@jamespa “The answer is …….. 42.😉

Posted by: @paul1098

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@cathoderay https://www.stelrad.com/starsapp/login - you have to setup an account and I can't remember how onerous it is as it was 10 years ago when I started using it.

MCS and British standards do have air change values that generally seem to be 0.5 (I assume that's room volumes but not fully checked that yet) they also include for chimneys and extractors so comprehensive.

Just had my first quote of over £18k so if that's the general cost of an ASHP + central heating install for a small 2 bed cottage then my journey will be stopping and i'll go for LPG.

We got quoted 33 grand for our victorian solid stone pile by a heatgeek engineer who to be fair told us not to bother, so I bought and installed some air/air kit and did it that way instead. No grant at the time (and still no worthwhile one now) but cost me slightly over 1/10th of the cost of air/water in this big solid stone building. Kept the gas as backup although all has actually done is heated some water. 

What comes out affordable can depend heavily upon the building and what is already present. For us the existing plumbing was mostly inadequate, some of it inaccessible and the building has a high heat loss so would have involved a lot big big radiators to make it work.

With air/air we just heat the rooms where the fires were traditionally and leave some doors open. It then works the way the victorian designers intended.

Posted by: @jamespa

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I will post the full explanation later if nobody else does and anyone is interested (both of which seem unlikely)

I'm certainly interested in it. I tried to work out how the units in the formula got to what is a single count of watts, but failed. The formula is delta t (in °C) x a constant x a volume (in cubic metres) x the ACH which despite being a single count is in "m^3/hr per m^2 of [e]xternal area" which I think resolves to m^2/m meaning we have °C x m^3 x m^2/m...at which point my brain gives up! 

Posted by: @cathoderay

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Posted by: @paul1098

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That included the BUS. It included for 5 rads, HWT, controller and central heating pipework.

Just to be absolutely sure, total cost is just over £18k, of which you would pay that less any grant you get?   

@etchedpixels This house is fairly straight forward as far as plumbing goes but it's not a typical ASHP job as it includes all the central heating pipework. The existing hot water tank needs to be replaced. All piping can be run between floors except the outside pipe to the ASHP. HWT is in the middle of the upstairs. I think all horizontal runs are within 1m of the HWT with a single drop to ground floor to feed 2 rads back to back on wall. Max rad is approx. 1500W.

@cathoderay 

The 'constant' has dimensions.  Its the volumetric heat capacity of air in Wh/(Km^3)

Thus if ach has the dimension 1/h it all works out.

I assume the 2.5 - 3kW required to heat the hot water cylinder is added to the total house heat loss to size the A2W heat pump as per a typical non combi gas boiler?

Posted by: @paul1098

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I assume the 2.5 - 3kW required to heat the hot water cylinder is added to the total house heat loss to size the A2W heat pump as per a typical non combi gas boiler?

I don't know what calculation is done by people fitting combis, but that one makes no sense for combis or heat pumps and vastly overstates the requirement unless your dhw use is quite extreme.

You aren't heating dhw continuously and you need only a finite amount so you need to think about the energy needs over a period such as 24 hrs.  So for example if you use 250l of water per day at 40C when mixed down at the tap, which has been heated from 10C, then the energy required is (specific heat capacity x mass x temperature rise) = 4.2*250*30=31500J=8.75kWh.

This must be delivered in the 24hr period, in addition to the energy required to heat the house.  Thus the extra capacity required is 8.75kWh/24h = 0.36kW.

So if the house has a loss of 6kW you need a heat pump capable of delivering 6.36kW.

Alternatively people often use the rule of thumb that the heat pump will spend say two hours per day heating dhw.  Thus the space heating requirement must be delivered in the remaining 22hrs so you need to multiply the house loss by 24/22 to get the required capacity.  So by this rule of thumb the required heat pump capacity for our 6kW house is 6*24/22=6.5kW.

If the dhw tank is outside the heated envelope of the house you need to add the tank loss, which is likely to be significant, to either of these.  If it's within the heated envelope it can be ignored because the 'loss' contributes to the space heating.

If you are basing house loss on a survey and haven't factored in the contribution to heating from electrical appliances and occupants, then this pretty much offsets the typical dhw requirement.  Typically electrical appliances will consume (and thus emit) about 300-400W total, and people 100W each.  So you can reckon on 500W of 'free' heating from these, roughly equal to the required uplift calculated by either of the methods above.

When your heat pump (or combi) is heating dhw it's entire capacity will be used for that purpose and space heating will stop.  The house will of course continue to lose heat during this period but it's thermal mass means that it will not cool noticeably.  The loss during the period it is not heated must nevertheless be made up which is where the extra (kW) capacity is actually used.

Posted by: @jamespa

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The 'constant' has dimensions.  Its the volumetric heat capacity of air in Wh/(Km^3)

Thus if ach has the dimension 1/h it all works out.

Ah, that explains it. Thanks!

Posted by: @cathoderay

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Posted by: @cathoderay

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Posted by: @paul1098

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That included the BUS. It included for 5 rads, HWT, controller and central heating pipework.

 

Just to be absolutely sure, total cost is just over £18k, of which you would pay that less any grant you get?   

 

Yes

Then I agree with others, £18k is on the high side, but I don't think absurdly so, given it presumably includes all the new pipework as well. Getting other quotes will give you a better feel for cost. It is worth noting that cheaper quotes are often cheaper for a reason that you need to take note of. It is even more worth noting that good design matters greatly, even if it costs a bit more, as does the installer's style. You need to be able to work with them.

All that said, there are clearly other options.

You said earlier:

Posted by: @paul1098

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My reasoning for the move away from storage heaters was to make life easy when the house is sold.

...

The self imposed constraint i have is value for money, if the new heating system can be close to cost neutral when compared to the increase in house value then it's worth doing if not i'll stick with my storage heaters.

These two objectives are somewhat at odds with each other. If I was in your situation, I would want to be clear in my own mind whether I want to maximise profit (return), or set things up for an easy sale. I would also be aware there are a lot of things I need to consider when making that decision! 

@jamespa Your explanation makes sense.
I didn't include for hw heating in my main home as I run a thermal store but had thought something would need to be added to the space heating. As it transpires it's rather low.
I thought combi's had massive hw requirements because they need to heat it rather quickly because people get bored of waiting for the water to get hot?

@cathoderay You never know how good a contractor is going to be until it's too late. Due diligence is required and if your lucky personal recommendations are great but not always available. I agree about the price but top price doesn't always mean best design or installation in my experience.

The house wasn't bought purely to maximise profits and it would be nice to have a system that would sell the house quicker but I'm not going to spend £18k just for that.
If I can get a system installed for no more than £4k plus BUS then I'll go for it. If that turns out to be unrealistic then I'll not bother.

Posted by: @paul1098

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I thought combi's had massive hw requirements because they need to heat it rather quickly because people get bored of waiting for the water to get hot?

Yes that's true.  So a combi is basically sized to deliver dhw demand 'real time'.  For almost all houses this is way in excess of house loss or house loss +2-3 kW.  

Almost all gas boilers, combi or not, are in fact well oversized.  Most are also run at a flow temp that is way too high.

My 7kW house had a 28kW boiler before I replaced it with a heat pump. I had DHW storage so that was totally unnecessary.  As a result it cycled furiously.  It also ran at high ft, and the combination could be felt in the house.  In the year before I replaced it I turned down the ft as far as it would go, but despite that the heat pump is much more comfortable because it's properly matched to the demand.

A big part of the reason that the transition to heat pumps is challenging is that our current heating systems are not designed, and most of the industry has no concept of what it takes to design a heating system.  This actually increases running costs by typically 10%, but we haven't cared because gas is so cheap.  Our heating industry is now belatedly learning about design!