ASHP sizing estimate

[Omi]

I have ASHP quotes from four different suppliers and they do not seem to agree on the sizing required. I don't have the SAP calculations at this time so gave all of suppliers the following information:

• Floorplan and elevation drawings - should be able to estimate wall, roof, floor, and door/window areas from these.
  • Floor area: 210 m2
  • Roof area: ~200 m2
  • External Walls:  ~220 m2
  • Doors/Windows: ~48 m2
  • 4 bedrooms, 3 bathrooms.
     
• Performance target - Scottish Building Regs

  • For external walls:                             0.22W/m²K

  • For roofs:                                            0.15W/m²K (Insulation between joists)

  • For exposed floors:                           0.18W/m²K

  • For windows, doors & rooflights:    1.6W/m²K
     

• Construction - timber frame w/ external wall insulation
• Heating - UFH throughout
• System to provide 100% of heating and DHW
• Location of build - Scottish Borders

 

With this, the estimates I have are:

• Supplier XK: 12 kW + 180L
• Supplier GR: 12 kW + 500L
• Supplier RG: 10 kW + TBC
• Supplier BU:   6 kW + 210L (Outside air temp: -5 ℃)

 

Based on the information I've received from the suppliers, the one that come up with the 6 kW sizing (supplier BU) seems to have put in the most amount of effort into the process and sent through documents showing the calculation/assumptions made and therefore gives me the most confidence at this time. This is also the same supplier that sized and fitted @Redoctober's system.

 

Does the 6 kW system seem reasonable? I'm just a bit puzzled since it's half of what two others seem to have estimated.

 

 

 

 

 

 

Edited May 2, 2021 by Omi

[Bonner]

What about ventilation, are you planning MVHR? Uncontrolled ventilations losses will create significant heating load.

[Omi]

17 hours ago, Bonner said:

What about ventilation, are you planning MVHR? Uncontrolled ventilations losses will create significant heating load.

 

No MVHR, the design specification is currently 5m³/h/m² or better if that helps.

[A_L]

19 hours ago, Omi said:

Does the 6 kW system seem reasonable? I'm just a bit puzzled since it's half of what two others seem to have estimated.

 

What shape is your house, Bungalow/1.5storey/detached villa? Even an energy efficient 'cube' will need more than 6kW for CH alone if built to given U-values and 5m³ ventilation

Edited May 3, 2021 by A_L

[Redoctober]

19 hours ago, Omi said:

I

Based on the information I've received from the suppliers, the one that come up with the 6 kW sizing (supplier BU) seems to have put in the most amount of effort into the process and sent through documents showing the calculation/assumptions made and therefore gives me the most confidence at this time. This is also the same supplier that sized and fitted @Redoctober's system.

 

Does the 6 kW system seem reasonable? I'm just a bit puzzled since it's half of what two others seem to have estimated.

 

 

That is correct. BU did install our system which included the ASHP, UFH and DHW tank etc. The science and calculations are sadly beyond me but suffice to say, they would have applied the same process to our build ads they have yours and I have to say, we have not had any issues with it whatsoever. The house has sufficient heat as and when we require it and we don't run out of hot water - even when "guests" were allowed.? For completeness, ours is a 165sq m 1.5 storey house with a 5KW ASHP and 170l tank. Our air tight test, should it be relevant, came in at 4.9.

 

Have you enquired with the other companies as to why they feel it necessary to install a 10kw or 12kw ASHP? 

 

All the best . ?

[Omi]

19 minutes ago, A_L said:

What shape is your house, Bungalow/1.5storey/detached villa? Even an energy efficient 'cube' will need more than 6kW for CH alone if built to given U-values and 5m³ ventilation

 

It's a 1.5 storey detached house. Interestingly, when I used the RHI estimator, the kWh/yr it came up with was close to the figure estimated by Supplier BU. I understand that it's not the same thing as the kW demand but surely it would be related?

 

2 minutes ago, Redoctober said:

Have you enquired with the other companies as to why they feel it necessary to install a 10kw or 12kw ASHP?

 

Not yet but plan to do so once we have the SAP calculations done. As for yours, do you know how much heat yours is delivering? BU estimated our demand at 3.8 kW.

 

[Redoctober]

4 minutes ago, Omi said:

 

Not yet but plan to do so once we have the SAP calculations done. As for yours, do you know how much heat yours is delivering? BU estimated our demand at 3.8 kW.

 

 

I don't sadly - all this SAP business is beyond me. I just set the timers and temps and away we go. That said, I shall PM you some paperwork BU provided us in an attempt that they may assist you as it seems you understand this "stuff" far better than I do?

[Omi]

4 minutes ago, Redoctober said:

I shall PM you some paperwork BU provided us in an attempt that they may assist you as it seems you understand this "stuff" far better than I do?

 

Brilliant, thanks @Redoctober!

[Bonner]

2 hours ago, Omi said:

 

No MVHR, the design specification is currently 5m³/h/m² or better if that helps.

 

For comparison, my build is 260sq.m, similar spec but with MVHR. SAP maximum heat load is 4.5kW, I will be going for 8-9kW ASHP.