The house feels warm throughout and the temperature is maintained, what a wonderful thing working weather compensation is.

The main and immediate difference to note is the volume level of the external unit, even though the new ASHP is a dual fan unit the Grant is whisper silent in comparison to the Peak.

Happy we made the leap to get it replaced as the Peak unit would have been a disappointment for years to come and it made recommending ASHPs to anyone difficult.

But with the Grant installed I can confidently say we will be recommending ASHPs to anyone who asks, extremely happy with it.

The installer came in today and removed the Peak cylinder and fitted the new Grant cylinder in its place. With the external unit being removed tomorrow. 

Ribble valley are coming in on the 7th May to fit the Grant ASHP. So we're just using the immersion heater for the hot water until then.

I setup the Grant controller with the cylinder and it is so much easier to use. 

Have you organised anything with your Peak system and installer or are you persevering with your 16kw unit?

Just waiting on timescales from them once they’ve sourced the correct cylinder we’ve asked for, to work with the new Grant 12kw ASHP that Ribble Valley Heating are currently booked in to install mid May.

We haven’t had an exact figure back from them on how much we’ll get back but it covers the cost of the new replacement cylinder and should be a reasonable amount to go against the new Grant ASHP.

They will then deal with Peak directly to recover their costs as best as they can.

Yes, here's mine. From the service manual parts numbers (compressor etc), the 12-14-16kW models are identical, and all are actually "much more than 16kW at 7/35°C heat pump" with a software limit to the official power on the label (in my case 16kW).

Since the three models are the same, in the same conditions (air, water, power) the COP is the same and defrost is also the same. The more powerful models just allow more watts at worse COP, which may be useful. Minimum power is the same. The heat exchangers are the same, so a bigger label won't magically pull more watts from the air at the same deltaT or cause less frost to form...

Sometimes below this table you get a tiny footprint *includes defrosting or *does not include defrosting, but that's at a specific temp/humidity, not necessarily yours.

Personally, since I have about 500 tons of stone walls and slabs with exterior insulation, I oversized it by about 2x, so it runs at high COP during the day when air temperature is higher, COP is higher, and electricity from PV is free. 

When I check the Grant tech specs, to try and compare the modulation range against the Vaillant table you provided, it states a minimum modulation of 3.2kw heat output, the missing info is at what external temperature this is for.

At a wild guess a low external temperature in order to flatter it!  If they were proud of the figure they would bother to tell you the information you actually need!

Regarding falling between model sizes I was in a similar position, being apparently stuck between model sizes.  In the end I put effort into refining my heat loss estimate by using information from gas consumption.  Surveys said 16kW, my own calculations 10.5kW, measured consumption was 7kW.  The last is a perfect fit to the Vaillant 7kW, and turns out to be one of the best decisions I made.  The difference between 10.5kW and 7kW is accounted for entirely by air change rate.  If this calculation hadn't worked out I would perhaps have gone with the Mitsubishi 11.2 for which full modulation data is available and wouldn't have touched any heat pump for which I couldn't obtain the modulation data on the grounds that, if they dont or wont provide it, they have somethin g to hide.

Of course this may well not apply to you but its worth observing that following scatter plot, taken from heatpumpmonitor, shows measured loss vs floor area for about 250 systems.  Notably only one system is both <200 sq m and >9kW

When I check the Grant tech specs, to try and compare the modulation range against the Vaillant table you provided, it states a minimum modulation of 3.2kw heat output, the missing info is at what external temperature this is for.

This is an important detail that I am looking at while trying to decide what ASHP to go for as a replacement for our Peak unit. Following my own heat loss calculations in Heat Punk it has come out with a 8.5kw heat loss at -3 for our property. This differs from the Heat Geek calculation of 10.3kw as this was done with an internal temperature target of 21 degrees and we only want to target 18 degrees. This would be with 40 or 42 degree flow temp at -3 to achieve our target temp.

This heat loss looks to put us in between model sizes either too small or slightly oversized, so the minimum modulation for the heat output of the ASHP will hopefully allow us to avoid too much cycling when the external temperature starts to increase.

Looking on the HeatPumpMonitor website their Heatpump database includes minimum modulation tests with real world installs. At 15 degrees external the Vaillant 12kw will bottom out at over 5.5kw heat output while pulling ~900 watts. The Grant 12kw also at 15 degrees external bottoms out at ~3kw heat output with a draw of ~500 watts.

Vaillant page:https://heatpumpmonitor.org/heatpump/view?id=5

Grant page:https://heatpumpmonitor.org/heatpump/view?id=34

@mk4 hoping we get something sorted out soon!

From my current understanding matching the ASHP size exactly to the heat loss of the property wouldn’t take into account any defrost cycles for the unit once external temperatures get down into the low single figures and below. Something we experienced with the initial Peak 7kw unit that we had installed in September, that struggled at 5 degrees external temps.

So while there is a 9kw Grant unit available that would cover our 8.5kw heat loss at -3 it leaves no overhead for defrosts, or if we decide to increase the internal temperature from 18 degrees at all. The next size that Grant offers is the 12kw unit.

If we look at Vaillant instead they offer a too small 7kw unit, then up to the 10/12kw units. With the 10kw unit being a restricted 12kw, it offers the same ~5.5kw minimum modulation heat output. So with the larger units the minimum modulation output starts to become important otherwise it would start to cycle at a lower external temperature rather than reducing its output down and maintaining the internal temperature.

Guys, I have been reading your fascinatingly horrid stories and I am rooting for you! 

No idea how Grant labels their heat pumps, some manufacturers go for the bigger number, others are more honest, but make sure you're comparing apples to apples and check the actual capacity of the heat pump at your actual air and water design temperatures.

Related to the above, a question also addressed to the savant of the forum engaged in this thread @jamespa (if I am highjacking the topic, please ignore the question) 

I have watched over and over in the forum statements along the lines of  “our heat loss survey showed 8kw losses so we were proposed and had installed a 8kw ASHP”. Hardly anywhere have I seen capacity tables being discussed (of course I may be wrong because I am very new to the forum and I have not read everything - far from it).

Nominal ASHP capacities are usually quoted for a combination OAT 7 & LWT 35. If the design point is -2, the ASHP may well not be able to deliver the nominal capacity of say 8kw at -2. Or it may approach 8kw with a LWT that is not functional for the house (cannot be used effectively with the radiators).

Capacity tables is quite high in the list of topics discussed with installers over here. And I guess also one of the reasons we end up with comparatively much larger (and probably oversized) units. I know design points supposedly represent the extreme and rare occasion, still I wonder if this is a topic routinely discussed. 

You are absolutely correct that the capacity table MUST be consulted.  Heat pumps vary in how much, and in which direction, they depart from the 'sticker' capacity and in most cases this is also a function of both flow temperature and outside air temperature (and where it isnt a function of those things its presumably because its 'clamped' in the firmware).  From time to time, especially when people are close to making a decision (or have made one and are seeking advice afterwards), this does get discussed on the fourm but many of the conversations are at a level where this level of detail doesn't (yet) enter.  However its always in the back of my mind, at least, I cant comment for others.

Most of the UK designs for -2 ~-3, so quite often the sticker capacity is not that far from the actual capacity at design parameters, especially if you are designing at FT45 or less.  That doesn't mean it should be ignored of course, but it is not a reason for oversizing (by oversizing I mean relative to actual capacity not sticker capacity).

Installers should always consider the capacity tables of course, that said many installers stick to one or two brands and operate in a limited geography so will know what these deliver at the design OAT applicable to their area.

Guys, I have been reading your fascinatingly horrid stories and I am rooting for you! 

No idea how Grant labels their heat pumps, some manufacturers go for the bigger number, others are more honest, but make sure you're comparing apples to apples and check the actual capacity of the heat pump at your actual air and water design temperatures.

Related to the above, a question also addressed to the savant of the forum engaged in this thread @jamespa (if I am highjacking the topic, please ignore the question) 

I have watched over and over in the forum statements along the lines of  “our heat loss survey showed 8kw losses so we were proposed and had installed a 8kw ASHP”. Hardly anywhere have I seen capacity tables being discussed (of course I may be wrong because I am very new to the forum and I have not read everything - far from it).

Nominal ASHP capacities are usually quoted for a combination OAT 7 & LWT 35. If the design point is -2, the ASHP may well not be able to deliver the nominal capacity of say 8kw at -2. Or it may approach 8kw with a LWT that is not functional for the house (cannot be used effectively with the radiators).

Capacity tables is quite high in the list of topics discussed with installers over here. And I guess also one of the reasons we end up with comparatively much larger (and probably oversized) units. I know design points supposedly represent the extreme and rare occasion, still I wonder if this is a topic routinely discussed. 

ASHPs do have a minimum compressor speed. The minimum heat output at that speed depends on air and water temperature.

12kW Vaillant with 45/40°C water has a minimum power of 3.6kW at -2°C air, 5.2kW at 7°C air.

That's because Vaillant labels according to power at -7°C air, so the same machine is a "18kW heat pump at 7°C air / 35°C water" and "12kW at -7°C air / 45°C water".

No idea how Grant labels their heat pumps, some manufacturers go for the bigger number, others are more honest, but make sure you're comparing apples to apples and check the actual capacity of the heat pump at your actual air and water design temperatures.