Veissmann ASHP buffer required for cooling use??

[MrWhat]

Hi All,

 

I am chatting to an engineer with regards to getting an ASHP via the MCS grant.  He has recommended a 8kw Veissmann Vitocal 150-A heat pump and has told me that it will be necessary to have a buffer installed if i want to use the cooling function of the pump.  Is anyone aware if this is the case?  My underfloor heating capacity is 228 litres.

 

[Dave Jones]

remove all zoning controls (if any fitted), 8kw sounds very large, what do the heat loss calcs say the engineer has provided to you backing up his design of 8kw being required ?

 

 

[JohnMo]

Zero idea why you would need or want a buffer. Other comments as per @Dave Jones.

 

 

[MrWhat]

I have no zoning.  My heat loss calcs are showing worst case 5kw which was LoopCAD and as low as 3.9kw (1 air change per hour)  4.7kw (2 ACPH) using Jeremeys spreadsheet from this forum.  Obviously air changes per hour are a guess at this stage .  He has based this on the build information i have given him at the moment.  He has said that the heat pump looses 25% efficiency on defrost cycle which is the reason for recommending the larger pump.  Its a 5 bed 350m2 ICF build

[JohnMo]

Our house sounds a similar build, we have lots lots glazing 192m² and we have 4kW heat pump. Your calculations sound in the right ballpark.

 

Do not over size, you will take an efficiency hit. important to look at min modulation at around 7 to 10 degs, you basically want to output half your design heat load. So about 2kW in your case.

[SimonD]

41 minutes ago, MrWhat said:

I have no zoning.  My heat loss calcs are showing worst case 5kw which was LoopCAD and as low as 3.9kw (1 air change per hour)  4.7kw (2 ACPH) using Jeremeys spreadsheet from this forum.  Obviously air changes per hour are a guess at this stage .  He has based this on the build information i have given him at the moment.  He has said that the heat pump looses 25% efficiency on defrost cycle which is the reason for recommending the larger pump.  Its a 5 bed 350m2 ICF build

 

Viessmanns always sound too big because of the way they label them. Go and double check the specs directly from either Viessmann or ViessmannDirect because IIRC the 8kW 150-A has a rated output of 4kW at A2/W35, 5.6kW at A7/W35 and 6.5kW at -7/W35. At 7C it'll modulate down to 2.1kW. The output range is 1.8-6.8kW. The 6kW unit does about 5.5kW? at -7/W35 I think. Somewhere there are nice charts published by Viessmann showing performance throughout the operating range at min and max outputs which honestly show the reduction in output between 5C & 0C. 

I don't think your designer is that far off, but slightly hedging bets on the defrost.

[Dave Jones]

3 hours ago, MrWhat said:

I have no zoning.  My heat loss calcs are showing worst case 5kw which was LoopCAD and as low as 3.9kw (1 air change per hour)  4.7kw (2 ACPH) using Jeremeys spreadsheet from this forum.  Obviously air changes per hour are a guess at this stage .  He has based this on the build information i have given him at the moment.  He has said that the heat pump looses 25% efficiency on defrost cycle which is the reason for recommending the larger pump.  Its a 5 bed 350m2 ICF build

 

have you got MVHR or trickle vents ? ICF should be very good for no leakage would have thought.

 

Whats your heating req in m2 in watts ?

 

gut feeling is your 50% overspeccing the ashp. it should never have to run a defrost cycle if used properly. mine never has.

 

[JohnMo]

4 minutes ago, Dave Jones said:

defrost cycle

This is location specific and depends on the dew point locally, and choice of heat pump. My older R32 (6kW) has a much smaller condenser (finned radiator in ASHP) than my new 4kW R290 unit. As a result in exactly the same position needs to defrost way less than previously, moving from once per hour to once every few hours in similar conditions.  We have a large body of water next to the house so local humidity levels are higher.

[MrWhat]

I have MVHR so no vents.  I would hope that I should achieve a 1 ACPH.  If heating per m2 is area / heating load it would be 12.6 watt per m2.

 

We are in north Devon in a valley by a stream.  Its rarely that cold.