ASHP Modulation split dwelling

[jamesdiyer]

Hi all

 

Great resource here and knowledgeable people, so thought I'd sign up for your thoughts.

 

We don't have gas, and I want to move away from our big wood boiler. ASHP seems a good option however...

 

We have three floors. 

Floor 1: Living space (has fireplace)

Floor 2: Bedrooms

Floor 3: Independent annex (could have fire place)

 

We will see -7C or so in winter. The annex will not always be used so does not necessarily always need be headed - though it might need be heated somewhat always, so that the delta isn't too great and can be heated to 19C if needed for use. Floor 1/2 are always lived in and need heating. 

 

So I come to modulation. Floor 1/2 seem to need around 10kw and floor 3 around 4kw. So to power all three we would need 14kw (give or take).

 

The issue (in my mind) would be on mild days (mornings can be very cold by we are south facing so by mid day quite pleasant) where floor 3 does not need heating (empty) but floor 1/2 do and a 14kw heat pump is needed to run at a low %.

 

I am struggling to find good technical data on minimum power input/output for some pumps (as I've seen noted here in other discussions), which seems to be my critical point.

 

I do wonder what the optimum solution is. 

• Two heat pumps?
• One unit and heat Floor 3 anyway on mild days
• One unit and accept some cycling will happen on mild days
• Don't use ASHP on mild days but use floor 1 fire place for heat.
• Use heat over night and morning and turn off during daytime letting internal temp reduce. 

 

I guess overall I lamenting the lack of min output modulation infomation. Maybe I am over worrying a non issue. 

 

 

Thanks

 

[JamesPa]

Heat rises so, unless there is a fairly serious thermal barrier between the second and third floor, I don't think you will maintain much of a temperature difference and thus your whole house heat loss probably wont be that much less if you don't heat the top floor than if you do. 

 

There is a good argument that zoning really does'nt help much or even may hinder with ASHPs, because the whole house heat loss doesnt reduce much (the thermal envelope is still the same) but you have to turn up the flow temperature to compensate for the fact that you are essentially trying to heat the same amount with fewer emitters.

 

With a long, thin, property then it might make sense, with a top floor you don't want to heat, its very doubtful that zoning will save anything material and it may end up costing you more. 

 

What sort of insulation standard is the house and how thermally isolated is the top floor?

 

Most major manufacturers do give minimum outputs but often buried in technical data books which can be found but not always easily.  Sometimes you have to ask their tech support lines.  Those who dont should probably be avoided.   

[sharpener]

You don't say what emitters you have got on your wood-fired boiler system which would  be useful info.

 

My initial thought is not to try and size an HP for the full duty for the reasons you state. As @JamesPa says, if Floor 3 is indeed above Floor 2 you may find depending on the construction of the floor that is is heated from below anyway.

 

Maybe plan to heat the core from the HP and in extreme low OAT either run the fireplace, or if necessary add an A2A HP for Floor 3 later.

 

The problem may be in convincing an MCS installer to size it like that, bc to get the BUS grant it must be designed to heat the whole house from the HP and installed by an MCS installer.

 

I eventually got mine to adjust their assumptions a bit, their initial quote for my 200 sq m barn conversion was for a Stiebel Eltron 15kW unit. Here is some data for the Vaillant Arotherm Plus 12kW which is what I am now planning to fit.

 

Water temp 40 - 45C. Other tables available here.

 

 

You will see the modulation ratio varies with air temp from 25 to 50%, that is pretty typical. Mitsubishi publish similar tables, but they are hard to find for low end products. Avoid Samsung, they quote a wide turn-down ratio but use hot gas bypass to achieve it which wrecks the efficiency.

 

The Vaillant spec is quite generous and it actually puts out 12.5 kW at your -7C. As you can see from the table below it will even do 14.7kW at OAT = -2 if your emitters are sized for W = 30 - 35C.

 

 

There is also a 10kW version of this HP but it is the same mechanically with the output restricted by limiting the compressor speed to 85 rps so not much cheaper and the ratio is worse.

 

 

 

 

Edited March 5 by sharpener

[JohnMo]

Although heat pumps modulate, that isn't the only means they match output to heat required, they also cycle. Your cycles really need to be a minimum on time of around 10 minutes.

 

So for example if your heat pump modulates to 6kW, you need an engaged volume of water of around 100L. This can via limiting zones (no small zones) or via a volumiser.

[jamesdiyer]

Thanks for the input all some good thoughts. 

 

We have 3 pannel radiators on 22/15mm pipe from the wood boiler, so moving to ashp they should be ok, they're sized ok (having modeled on heatpunk at a 40-45c flow rate).

 

The house is 800mm stone wall. The floor 3 is stone, insulation, brick with well insulated rafters. The inner floors are non insulated concrete slab. Though I had considered insulating between 2/3. 

 

Each floor is about 85m2 mostly ontop top of one another. 

 

So yes, perhaps just ways heating the loft works out. Intend to run the system open with no buffer, it's a large system. Was more to shut off supply to floor 3 when not in use. 

 

 

Thanks the the modulation tables and cycling information, I figure I can work out system volume and see. 

Edited March 5 by jamesdiyer

[sharpener]

22 minutes ago, jamesdiyer said:

Was more to shut off supply to floor 3 when not in use. 

 

Lots of ways to do this. If the pipe runs are suitable a two port valve is simplest, could power from a Shelley or similar wireless gadget to save a long/difficult wiring run. Set up as a separate Circuit on the Vaillant controls, can then have its own time/temp schedule. Or wireless zone valves or TRVs on the rads, Tado are probably better than my Honeywells (could hardly be much worse!).

[JamesPa]

23 minutes ago, jamesdiyer said:

The inner floors are non insulated concrete slab.

Thats going to be a pretty good conductor vs your walls and roof which are pretty good insulators.  This being the case I seriously doubt you will maintain much of a temp diff between top floor and floor below, so you may well get the lowest cost by leaving the emitters on the top floor on, allowing you to reduce your flow temp to the absolute min.  Otherwise you risk having to adjust your flow temp up to satisfy the demand of three floors from the emitter area of two.

 

Unfortunately conducting meaningful experiments isnt easy because you don't have control of the variables (namely the OAT).  I would be tempted to adjust the WC curve with all floors heated, doing this for a year until you are comfortable its as good as its going to get.  Then turn off the emitters upstairs and see if you have to increase the flow temp up to compensate.  If you do have to, you are probably better off leaving the top floor heated.

[sharpener]

1 hour ago, jamesdiyer said:

So to power all three we would need 14kw (give or take).

 

Did that include any allowance for heating the DHW? It can have a material effect on the sizing depending on # of occupants and usage pattern. Also an MCS installer may insist on changing the tank, what have you currently got and importantly what is the coil surface area?

[jamesdiyer]

Thanks. I could insulate between floor 2/3 but only with about 50mm board. Floor 3 will have wet UFH (it's a shell currently to be renovated). 

 

Yes appreciate the thoughts. I'm uninclined to undersize the ASHP as when a big storm rolls in here it will be brutal for 5-7 days, so even with internal fire place to take up some slack, it's a risk. I'd rather be right or just over sized, and look to improve insulate as best possible. 

 

I will have a look at modulation on different brands, but it sounds like it will be similar. I'd be tempted to buy the best modulating one. But ultimately will defer to installing company due to then being able to invoke warrenty if badly sized. 

 

I'm outside UK so MCS etc isn't a concern. 

Edited March 5 by jamesdiyer

[jamesdiyer]

Thanks. It's not 14kw with DHW. Currently got 250w immersion which would likely be kept as a backup and install a new tank linked with ASHP. Normally it's 2 adults and 1 kid but can go up to 5 adults and 4 kids if floor 3 annex is inhabited. 

 

 

[JamesPa]

27 minutes ago, jamesdiyer said:

Thanks. It's not 14kw with DHW. Currently got 250w immersion which would likely be kept as a backup and install a new tank linked with ASHP. Normally it's 2 adults and 1 kid but can go up to 5 adults and 4 kids if floor 3 annex is inhabited. 

 

 

Have you got any figures for your current gas consumption  to sense check the installers figures.  They can get it terribly wrong.  14kW is actually a rather high figure (but of course I dont know what your climate is or the floor area so this statement could be nonsense!)

Edited March 5 by JamesPa

[jamesdiyer]

Spanish Pyrenees at 1000m. So cold like Scotland. Each floor is about 80m2. No gas to measure, we have a huge wood boiler, but running it at 50C keeps the house quite warm. I feel I modeled it quite accurately in heatpunk (I'm a civil engineer so not unfamiliar with it) and got 12k iirc. I said 14 give or take because I modeled at 18C everywhere and didn't include one currently unheated room in modeling, plus hadn't accounted for DHW if needing to. 

Edited March 5 by jamesdiyer

[sharpener]

15 minutes ago, jamesdiyer said:

plus hadn't accounted for DHW if needing to

 

47 minutes ago, jamesdiyer said:

can go up to 5 adults and 4 kids if floor 3 annex is inhabited

 

A full 200l tank of water takes about 9 kWh. For 9 people you should allow 9 x 45l so at least twice that. With a 12kW HP it would spend 1 1/2 hours in 24 heating the water which is a big dent in the heat avail for space heating.

 

51 minutes ago, jamesdiyer said:

Currently got 250w immersion which would likely be kept as a backup

 

Gonna take a v long time.

 

[jamesdiyer]

16 minutes ago, sharpener said:

 

 

A full 200l tank of water takes about 9 kWh. For 9 people you should allow 9 x 45l so at least twice that. With a 12kW HP it would spend 1 1/2 hours in 24 heating the water which is a big dent in the heat avail for space heating.

 

 

Gonna take a v long time.

 

 

Thanks good food to consider when specifying a ashp size, and good reason to not under spec. Yes, when I say backup, it could mean a 250l tank running separately for the loft annex or running in tandem with the main system. Night electric is cheap here so easy to heat up over night.