter, in a new social housing project, and then replaced because they were undersized. To be fair to Calorex and the architects, last winter was a little extreme and the ASHPs were just a bit too marginal even for a normal winter.

So, as they are the smallest (4.5 kw) units, I'm pairing two together, in parallel, and keeping open the option to convert them from R134A gas to R407C. This gives an instant additional 25% in heat output at low ambient temperatures - albeit at the cost of a lower maximum heat output. This costs around £300 per pump incidentally so, to begin with, I'll see how 2 x 4.5kw works.

By connecting them in parallel, and setting different on/off temperatures on each ASHP's control panel, I hope to have one unit taking the lead/primary role and would expect it to be on much more than the second (less demanding on the compressor with fewer on/offs and more efficient). The second could perhaps be dedicated primarily, though not solely, to the DHW.

Do you think this could be workable?

Andy

PS, I'm a great believer in getting the floor-slab temperatures a little higher than normal during the 10 cheap hours of my Economy 10 electricity tariff (5 five hours at night, 3 in the afternoon, 2 in the evening). Usually,  I can then have the Heat Pumps off for the remainder of the hours which are charged at nearly 3 times the cheap rate. This works especially well with the woodburners topping up room temperatures in the evening.




Billi : just reduced  the size of fonts   

[dhaslam]

It sounds reasonably OK  but  one thing to be aware of is that ASHPs have a relatively short life compared to ground source.  My second one has now failed and isn't worth repairing.   Probably the severe winter hastened its end even though it wasn't used in the  coldest period.

[andyrob]

Sorry about the font, Bill, my very screen and old eyes made me think I was doing the right thing.

DHaslam, thanks for the warning  (Quote "My second one has now failed and isn't worth repairing") - what make were yours and how long did they last? It's not good news but that's one reason why I bought the Calorex units, a well established company (which means nothing in terms of the future of course) but maybe I should keep one of the three others I bought as a 'spare'. The other two are going to be used for a barn conversion next year so I'll have that as a spare for them as well.

Just as a fly-in-the ointment comment, remember that ASHPs are not included in current PDRs (Permitted Development Rights) so - strictly speaking - you must have Planning Permission to install them. I know of dozens of installations that have been done without PP - by local authority schools; the council didn't realise or didn't care! Strangely, there's no need for PP for air-conditioning units and I think that's why the mistake was, and still is being made. It could be a good excuse if anyone feels the wobbly arm of a planning officer on their shoulder.  I understand that ASHPs on the MCS approved bproduct list will be exempt sometime in the future.

Perhaps I could ask a quick supplementary question?

Is it worth putting the ASHPs in a well-vented shed - like the one people of keep their wheely bins in? Just to keep the wind and rain off, but obviously keeping lots of air flow. This might keep the fan noise down too which is why PP is still required.

Andy

[rondurrans]

I found the following report useful when I was looking into a ASHP:-

http://www.energysavingtrust.org.uk/Media/node_1422/Getting-warmer-a-field-trial-of-heat-pumps-PDF

[dhaslam]

The first one I had was one of these.   I leaked  fluid after arriving and couldn't be repaired.   
www.gosayheatpump.com/en/productpage.php?id=50

The next one was a Trianco Activair 5 kw.   It was just two years old when it gave errors.  I didn't investigate  further but it seemed similar to the problem with the other one. 

[StBarnabas]

The first one I had was one of these.   I leaked  fluid after arriving and couldn't be repaired.   
www.gosayheatpump.com/en/productpage.php?id=50

The next one was a Trianco Activair 5 kw.   It was just two years old when it gave errors.  I didn't investigate  further but it seemed similar to the problem with the other one. 

DH
sorry to hear about your ASHP as you know I also have a Trianco ASHP, which seems fine though it is pulling more power at c 1.2kW than the 850W it is supposed to. One thing to be aware of is that recent UK field trials of ASHPs in field conditions have yielded average COPs of less than 2. They are worst in the autumn and spring and give the best COPs in winter - exactly the opposite of what simple theory would predict.

 

[titan]

 One thing to be aware of is that recent UK field trials of ASHPs in field conditions have yielded average COPs of less than 2. They are worst in the autumn and spring and give the best COPs in winter - exactly the opposite of what simple theory would predict.

 

Sounds interesting do you have a link

[StBarnabas]

Hi Titan
I don't have a link yet as the data has not been published. Prof Chris Underwood (Northumbria Uni) is one of the few people who is currently analysing the data. I will check with him in a few weeks time if the analysis is anywhere near complete and a likely publication date. The results are considerably worse apparently than the earlier Energy saving trust field trial http://www.energysavingtrust.org.uk/Generate-your-own-energy/Heat-pump-field-trial. The likelihood is that heat pumps don't like being switched on and off - they take some time to come up to steady state efficiency. In the autumn and spring if the system has not been installed properly the heat pump can cycle on and off rapidly. (Very few domestic heat pumps have proportional control). This would kill their efficiency. I will try to post more details and a link when it goes public domain.

[andyrob]

Cycling on and off

Yes, exactly why I was proposing using two small ASHPs. The same kind of principle as the twin turbos on diesel BMW 335/535 cars.

My IVT GSHP has a count-down that prevents a switch on until 15 minutes have elapsed after switch off, but this is the wrong solution for the problem - you must size the heat pump to the heat demand - not too small, not too big.

Too small and you are chilly on the coldest days. But at least the pump is running efficiently.

Too big and the pump does its stuff and then turns off. If you have a poorly insulated house the pump has to come back on again soon afterwards, especially if you're not using UFH and a big thermal store (screed slab under the floor covering which acts as a big heat sink and evens out the heat demand from the boiler/heat pump).

The solution for this is almost always a largish tank of water (thermal store) in series with the UFH/radiator and DHW circuits - ie no coil within the tank. My GSHP tank is 300 litres and I'll use a similar one in my "retirement" home project with the ASHPs.

The other thing to remember is that that most people who know much more than me recommend sizing a heat pump to be able to provide the required heat load for the coldest 90% of days - and for the other 10% cartridge electric heating elements built into the better GSHPs (mine has 3x3kw) kick in to help the compressor on those 10% days. (This is why so many heat pumps were thought to be undersized last winter which had many more days that would be classified as 10% coldest days over the long term - and hence why I like woodburning stoves: very good at responding to a chilly evening, especially if you don't have weather compensation in your software which, by monitoring external temperatures, anticipates a colder house a few hours before it really is).

I didn't like the electric elements idea though, hence I disconnected them all (not possible in the IVT control panel software unfortunately but quite simple using the hardware links inside the cabinet). You can choose to have 3kw, 6kw or 9kw but they can't be separately timed to the Economy 7/10 times. These electric heaters can obviously add considerably to heat pump running costs because of this but if, like me, the pumps run only/mainly in the economy periods this wouldn't actually be too much of a problem. You just don't get the COP of 3-4 or whatever. At 1 it's still better than gas and oil though!).

and St Barnabas - yes, I'd read the EST field trial report and what was significant was that the users in the trial (who were often tenants/old/disinterested) found the different regime and mind-set that's needed for heat-pump heating [and especially HP with UFH] was too different for them to operate their systems properly.

I once knew a school whose caretaker turned on their gas-fired UFH at the usual Monday morning time (for a radiator system) of 6am. Floor temperatures became so high (to get air-temperatures up in time for 8.30) that the floors were too hot for children to sit on during the morning assembly time. Monday morning heat loads in a building that has cooled down over a weekend (even worse after a cold holiday) are around double of what they are on a Tuesday and each day this heat requirement continues to drop. Hence the need to keep UFH heating ticking over, even when the building is unoccupied - or at least turn it on well before you would for a radiator system.

Andy

[andyrob]

Correction to para 6 - but I can't edit my previous post now.

" ........ sizing a heat pump to be able to provide the required heat load for the coldest 90% of days"

this should read

" ........ sizing a heat pump to be able to provide the required heat load only for the warmest 90% of (heating season) days"

ie the heat pump shouldn't be designed to cope (by itself) with the coldest 10% of days

[martin]

"At 1 it's still better than gas and oil though" - could you please clarify that statement? - in what way "better?"

[andyrob]

The COP of electricity is 1 because, as I understand it, using 1 unit (1kw) produces that amount of heat/power.

Burning gas or oil, even with an new, efficient boiler in condensing mode - which isn't often - efficiency is 85% or less (ie the COP is .85) - ie 1 unit of fuel produces 0.85 units of heat. The rest goes up the chimney.

Of course, cost is another matter.  Comparing prices is difficult with cheap rates, leapfrogging prices etc; add to that the cost of a system over its lifetime - maintenance etc - then, again, electricity can start to look 'better'.

I hope we don't start looking at the carbon content of all this - which is complicated and, though very important, more a national policy issue than a domestic one. My main issue is, like thousands of rural area dwellers, that I don't have the option of mains gas.

Andy

[martin]

I thought so, you're airily dismissing the fact that the electricity has to be produced (probably from fossil fuels), and discount the inefficiencies involved in it's production and transmission - and of course, not a thought for the fact that if we all get ASHPs as suggested by Mackay (and charge our leccy cars) the whole tottering grid will fall over a lot quicker..........
As you may have guessed, I'm not a fan of ASHPs

[titan]

Hi Titan
I don't have a link yet as the data has not been published.

Thanks, St.B it will be an interesting read. Although it looks like the results may mirror the EST report in that the heat pump is just one part of the whole heating system and is not just a direct replacement for a conventional boiler, the whole system needs to be designed for heat pump use and operated differently to a conventional set up for maximum efficiently.

[andyrob]

I thought so, you're airily dismissing the fact that the electricity has to be produced (probably from fossil fuels), and discount the inefficiencies involved in it's production and transmission

Hallo Martin - I was told to expect you!

As one who is passionate about the future of my grandchildren, and having worked across the East Midlands for the past four years promoting the National Strategy for Sustainable Schools, I can assure you that I don't underestimate the importance of the carbon content of grid electricity. I just felt that here, this Green Construction/DIY thread, wasn't quite the right place to discuss it.  But ......

I think we have to make decisions over our individual domestic needs in the context of what we have, which in my case is no mains gas and not much garden for an oil or LPG tank. let alone a ground loop.

Renovating a 250 year old house, I'm not sure what better I can do than rip out the interior, dry line and pack the roof with insulation  - and then use ASHPs and multiple woodburning stoves, burning fallen trees (including some I planted myself) that I replace with saplings for others to burn and which absorb the carbon my woodburning gives off.

I will use lime screeds, sheep's wool and recycled/recyclable materials - as much as I can afford in my renovation. I will have 8kw of solar PV (I've mentioned the good roof) because I want to produce all the electricity I use, on-site,  not least for the very reasons of power station and transmission losses Martin quite rightly mentions, but also because of peak oil/gas arguments and the carbon emissions of fossil fuel power stations.

It's a shame that FITs have distorted the arguments I had for not using PV because I had planned to use a locally grown, rape-seed oil-powered CHP unit for both heating and electricity - but the relative economics of that have changed because of national policies over which I have no little or no influence.

Andy