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ASHP v gas boiler


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Looking for some advice, we have gas on- site and trying to work our whether we should put in a gas boiler with pv panels or go down the route of an ASHP. We will have ufh downstairs and rads upstairs and the house is 300sqm. I’ve heard such mixed reviews about the ability of an ASHP to heat that size of house efficiently and cost effectively. I know there is a big difference in capital expense but keen to canvas people’s views and experiences of other issues other than costs.

 

Thanks 

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@Jenjen what is the spec for the walls etc as that influences the heat requirement and will have a knock on to your heating design. 

 

If you’re airtight and decent spec then I doubt your rads will get much use. Downside of rads with ASHP is that you need to oversize due to flow temps. This will then be false economy if you don’t need them ..!

 

What are your plans for hot water ..? That will dominate the design. 

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As @PeterW comments - this is mainly backwards.

 

Your heating system spec depends on your heating need which depends on the fabric and context of your building, as well as the size. It is quite possible to heat a normal sized house with a single fan heater, if built for that.

 

So you need to start by estimating your spec.

 

The one thing I would say is that if you go for a a gas boiler, site it such that an ASHP replacement is possible in without gutting everything. That mainly means oversized rads or ufh.

 

Ferdinand

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On 28/05/2019 at 21:37, Jenjen said:

I’ve heard such mixed reviews about the ability of an ASHP to heat that size of house efficiently and cost effectively.

 

 

I hear the same, it is odd that beyond the scope of this forum the prevailing folk law in the building industry is that ASHP = shivering disappointment. This seems to be because they are retrofitted into unsuitable properties on green initiatives and that general UK building construction standards have not caught up with the exacting standards of the Passiv House pioneers here.

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Just now, epsilonGreedy said:

 

I hear the same, it is odd that beyond the scope of this forum the prevailing folk law in the building industry is that ASHP = shivering disappointment. This seems to be because they are retrofitted into unsuitable properties on green initiatives and that general UK building construction standards have not caught up with the exacting standards of the Passiv House pioneers here.

 

Speaking as a restorer of (mainly 1970 or earlier, often pre-1920) houses for rent I agree with that.

 

I am now probably confident enough to put one into the next house I do, but I would want to be restoring to an EPC number of mid-70s to do it. I am not sure how it would be perceived in the sales market, however.

 

Ferdinand

 

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16 minutes ago, epsilonGreedy said:

 

I hear the same, it is odd that beyond the scope of this forum the prevailing folk law in the building industry is that ASHP = shivering disappointment. This seems to be because they are retrofitted into unsuitable properties on green initiatives and that general UK building construction standards have not caught up with the exacting standards of the Passiv House pioneers here.

 

Nope .... it’s because they are fitted as a like for like KW rating by people who used to sell solar panels or the last green fad the government cancelled ...

 

There is no issue at all putting an ASHP in to replace a gas or oil boiler as long as the limiting factors are understood. The key is flow temp vs flow rate with an ASHP which unless you get to the very large capacity (ie 30Kw 3 phase units) you cannot get the delta temperature/flow rates that a traditional rad system needs. DHW provision is another completely separate issue and needs to be revisited with ASHP but again as RHI majors on the heading load first, it all gets very messy. 

 

As long as the heat lost < heat input then an ASHP is fine - it’s when you can’t input the heat fast enough (within a time period) then the problems occur. 

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14 minutes ago, PeterW said:

 

Nope .... it’s because they are fitted as a like for like KW rating by people who used to sell solar panels or the last green fad the government cancelled ...

 

There is no issue at all putting an ASHP in to replace a gas or oil boiler as long as the limiting factors are understood. The key is flow temp vs flow rate with an ASHP which unless you get to the very large capacity (ie 30Kw 3 phase units) you cannot get the delta temperature/flow rates that a traditional rad system needs. DHW provision is another completely separate issue and needs to be revisited with ASHP but again as RHI majors on the heading load first, it all gets very messy. 

 

As long as the heat lost < heat input then an ASHP is fine - it’s when you can’t input the heat fast enough (within a time period) then the problems occur.

 

 

Ok when armed with this additional knowledge how would you design an ASHP installation for an improved 4 bed 1800 sq ft 1970's house which looses say 10kw of heat when it is freezing outside?

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Mains gas seems to be more cost effective than ASHP. I'm in the same position and undecided, tho leaning to gas as it's a cheaper capital investment. Long term, running costs seem to be similar. You also have to consider hot water production. ASHPs have lower COPs if you want to heat water to a useable temperature. I'm sure somebody else can explain better than me. Gas is ideal for heating hot water to 60c.

 

But you also have the environmental aspect... Gas is still burning fossil files and always will... But with a heat pump you can theoretically run on 100% renewable energy.

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21 minutes ago, Conor said:

Gas is ideal for heating hot water to 60c.

 

why would you  want  dhw at 60c --you  can,t put your hands under it ,just going to be putting more cold in it to be able to use it?

all ashp have a dedicated dhw function ,where it swops from under floor to dhw tank when DHW tank temp drops -but it will not heat to 60c more like 52-55c or less if you want -,but it will automatically cycle ,now and then higher to kill any legionella possibility 

at this time gas boiler will be cheaper --until they raise taxes on fossil fuels

Edited by scottishjohn
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4 minutes ago, Conor said:

Mains gas seems to be more cost effective than ASHP. I'm in the same position and undecided, tho leaning to gas as it's a cheaper capital investment. Long term, running costs seem to be similar.

 

 

Did you struggle to get a passing SAP design with mains gas heating?

 

I tend to agree with you about running costs however lowest running cost does not imply an optimum SAP score because CO2 generation is arithmetic poison in the SAP formula and scheduled to be a larger factor in the next standard as electricity will get a lower CO2 penalty due to the growing percentage of renewable generation.

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10 minutes ago, scottishjohn said:

why would you  want  dhw at 60c --you  can,t put your hands under it ,just going to be putting more cold in it to be able to use it?

all ashp have a dedicated dhw function ,where it swops from under floor to dhw tank when DHW tank temp drops -but it will not heat to 60c more like 52-55c or less if you want -,but it will automatically cycle ,now and then higher to kill any legionella possibility 

at this time gas boiler will be cheaper --until they raise taxes on fossil fuels

 

Apologies, thought the temp required for legionella was closer to 60c. Agree that gas is currently cheaper than ASHP but it will change bin the future, and as heat pumps are an emerging technology, I see them becoming cheaper, more reliable and more efficient in the next few years. On our tight budget gas is the preference. If we go gas, I'll make sure we can change to a heat pump easily in the future.

 

 

6 minutes ago, epsilonGreedy said:

 

Did you struggle to get a passing SAP design with mains gas heating?

 

Not finalised, but sitting at 85B before we add in PV. (Min we need for Ecology mortgage)

 

I'm not really worried about SAP, as they bare little resemblance to actual real world running costs.

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14 minutes ago, Conor said:

Apologies, thought the temp required for legionella was closer to 60c.

 no problem

i as long as your dhw tank is cycled above the threshold periodically ,you don,t have to store water at high temp all the tine  and save money 

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