You can't measure heat loss with a website.  But you can get a good estimate if you know your fuel consumption.

Where are you located, what is your fuel consumption, how do you currently heat your house?

@jamespa Since we have a heatpump currently; annual electric use is about 6000 kW, and deducting the lowest monthly use of 130 kW x 12 gives a guesstimate of 4400 kW.

Does that help?

What I am looking for is the ability to calculate what we need independently based on figures from the EPC. to try and get a reasonable sized AHP for our needs.
And to reconcile different size ASHPs from different installers.

Posted by: @gutoffowc

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What I am looking for is the ability to calculate what we need independently based on figures from the EPC. to try and get a reasonable sized AHP for our needs.
And to reconcile different size ASHPs from different installers.

Ultimately, the EPC is about as accurate in estimating your home's energy efficiency as relying on Nostrodamus for your next investment tip. Even if the surveyor was actually experienced and knew what he or she was doing, there are still so many assumptions written into the process that the results are largely meaningless, and if the surveyor is newly qualified and inexperienced then all bets are off. As a result, using the EPC to estimate reasonable heat pump sizes is doomed to failure, and the only value I can see in that approach is that if an installer is prepared to use the EPC at all, you now know they're probably well worth walking away from.

The only online tool I'm aware of that can do an even half-decent job of providing a heat loss estimate is Heatpunk. Even then, that requires you to make accurate measurements of each room's dimensions, accurately assess the materials used and thickness of walls, floors, ceilings, windows, doors and roofs and so in effect do your own heat loss survey in detail. Even then, it'll only come out with a ballpark that's good enough to compare proper surveys against. That said, it does allow you to model different outside temperatures and different flow temperatures so you can see the effect of maximums/minimums as well as average temperatures on the heat pump sizing requirements.

In short, forget the EPC entirely and either get a proper heat loss survey done (one which you can read through to see what assumptions have been made) or do a survey yourself. It's a classic case of garbage in, garbage out. If you're only after a very rough and ready starting point, as @jamespa says the past consumption is still a far better option than the EPC.

Posted by: @majordennisbloodnok

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. If you're only after a very rough and ready starting point, as @jamespa says the past consumption is still a far better option than the EPC.

Annual consumption is almost certainly better than an EPC.  If you have daily consumption (which you can plot Vs OAT) and provided your house is reasonably warm most of the time I would argue that this is as good as, maybe better than, a 'proper' survey for whole house sizing.   As a minimum measured consumption, even if only annual, should be used to sense check surveys to weed out those that are 50-100% out, which does happen.

@majordennisbloodnok Thanks for your reply. I agree with most of what you say, but I would like to 'normalise' the calculation, so I can have a baseline for comparing different installers.The original EPC was done when the house was newly built, so I am assuming U-values are reasonably accurate, and somewhere in our files I have a drawing which shows the fabric construction.
So I will probably be able to input fairly accurate figures, and make allowance for reduction in performance of double glazing etc.

It's worrying when two different quotes have the same brand of ASHP but one is almost double the other. I've read Mars' blog on undersizing, but we have no back-up heating system to rely on for shortfall of heat output.

Posted by: @jamespa

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Posted by: @majordennisbloodnok

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. If you're only after a very rough and ready starting point, as @jamespa says the past consumption is still a far better option than the EPC.

Annual consumption is almost certainly better than an EPC.  If you have daily consumption (which you can plot Vs OAT) and provided your house is reasonably warm most of the time I would argue that this is as good as, maybe better than, a 'proper' survey for whole house sizing.   As a minimum measured consumption, even if only annual, should be used to sense check surveys to weed out those that are 50-100% out, which does happen.

what does OAT stand for?

Posted by: @gutoffowc

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@majordennisbloodnok Thanks for your reply. I agree with most of what you say, but I would like to 'normalise' the calculation, so I can have a baseline for comparing different installers.

...

@gutoffowc, I can understand your target and agree entirely. All the installers are eventually competing to achieve the same thing - to provide your home with heat and hot water. Whilst the quotes may be based on different approaches, they are in the end based on the same home and the same set of customer requirements.

Posted by: @gutoffowc

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...

The original EPC was done when the house was newly built, so I am assuming U-values are reasonably accurate, and somewhere in our files I have a drawing which shows the fabric construction.
So I will probably be able to input fairly accurate figures, and make allowance for reduction in performance of double glazing etc.

...

This bit of your last post is the bit I have the big problem with, and particularly the comment of "...assuming U-values are reasonably accurate...". That's a huge assumption to lay on even a well-surveyed EPC, and at worst will be plain wrong. If you have detailed plans that show the fabric construction, it'd be FAR preferable to use the HeatPunk online application and plan out your property yourself using those detailed plans as reference. At least in that situation you will be able, if you find you were mistaken on any particular point, to amend parts of the fabric of the plan and see what effect that has on the heat loss calculation overall. The result you get from that software will also be one you can have a bit more confidence in since you can't get a result without learning something about your property as you get to grips with the way the software works. That better understanding of your property and of what makes a big difference to the heat loss will be invaluable when you then talk with the installers about their efforts. It's even quite likely they'll use HeatPunk to create their own heat loss survey, so you'll be able to challenge their assumptions intelligently.

Posted by: @gutoffowc

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...

It's worrying when two different quotes have the same brand of ASHP but one is almost double the other. I've read Mars' blog on undersizing, but we have no back-up heating system to rely on for shortfall of heat output.

I totally agree. However the brand is in many ways almost irrelevant. Is one installer quoting for several more radiator upgrades than the other? If so, are those upgrades necessary? If not, why are they trying to push them through? If on the other hand they can be justified, why is the other installer not including them? These are legitimate questions to ask and precisely why getting the quotes back is only the starting point for the next round of discussions and research.

We have on this forum seen examples of undersizing but we've seen many more of oversizing. It's worth bearing in mind, however, that even when chasing the sweet spot there is some wiggle room available. Our installer calculated a heat loss of about 6.6kW and so originally specified a 7kW Vaillant. However, supply chain issues at Vaillant landed us with a problem and so we were given the option of going for an 8.5kW Mitsubishi. We went with the latter and, although slightly oversized, it has still proven to be a good and reasonably economical fit (yearly SCoP of around 3.5). Fortunately for you, you already do have a heat pump and so that should give you a good starting point. Do you have the specs? Do those specs give an estimated CoP at various outside air temperatures?

Posted by: @gutoffowc

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what does OAT stand for?

Sorry OAT = Outdoor air temperature, IAT = indoor air temperature, FT = flow temperature, DT = difference between two temperatures, often flow and return but there are so many DTs in a heating system (because DT is the engine for heat transfer) that its always worth checking if there is any doubt,   

Posted by: @majordennisbloodnok

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Posted by: @gutoffowc

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@majordennisbloodnok Thanks for your reply. I agree with most of what you say, but I would like to 'normalise' the calculation, so I can have a baseline for comparing different installers.

...

@gutoffowc, I can understand your target and agree entirely. All the installers are eventually competing to achieve the same thing - to provide your home with heat and hot water. Whilst the quotes may be based on different approaches, they are in the end based on the same home and the same set of customer requirements.

Posted by: @gutoffowc

↑

...

The original EPC was done when the house was newly built, so I am assuming U-values are reasonably accurate, and somewhere in our files I have a drawing which shows the fabric construction.
So I will probably be able to input fairly accurate figures, and make allowance for reduction in performance of double glazing etc.

...

This bit of your last post is the bit I have the big problem with, and particularly the comment of "...assuming U-values are reasonably accurate...". That's a huge assumption to lay on even a well-surveyed EPC, and at worst will be plain wrong. If you have detailed plans that show the fabric construction, it'd be FAR preferable to use the HeatPunk online application and plan out your property yourself using those detailed plans as reference. At least in that situation you will be able, if you find you were mistaken on any particular point, to amend parts of the fabric of the plan and see what effect that has on the heat loss calculation overall. The result you get from that software will also be one you can have a bit more confidence in since you can't get a result without learning something about your property as you get to grips with the way the software works. That better understanding of your property and of what makes a big difference to the heat loss will be invaluable when you then talk with the installers about their efforts. It's even quite likely they'll use HeatPunk to create their own heat loss survey, so you'll be able to challenge their assumptions intelligently.

Posted by: @gutoffowc

↑

...

It's worrying when two different quotes have the same brand of ASHP but one is almost double the other. I've read Mars' blog on undersizing, but we have no back-up heating system to rely on for shortfall of heat output.

I totally agree. However the brand is in many ways almost irrelevant. Is one installer quoting for several more radiator upgrades than the other? If so, are those upgrades necessary? If not, why are they trying to push them through? If on the other hand they can be justified, why is the other installer not including them? These are legitimate questions to ask and precisely why getting the quotes back is only the starting point for the next round of discussions and research.

We have on this forum seen examples of undersizing but we've seen many more of oversizing. It's worth bearing in mind, however, that even when chasing the sweet spot there is some wiggle room available. Our installer calculated a heat loss of about 6.6kW and so originally specified a 7kW Vaillant. However, supply chain issues at Vaillant landed us with a problem and so we were given the option of going for an 8.5kW Mitsubishi. We went with the latter and, although slightly oversized, it has still proven to be a good and reasonably economical fit (yearly SCoP of around 3.5). Fortunately for you, you already do have a heat pump and so that should give you a good starting point. Do you have the specs? Do those specs give an estimated CoP at various outside air temperatures?

 

There will be no changes internally; ground floor is all UFH, upstairs is rads. Only bathrooms are undersized IMO.

It was we hoped going to be a simple job of removing the existing heat pump, fitting a new one, with a new base as required; connecting up and installing new conrtol systems.

The existing ESP Classic II 13kW gives CoP of 2.5 at -7 OAT at 45 degrees, 
daily max electricity consumption; I estimate to be 30-35kW