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Author Topic: Retro-Fitting an ASHP as a replacement for an LPG CH Boiler  (Read 4522 times)
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« on: July 19, 2012, 06:43:32 PM »

My neighbour who lives in a 3 bedroom bungalow heated by an LPG boiler is considering replacing the boiler with an air source heat pump.
House construction is double skin brick with suspended wooden floors throughout and has been insulated in walls and attic under government assistance schemes.  Crawl space under the floors is minimal (non existant)

I am concerned that she may be influenced in her decisions of choice by salesmen more interested in their profit margin that suitability for purpose.

It is my understanding that an ASHP unit can achieve at best a 50 degC differential between radiator circulation fluid (water) and the external environment.  If this is so, then in the depths of winter her radiators are going to be somewhat cool when the air temperature is sub-zero.   She is talking about increasing radiator surface (replacing singles by doubles) but her wall space is fully utilised so there isn't scope for increasing panel area.

Could the more knowledgeable amongst you please advise what the real pros and cons are of a retrofit ASHP as a replacement for an LPG boiler.  Obviously electricity is cheaper than LPG but an inadequate system which is cheaper to run is of no benefit.

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« Reply #1 on: July 19, 2012, 06:53:19 PM »

I'd be extremely sceptical about the value of an ASHP - it really needs to have underfloor heating installed to make the best of the low grade heat it produces, and even then............ it would probably cost a small fortune to install anywhere near properly and to run, and she'd need an alternative form of heating for when it actually got cold........... in my view "double" radiators aren't going to help a lot either!

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« Reply #2 on: July 19, 2012, 07:07:35 PM »

If the LPG boiler is serviceable I would be tempted to leave it in the circuit and put in the air source heat pump.  The air source heat pump is going to be good when the outside air temperature is above 6C and probably cheaper to run than the LPG boiler.  However when the air temperature drops below 6C then the LPG boiler should be used as the COP of the air source heat pump will not be adequate to heat the house.  It really depends on how many days a year your air temperature is below 6C (You should be able to estimate that)

We have an air source heat pump but use a wood burner when it is really cold outside in France, so same sort of idea.  Air source heat pumps are ineffective on really cold days so you need backup.

Of course any further insulation you can do will help.

Nowt currently, Aberdeen.....well actually very well insulated extension with passive solar that seems to heat the house....
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« Reply #3 on: July 19, 2012, 07:16:16 PM »

some extent it may depend where in the country - I'm in Hertfordshire and have a 2 bed bungalow with newly installed radiators. I can get away with a flow temp of about 35-40 degrees, and didn't seem to have any problems maintaining that during february this year (heat pump was running at about 25% of full load). Adding small fans to the radiators would be a simple way of getting more heat out of them without the expense of installing dedicated blower units - but regard this as a fall-back option rather than a design plan.

Provided the heat loss calculations add up (and they shouldn't be too hard to do), an ASHP should work. I'm not finding mine significantly cheaper to run than storage heaters (20% saving maybe), but the comfort factor is a massive improvement (as is the high flow shower). Given the limited options available, an ASHP isn't necessarily a bad idea. Find an installer who actually understands what he's doing though - most have just spent some money on a course.

Hitachi Yutaki RHUE 4.0AVHN ASHP
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« Reply #4 on: July 19, 2012, 09:38:02 PM »

A lot depends on the cost as to whether there would be any long term financial savings.    If off peak electricity  can be used and  the heat pump is only used within its  efficient range then there should be good saving on running costs but perhaps not enough to justify the capital cost.    ASHPs, being exposed to weather, do not last as well as ground source systems  so the capital cost  needs to be absorbed over a shorter period.   

It is unfortunate that the  floors of the house cannot be insulated easily but it would be best to  compare to do a financiial comparison of other energy savings that might be made like solar water heating,  PV,   Heat recovery ventilation etc. 


DHW 250 litre cylinder  60 X 47mm tubes
Heating  180,000 litre straw insulated seasonal store, 90X58mm tubes + 7 sqm flat collectors, 1 kW VAWT, 3 kW heatpump plus Walltherm gasifying stove
« Reply #5 on: July 20, 2012, 09:15:01 AM »

Construction sounds similar to us but ours was not really a retrofit. LPG, pipes, rads, tanks etc the whole lot was removed as part of major renovations.

ASHP installation was carefully designed and installed and as such never looked back. We have mostly fan convectors with some UFH. We went this route as lower flow temps are required than huge regular rads which keeps the unit as efficient as possible. Also has very fast start up times from cold.

House maintained at 20c regardless of weather/temp outside with bills a fraction of LPG.

So yes expensive to install but far cheaper to run plus I can shop around for the best leccy deal every year instead of getting raped for LPG in unfair contracts. Also gets a boost from the PV investment.

We also have log burners which once going strong can heat most of the house to silly temps but tend to just use them in the eve for cosy nights as can't be arsed with the constant cleaning and lugging fuel about.

My advice would be... don't just remove the LPG boiler and shove in an ASHP. It is unlikely to work particlarly well even if some rads are changed to doubles. You really need to start from the ground up or not at all.

Lastly, find a company without a salesman. Look for a local MCS installer with a solid grounding in plumbing/heating and go look at their installs and speak to end users.

If I wasn't in a position afford a proper ASHP install I'd be on oil now as LPG IS the work of the devil  Wink
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« Reply #6 on: July 20, 2012, 11:15:13 AM »

bungalow = lots of roof area for solar thermal, use the heat pump to recover heat from a water filled heat store amd keep the LPG as top up
« Last Edit: July 20, 2012, 11:17:29 AM by derekmt » Logged
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