This forum has been a huge help for us thinking through our renovation project, but we are stuck on choosing a CH and DHW heating option and thought we'd post to bounce our ideas around and see what the community thinks.

We have taken on a 19C random stone rubble end-terrace in Devon in need of deep refurbishment. 600mm rubble filled walls, no central heating, off gas network, concrete floors, cement external render, south east facing. It is 66m2 and we are planning a replacement 40-50m2 extension (see sketchup pic of proposal).

We are renovating the main house first before undertaking the replacement extension. So far we have stripped hard plaster, installed 5kW wood burner in the living room, installed 30mm expanded cork IWI to living room using lime (not enough room for more unfortunately).

We are looking to install cork flooring on ground floor to avoid replacing the floors, installing double glazed casements, moving bathroom from extension to ground floor and as we have to cut into the slab for drainage replace a small portion with insulated ufh. The extension will be dual pitched, timber frame, insulated as much as poss, would suit ufh and has capacity for 7/8 SE facing panels on the roof (PV or ST). The bathroom will be happening in the next few months so DHW will need to be installed soon and I think we'd struggle to fit a thermal store in the main house.

We are stuck choosing a DHW and CH system, we've whittled it down to two options we'd really appreciate some advice on:

Cheap and simple option: immersion unvented cylinder for DHW boosted by PV or ST on extension roof when it happens. Woodburner centrally in extension. Electric on demand ufh in extension (maybe 30m2 so 3kW@100w/m2) and radiators upstairs in main house. We can store wood and we like the aura of a burner, but running two burners could become tiresome when their charm wears off and then relying on elec can be expensive and inefficient. We can install the ufh under deep screed so it works as a thermal store and can work during the day with PV although we might be asking a bit much of 7/8 panels (there's a future gara/shed we could put maybe 3 on). E7 could help keep costs down, but I need to do more maths.

Expensive option: unvented cylinder for DHW boosted by PV. ASHP and second cylinder running wet ufh heating in extension and extended to large rads in main house. We can probably find a spot for the unit, but I fear it will be expensive and not as efficient as promoted, as well as having ongoing maintenance costs. The cost of ASHP and PV does erode the budget somewhat drastically and while the house should do us for a long while, we'll likely want to move at some point.

That's where we've got to with our thinking.

Any thoughts or ideas from the community would be greatly appreciated as we try to navigate this minefield.

Cheers

Thanks Peter, that's really helpful insight.

There's the option of doing a small amount of EWI later in the project, and combined with 30mm IWI I'm hoping we should get to a good level of insulation. Extending the eaves is problematic because we are terrace (not immediately apparent from the image)

We haven't got the headroom for 30mm insulation unfortunately and are not excavating the floors, just because of the potential problems this could cause with foundations and the cost. So we're squeezing in 10mm which should help take the edge off. We are excavating drainage for a downstairs bathroom, which is where foundations are likely deeper so we'll try to get an insulated slab in there with ufh depending what we find.

I think I'm leaning towards what you've recommended, but to mitigate cost a little, installing rads and underfloor heating so they can be connected to an ASHP at a later date when funds allow and/or I can find a cheap one somewhere.

Any other thoughts, much appreciated.

Having dug around the chimney hearth and alcoves we began finding the foundations quite shallow - about 25cm down. The insulated slab build-ups require about 60cm excavation from ffl (from memory). This does risk disturbing our foundations and I really don't want to be getting into underpinning. But there's the option to revisit this when we excavate the drainage channels and see how deep foundations at the other end of the house are.

I think your heat requirement estimations are about right. We heated the main house to an ok level (16-18C) over winter (temperatures around 1-5C) with our 5kW wood burner burning hot for an hour or two and then dying down. The place has very little air tightness. Generic calculators put the main house at around 7-10kW uninsulated, for which I've assumed our rubble cavity walls are about as insulative as a standard brick cavity - they could be a bit better if the SPAB u-value research is anything to go by.

Posted By: ag1Having dug around the chimney hearth and alcoves we began finding the foundations quite shallow - about 25cm down. The insulated slab build-ups require about 60cm excavation from ffl (from memory). This does risk disturbing our foundations and I really don't want to be getting into underpinning.

One possibility, apart from just leaving the area below the wall alone (45° slope and all that), is to dig out a small area and add some extra foundation material there (concrete with rebar sticking out might be a favourite), wait for it to go off and then dig out the next small section. Rinse and repeat. Can start in more than one place at the same time, of course.

Posted By: ag1I have to say there does seem a lot of unnecessary complexity and expense in heating a tank of water and circulating it round a house.

Hi Alex,
I reached much the same conclusion, and when my log-gas biomass breathes its last I may rip out all the wall hung radiators, or at least partially.
I haven't noticed any difference in the air quality but then I live in a fairly roomy open plan, and non airtight home. Even if that were the case, a simple automatic air humidifier would be all it takes.
Mine is a ducted system, largely chosen for aesthetic reasons, but wall hung indoor units are becoming more and more design attractive. Ceiling cassettes which once were only suitable for commercial suspended ceilings with 600 x 600 tile modules, now I think, are available for smaller domestic joist spacing, I can't remember which manufacturer though.
Look at COPs, some can now achieve in excess of 4 and even 5 for both heating and cooling. Multi splits are OK provided you are happy with one central control, otherwise go for two or three smaller singles.
On start up there is some noise but not too intrusive, once running and the home is up to set temperatures they modulate and are fairly unnoticeable.
Much of the pipework can be accomodated in external ducting and in ceiling voids,- plan carefully.

I'm waiting until mid June to get an annual electricity comparison 2018/2019 ( pre heat pump ) to 2019/2020.

Cheers both.

Will, did you get building regs sign off for that floor relpacement? We had been advised we'd need to excavate and then build back up with a terrifying amount of scalpings. But from what I've seen the soil is very compact down there (barely takes a thumb print), but with some clay content. 30cm wouldn't actually be that bad. Did you go for an insulated slab or floating floor with that build-up?

Thanks Will, it would have to be without building regs if we were to do it like this then. Not impossible, but would make things harder if/when we sell.

Does anyone know of any websites for comparing different single and multi split systems? I've got a handle of the basics, ventured on to manufacturer's web and found them full to the brim with inane marketing jibber jabber and mostly useless for getting to grips with different systems. With the exception of Fujitsu who actually seem quite sensible. I know we're going to have an installer to help us design and spec, but I do like having a pretty good grip on things first so I don't get the wool pulled over my eyes. Would also be good to have an independent body's view on efficiency, if there is such a thing.