My energy delivered in kWh was as follows:

Dec 2777

Jan 3468

Feb 2574

Mar 2390

House is heated to about 21 and it's loss, measured by gas boiler/heat pump, is 7kW.  I can rationalise this with surveyed figures by assuming ach=0.5

Unless you don't keep yours warm, your loss is quite a bit less, possibly a match eg to the 5kW Vaillant or if not the 7.  

Assuming you do keep the house reasonably warm then if you have detailed records plot daily consumption against average daily outside temperature (or degree days) to get a better indication.

Our gas consumption (heating, showers, gas hob as electric oven) was: 

December 25: 1914 kwh

Jan 26: 2231 kwh

Feb 26: 1667 kwh

Mar 26: 1767 kwh

I described our house insulation before here.  We also converted the loft 5 years ago (long story, but we switched to a self managed conversion). But I can confirm the insulation is better than building regs require. 

You are very welcome.

Daikin 'customer support' was essentially non-existent when I was looking for my heat pump.  Basically the answer to any question was 'ask your installer'.  I simply don't understand this attitude.  Fortunately other manufacturers are available!

But as I counted 4 tanks in total in our neighbours', it seems we will need more space than for the hot water tank. And if we need so much, it could be a showstopper..

You need one hot water tank.  Its typically about 500mm diameter and  perhaps 1.3m high, although it depends on what capacity you want (see comment below).  It can go either upstairs or downstairs, in a garage, in a loft, or even in a purpose built insulated shed or (as in older homes) in a purpose built cupboard which is then used for 'airing' clothes/towels etc. You ned to be able to rout a pipe to the outside world with a continuous downward slop, so if its near an outisde wall it helps (there are workarounds though).

There are two expansion vessels about 30cm dia and 30cm high, normally one red and one white.  These can go more or anywhere connected to (a) the central heating (the red one) and (b) the DHW (normally white).  I have one in the loft and one in a garage.  They are often tucked away at high level, or at low level in some available space.

You may need a volumiser, it depends on your house loss in relation to the size of heat pump you can conveniently have.  50L is usually plenty, it needs to go before the pipework divides to feed the rads/UFH.  They can be wall mounted and in more or less any orientation.

Anything else is superfluous, probably a buffer tank, which you do not want under any circumstances.

Thanks, it is amazing how this info is hard to get even talking to a manufacturer! Daikin in my case, which also has an indoor unit. I think we could cope with it, but it will take some convincing..

Octopus is now giving night electricity for 3.5p per kwh. If this stays, the maths will be very appealing. With another battery..

But as I counted 4 tanks in total in our neighbours', it seems we will need more space than for the hot water tank. And if we need so much, it could be a showstopper..

You need one hot water tank.  Its typically about 500mm diameter and  perhaps 1.3m high, although it depends on what capacity you want (see comment below).  It can go either upstairs or downstairs, in a garage, in a loft, or even in a purpose built insulated shed or (as in older homes) in a purpose built cupboard which is then used for 'airing' clothes/towels etc. You ned to be able to rout a pipe to the outside world with a continuous downward slop, so if its near an outisde wall it helps (there are workarounds though).

There are two expansion vessels about 30cm dia and 30cm high, normally one red and one white.  These can go more or anywhere connected to (a) the central heating (the red one) and (b) the DHW (normally white).  I have one in the loft and one in a garage.  They are often tucked away at high level, or at low level in some available space.

You may need a volumiser, it depends on your house loss in relation to the size of heat pump you can conveniently have.  50L is usually plenty, it needs to go before the pipework divides to feed the rads/UFH.  They can be wall mounted and in more or less any orientation.

Anything else is superfluous, probably a buffer tank, which you do not want under any circumstances.

We had a few initial estimates a few months back, but with little detail. I am wondering the minimum number of tanks that we need to consider and does MCS mandate a minimum hot water tank size for a a 4 bed..

Yes, but there is leeway if the mandated size is not practical so its not absolute.  I think its about 200l for 4 bed (thats what my water tank is and I have 4 bedrooms).  Obviously you need sufficient capacity for your DHW needs but there are games you can play with this also if (say) 200l wont fit.

Hope that helps you find space.   In a 4 bedroom house, even if the bedrooms are quite small, it should be possible if you think a bit out of the box..  

If you say a bit more about house size and construction we its possible to ballpark the capacity of heat pump you need, better still if you have records of gas usage.  This is anyway a good sanity check on the sometimes excessive figures that some surveys produce!

Hi James, we are now evaluating installing a heatpump. We had a look at a neighbours install that have a Vaillant which they was installed above their disused garage, with the tanks in the  garage. We do not that type of spare space. Also as we would be replacing a combi, we need to find space for the hot water tank. But as I counted 4 tanks in total in our neighbours', it seems we will need more space than for the hot water tank. And if we need so much, it could be a showstopper..

We will not have UFH, but will probably have some rads replaced. Pipework should not need changes. Also since we insulated the house to modern standards, I am not expecting a very powerful ASHP. 

We had a few initial estimates a few months back, but with little detail. I am wondering the minimum number of tanks that we need to consider and does MCS mandate a minimum hot water tank size for a a 4 bed..

I believe we always need hot water (200L?), volumiser (50L?, hopefully horizontal is available) and expansion vessel (12L)? Hopefully we do we need to find space for more...

Our current system runs either at a set flow of 35 or WC with the thermostats set to 23, WC mode 24/7 no setbacks and we adjust the WC to aim for room temps of 20. The issue is when have errors on the LG controller 90% of plumbers / engineers insist the strainer is blocked (it isn't), the few that have come and looked at the system just seem in shock at how poor it's been plumbed in and don't want to get involved with it. I appreciate the offer but we have already made the decision to cut our loses and go for a new install.

Fair enough, once a decision is made its best to stick with it.  Just be certain that your new installer doesn't make different mistakes

If the errors are flow rate errors and the strainer isnt blocked it may be a control problem.  With lots of zones (or active TRVs) you can get zones shutting down reducing throughput which causes flow rate errors.  The solution is to operate in a way that means the zones dont shut down, or if they do its only a minority of the total.  Of course because its oversized its expecting a much higher flow rate which is also a problem, so getting a lower capacity unit should deal with this.

@jamespa - Ideally we would leave it in WC mode 24/7 but it would be nice to have the room influence for solar gain etc, I'm not chasing 100% efficiency just a balance between efficiency and comfort. If I can get it to work with the heatmiser that would be ideal - in this scenario let's say the sensocomfort is the thermostat in our main bedroom which controls the upstairs zone, if we have WC on and the thermostat set a few degrees above our desired temp, if we get solar gain and the thermostat turns off the upstairs zone, will the ufh downstairs still get flow from the heat pump?

No, thats the point I have been trying to make.  If you turn the heat pump off you turn it off, just like if you turn a boiler off, you turn it off.  

Assuming you have TRVs upstairs then I would deal with excess solar gain using these so that you don't conflict with the downstairs, but dont overdo the number of rads you have active TRVs on otherwise you will be getting flow rate errors still - presumably only a minority of rooms suffer/benefit from excess solar gain?  That said, unless you work at night, does it matter if a bedroom overheats slightly during the day?

I'm not chasing 100% efficiency just a balance between efficiency and comfort

No need to worry much about 'balance' as, in an unusual quirk of physics, maximum comfort and maximum efficiency can pretty much be achieved simultaneously in most cases by operating as suggested.  Forget needing lots of controls to achieve comfort, you don't.  All you need is to balance heat loss from the house with energy supplied by the heating system, which results in a stable temperature.  Properly adjusted pure weather compensation comes very, very close indeed to achieving that, as well as being most efficient.  The only 'losers' in this game are the manufacturers of third party controls which are in most cases unnecessary and often counterproductive.

Our current system runs either at a set flow of 35 or WC with the thermostats set to 23, WC mode 24/7 no setbacks and we adjust the WC to aim for room temps of 20. The issue is when have errors on the LG controller 90% of plumbers / engineers insist the strainer is blocked (it isn't), the few that have come and looked at the system just seem in shock at how poor it's been plumbed in and don't want to get involved with it. I appreciate the offer but we have already made the decision to cut our loses and go for a new install.

My UFH manifold and UH8 is in a different room to the cylinder, looking into it I think if I want to get the full vaillant setup and controls I would need to replace the heatmiser with something like the VR71 - this would require me to wire an ebus between the manifold room and cylinder room?

You dont need to do any of this and by far the best thing is to make it more simple not more complicated, particularly in a new build house of the size yours is.  Assuming you haven't got a buffer, just disable any room influence due to the heatmiser and any other external controllers and TRVs by turning all the thermostats/TRVs up to full.  Then turn down the weather compensation (heat) curve on the Vaillant until the house settles at the right temperature overall, and balance the system so the rooms settle at the right relative temperature.   

Having set it up this way you can either leave it as is or decide to reintroduce room influence in rooms where its eg to control excess solar gain.  If you do you the latter can experiment with where to where to put the wireless controller (Sensocomfort) that comes with the Vaillant, otherwise it doesn't matter where you put it because its not taking any notice of the room temperature.

Please forget everything you thought you know about controlling central heating and get over the idea that you need lots of controls, you dont and they arent particularly effective in anything other than a mansion, because houses have memory, rooms share heat and too many bosses simply doesnt work.

Its almost certainly the case that your most comfortable and cheapest way to run the house is to set it up to run it without room influence, on pure weather compensation and balance the rads/UFH to get the desired relative temperatures between rooms.  My house is bigger than yours and thats how I run it, many here do the same.  Its never been more comfortable

The thermostat(s) and/or Vaillant controller can be used as a limiting device, set a couple of degrees above the desired temperature, to switch off in the case of excess solar gain.  I did this at the very end only of the first season with a heat pump, when solar gain becomes significant in comparison to house loss.  This season it hasn't been necessary, I have operated on pure weather compensation throughout.

It might be a good idea to read this introductory article   which explains some of what I am saying above with a bit more context.

As an aside and without knowing how you are operating your LG, I would hazard a guess its because you are are using too many controls that you currently have a problem, not because its oversized.  You probably will find a smaller Vaillant easier to get on with and it probably will be more efficient, but Im far from convinced that this is the root cause of the problem you describe.  If you wish to explore that thought further rather than spending several £K on a new heat pump I am happy to do so, but only if you are serious because I dont want to waste time if you have already made a decision!  Bear in mind that its interests of the controls industry to sell you more controls, and in the interests of the plumbing industry to sell you new systems.  Just saying!

Seems like this is becoming more expensive and complicated by the day!

My UFH manifold and UH8 is in a different room to the cylinder, looking into it I think if I want to get the full vaillant setup and controls I would need to replace the heatmiser with something like the VR71 - this would require me to wire an ebus between the manifold room and cylinder room?