ASHP or MAINS GAS?

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ASHP or MAINS GAS?

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Steve Williams

Hi and thanks for reading my query

I am renovating an terraced Edwardian house which will be well insulated. The gas boiler is old and the entire Central Heating system is a mix of ugly rads, normal piping and microbore, in short it needs replacing.
I will be fitting UFH regardless of the ASHP/Gas question as I prefer it and the floor boards need lifting and insulation installed anyway.
We have a mains gas supply.
Is an Air Source Heat Pump a better choice than gas?

Steve
 
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sussexflange said:
Would one of these air source heat pumps be efficient with the overlay type of underfloor heating or is it best to run it with more traditional in screed type? Property was built early seventies so i imagine no insulation under screed. I dont want to dig the floor up really, so i suppose its a choice between the overlay and rads.
Click to expand...

Hi , we have been fitting overlay for around six years now, it's one of our best systems, the output is around 90w/sqm with a run temp of 45deg. Most suspended floor systems require 50-60deg which is too high for ASHPs , the problems we face with ASHPs is that when it's below freezing outside the ASHPs output drops so the higher temps are expensive to attain.
The heat up time is quicker than solid screed but the cooling down is quicker.
The ASHPs in temps below 0deg will go into defrost quite a lot. When this happens the ASHP stops supplying the under floor and reverses the hot water from the heating system around the evapourator to de frost it. This can take ten minuets so not only is the heating down for ten minuets it also robs your house of heat, once the ASHP re starts to heat then it will take 15mins or so to reach the temperatures it had before going into de frost, so the ASHP may only be heating for 35-40minuets in the hour. Not the sort of thing the manufacturers mention a lot although they now recommend fitting a buffer to overcome this.
 
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The general consensus is that an ASHP will work best with underfloor heating hence me thinking abut the overlay system, but i was worried about the thickness of the insulation. Those boards i looked at are only 20mm compared to the usual 50-75mm or whatever it is, dont want to fit it if its gonna cost a fortune to run. I have thought about maybe fitting rads and some insulation boards for the floor.

Its only a 2 bed bungalow, well insulated roof and cavity wall insulation, small garden though so definitely no room for oil tank or lpg, i would have had gas in a heartbeat if it was available.

I had thought about a wood burner, but i dont think logs are that cheap atm, and there is no chimney, so i think a flue in the living room woud look a bit naff.

Groundheat you talk about them perfoming badly below freezing, but eaton and a couple of others have posted positive results from their ASHP. Seems to be alot of conflicting info about this tech.
 
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Don't install cheap crap. The PV cells do have issues with dielectric and power drop off but that's because they are manufactured to within 0.0000000005mm tolerance in someone's shed in ahi zen.

Solar thermal is well proven and when done correctly will out last 15 year warranty.

Mitsubishi ashp tend to out perform. Bit like saying combi won't ever work........ Because all u seen is a fer falcon!
 
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Wouldn't say they would perform badly. But it is a limitation in the technology currently. The COP and output really begins to drop off in freezing temperatures. Which means it really begins to lose efficiency when you most need it. Hence why a decent auxiliary system would be preferable.
 
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Hi Guys ASHP vs Gas at present ASHP is an expensive option even with the grant, for UFH gas really is not an option unless you run the boiler at a lower temperature (unless you like dancing over floors as the floor will be hotter) but efficiency will then be rubbish. GSHP could be an option but again the installation costs could be bad. I am looking at other options one being the Newish Thermodynamic panel System (not sure if it qualifies for any RHI grant though because it works even at night but at present only for hot water).
 
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Blending valve controls temp of floor. U can have the flow into ufh at 100c if u wish and blending valve will keep floor temp within specified range.
 
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Even with the RHI, ASHP's don't stack up against Nat gas.
Our advice to everyone is Insulate, Insulate, Insulate.
For an ASHP to get a CoP of >3 you need to run either ufh or rads at less than 45°, to run rads at that temp you need them to be sized 3x larger than normally required.
The gudelines on sizing heatpumps and emitters is both clear and comprehensive - you can download it all from the MCS website.
When you add in the costs of ugrading the rads it will never add up against Nat Gas, we are struggling to make a financial benefit to clients even when that are on oil.
For oil, the most sensible Green option is a Biomass boiler.
The RHI payments are open to anyone on or off gas grid - there are no restrictions, however they have been set at a level such that an effcient Nat Gas boiler will be the cheapest option, DECC are desparate to avoid a green rush away from gas, hence the payments are designed to make OFF gas grid people CONSIDER a green alternative when their boiler needs replacing, not as a wholesale green update. - They've learned from over generous PV FiT payments!
Also see this paper why buffer stores are essential on any form of heat pump (I now it relates to biomass, the same principle on sizing applies though)
ctc810-insights-into-biomass-heat-installations.pdf
Net effect you'll put an ASHP in to a on-grid property because you want to... however you need a CoP of much > 2.5 to really be greener than nat gas...
See here for the deemed CoP's at various output temps: http://www.microgenerationcertifica...y_Information_2_-_Heat_Emitter_Guide_v2.0.pdf
See this for the actual EST field trial results of ASHP's as installed : [DLMURL]http://www.energysavingtrust.org.uk/content/download/80377/973482/version/2/file/The+Heat+is+On+web.pdf[/DLMURL]
 
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well ive got a 14kw ecodan unit coupled with 4kw of pv and its fekking wonderful,,,,replaced it 2yrs ago from an oil boiler system and it heats my 6 bed chalet bungalow and the water very nicely
 
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I'm running rads on all mine at 50c, most don't need changing has insulation has been upgraded through years (my own house needed them all done) find all systems have run amazingly well, against oil payback without any grants is working out around 3-4 years, most places we go an oil boiler, new tank and cylinder will come in at 5-7k, heat pump fitted for 8-10k normally isn't a huge amount extra, with rhpp £1300 grant some come in very similar to oil install if tank and cylinder needed changing. Then customers are quids in, some are just taking the **** with install prices etc, seen costs of £12k just for a 9kw, no cylinder, rads etc, just unit, pumps, pipe bit of wiring etc.
 
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@Eaton, thanks for that

I could be wrong, the way we see it is the problem with running rads at 50° comes with the SpF figures for the RHPP / RHI - firstly the heat emmitter design needs signing off and even at 50° they need to be 2.4 times bigger than normally required for that room / space. We find that in 95% of cases radiators are wrongly placed and wrongly sized (too small or too large) so unless you do a radiator refit, then the system wont be MCS compliant so wont qualify for RHPP/ RHI If now add in the costs of replacing a bunch of radiators and you see why biomass adds up (no rads to replace!)

Unless I've missed something?
 
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Worcester said:
@Eaton, thanks for that

I could be wrong, the way we see it is the problem with running rads at 50° comes with the SpF figures for the RHPP / RHI - firstly the heat emmitter design needs signing off and even at 50° they need to be 2.4 times bigger than normally required for that room / space. We find that in 95% of cases radiators are wrongly placed and wrongly sized (too small or too large) so unless you do a radiator refit, then the system wont be MCS compliant so wont qualify for RHPP/ RHI If now add in the costs of replacing a bunch of radiators and you see why biomass adds up (no rads to replace!)

Unless I've missed something?
Click to expand...

On my heat loss calc it tells me the watts needed per room and then average rad size giving a % of +or- of the current rad eg
room size is
current rad size is
watts
current rad size ie 10%+
then you play around with rad sizes to get what you want ie +%

there is never an issue with over sizing! I've oversized mine to run at 45c and set heat pump to run weather comp curve to suit. Mcs only want you to prove rad size to suit that particular room, for eg my towel rail is supposedly 30% undersized yet room hits 21c at minus 4! Personally not had an issue but it depends on your heat loss calc I suppose. Got a 2 bed house to start Monday, 16kw, 210l unvented full heating system (storage heaters out) new fuse board total price £9.5k inc vat, some say that's cheap but I'm making very good money on that but I do buy at cheaper price than most
 
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16kw! 2 bed castle!?

its good to hear positive feedback about heat pumps after so many numpty solar p.v companies started bashing them in wrong!
 
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Mines 118m2 1975, cavity insulated, loft insulated and double glazing about 20 years old, units using about 65% capacity! was on borderline of a 9kw but would like to add a utility, bedroom/ensuite ext in few years so went with 16 for future proofing.
 
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Yes I installed a bran new 15 Kw Dream ASHP in Sept 2012. It`s working brilliant my whole house has been so cosy, a 4 bed semi & attic conversion, with a 6 Mtr UHF conservatory and heated Koi Pond.
Well worth the capital outlay of £1,500 plumbed into my flow & return, I have the system wired to operate via an optimizer/stat. I set that to maximise the COP factor so that the ASHP does not ice over, (it costs to melt ice) when this happens the COP factor is not very good, as on all ASHP`s. That is why I use my Dream as a Hybrid when the dew point is likely cause less efficient use. The Hybrid use has enabled me to only use 20% of the gas I used on many previous years, that's now 80% of my heat coming from a more eco source, AIR. The Leccy use has not risen by very much ( I don`t think all my LED lamps have saved that much) so my outlay payback should be on track in about 4 years. The ASHP is controlled by my optimisation / stat wired so that when it`s inhibited, the Gas boiler will take over if frosty. Although the ASHP output temp is set lower, that has proved to be acceptable as I have some oversized rads & UFH thermal store, when it`s colder the gas boiler (at a higher temp) overcomes the greater heat losses. The "shoulder" months are when the COP values really pay off at 3, 4, or nearly 5 times the input Kw, so even allowing for high leccy prices over gas there is quite a good saving. Some have Mitsubishi, Toshiba or ( mine ) a Sanyo with the more efficient defrost system of a 4 way valve instead of elements to melt the winter ice. As you can tell I have had a mainly electrical career for well over fifty years so I have a relay for all zone digistats to be "ON" via the heat source in use. Many of the quotes that I had were horrendous £14 K to over £20 K for a system that is little better (if at all ) for under £2K as a pensioner I am well happy for such an efficient system. Hope you have found these comments helpful. I have No connection other than a very satisfied pensioner with a system that works well.
 
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eaton said:
On my heat loss calc it tells me the watts needed per room and then average rad size giving a % of +or- of the current rad eg
room size is
current rad size is
watts
current rad size ie 10%+
then you play around with rad sizes to get what you want ie +%

there is never an issue with over sizing! I've oversized mine to run at 45c and set heat pump to run weather comp curve to suit. Mcs only want you to prove rad size to suit that particular room, for eg my towel rail is supposedly 30% undersized yet room hits 21c at minus 4!
Click to expand...

That won't be a towel rail then, it'll be a radiator.
 
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That's not a room it's a shower cubicle :)
 
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