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4 posts

Wannabe Geek


#261316 20-Nov-2019 17:41
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Hi, we are researching a retrofit central heating system in our home and trying to compare diesel v air to water heat pump for the heat source. We are in cold Dunedin so whichever system will have plenty of months in operation. Can anyone compare running costs for us? Seems to be contradictory info online in NZ where few people have central heating at all. Thanks.

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9261 posts

Uber Geek


  #2358393 21-Nov-2019 08:38
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Diesel, assuming 100% conversion, has an energy density of about 10 kwh (ie units) per litre.  So at $1.50 / litre that's about 15c per unit. (assume a little more as there will be losses and inefficiencies).

 

If you assume a COP for a heat pump of 3 on average in practice (ie less than the headline marketing figure) and a price of $0.30 / unit, then that's about 10c of electricity used to produce the same heat output as 15c worth of diesel.

 

Diesel will be more expensive.  Dirtier, less sustainable, and probably less reliable as well.

 

OTOH if you're thinking of using a heat pump to heat water for radiators, talk to an expert.  I thought this was a great idea, experts convinced me I was dreaming as the higher the temperature differential the less efficient the heat transfer, beyond about 50 degree water temp isn't economically achievable and that's still not hot enough for central heating radiators.  Ordinary heat pumps, ducted or on wall/floor are very hard to beat.  Sure the COP will drop in sub-zero temps, but on average they'll be the most efficient method to heat a house.  (unless you can get free firewood).


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Ultimate Geek

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  #2358483 21-Nov-2019 10:01
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Fred99:...OTOH if you're thinking of using a heat pump to heat water for radiators, talk to an expert.  I thought this was a great idea, experts convinced me I was dreaming as the higher the temperature differential the less efficient the heat transfer, beyond about 50 degree water temp isn't economically achievable and that's still not hot enough for central heating radiators.

 

Correct.  The heat output from radiators falls off quickly as the water temperature is reduced. At 50 degC water flow temperature the output of the radiators will be less than 40% of the nominal rating, so you need a very large radiator area to get the output you need.





McLean


 
 
 
 


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Ultimate Geek


  #2359385 22-Nov-2019 16:51
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As above, you need underfloor heating or special radiators (either massive, or fan forced) to run a hydronic system off heat pumps.

Fred69 calculated 15c/kw cost of diesel. If your boiler was 90% efficient, that would give a cost of heat of 16.7c/kWh.

I pay a marginal cost of 15.72c /kWh for electricity (standard user in Auckland). As there seems little point in installing a new diesel heating system in NZ, unless you have a special use case. Little value in paying the capital cost of a diesel boiler & tank age etc, monitoring fuel, ordering deliveries etc.

 


LPG seems to be the combustion fuel of choice in the south island. It's similar price but cleaner burning a lot easier to deal with.

 

Heatpumps are the way to go economically these days. I would look into ducted / high wall / floor counsel / hydronic  underfloor system driven off a air sourced heat pump.

Baring this, it seems it would be more economic to use an electric restive electric boiler (laing or similar) in your hydronic system, than a diesel boiler. Alternatively you could ditch the hydronic system and replace the  radiators with panel heaters.


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