Is ASHP still the way to go?

[roblane321]

Perhaps not quite the only question but it gets me started.

Have been working on retro fit / converting an old single skin block outbuilding at home (cornwall) having been given PP to turn part of into an Annex. The time arrives to make a final decision about how to provide heating / dhw

 

For context this is a single double bed, bathroom (shower only), entrance hall and kitchen diner for a total of about 50m2 (13 x 4) floor space. We've put on new roof with 100 between rafters and 50mm PIR. Wall insulated inside and out 50mm + 70mm PIR and then timber clad.

Original floor slab has (only) 70mm pir on top currently leaving 65mm for further build up (50mm ? screed + 15mm finished floor). Doors, windows and roof lights all 3G.

 

Yes it would no doubt have been better to knock the whole thing down and start again / especially to get more depth of insulation in the floor but that's not where I am now

 

Expectation is permanent single occupancy - a genuine granny..

 

The plan had always been to go UFH and with my eco futures head on preferably with ASHP despite not being eligible for BUS but the quotes (12k for 5kW ASHP / UFH system to 25 K same inc all the DHW work) have me seriously wondering if I should consider alternatives or just get on with it...

 

We've no mains gas and I'm not interested in oil (main house is oil and at some point we'll try to change from that to get BUS). so what does that leave?

 

• Bottled gas with combi boiler for all DHW and UFH?
• Bottled gas with combi boiler for all DHW and Rads  substitute screed for other 40+mm floor PIR for floor?
• All electric (No UFH except in bathroom  substitute screed for other 40+mm floor PIR for floor 
• Stick with ASHP UFH but do as much as I can myself and just get in whatever necessary external skills (Not entirely sure what that would really mean).
• Stick with ASHP but switch to rads and add another 40mm PIR to floor

 

 

 

Ignoring the upfront cost I assume ASHP is the most cost effective from a run state expenditure (~1300 v's ~3K for all electric @ 26p\kwh?)

 

From reading on here for the UFH design seems like it would probably best just to go for a single zone?

 

Any comments / thoughts / alternatives..about things to avoid from design perspective? (and so leading to requesting spec changes in h/w)

 

Ta

 

Heating.pdf

[Temp]

Ideally you want a bit more insulation in the floor if you have UFH.  We have 80mm and would fit >120mm if building again. It works but in our first winter we had to turn up the flow temperatures - not an issue as we have an oil burner. 

 

What will the floor coverings be? With UFH Stone and tile works best, then Engineered wood and lastly carpet.  The TOG value of carpet and underlay should be kept below 2 ideally. Special underlays with TOG of 0.6 are available. Carpet with TOG of 1.4 isn't exactly thick shag pile. Some companies make two versions of their carpet with foam rubber or Hessian backing. The Hessian backed versions have lower TOG but may feel a bit hard under foot. Go test some combinations of underlay and carpet. Beware the small print on order forms allowing them to supply either version at their discretion.

 

Small rooms tend to have proportionately less exposed floor area. Proportionately more is hidden under beds, sofas, book cases, kitchen units etc  than with large rooms. This can reduce heat output from the floor. You might want to think about reducing the UFH pipe spacing to fit more pipe in the floor.. Costs more pipe but its a lot easier to turn down the flow temperatures than fit more pipe later. 

Edited June 25 by Temp

[Temp]

1 hour ago, roblane321 said:

 

From reading on here for the UFH design seems like it would probably best just to go for a single zone?

 

Yes but I would still consider fitting one loop per room. By all means use one central stat so its all one zone but aving a loop per room makes it possible to separate it into multiple zones later if necessary by just adding stats. It also allows you to balance the system if one room is too hot/cold by tweaking the manifold flow rates.

 

In our case we tend to heat the bathrooms for more of the season than the bedrooms so we have the master bathroom on its own zone. The stat is on the wall outside the bathroom with a remote sensor in the bathroom.

[JohnMo]

Get E7 electric, direct unvented cylinder heat by immersion on E7. Heat house with a couple of storage heaters

[roblane321]

@Temp Finished floor will be 9mm laminate with stone / ceramic in the bathroom / hall.  Multiple loops - I see the logic yes  - thanks

 

[roblane321]

@JohnMo Thanks  John - could you elaborate on your thinking a little for me?

 

e.g. Is that purely down to losses through the floor?

 

 

In case its relevant : From electrics standpoint  the house and annex are currently on the same meter. I work from home full time in main house which has small 2KW PV array (21p FIT) that helps with usage through the summer as does Solar thermal. I wonder if we could get the two buildings on different meters? On the other hand we have room for more PV on the main house and have been considering adding separate system with battery so E7 would work better then if charging the battery overnight through the cold months?

 

Sadly the annex is very heavily shaded so no PV option.