@judith Hi Judith and thanks for the reply.

The final design and specification on the system hasn't been confirmed, I'm not totally convinced with the survey/forecasts, I take your point on the size of the heat pump - the 11kw is heat output i think, I'm happy to slightly over specify but will take advice on the best to choose and would like to know what the best operating range is for them.

I understand what you mean about the LVT and adhesive, its something we'll look into carefully before specifying.

Understand what you're saying about batteries and peak electric - I wasn't thinking of a hot tub - I've the physique of Homer Simpson, but would consider dumping the power into the cars. I think in the high summer we can use the excess on Aircon, we have an office at home that my wife and I use and its on the south side of the house.

Thank you. Simon,

@jamespa 

agreed James, I plan on exporting as much as possible and then taking what I need through the night.

thank you, S

Posted by: @sjc123456789

↑

The house is less than 200sqm, upstairs is smaller than down, single storey extention.

We've had 2 surveys done, one by the Heat Geeks which has gone cold - long story, and we've got a second one. I'm not 100% convinced either fully understood what we were doing and put the time in to model it properly, but both came back with a heat loss of between 10 and 11 (forgive me I'm not too technical).

Yes, the upstairs is designed to work at the lower temperatures we're expecting with the UFH downstairs. I didnt put it in the spec, but we might well have aircon fitted upstairs as its usually too hot for us (we have an office upstairs, which needs aircon) but we might move the office downstairs.

I understand where you're going with points 3 onwards, I'll do some research and come back to you.

I'm really struggling to imagine that the house you describe with the insulation you describe is 10-11kW unless you the design temperature where you are is <-5.  Is there any way to defer the heat pump installation a year and get some measured numbers?  Alternatively another survey once the insulation etc is done so the surveyor can verify the fabric upgrades (some, perhaps many, refuse to account for fabric upgrades they cant see).   Alternatively if you have a decent slab, which I think you are planning, you could probably get away with an oversized machine (but 12kW might be a bit too much) and batch heating.

I had two full surveys (3 hrs each) on my 200 sq m 1930s house with partial fabric upgrades, both coming out at 16kW.  Unlike you I wasn't in the middle of fabric upgrades, so they could have taken the correct fabric into account (and not made several other errors).  The actual loss is 7kW, confirmed by measurement both from my (now removed) gas boiler and my ASHP.  If I had believed the surveys I would have been one of the unfortunate people on this forum complaining about poor performance!  My case is probably at the extremes, but not unknown.  Caveat Emptor!

Posted by: @sjc123456789

↑

Yes, the upstairs is designed to work at the lower temperatures we're expecting with the UFH downstairs. I didnt put it in the spec, but we might well have aircon fitted upstairs as its usually too hot for us (we have an office upstairs, which needs aircon) but we might move the office downstairs.

Good to the first point.  Exactly what's needed.

Have you considered that aircon is just another ASHP which BTW can also heat.  Do you need another or would you be better off with 'light cooling' by fitting a small number of fancoils operated above dew point temp (UFH will also work in this mode and reportedly does a pretty reasonable job, although I haven't personally experienced it).  It wont give you anything like the USA chiller effect, but for the number of days its needed it may well be enough.

@jamespa 

Hi James, I'll have a good look at the report and find you some proper details - apologies.

Its not like a newbuild slab, the house is made up of the original house then a couple of extensions  so the floor is a bit mix and match, the floor surface is all level, and there will be 100mm of insulation and then the heat spreaders all over the house. the cavity under the floor varies in depth, old and new. I think we've about 25 to 30mm surface we can put on top of the spreader plates.

Yes, on the aircon, we've looked to see if we can achieve something with the ASHP but we cant get what we need. The office is SSW facing and gets hot in the summer - agreed we wouldnt want it for many months, but it will be worth the investment i think.

Just to have cooler bedrooms will be great too - although again its probably only 2 to 4 months.

i'll give you some proper figures if i can. thanks Simon,.

Posted by: @sjc123456789

↑

@jamespa 

Hi James, I'll have a good look at the report and find you some proper details - apologies.

Its not like a newbuild slab, the house is made up of the original house then a couple of extensions  so the floor is a bit mix and match, the floor surface is all level, and there will be 100mm of insulation and then the heat spreaders all over the house. the cavity under the floor varies in depth, old and new. I think we've about 25 to 30mm surface we can put on top of the spreader plates.

 

Yes, on the aircon, we've looked to see if we can achieve something with the ASHP but we cant get what we need. The office is SSW facing and gets hot in the summer - agreed we wouldnt want it for many months, but it will be worth the investment i think.

Just to have cooler bedrooms will be great too - although again its probably only 2 to 4 months.

 

i'll give you some proper figures if i can. thanks Simon,.

All noted.  Given the construction I would be wary of relying on 'batch heating' so a heat pump well matched to the loss is desirable.  If you are intent on fitting aircon as well then that offers an opportunity to take a bit of a risk with sizing of the heating - if it turns out to be a bit undersized on coldest days, just turn on the aircon! 

Remember that the load on most days will be half the peak, so really its relatively rare that you need full capacity.  If your heat pump is say double the actual peak, that makes it four times the typical which means it will be cycling most of the time, an undesirable state of affairs.

@jamespa hi James, yes, I want them to confirm the surveys before finalising things.

thanks Simon,

@jamespa 

Agreed on the batch heating and aircon.

Isnt specifying the ASHP the same as choosing the engine option in the car - I'm sure I need the 650bhp twin turbo, although I only need 27bhp to maintain 70mph on a flat motorway. 🙂

thanks Simon.

Posted by: @sjc123456789

↑

Isnt specifying the ASHP the same as choosing the engine option in the car - I'm sure I need the 650bhp twin turbo, although I only need 27bhp to maintain 70mph on a flat motorway.

Sadly no.

ASHPs (and boilers in fact) are most efficient if they are working and producing heat for long periods, preferably continuously.  Unfortunately ASHPs have a limited 'modulation range' - the ability to 'turn down' when the load is reduced.  Most have a practical range of 3 to 1 or thereabouts, some less.  When the load is less than the minimum they can't turn down to match it so they are forced to cycle, ie switch on and off, to reduce the average output to match, a behaviour called cycling.  

This is not efficient.  To use the car analogy its like your 100mph car has a minimum speed of 35, so if you want to travel at 20 you have to do it in stop start mode.  If you are travelling in a built up area this is pretty inconvenient and also inefficient and you would be better off with a car that has a max speed of 70mph or even less!

Boilers have the same problem.  They have a much larger modulation range than ASHP but are typically really grossly oversized and thus also often cycle much more than they should, which reduces efficiency, but we tend not to care so much. 

All ASHPs and boilers will cycle at some point (when the OAT is high enough) but if an ASHP is cycling at typical winter temperatures of around 7C in SE England (where it spends most of its time operating) then the performance degradation can, it appears from the limited evidence we have, be 10 % or even a lot more if the oversizing is too much.  Some ASHPs are better at dealing with it than others and extra system volume helps (hence volumisers), because it increases the cycle period, thus giving the ASHP more time to get into an efficient operation state.  Cycling also increases wear on the compressor, which is why most ASHPs have a mechanism to stop 'short cycling', generally taken to be cycling where there are more than 6 compressor starts per hour.

You can reasonably assume that your ASHP will do most of its work at half the design loss, so an ASHP in a house with a 7kW loss will do most of its work operating at around 3.5-4kW.  Ideally you don't want the ASHP to be cycling at that temperature, which places an upper limit on the ideal capacity.

There is a further wrinkle to be aware of, namely that ASHP manufacturers often produce fewer hardware variants than they produce models, and many of the models are simply higher rated models derated in firmware.  This derating reduces the max output but doesn't reduce the min output relative to the higher rated model on which it is based.  So for example the Vaillant 10kW is a derated 12kW, and the 3kW a derated 5kW.  The 12 and 10kW models have the same min output, as do the 3 and 5kW models.  The main Daikin range has only 2 real hardware variants, ~8 and ~16kW.  They sell many other sticker ratings but they are all derated versions of either the 8 or the 16, an even coarser approach.

Realistically in your case the question probably reduces to 'is 7-8kW sufficient?'  If it is then a wide selection of models around 7-8kW actual (ie not artificially derated) capacity exist.  If its not you are into the 10-12kW class which are incidentally often, but not always, double fan.  I faced exactly the same dilemma but fortunately was able to collect measured data from my gas boiler to resolve it, perhaps not beyond all reasonable doubt but certainly better than balance of probabilities.  I took the risk and went for the lower rated model, knowing that if it was a bit underpowered I could resort to either a backup heater or the log fire on the few days when its really cold.  The gamble has paid off for me, the system copes down to -5 and only starts cycling at +11 (which is about the best that can be expected in the UK given current ASHP modulation ranges).  Of course that doesn't mean the same gamble would pay off for anyone else.

Hi James, thank you for the reply, that makes a lot of sense - very much appreciated and plenty to think about. I'll break out the spreadsheet.

I'll be honest, it didn't occur to me until you pointed it out about the model makeup and manufacture of the ashp and how they are ranged.

I'll check what we looked at with the Aircon for upstairs (Inc office up there). I understand we can use that for heating also if necessary, as the ashp can cool to some extent if needed - just have to work out efficient ranges.

My wife is also looking at an electric "feature" fire, we got one for a rental a while ago and it's not offensive, so we will have a couple of other heat sources.

We'd both like a wood burner, but to be honest it just wouldn't work for us I'm afraid.

On heat sources, the towel rails/radiators in the bathrooms are hybrid with electrical heating too.

Lots to think about, hopefully the solar generation and batteries will give us a bit of headroom to ensure we can run it all in the sweet spot.

Thank you. Simon

Posted by: @sjc123456789

↑

@batpred Hi Batpred, thanks for the reply.

The current system is made up of a 2 x 5kw inverters in parallel to give 10kw. The 16kw of batteries have a throughput of 5kw each, so i think the limiting factor is 2 * 5kw - I'm no electrician as you've probably guessed. The export is clipped to 8kwh

The equipment is Fox, not sure on the model on the inverters but the batteries are 4300's

I'm paying 7p at night (23:30 - 05:30) for electric and export at 15p.

Just thought of mentioning that Octopus has some special tariff, Intelligent Flux, that involves giving them some control over the system and from memory, I think it works with Fox. Those peak export rates are unbeatable..

Posted by: @sjc123456789

↑

I was under the impression that the panels were the cheaper part of the system, the current system's ROI is about 8 year, I've not fully worked it out for our new system, but I will.

Perhaps if you are adding PV, it is a lower cost per kw than usual. Being able to export at peak times with an agile or similar tariff may work well. In some cases, like where more storage can be added for around £70/kwh, it will help both net0 and ROI! 

We have recently completed rather similar on a slightly larger floor plan, but minus UFH.

Before deciding battery capacity, get your heat pump unit decided first. Go onto heat pump monitor and find your unit and you can see representative usage over different outside air temps. You will be shocked at how much power the larger units consume on a cold day. I’ve seen folks using my unit consume approaching 200kwh a day, which gives me indigestion just thinking about it. We are frugal with our heating but at -5C we still chewed through 80kwh. My point being you’d have to have very large capacity batteries to get you through 24 hrs.

Our compromise is to have smaller batteries and cycle them 3x a day on a multiple time TOU tariff like OE cosy.

Be so careful with your choice heat pump contractor, I thought I was cautious but I still had troubles. Furthermore, assume they will stop trading within weeks of finishing your install. Mine did. 

Posted by: @papahuhu

↑

.. You will be shocked at how much power the larger units consume on a cold day. I’ve seen folks using my unit consume approaching 200kwh a day, which gives me indigestion just thinking about it. We are frugal with our heating but at -5C we still chewed through 80kwh. My point being you’d have to have very large capacity batteries to get you through 24 hrs.

200kwh..  Would that be warming an outdoors tent or a castle? 😀  I prefer the image of a castle with cold stone walls..

Now seriously, ASHP in most reasonably well insulated homes with a suitably low flow temperature seem to consume a lot less?