Hello,

The heat loss could well be at 80kw having worked on similar systems / properties. As other replies have mentioned the hot water demand seems crazy high and something I would definitely question.

From experience working with M&E contractors, having companies install certain aspects of the system, I.e, a company specialising in heat pumps and only installing up to a certain point and an M&E contractor installing the distribution side, I believe never really works, or you get issues and the blame gets pushed from one to another. I would look to get a company in that will design and install the whole heating / hot water system.

@greenlover75  slightly off topic but has your architect considered vacuum glazing?  This fits in existing frames and has been approved for listed properties in some cases.  The vacuum units are 8.4mm thin with a 0.4mm gap and give triple glazing-like performance. Search for Landvac.

Also (as mentioned above) have a look at Jaga Strada fan-assisted rads, if you are allowed to change from the existing cast-iron.  They are efficient at low flow temps and emit relatively more heat for their size.

I have installed both the above products and can recommend.

Posted by: @greenlover75

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 However, isoenergy, even prior to heat loss were sugegestign 80kW, so the fact we are now at 100kW did not seem to be so wrong?

I should clarify that it's possible, given size and lack of insulation, that 80kW could be right, but not 25kW.  For me this is the red flag that undermines the figures.

@jamespa Agreed.  Just doing a few more of my own calculations.  

They have specified a 400 litre tank with a 9kW immersion, which would seem ok for a 10 person max. occupancy (with recommended figure being 35-45 litres)?  I have thought whether we should go with a lower size tank (say, 200-300 litres) as most of the time likely to be only 2-4 of us in the house.  However, looking at a hot water tank that is 180 litres, I am "only" getting around 60kWh per month of lost heat and therefore the "over-sizing" does not seem to cost that much, vs. the issue of not enough water at Xmas, say.

However, if you then back-calculate the energy required, indeed a 25kW input would heat that water in 45 mins, which is more akin to how we would expect an oil boiler to work.  So using the view that we would use overnight tariff to heat the water (so, 8 hours, say), then that would only get to 2.4kW system?

Thanks for highlighting this issue, I will go back to the M&E guys and challenge them more on the hot water system.

I guess the heat loss calcs are much harder to challenge?

You could see if https://wiltshire.energy/ are interested. They installed my system (albeit only a 4 bedroom house) and did a good job compared to many installations I read about.

@downfield Thanks for your suggestion.  The house is grade I, with some windows from the 1600s so really tricky.  We have outline permission to put in metal frame secondary glazing but this needs to be specially designed for each mullion insert, so makes the cost of heat pumps look small.  Our current plan is to move in and then see how it goes on this - may be an annual Xmas present to each other to do a few windows at a time!

Will look at the fan assisted rads - thank you.  The house never had radiators in the top floor so we probably have flexibility here.

@jpreston1 Thanks

Posted by: @greenlover75

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They have specified a 400 litre tank with a 9kW immersion, which would seem ok for a 10 person max. occupancy (with recommended figure being 35-45 litres)?  I have thought whether we should go with a lower size tank (say, 200-300 litres) as most of the time likely to be only 2-4 of us in the house.  However, looking at a hot water tank that is 180 litres, I am "only" getting around 60kWh per month of lost heat and therefore the "over-sizing" does not seem to cost that much, vs. the issue of not enough water at Xmas, say.

The MCS guidelines specify a certain volume per bedroom (I cant remember exactly what) so they will likely have got 400l from that.    The penalty for having a larger tank than you need is fairly small, so if you have the space fine.  

Posted by: @greenlover75

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However, if you then back-calculate the energy required, indeed a 25kW input would heat that water in 45 mins, which is more akin to how we would expect an oil boiler to work.  So using the view that we would use overnight tariff to heat the water (so, 8 hours, say), then that would only get to 2.4kW system?

Normally with a heat pump (or a boiler operated on weather compensation) you heat either DHW or space using the entire capacity of the ASHP/boiler.  You never heat the two at once because the two are require different flow temperatures.  This means that you cant afford to spend 8 hrs reheating particularly with a lossy house, but you wont need to because more than 2.4kW is being directed into the tank. That said most of the time you will be reheating a small fraction of the total volume so you wouldn't be taking much time at all.  Furthermore, given your total system capacity, and the fact it will be made up of multiple units you, could arrange to do both simultaneously with appropriate control/plumbing.  In fact I doubt you will have a choice because trying to force 75kW into a 400l tank is simply not going to happen.

Frankly, if your typical consumption is 200-300l and if 80kW is anything like right, the average DHW load is 'in the noise'.   You should have bags of capacity even at Christmas.  Even if you dont and need a boost you could just turn on the immersion (Its (presumably) not going to break the bank given that your annual heating bill is likely to be ~£10,000+).  However it does need separate attention because of the unusually large relationship between actual DHW need and your heating load, which means that the plumbing design will need some thought.

All of this and the sheer scale of the apparent heating demand shows that this is a specialist design that needs some clear thought based on a very solid determination of the heat loss and an clearly thought through analysis of your DHW needs, by someone who has a solid understanding of heat pumps in a domestic environment, but who also understands larger scale installations.  From my perspective your M&E consultant has already demonstrated, with the figure they have proposed for DHW, that they simply aren't capable of that.  

I have personal experience of 'M&E consultants' completely messing up an ashp design, albeit on a smaller scale.  The system proposed for a Community centre that I was having refurbished was absolutely atrocious, hopelessly over specified, overcomplicated, and costing at least twice as much as it should.  The architects to whom they were subcontracted hadn't got a scooby, and the M&E consultants mysteriously disappeared as soon as I started asking questions.  The system was redesigned (before it was installed) on my instruction and by another M&E specialist, and now works well.

In the end its your decision, but if I were in your place I would not even contemplate letting them go ahead without a 'second opinion' (or at all).  If the install goes wrong/doesn't work you will find that everyone has covered their backsides with reference to somebody else and you will be left with a very expensive catastrophe.

Posted by: @greenlover75

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@downfield Thanks for your suggestion.  The house is grade I, with some windows from the 1600s so really tricky.  We have outline permission to put in metal frame secondary glazing but this needs to be specially designed for each mullion insert, so makes the cost of heat pumps look small.  Our current plan is to move in and then see how it goes on this - may be an annual Xmas present to each other to do a few windows at a time!

 

Will look at the fan assisted rads - thank you.  The house never had radiators in the top floor so we probably have flexibility here.

Underfloor heating? - given you are putting insulation in between the floors its got to be worth considering although LB constraints may make it impossible.  That said if the upstairs never had radiators you might be able to argue that UFH means that the original appearance is not disturbed by modern looking radiators!

Posted by: @greenlover75

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@editor Thanks - that would ideal if you could.  As I said, we originally wanted to use isoenergy who had a very long track record in installing such systems in large old houses.  So, some recommendations of similar companies would be ideal. 

I’ll connect you via email (using the email address you registered with on this website) with some top installers tomorrow.

Hi @greenlover75,

We are picking up quite a lot of Iso projects and a few of their engineering staff as their Southern office was nearby. One of our in house installers lives in Swindon as well, so not a million miles away.

I completely agree with Damo, M&E consultants are an expense you can do without in most cases.  They have been the bane of my professional life, always ott, no regard for efficiency and usually recommend the equipment that has the best kickbacks.

@greenlover75 intro emails sent. Good luck with your project and please keep us posted on progress.