nfoote

Ok great, seems pretty clear cut, thanks!

We're currently undergoing an extension to our detached house which we have planning permission for; converting the GF garage into living space and demolishing an existing FF extension above the garage and replacing it with a larger FF extension the entire length of that side of the house. The original house was built with a room in the roof space and the previous owners added a rear dormer for another bathroom. While the new FF extension roof ridge is lower than the original house, to get the extension its subservient appearance, we've realised now that the roof trusses are up there is still a lot of potentially usable space in that new loft area if we cut into the existing in-roof rooms and re-organised things a bit as a large combined space. The builder has already added proper floor joists through the area thinking it would have been storage. All of that could be completely internal works, but wanting to make the most of every possibility I'm now wondering if we're able to add another rear dormer into the new extension roof. It'd be smaller than the existing one but would add more usable full head height. I'd think we could do that under permitted development at some later point, but while the builders are here and the scaffolding is up it would make most sense to just get it done now, even if we don't actually fit out the internal space as a room yet, just get the dormer done and dusted. So question is, can we add things into the scope of the build that could be done under PD later on, even though they weren't included in the planning permission detailing the current build?

Yes GSHP proposal is a borehole, the Ecoforest would be 3 holes at 125m deep while the Nibe only needs 2 holes, thus being a bit cheaper. Our 2020 monthly energy usage is as follows; E kWh G kWh January 671 6903 February 553 6288 March 573 6348 April 539 3232 May 496 1603 June 502 790 July 493 716 August 492 649 September 500 919 October 567 3236 November 527 4370 December 654 6582 2020 Total 6567 41636 Airtightness is probably going to be hard. We're replacing most of the windows and doors but are not sure what to do regarding wall insulation. Apparently at least some of the GF cavity is filled with rubble so perhaps internal wall insulation for GF and FF is the way to go, although loath to lose the floorspace. External insulation might be hard being in a conservation area and the FF being mock tudor.

The heat survey they did including measuring the rads we already have, all but a the bathrooms and one other apparently were large enough to handle a lower temp, which seems to make sense as we don't run the thermostatic valves in most rooms beyond 3/5. That's what I'm feeling really. Chucking in the bit about the CCL which took all of 20 seconds on Google to determine doesn't apply to domestic didn't help their case ...

I guess the logic there is the GSHP is more efficient, under the Nibe at SCOP of 4.17 vs a max of around 3.5 on a ASHP? Plus the potential noise from a ASHP with our only placement options being right beside the lawn or on the patio doesn't seem attractive.

Yes we have the sales guys calculations, basically they're coming up with (including the extension which they have the plans for) heating at 37000kWh + hot water at 8000kWh using a SCOP of 3.54 at flow temp of 50 degrees, with gas at 4.5p/kWh and electricity at 12p/kWh coming in at £1746pa on a 22kW HP vs £2247 on a 92% efficient gas boiler. We seem to be running the house a bit cooler than their models predict but either way the big difference appears to be their gas and electricity pricing assumptions, we've paying far less for gas and a fair bit more for electricity, which completely wipes out their predicted £500 saving and turns it into an actual cost! Yes, we're on mains gas. The thing is I personal want it to work, it sounds cool and is greener etc, but the numbers at the current prices aren't stacking up. Other members of the household however seem fixated on the future, possibly rightly so, and that gas prices will rise thus creating the running cost savings the sales guy predict.

Sorry if this is not the right place for this, we're just desperate for some outside opinions that aren't so close to the problem and aren't trying to sell us something! So we're pretty torn over what to do regarding installing a heat pump, probably a GSHP, or not bother, during construction of our extension, which requires moving our entire boiler/pump room anyway. We've got a 300m2 1890s detached house in the South East. Empty cavity walls on the ground floor and solid walls on the first floor. Loft has been insulated. We're building an extension at ground and 1st floor to one side which will be insulated to current regs, plus the kitchen has already been extended 10 years ago to the rear. So basically 3 out of 4 elevations on the first floor remain original, while 2 out of 4 at the ground floor are original, that is to say poorly insulated. We've had HP sales guys around who have done their heat lose survey and they're coming up a heating + hot water demand of 46,000kWh and us needing a HP with in excess of 17kW capacity. They're suggesting either a 22kW Ecoforest unit or a 12kW Nibe unit while keeping the gas boiler in a hybrid system (ie boiler takes over at a certain temperature when the HP won't handle the load). The Ecoforest is coming in at around £52k with £31k RHI and the Nibe at £44k with £33k RHI. The sales guys headline figures using electricity and gas pricing they've pulled out of the air come up at saving around £500pa on running costs. Not a huge amount with £10-£20k install costs remaining to pay off. I've followed along on the calculations using our actual figures from last year (which don't include the new extension obviously). In 2020 we used 41636kWh of gas, basically all for space and water heating. We're currently paying basically 16p/kWh for electricity and just under 3p/kWh for gas. If the boiler is 92% efficient and a Nibe GSHP has a SCOP of 4.17 we're needing about 9100kWh of electricity instead of all that gas. All in all I'm coming up at actually costing around £300 more a year! We're struggling to see what the point of installing a heat pump in the house, given its age, and retro fitting etc etc. The sales guys are touting things like the Climate Change Levy coming in to tax gas thus making it far more expensive (as far as I can tell the CCL is business only tax?). Attempts to insulate would benefit both gas and heat pump in terms of lowering running costs, although under a hybrid heat pump system the backup gas boiler would kick in less often I guess. We've also looked at PVs (might get a 3000kWh system in, not much free energy for the install cost of around £7k) and storage batteries to work with both PVs and variable tariffs like Octopus Agile (which I worked out given a sample few days over Jan 2021 might save us an average of £1 a day, again, maybe 20 years to pay off install cost at todays prices, far in excess of their warranty). On the flip side, gas prices are apparently set to rise even without taxation and we might have to install a secondary boiler in the extension (actually an annexe) anyway so there's £5k install cost anyway and more running cost. Or, do we wait a few years to see if the GSHP install costs come down or the efficiencies go up? The fear there is the government subsidies might disappear. Does anyone have any advice, we're pretty conflicted!?!