@carpenterstation

Posted by: @carpenterstation

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I’d agree with all of the above. Especially sequencing. But you may need to move fast to avail yourselves of VAT and BUS.  Our install was massively over specified (16 kWh on a 450m2 floor plan).  House is quite well insulated - but old so not perfect and we have never needed anything like 16kw.  The whole house has never consumed more than 5kw in a 30 minute period - so I think that means 10kw would have been big enough!

I think you must mean “The whole house has never consumed more than 5kWh in a 30 minute period.”  That means an average electrical power over the half hour time slot of 10kW.  

Of course, it may have been running harder than that for part of the time, so 16kW is probably fine.  So long as the heat pump is not overconfigured by more than 30%, the efficiency impact should be small.  As you say, it is good to have some power in reserve for that really cold night.  Also, it mean that your hot water will get up to temperature faster.

@grahamf I did mean the whole house has never- yes! But I didn’t need the tandem heat pumps. One would have been fine and half the cost. It’s true that I have loads of margin - but I paid for that!

@editor sorry should have been clearer - the whole house - in 30 minute intervals has never exceeded 5kw (hence 10kw for an hour - ergo a 16kw system is at least 60% too big (and is 2 heat pumps, not one)

@carpenterstation you have tandem heat pumps?!  Sorry I missed that.

I was picking you up on your units.  You used 5 kilowatt hours of energy in a half hour period, meaning a power of 10 kilowatts (no hours as it's power).

Given that heaptump calculations are basically expensive guesswork for many houses I'd also ask if the oil usage numbers on worst days correlate at least roughly with the numbers coming out of the magic guessing machine. They should reasonably correlate unless the house is currently cold even with the heating on.

At that kind of heat loss I'd be looking at the insulation question in detail. 1990s houses may be better than unimproved older ones but they don't necessarily have decent insulation. The loft insulation of the time tended to be token, the floors concrete and uninsulated - which is hard to fix cheaply. Some of that may have a far better payback, particularly loft insulation and air tightness as that's where much of the heat goes and can easily be stopped. If you have a lot of sea facing wall with high exposure to wind then I'd be taking a good look at windows and air leaks on that side as well.

I think there's a broader point worth drawing out of this discussion, because it's surprisingly easy to confuse a heat pump's rated capacity with the electricity a household consumes, and the distinction is important when we're trying to establish whether a system has been oversized.

A heat pump's kW rating generally refers to its thermal output, or how much heat it can deliver under specified operating conditions, rather than how much electricity it draws from the grid. The appropriate capacity should be determined principally by the property's calculated heat loss at its local winter design temperature. Electricity consumption, meanwhile, tells us something quite different. A reading of 5kWh over 30 minutes represents an average electrical demand of 10kW during that period, but it doesn't establish that the house requires 10kW of heating. 

None of this means a system cannot be considerably oversized, and I think homeowners are absolutely right to question the capacity they're being sold, particularly when the equipment represents such a significant investment. But I'd be reluctant to draw conclusions about the degree of oversizing from electricity readings alone, without understanding the actual heat delivered and the outdoor conditions at the time.

This is also why I agree with @etchedpixels about using historical oil consumption to sense check the heat loss calculations in the original question. It's not a perfect measurement, particularly with an older boiler whose efficiency is uncertain, but it provides another useful piece of evidence against which to judge the proposed design.

@seaviews 

I cannot emphasize enough that there is a right order to do things here, which is to retrofit insulation upgrades first, and only once that is done, to get a heat loss survey and make decisions about a heat pump. Doing these two steps in the reverse order will cost you a large amount of money on inappropriate and unnecessary equipment that is likely to include: a heat pump that is substantially over-sized compared to what you will later need, more costly installation for that over-sized heat pump, upgrades to much larger and more expensive radiators than you will later need (for example K3s when equivalent K2s would have been fine, or even keeping your existing ones), and pipework upgrades that may not actually be necessary. 

I would strongly recommend that you find a completely independent retrofit assessor (not tied to any company selling insulation, heat pumps etc.) and ask them to make an assessment of your property and a plan for a set of upgrades. This will cost you a few hundred pounds and will be money very well spent.

I used a not-for-profit community interest company in Yorkshire called "Rethink Retrofit" https://rethinkretrofit.org.uk/ The woman who advised us was absolutely excellent and the value-for-money / effective hourly rate for all the work that she put in was very good.

I suggest that you search for something like "community interest retrofit assessment company devon" using google and check out some community led groups that can provide impartial retrofit advice. I think engaging with one of them would be an excellent way forward, particularly as you say you are "retired and spend the entire day at home. We would appreciate your thoughts and advice as we feel confused and overwhelmed...."

Don't rush into anything...

Thank you everyone for your very helpful and informative responses, I am so grateful for such detailed guidance and suggestions. Based on the responses we have now sent a set of new questions to the installer we have been working with (on refining their quote) for more technical clarifications. Our fear is that a sufficient heat loss study has not been conducted. I recall the Mitsubishi installer carefully calculated each radiator component on his visit/ survey byt neither of the other 2 installers did this, although our preferred installer did have the original plans for the house.

We have also contacted 2 companies to request loft insulation quotes. Meanwhile we are also waiting for our up to date energy certificate which we hoped would perhaps help contribute to our decision making in this whole process.

To clarify the radiators and pipe work are in reasonable condition for their age although old and of course older style dimensions, as the house has only been used as a summer house until we moved in. Our architect thinks we might get 10-15 more years out of them. As it is likely to be a costly job to replace we thought we would wait to replace them for now. As well as having the system drained we had requested inclusion of the system being cleaned as part of the installation.

The brand and model of heat pump proposed by 2 of the companies is the Saunier Duval GeniaAir Max ES 15kw (and our favoured one to date). And by the other installer it has been proposed either; Mitsubishi WZ140VAA-W with Bygold treatment or Mitsubishi IZUMI HIDRO R290 PUZ- WZ140VAA-W also with Bygold treatment.

@seaviews, thank you for coming back with the additional information. I'm pleased you've taken some time to ask further questions rather than feeling pressured into making a decision, particularly given the scale of the investment and the fact that you're planning improvements to the property.

Your concern about the heat loss calculations is the part I'd concentrate on now. The fact that the Mitsubishi installer measured your existing radiators is encouraging, although that alone doesn't establish that a comprehensive heat loss assessment was completed. Equally, having the original architectural drawings is useful, but it doesn't necessarily tell an installer how the building performs today, what insulation is actually present or where heat is being lost. I'd want to see the room-by-room calculations themselves, including the assumptions made about your walls, roof, windows and ventilation, rather than simply being reassured that the calculations have been done.

The EPC will be useful for identifying potential improvements, but I wouldn't place too much weight on it when determining heat pump capacity. It isn't a substitute for a proper heating design, and the distinction becomes particularly important when you're about to improve the loft insulation. I think @rob-of-york and @jamespa made valuable points earlier about the sequence of work, and I'd be reluctant to finalise a 15kW installation before understanding how those planned improvements might affect the property's heat demand.

On the radiators, I'm actually quite encouraged that they're in reasonable condition. I certainly wouldn't replace them simply because they're old, particularly if doing so would place additional financial pressure on you. Cast-iron radiators can work with heat pumps (I've been told), provided they're capable of delivering sufficient heat at the proposed operating temperatures. Their age and physical condition are one consideration; their ability to heat each room efficiently is another. I'd want the installer to demonstrate that the existing emitters are suitable, rather than assume that replacement is either essential or unnecessary.

The condition of the old pipework and radiators also makes me interested in how the system will be cleaned and protected. That's sensible work to include, but I'd ask exactly what cleaning procedure is proposed, whether the condition of the existing system has been assessed and how the new heat pump will be protected against any remaining debris or corrosion. With an older installation, I'd rather see a carefully considered approach than an aggressive cleaning process undertaken without understanding the pipework's condition.

As for the proposed equipment, I have to admit that Saunier Duval isn't a heat pump brand I've encountered, so I'm not going to pretend I can give you an informed assessment of its reliability or performance. Having looked into it very quickly, it's part of the Vaillant Group, which is reassuring in terms of the company's background. I'd still want to understand what local technical support, servicing and spare parts availability would look like for that particular model. When it comes to heat pumps, I think that selecting an established brand is a far safer bet than a no-namer. I'm not saying that Saunier Duval is bad, but I'd be tempted to go with a Vaillant instead. 

I also hadn't come across what your quotation calls Bygold treatment, although I understand this is probably Blygold (after a quick google search), a corrosion protection coating for heat exchangers. Given that you're only 200 metres from the sea, I think it's encouraging that at least one installer has considered the exposure to corrosive, salt laden air. However, I'd want confirmation that the proposed treatment is approved by Mitsubishi, whether it affects the manufacturer's warranty and what maintenance will be required to preserve the protection. I'd ask the Saunier Duval installer precisely what coastal protection their proposal includes too, because this isn't an issue I'd want overlooked.

One other thing worth exploring is the Mitsubishi model designation and whether a factory coastal protected version is available, rather than relying solely on an additional coating. The installer should be able to explain the differences and provide written confirmation of the warranty arrangements.

But I'd come back to one central concern. You now have quotations naming specific machines, yet we're still uncertain about the heat loss those machines have supposedly been selected to meet. Before comparing brands, I'd want the predicted heat loss of the house, the proposed design flow temperature, confirmation that your existing radiators and pipework can support that design, and an explanation of why a 100 litre buffer is considered necessary. I'd also ask for the heat pump's output and minimum modulation at the proposed operating temperatures, because those figures would help us understand whether the larger machines could operate efficiently once the insulation improvements are completed.

One final question. Does the 3,000 litres of annual oil consumption represent a full year of living in the property permanently?

I wouldn't dismiss any of the proposed manufacturers at this stage, and I certainly wouldn't conclude that all three installers have got the sizing wrong. But neither would I take three recommendations for approximately 15kW as sufficient evidence that 15kW is appropriate.

You're making good progress by asking these questions, and I think the next set of answers will tell you a great deal about the quality of the proposed installations. I'd be particularly interested in what your preferred installer comes back with regarding the heat loss calculations and buffer arrangement.

@seaviews 

As well as loft insulation, there are other aspects of retro-fit insulation that can make a substantial difference to the heat loss of your property.

For example, if you have a timber floor (floorboards) on the ground floor and there is sufficient crawl space below the floorboards, then you can get underfloor insulation fitted. This is just 150mm Rockwool (loft insulation) netted up between the joists. It makes a huge difference to the heat loss through the floor. The job involves minimal disruption, since only one or two entry points are needed. In our house it was sufficient to roll back 1m of the living room carpet and cut 3 or 4 floorboards to provide access.

If your window frames are in good condition, then another excellent retrofit solution is to simply replace the old double glazed units (DGUs) i.e. the glass rather than the windows themselves. Using Planitherm One toughened glass argon filled units with warm edge spacers gives a U-value of 1.0 for the DGUs compared to over 2.5 for your original 1990s DGUs. In other words the new glass lets only 40% of the heat through that your existing windows do. This results in a whole window U-value (including the frames) of about 1.5, or roughly half that of your existing windows. Replacing only the DGUs is far less expensive. We went down this route and had 60 DGUs replaced at an average cost of £75 each (£4500 in all). This was less than 25% of the cost of complete new double glazed windows. Two additional benefits were cooler rooms in summer, due to less heating effect through the glass, and better noise suppression.

@editor Thank you very much for this comprehensive reply. I hope to have time to review and respond in more detail soon. For now I wanted to mention that the response we got around the reasoning for the buffer tank also references the filtering of the water to protect the system "In the case ...... where there is already an existing installation, it is fundamental to install the buffer tank not only to guarantee the boost on colder days, but because it works as an air and sludge separator (there is always dirt and air bubbles in existing old pipes and it is not good that they enter the primary circuit of the heat pump system). In that way, we increase the useful life of the exterior unit." I'm curious as to your thoughts about this? 

And yes the 3000l of oil does represent full time living for heating and hot water. We can't be sure of the exact figure as there is no indicator and it is a very large tank that was pretty full when we moved in. So we have estimated based on how much oil we have bought over the last 2 years and approximately how much was in the tank on arrival...

Unfortunately our floors our concrete so not possibility of using underfloor heating without digging them up, which we may do in future subject to funds. Regarding replacing the glass in the windows, this is very interesting and not something I have heard of before. I will look in to it thank you!