Welcome to the forums. Can you please share what the heat loss for the property is?

So my heat loss is right on the limit of the 7kw unit at 7.2, so granted it's over the max of the unit but marginally and I believe the system was designed to operate at at 21 degrees at -3. 

All the required rads that needed boosted in size have been part from the 1 in my ensuite, and that was just to avoid the mess on the tiles etc, however the ensuite is plenty warm enough. 

The heat curve has been at every possible curve, it's been fully weather comp, it had extended switched on, it had a target max flow temp of 65, all to which is not helping. 

The house was built in 2010, so is relatively up to date with insulation etc and the extensions that have been added by me are very well insulated going above and beyond the regs, I get that it's a large house but and the distance from ASHP to the farthest away rad is about 20 meters in a straight line so by the time you factor in ups and down and lefts and rights it's clearly way more that that, it just doesn't get warm. 

Like I say the installers have been out many times and are great, they had to refer it back to vaillant to get the unit checked, the guy came and said it's is 100% perfect, he thinks the house is just way to big. 

Sorry to hear that. I have the 7kW Vaillant and, apart from an unfortunate increase in noise when its defrosting, its been excellent.  Of course if the unit doesn't have sufficient capacity then it doesnt have sufficient capacity, but its unusual for surveys to underestimate the house loss! 

A couple of questions just to validate Vaillant's conclusions (if you wish to do that)

• Do you have any data to show that the unit is operating at its maximum output most of the time (to prove that its the unit not the emitters that are the problem)?
• I presume your TRVs and zone valves are all fully open, the unit is operating 24x7 and any external thermostats are set well above the desired temperature so they are constantly calling for heat?
• Can you confirm you dont have a buffer tank or low loss header? 
• Whats the floor area?
• 22 rads and 3 UFH circuits is a lot, do they all run at the same flow temperature or is it mixed down for the UFH?

@jamespa hi mate,

yes I have a  monitor to measure the energy consumption of the HP, it shows max nearly all the time when the temp drops below 4, 

Yes, all TRVs are fully open. 

Yes, there is a buffer tank fitted as per design by the installers. 

The floor area is around 280m2 for quick maths 

The underfloor heating is operating at the same temp, the underfloor heating area is fairly minimal at around 25m2 and does operate on its own thermostat, so cuts off when the rooms are at 21.

Looking forward to your response. 👍

@skidoc my initial instinct is that you've got flow issues, a poorly designed system and I assume you have a lot of micro-zoning and on-off stats. 

Posted by: @skidoc

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Like I say the installers have been out many times and are great...

I used to say the same thing about our installers. They were "great" because they kept coming out, but they never had the skillset or competence to resolve our issues, hence the 2.7 SCOP, with rads that still don't come to temperature. 

All signs point to poor design I'm afraid. Not dissimilar to you, our system starts to struggle at 3C and less. The heat pump is doing the job, it's the rest of the system with the zoning and hydraulic separation that adversely affects the performance.

@editor I know what i think should happen but I'm not heating engineer, my feeling is that it should be a split system with a 3.5 unit to do upstairs and a 5.0 unit for downstairs/water, this I think proves for the easiest fix but deffo not the low-cost /no-cost fix which I imagine is what the installer/vaillant will be looking for. 

My heating system is absolutely fantastic when the temp is above 4 and even good through a short (12hr) cold blast especially as I have batteries installed that should keep the whole house running all-day everyday, but getting stuck in a cycle of nearly 4kw/h soon drains my batteries making it not cost effective or even worse whereby it's actually costing me money to cool my house, deffo not the desired outcome. 

@skidoc  I dont think there is a particular reason to argue for a split system and it would almost certainly require planning permission.  Its also unlikely to be favoured either by Vaillant or your installer!  I wouldn't get distracted too much by the UFH as its less than 10% of the total area, its the radiators we need to focus on.

I tend to agree with @editor that this is probably a system design issue which, if this is true, means that replacing the heat pump may not fix it, albeit that a larger capacity pump may be required in addition to fixing the system design.  Fortunately this would be an easy swap if needed provided you have the space for the taller unit (the footprint is identical).

Given that your installer put in a buffer tank (unless it ahs only 2 ports), then I, like @editor, have concerns about his ability to diagnose and fix the problem.  Also turning the flow temp up to 65C is clearly not going to fix the problem unless the emitters are well undersized.

 So a few more questions if you want a few more ideas:

• How many ports (connections) does the buffer tank have, 2,3 or 4.  Please can you post a photo of the buffer tank?
• What is the diameter of the pipes feeding your heating system?  Plastic or copper?
• What do you know about the secondary pump (the pump on the radiator side of the buffer tank)?
• 4C is suspiciously the point of onset of defrost which suggests that the problem may be connected with that.  Do you observe defrosts and how often?  Is this what you are referring to when its 'getting stuck in a cycle of nearly 4kw/h'
• Do you have any plots or series of data (almost anything) we could look at, you obviously measure energy in, do you have a plot?  Is there anything else you can plot?
• Which WC 'curve' are you currently operating on?
• Have you got a system diagram/could you sketch the system?
• When its operating at max output, whats the flow rate (you can find this in 'live monitor' on the wired interface, you cant get this from the sensocomfort.

Dont feel obliged to answer, however if we are to attempt to help we need to know some more about your system.

@jamespa I will do my best to answer these questions and very much appreciated the help, I'll do my homework and return asap.👍. Thank you.

Posted by: @jamespa

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• 4C is suspiciously the point of onset of defrost which suggests that the problem may be connected with that.  Do you observe defrosts and how often?  Is this what you are referring to when its 'getting stuck in a cycle of nearly 4kw/h'

There was a recent thread over on the openenergymonitor forum discussing the issue of Vaillant 7kW unit not being able to deliver anywhere near 7kW (sometimes only 5-5.5kW) at temperatures conducive to frosting (i.e, below 4C)

If the heat loss is accurate at 7.2kW, the Vaillant 7kW unit is likely going to be undersized and will struggle to cope. The figures Vaillant quote are peak output figures, not integrated figures allowing for defrost cycles.

@old_scientist I saw this thanks.  I am currently treating this article with a bit of scepticism.  I could imagine that the figures in the capacity table are gross (ie ignoring defrosts) but am struggling with the suggestion that the 7kW do only 5kW not least because it wouldn't be enough for my house which is doing fine on the 7kW unit, but also because several well respected installers seem to rave about this particular machine.  So Im keeping the possibility in the back of my mind while pursuing causes which, prima facie, seem more plausible.

This is just an observation, but after recording most podcasts with installers, we usually chat afterward, and while nearly all of them install Vaillant heat pumps in some capacity, they consistently say that the reported COP/SCOP figures from the control panel are overstated as are the performance data sheet specs for these units.