@novemberromeo As a budget I make the figure around 15kW required so 20kW is madness.

I passed your dilemma around to a network I am part on  Twitter with highly knowledgable heating engineer and, after a bit of a furore, Mars has seen the comments.

@bobbt9866, thanks for pulling some strings. I've sent the results to the engineer in question and hopefully we'll hear back from him soon.

@novemberromeo 

Hi,

Based on calculations I have carried out on ASHP data, a good rule of thumb would be that an ASHP should be sized to be approximately 1.5 times the heat loss at -3C.

@novemberromeo,

My heat loss calculations show a total area of 128 square metres, heat loss of 6,400w, heat loss W/m2 50, space heating requirement of 12,537 kWh/yr, hot water demand 3,821 kWh/yr.

I ended up with a 14kW Ecodan.  The supplier said I could have had the smaller 11.2kW one but it would be borderline.  It seems to work well so far

@novemberromeo The variance in W/M2 is significant, 40 W/m2 vs 64w/m2 shows that clearly the fabric of the building has been miscalculated by one or potentially both of the companies.

It would be worth getting a detailed account of the inputs, wall construction, roof insulation, glass area etc and comparing them. Remember you have paid for this to be undertaken and can expect a degree of accuracy.

You mention air to air heat pumps, given cooling in the summer is of value to you is it not worth a whole house system? We use air to air in a 30sqm conservatory, they do a great dual purpose of heating in the winter and cooling in summer in comparison to the underfloor heating. 

wow, what a mess. whoever is sizing this up seems to be making some horrible assumptions. sounds like you need someone who does this properly for a living to do it. 

Posted by: @grahamh

wow, what a mess. whoever is sizing this up seems to be making some horrible assumptions. sounds like you need someone who does this properly for a living to do it. 

That's strange, I thought that the people carrying out the heat loss calculations were actually 'employed to do that for a living'. If not, then they should not be charging for providing a service for which they are obviously not qualified.

Since the companies involved are allegedly 'MCS accredited' and also the help so far provided by MCS, would indicate to me that MCS are about as useful as a chocolate fireguard.

If the government think that they can convince millions of people to adopt ASHP's when the people hear stories like this, then obviously the ministers involved are even more delusional than I first thought. 

Indeed, I thought the whole point of using MCS certified installers to provide a service was to get some level of guarantee that they do things correctly, and as such you'd expect three heat loss surveys to show similar results.

I tried emailing my query to the MCS helpdesk to see if I'd get a better response than I got on the phone, unfortunately not:

Thank you for getting in touch. Sorry to hear about the issue you are having with instructing an installer for an air source heat pump installation.
 
MCS is a standards organisation for small scale renewable energy products, installers and installations. 
 
MCS is not able to offer technical design advice. MCS can respond to queries relating to MCS Standards and the requirements therein.
 
To become certified an installer must have relevant qualifications for their chosen renewable energy product. A certification body will assess the company to ensure they meet the MCS standards to gain certification, followed by annual assessments.
 
Once MCS certified, the installer can choose which MCS certified products they wish to use, some of these require further training from the manufacturer to ensure installations are correct and efficient.
 
The company will also require membership with a consumer code, to ensure the installer and the consumer are protected throughout the installation process.
 
Each quote you have received will be based on the efficiency and performance of the installers chosen heat pump.
 
As we are unable to offer advice on system design, we can only recommend to get advice from technically trained installers. If you are unsure on the advice already provided, please query this further with the installer ass they should be able to justify their proposal.
 
If you are still not confident with the advice provided from your chosen installers, we recommend a minimum of 3 quotes and can only recommend seeking further advice from an alternative installer.
 
Again to reassure you, as the installer is MCS certified, measures are in place to protect both the consumer and installer as part of your installation.
 
I appreciate this does not give you clarification on which system may be best for your property, however as mentioned above MCS can only comment on the MCS standards.#

You don't really want to be spending that amount of money on the basis of gut instinct, do you...

How did you get on with the Stelrad calculator?  I found it quite easy to use.  You can use the MCS one yourself but it's quite fiddly.  It would be interesting to see which of the professinal assessments you're closest to.

To give you something to compare, our 14kW Mitsubishi with 250l cylinder was about £12k excluding radiators and pipework, £16k including. We are getting about £11,200 RHI.   

@novemberromeo Hi Nic,

As suggested previously by Matt, I think you should ask for a detailed breakdown of the heat loss calculations so far performed. It may then be possible to see where they differ, and if necessary question how they arrived at their assessment.

Posted by: @mattengineer

@novemberromeo The variance in W/M2 is significant, 40 W/m2 vs 64w/m2 shows that clearly the fabric of the building has been miscalculated by one or potentially both of the companies.

It would be worth getting a detailed account of the inputs, wall construction, roof insulation, glass area etc and comparing them. Remember you have paid for this to be undertaken and can expect a degree of accuracy.

I think this is exactly what's happened.  All three companies have used different inputs for the wall insulation, roof insulation, glass heat loss, etc. 

We have a lot of glass in our house, full length windows and doors in most rooms.  I see one company has used a U-value of 1.8 and another 2.4.  With similar differences for roof, ceiling and floor insulation I can understand how the differences can arise.

The question is, how to tell who is right and who is wrong? I have no idea what insulation is behind the walls or in the roof, nor do I know the U-value of our glass.  Presumably they're all just guessing!?

You mention air to air heat pumps, given cooling in the summer is of value to you is it not worth a whole house system? We use air to air in a 30sqm conservatory, they do a great dual purpose of heating in the winter and cooling in summer in comparison to the underfloor heating. 

A good point.  We are using our air to air heat pumps exclusively for house heating at the moment, to minimise our oil usage, so we can save the limited remaining oil for hot water only.  The house is nice and warm with our air to air heat pumps.  We have them in most rooms but not all.  Whilst it's fine for now (and the occasional top-up when the underfloor heating is on), overall I do prefer UFH to the air to air units because walking on a heated floor barefoot feels nice.  And of course, we need hot water for the showers, taps and baths too...

Posted by: @kev-m

You don't really want to be spending that amount of money on the basis of gut instinct, do you...

How did you get on with the Stelrad calculator?  I found it quite easy to use.  You can use the MCS one yourself but it's quite fiddly.  It would be interesting to see which of the professinal assessments you're closest to.

To give you something to compare, our 14kW Mitsubishi with 250l cylinder was about £12k excluding radiators and pipework, £16k including. We are getting about £11,200 RHI.   

I really don't want to be spending that amount on gut instinct, but equally I don't want to run out of hot water and have to wait weeks before we get it back again.  We have two young children so that would be a disaster situation!

I tried the Stelrad calculator, thanks for the suggestion.  As a comparison, I tried just doing our kitchen room.  I measured everything up and added the doors, windows, etc but then stumbled on the point I mentioned above about not knowing which inputs to use for the fabric of the building.  I really have no idea if the wall insulation is 100mm thick, 300mm thick or 500mm thick and not sure I could find out without making holes in the walls and generally making a mess!?  Depending  on the inputs, you really do get a massive variation in the heat loss output, which I think explains the differences from the professionals.  But like I say above, is there a practical way to find out what inputs to use, or do the professionals just take an educated guess?

They make an educated guess, but in theory they are a lot more educated in heat loss than you!  If they have done heat loss calcs can't you ask them what their assumptions were?  After all, you paid them.

You should be able to measure how thick the walls are and tell whether they have a cavity, internal insulation or whatever.  U value of windows doesn't vary much, i.e. all uPVC double glazing is similar.  If it's recent conversion, can you get plans from your local authority planning portal ot even look at the building regs applicable at the time?