What is your heating pattern? If the house is remaining warm most of the time (whether it not the heating is on) 1800 litres suggests 7kW or even a bit less is about right.  Depending on current pipework, if this is the case, it's quite likely that repiping isn't necessary. 

Can you provide more details of house in particular floor area and shaoe/summarise current pipework/layout.  Do you know the estimated loss of the most demanding room (from heat loss reports), this tells you about the need or not to repipe.

What is the design oat where you are, need that to check the output (max and min) of the 10kW midea which may well be less than 10kW!

Reject any quote with a buffer tank.

Is anyone guaranteeing performance (heat geek often do)?

Posted by: @penval

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We are at the stage of getting quotes for an ASHP retrofit. I recently posted about potential issues with microbore pipes and a huge thank you to everyone who gave their views it was very helpful and I am, to some extent, wiser than I was when I posted the initial question. 

We have just had our second quote and it's very different to the first one. Quote one proposes a complete re-pipe the reasons for which I now understand. The second one is proposing a high output ASHP specifically a 10 Kw Midea. The heat loss calculation for the house (which is a self build to 2006 regs but has an uninsulated floating floor downstairs) is 7.5 KW, we have a 250 Ltr cylinder and 3 bathrooms. The only reservation seemed to be the recovery time for the cylinder which is currently 20 mins but would be closer to an hour. Proposal has no buffer tank as part of the design. I would add that despite the floor downstairs it's a warm house which used 1600 litres of oil last year, in the past if it's a very cold winter it usually uses 1800 litres.

Obviously the cost is way less without the re-pipe but we're not comparing like with like - we still have one quote to come. I don't want people to make a decision for us but I would like a few thoughts on what I'm looking for here. All comments gratefully received, many thanks in anticipation.

I am about to install and review a new tech on the market by Adia Thermal, I have 10mm microbore in the dot and dab walls so I defiantly do not want to remove all the plasterboards, there is a 22mm main feed split at airing cupboard and as a run around leading to the 10mm mico. all the radiators in my home are about 10% undersized and I am not changing them or the existing hot water tank which is 5 years old stainless steel, its good thermally but has a slightly undersized coil.

Now Adia claim with a clever learning process within the controls the ASHP system can be controlled to operate efficiently and balance its self perfectly over a period of 2-4 weeks, take advantage of the TOU tariff and choose to run the system harder in cheaper periods and more gentle in peak. This systems can balance every room and maintain every room with each room having a smart TRV talking back to the Adia hub and then to the ASHP, by controlling this way they say they can make slightly undersized rads work, the under sized coil work in the existing tank and still run the system right to save or at least tun the system efficiently, There are now several installed in trials and the results are so promising they launched to market around a month or so ago. How this magic works is by running the ASHP hotter its corrects all of the undersized rads and coil in the water cylinder, if this is done during an off peak period it will not cost more and once the fabric in a reasonably insulated building is near the desired temperature it would not take the full output of the radiators to keep it at a level of comfort, so runit hard for 2hrs during off peak and trickle all day on higher costs = balance low cost solution. 

My home 1999 build calculated out at 4.47kW peak, my gas usage for 2 years on heating calculated out at 4.41kW, so I was happy with my heat loss at 5kW, I have chosen a Vaillant 5kW system as it puts out above 6kW -2 Deg C which is enough spare although tight (for periodic use - running hard then soft is the same as periodic use) but I felt the 7kW which is really an 8.3kW output was over sized. My choice for the lower kW heat pump is also whole house smart system driven, you see if I choose the 7kW there is a chance the system on DHW may pull very close to my inverter 3.6kW output on the solar and I want to try and run all of the ASHP work on my solar and batteries if not all as much as I can, if we peak over electrical draw of 3.6kW or someone uses something else in the house we will draw from the grid.

If you are in no rush to choose and can wait for me to review this tech it might be a solution for the microbore and there may be no need to change your hot water tank or change to much pipework if you did not want to. It does take longer to heat the tank with a smaller coil but I will leave mine heating up during the day when its sunny or on the batteries after they self charged in the of peak period. That said, it will be the 1st time I am installing this to test it and this is why my poor house is a test bed again as I would rather sell something I lived with than just say it works, which I am sure it does as they won some awards it so good.

Good luck with your choosing.   

@ashp-bobba 

That is a very interesting new angle on how to use the Adia technology, basically to 'deal' with some edge cases where the alternative is massive disruption or taking a risk that loss is in fact overestimated.

I'm looking forward to hearing the results!  With a few hours of cheap leccy and this level of intelligence, I can see that quite a few iffy cases suddenly become clear.

Posted by: @jamespa

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@ashp-bobba 

That is a very interesting new angle on how to use the Adia technology, basically to 'deal' with some edge cases where the alternative is massive disruption or taking a risk that loss is in fact overestimated.

I'm looking forward to hearing the results!  With a few hours of cheap leccy and this level of intelligence, I can see that quite a few iffy cases suddenly become clear.

I am going to review the whole thing, every inch, how hard or easy to install, how it fits or it will not fit, what it costs and so on. we install on Monday next week for the week, it should only take 3 days but I gave us the week so we can film and comment, review and discuss.

Many thanks to all. Really appreciate and will try to answer questions asap. We have one more quote to come so time to read review of the install. 

IMO "Oversizing the heat pump" is a bit of an over simplification.

What really matters is what water flow rate the heat pump likes, and its modulation range. Oversizing becomes a problem if the heat pump wants a flow rate the piping can't deliver, or it can't modulate low enough so it cycles a lot. That depends quite a bit on manufacturer, model, etc.

Quite often several models from the same manufacturer quote different power numbers but they're the same heat pump with different software settings, ie different upper limits on the modulation range, but the lower limit is the same. In this case, if the required power is below the lower limit, the smaller heat pump will cycle just as much as the bigger one.

Then there's what the heat pump decides to do with the flow. For example my LG 16kW has a modulation range of about 3.5-16 kW. While the nominal flow specified in the docs is 2.7 m3/h which is huge, it doesn't care much about it and it is perfectly happy working at half that, deltaT simply increases. This wastes a bit of efficiency but it's not a nightmare scenario. It will only throw an error and shut down if flow is ridiculously low and deltaT gets over 20°C. On the other hand, some other brands' software may be a lot pickier about flow and decide to throw errors and shut down much more easily.

Another example, I don't know about Midea so I'll give numbers for LG. They sell two monoblocs, 16kW and 9kW. Then the 12-14kW are the 16kW with software limit, and the 7kW is the 9kW with software limit. In this case, whether you get the 7 or the 9, it's the same heat pump. But going to 12kW would mean an increase in the lower limit of the modulation range, which may be an issue. What I mean is, just because you pick the heat pump size "just right" doesn't mean the low end of the modulation range lands where you need it.

@bobflux

I agree that its a simplification but I think its more serious than flow rate.  An overestimate of house heat loss can result in unnecessary re-plumbing, excessive cycling at relatively low temperatures and the heat pump operating permanently at an inefficient point of the compressor curve.  The first is very disruptive and the 2nd and 3rd can significantly reduce COP.    In worst case it can result in a system that is very difficult to control, as we have seen with a couple of low loss (3kW) houses that have been fitted with massively oversized heat pumps or, at the other end of the spectrum, an unnecessary cascade with all the complexity that involves, which we have also seen more than once here.   So IMHO oversizing should be taken as seriously as undersizing.

Your point about ranges is well made.  Daikin likewise only do 2 hardware variants, 8kW and 16kW, everything else is simply software limited.  The Vaillant 3kW is just a limited 5kW and the 10kW a limited 12kW., others do similar.  Reference should always be made to the output table (sometimes difficult to find!), which should specify minimum as well as maximum output.  Also don't forget that the interesting comparison is minimum at high OAT/Low FT vs maximum at low OAT/High FT.  This is often only a 3:1 ratio or a bit less unless (sometimes dirty) tricks have been played by the HP manufacturer artificially to extend it.  Of course with software limited variants it will be less still.

Yes it's a shame the manufacturers make this vital information so difficult to access!

No buffer, check.
No re-pipe, check.
6 kW would add up better imo, 10 kW doesn't at all. Big pump bringing a 200-day-per-year risk of too little demand and HP hiccoughing, small pump a 10-days-per-year risk of having to boost the living room with an oil radiator or like.
With 10% undersized rads, 45°C FT is reached soon enough. At such FT you can just loop the DHW into the system temporarily and suffer no waiting times in cold weather. Limitation indeed, you must find a controller which can manage such scenario. Details we would need to discuss here, but I think it's feasible.

Second thought, sorry:

What I said about 6 kW instead of 7.5 kW against 10 kW, was under the assumption, that at 1600 or 1800 l oil per year the indoor temperature would be around 20°C every day with the usual set back at night.

Without knowledge of this being a really good idea, with 1600 l on my own flat I would go to 4 kW at -2°C and think this would work out gladly.
A lot of attention then I would put to the temperature range above freezing point -- if there was any evidence that the HP might put too much focus on defrosting, I would switch to the next bigger model.
This said, I would strive towards 4 kW @ -2°C (as already said), 3 kW @ 4°C and 2 kW @ 10°C.

Our house is not dissimilar to yours on paper. We had a heat loss calculation of around 7.4kW and previously used around 1600-1800L of oil per year, so same ballpark. We've had some insulation upgrades, and because we only heat to 20C, the actual heat loss is probably more like 5kW (I don't think we have ever delivered more than 120kW of heat in a 24h period)

We have a 12kW Samsung ASHP, which is actually a 16kW unit software limited to 12kW, and hence can only modulate down to around 4kW minimum output (see @bobflux comments above)

Whilst clearly oversized, we have not found that to overly limit it's efficiency (in fact we are really happy with it's performance). With temps below ~7C we can just about run continuously during the day, and we turn off overnight. Below 5C and we only turn off overnight for around 4h and back on at 4am during our cheap rate slot. If it's around 2C or below, we can run continuously. During milder spring and autumn periods like we are currently having, we may run for a few hours then turn off for a few hours as required, which doesn't affect efficiency. In warmer weather we may turn it on for an hour or two when we first get up, and maybe again for an hour in the evening which is sufficient, so almost like a conventional boiler. All the time we are running a flow temp of 32C due to large radiators, hence it is efficient.

My point is an oversized heat pump isn't necessarily the disaster you may first think, so I certainly would not discount a 10kW unit on a 7.5kW heat loss. And it has some benefits too - you are not continuously operating the unit at the top end of range where it is less efficient, and the DHW reheat times are shorter which you have already highlighted as a potential issue (we can reheat our 160L cylinder in around 20mins). Further, a 10kW unit may only achieve 10kW output under very specific conditions so you may not have as much headroom as the headline figure suggests when it's -3C outside. Overall, a nominally 10kW unit on a 7.5kW heat loss does not seem unreasonable to me. 

I'd go so far as to say the actual size of the unit, assuming it is large enough, potentially has less impact than the rest of the system design. The system pipework must be capable of sufficient flow to carry the required amount of heat. If the existing pipework is capable of doing this, it doesn't need replacing. Emitters should be as large as possible - bigger radiators equates to lower flow temperatures which equals lower running costs. Obviously there is a cost trade off between replacing all radiators with the savings that will come from lower flow temperatures. How low you can go with flow temperatures will also be limited by the size of your radiators and the minimum output of your heat pump. For example, the minimum output of our heat pump is ~4kW, and our radiators need a flow temperature of at least 32C to be able to dissipate that amount of heat, so that is the lowest we can effectively go. If we need less heat than that, we are cycling on/off as required. 

I definitely agree with others that 99% of homes do not require a buffer tank, although many will benefit from a volumiser (make sure you understand the difference). I would view a buffer tank as a major red flag and discount any quote/installation specifying one on that basis.