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This isn't for anything like a Passivhaus though - it's a retrofit done on a tight budget. If you look online for underfloor heating temperatures, you get links like this which suggest 45C flow and 35-40C return temperatures for a screeded floor, higher for a floating floor. Those will be based on their experience of supplying systems for typical UK housing stock, and seems to be supported by other sources as well - Baxi for instance suggest that a 7kW boiler would be needed (100W/m2). That gives you a floor surface temperature roughly 10C warmer than the air temperature to provide the required heating, which depending on floor coverings will probably give you a slightly higher temperature on the downwards insulation surface.

Umm... one thing to be aware of if you stick with radiators is that the temperature difference between the floor and the outside world will drop a lot - with underfloor I've assumed 30C because the floor will be heated to keep the rest of the house warm. If you're using radiators then 18C is more realistic since the floor will be the surface the radiators find hardest to heat. That moves the peak heat loss down to about 150W or so, and more importantly for most of the heating season it's about 50W or so since the difference between floor and outside temperature is so much lower without the floor being used as a heat emitter. In other words, the £2-300 you've calculated is only valid for underfloor heating - shift to radiators and the value drops to something in the region of £100 - £1-2 per m2. That's why I don't think anything extra is worthwhile unless you're chasing certificates - the cash benefit of doing so is pretty small already. That's why I was looking at other things - chasing the last few Watts is always a painful and expensive job.

I'm not sure how much going the extra mile on floor insulation would really help you. Shifting from 0.23 to 0.12 saves: (0.23-0.12) x (64m2) x (30+3 C) = 230W. at roughly the design low temperature condition (-3C for me) and assuming underfloor heating running at 30C for a heat pump. It adds up over a year if you're going for a particular standard like Passivhaus, but if you aren't then the value of additional insulation is really pretty small. I would also be wary of what is available with a ventilated underfloor cavity - the advice we got from our architects was that the ventilation itself makes improving the U-value up to around the 0.1 level almost impossible, but replacing the whole thing with an insulated concrete slab is horribly expensive, so I think you've done about all that is feasible on a retrofit. You've already discussed external wall insulation and airtightness - realistically EWI should help with airtightness a bit , but I think if you're trying to reduce heat demand then this is the place to go. One thing that might be worthwhile considering if you're chasing SAP points is a shower heat exchanger. That's harder but not impossible in a bungalow (you have to use the drain type rather than the vertical pipe type, which is more expensive and less effective), with something like 50% recovery being feasible. For my current bungalow (3 bed), SAP would probably interpret that as saving 1,000 kWh/year. Fitting it to the only bathroom which would presumably be being ripped out and replaced anyway is probably not too painfully expensive, particularly if it's an alternative to other means of compliance.

Maybe not - nitrogenous fertilizers are critical to high food yields, and they require energy to produce. If you're out of fossil fuels, that means the next best source may well be solar. Turning the energy from a solar farm on marginal agricultural land into nitrates which are then used on far better land will give you more food than ripping up the solar panels and growing without the nitrates.

Umm... can't you get the thermal power by shutting off the UFH pump and measuring the rate of temperature drop (or even just the total drop) in the buffer tank? 70 litres isn't a lot - you're looking at 0.08 kWh/°C - so you should be able to get a pretty good measurement of heat abstracted, particularly since the pipes will be small by comparison.

Thanks. So on that basis you'll never see heating related icing issues for a well designed modern house with slab heating (radiators may need more thought, but they aren't relevant in my case). That means it's only an issue for heating DHW from 40 to say 55°C: that's of interest to me because we're considering a demolish/rebuild on a house we previously only previously considered refurbishing, which means any complicated options aren't going to be financially viable - if you can do hot water off the HP, that's what'll happen. The Clark/Grant paper gives measured hot water consumption in real life being in the region of 25-40 kWh/m2/year - for a 150m2 house that's 13 kWh/day including losses in the primary circuit, etc. That seems to be a bit of an overestimate - for our current 100m2 bungalow the combi uses 160 kWh/month of gas when we're home all summer with the heating off (20 kWh/m2/year of gas - probably about 16kWh/m2/year for actual delivered hot water). Essentially that means the heat pump would only ever risk running in icing conditions for ~1h/day - after which it can defrost naturally - and on that basis I should be pretty safe using the claimed COP figures without making allowances for heat lost to the defrost cycle.

I'm not sure if that's actually a big problem in a well designed house - design low temperature around me is somewhere around -3°C, at which point heat demand will be about 1.5kW. Since it's hard to find a heat pump which can supply pumped heat at a low enough temperature to heat a Passivhaus slab directly (the Samsung units for instance use resistance heat in the 20-30ish temperature range for instance) then it's likely to be cycling on/off. At +2 in damp air, the cycle might be an hour at 5kW followed by 4 hours off to defrost. What you lose in a heat cycle to get rid of the accreted ice you might have gained in the first place by the phase change on the ice releasing latent heat into the evaporator - and even if you didn't the effect over the course of a year may well be quite modest.

One interesting thing I found the other day is a complete set of what are effectively COP tables (heating and electrical power, so you have to work out the COP for yourself, but it isn't hard) for Samsung heat pumps (attached). The impact of low flow temperatures is huge - if it is supplying water at 30°C the COP is still greater than 3:1 even at -20: supplying water at 55°C doesn't achieve this until the ambient temperature reaches about +10°C. This is where phase change materials like Sunamp or thermal stores are potentially very interesting - DHW flows should ideally not exceed 48°C, but stored water needs to regularly exceed 60°C for legionella protection. At 45/2°C (hot water/outside air) the COP is 2.75 - increasing that to 55/2°C drops it to 2, and resistance heating is needed to boost it after that - dropping effective COP down to about 1.8. Same thing applies in summer, although at 20°C the numbers are rather better - 4.85 and 3.2 respectively. That's actually something I want to have a play with in the next couple of weeks if I can find the time with a spreadsheet - heating degree day temperature records are available for download, and Sheffield Solar have downloadable PV generation data. That makes it quite easy to do a crude spreadsheet model of annual COP in a Passivhaus - you know peak consumption is about 10W/m2 of low temperature heat with no solar or internal gains, which gives you a W/°C value and hot water demand can be estimated as well. I've got a internal gains figure of ~10kWh/m2/year from my architects (call it 1W/m2). The idea is then to balance solar gains (assuming no gains when heat is not needed and using the Sheffield Solar data integrated over a day as a measure of solar resource) against this to give the 15 kWh/m2/year total consumption. That then gives a spreadsheet with a couple of years of real world data showing air temperature and thus COP coupled with heat demand on that day. A very similar calculation can be done for a GSHP, just taking the seasonal average temperatures and the associated COP values. I have a strong suspicion that the ASHP will come out with lower power consumption, depending slightly on how much hot water use I assume - the higher the hot water use fraction, the more an ASHP will benefit from the higher summer air temperatures pushing the COP up. Performance-Data-Technical-Data.pdf

Draft excluder, obviously!

Doesn’t that mean whenever the outside temp is below zero you’ll be on resistance heat and be warming up the outdoors?

Yeah, I'm not so concerned about the actual prices quoted - it's essentially a bill of materials and we didn't pay the guy enough to go around and get quotes for everything, and that has a lot of value in and of itself later in the process. What I am more worried about is that it's become very clear that the original plan of extending in stages and fitting out from income isn't viable - demolish/rebuild is clearly cheaper and gets a better result which will have more final value. The QS prediction for a dry shell upstairs, new windows and fitting external wall insulation downstairs was ~80k +VAT, and that would (eventually) get us to EnerPHit: a couple of the timber frame companies recommended on here quoted on the basis of rebuild only at less than that for an insulated Passivhaus shell, albeit with no windows. Sure I could and would trim costs back - no way would I pay £10k for painting, for instance - but it doesn't change the fact that plan A was the wrong way to go. I'm at a Tier 1 supplier, most of what we do goes to Airbus but not all. I qualified in Aero but have been Mechanical ever since.

Heh. I actually work on a subsystem where efficiency is at a huge premium so I spend my days chasing tens of watts around a system about which nobody knows very much. Sound familiar?

That's exactly what we originally planned to do - problem is that the downstairs layout as it currently is really needs a rejig to work properly, and the bay windows at the front are a bit of a nightmare for the upstairs, so you end up retaining 3 outside walls and forcing yourself to do a stick-built timber frame on the top while still paying VAT on everything.

Caravan in the garden isn't an option - the gap between the current house/attached garage and the boundaries on either side is only ~1m or so (the boundaries aren't quite square, and the house is slightly skewed on the plot - that or it's a slight rhomboid, I'd really have to dig out the measured survey to be certain), and the only road access is from the front. That means unless I can find someone with a large helicopter the caravan isn't getting in - and while the kids would be fine with it my wife would be climbing the walls within minutes. Project managing it myself is probably too much - I'm already doing one ~£5m project at work (technical rather than project lead thankfully) and have a horrible feeling that another one will soon end up in my lap. Combined with two young (and demanding) kids and a wife who is struggling a little mentally with going back to work that would be more than I've got the mental capacity to handle. Re-use of the existing foundations isn't a deal breaker if impossible (one of the timber frame companies quoted ~£20k for instance) - existing structure is masonry and we'd probably go timber frame which is significantly lighter so the loads would be at worst comparable. QS estimate was based on a simple upfront fee to calculate the cost of the spec he was given. It's gold plated to a certain extent - £12k for a kitchen for instance, although that is probably justified by the likely selling price for a house of that size on the street - but even if we assume it would cost us half the estimate that's still more expensive than knocking it down and starting over. Getting the spec down to something we could afford would leave us with a big, uncomfortable house - which isn't something we'd be happy with. Plot is long but narrow, all the other gardens are the same - ~15m across I'd guess (current house footprint is 9m wide) - in theory a pair of semis would be practicable, but we'd be left with a long thin garden and the garden is one of the things that I love about the house. For perspective, the tall leylandii you can see in the background of the photo below are ~10m on my side of the back boundary, with mature apple & plum trees beyond them.

A little bit of background for you, we did a fairly comprehensive refurbishment of our last house (keeping the walls, roof, windows and not much else) which gave us a house we absolutely loved. 18 months or so ago I started a new job which was a hideous commute (particularly with two young children), so sold up and finally bought a new place over the summer. I'm a principal mechanical engineer working in the aerospace industry, so have a good grasp of the theory but realistically with two toddlers am not going to get any significant amount of hands-on work done myself. What we've ended up with is a 1920s bungalow in a nice (large) village, with a huge garden and a 1970s extension on the back. Location is great for us - halfway between where we both work, close to nursery and schools and the kids love the garden.The original plan was essentially a glorified loft conversion, with stub walls to expand the usable floor area a bit since the footprint of the original house isn't all that large (55m2 or so). Doing so would also give us a chance to sort out some of the issues with the existing building - solid walls and ventilated suspended floors mean it's pretty chilly/draughty at times. Found an architect we liked, who came up with a scheme which seems to work and which we should be able to get planning for (very similar to what the house next door did 10 years ago, and they're also building 50 similarly sized houses at the moment at the bottom of our garden). That got modelled up in PHPP and it looked like EnerPHit (which had been an aspiration for some time) might actually be possible. Then we got the quantity surveyor report on the expected price for everything we would like to do (phased over a number of years), and that's where the plan sort of fell to pieces. The value came back at ~£400k including contingencies and VAT for a 150m2 end result - nearly £3k/m2 and it also came back with several quotes from timber frame companies saying that they could knock it down and build from scratch for less than the cost of just putting a new floor on, with one noting that they should be able to reuse the existing foundations to save money. As I see it we've got 4 options at the moment: Find quite a bit more money than we were expecting to and go down the demolish/rebuild route. This isn't totally impossible but will be very tough and delay things substantially - our current mortgage has some fairly huge early repayment fees for the next 18 months, for instance, but my wife has just gone back to work so we could probably borrow quite a lot more when it comes up for renewal. Emotionally this is what we'd like to do but I really am worried that we can't afford to do it. We should break even on this option in the long term, but getting sufficient cash to do the work would be problematic. Scale the plans back seriously and do a simple loft conversion complying to building regulations. I don't think this will give us what we're looking for though - in particular we want our bedroom to be on the same floor as the kids (currently 1 and 3), which wouldn't be possible without raising the eaves, which takes us back to Plan A. It would also leave the house uncomfortably cold in winter. Get planning permission then try to sell it on to someone with a bigger budget than us. Unlikely to add much value but would at least potentially mean the work so far wasn't wasted. Live with the existing structure as it is, giving it a bit of a wash & brush up (getting rid of the geriatric wallpaper in the bathroom for instance) and move on in a few years. I'm guessing that quite a few of you guys will have gone through exactly this sort of experience before, so I'm wondering what if any comments you have on our options.