Jeremy Harris

Best bet is to either put the cable in ABS trunking or conduit, so it's kept well away from the EPS by a barrier that won't allow the PVC plasticiser to affect the EPC, or use non-PVC cable where it has to run through EPS (NYY-J is OK running through EPS, for example). Cable ducting is also OK running through EPS, as it's usually made from something like PE. Whether a thin membrane would be impervious to the plasticiser in PVC cable I don't know. It's not something that's in the spec for the stuff, and personally I'd not risk it.

Concrete pads with bolt on post brackets would work OK. Just dig a hole, maybe add a bit of timber shuttering around the top, fill it with concrete (the ready mix bag stuff would do OK), level the top, then when cured drill the top and fit post brackets, like these: You can get galvanised versions of these that are a fair bit better. You can bolt them down with thunderbolts easily enough, or use one of the adhesive bond fixings (we have a row of posts fixed to the top of a wall that are just fixed to brackets like this that are thunderbolted down). You don't need to worry about getting the concrete pads all the same height, just get them pretty much in line. The posts can be trimmed down once they are in place to get the level right.

Yes, if it had been more than about £200 I'd have remembered it!

As above, a hand auger or post hole digger will make the holes OK, as long as the ground isn't too rocky. I prefer the hand auger; the chap that put in some of our fencing preferred to use a post hole digger. Both needed a pry bar occasionally to shift stones. A post hole digger looks like this: And a hand auger looks like this:

We had our plot valued by a local RICS valuer. He came out and looked at it (after we'd levelled it and got services sorted), looked at the plans, worked out the market value of the completed house, knocked off 5% because it was (in his words, not mine) an "eco house" and worked back to get the plot value from that. I can't remember what the valuation cost, but it wasn't overly expensive.

Shouldn't have any effect on the MVHR, as there would be no net change in the MVHR operating conditions. I quite like the idea of recirculating air for cooling, as it should be pretty efficient, a bit like the way setting a car heating/cooling system to recirculate tends to make it perform a bit better.

Modern inverter controlled heat pumps have zero startup current surge. In fact, our has a lower startup than a vacuum cleaner, by a long way. The DNOs are way, way behind the curve with this, and are still working to really old, pre-inverter technology, heat pumps, that did tend to have a pretty hefty starting current. Best bet is to try and educate them. They tried it on with me and I just sent them a detailed description of the 30 second inverter start sequence, how both the starting demand and power factor was dynamically managed by the heat pump control system and then I heard nothing more from them...

Our MVHR cools at around 10 to 12 deg C at the heat exchanger, which is about the same as the flow temperature I run through the floor. The floor surface never gets down to the flow temperature; it seems to sit around 5 or 6 deg C above it, usually around 16 to 18 deg C. Based on that, I'd say that a duct cooler run from our ASHP would probably work as well as the built in air-to-air pump does. The main snag with using the MVHR is the relatively low air flow rate, though.

You definitely will need a condensate drain. Our experience with the Genvex is that there is a constant trickle of condensate from the drain when it's in cooling mode.

Easy enough to do, and probably worth doing. Air cooling feels better, in that the effect is felt pretty soon after it comes on, whereas the slab cooling takes an hour or two to have any noticeable effect.

The window tape isn't the same as the airtightness tape used elsewhere, from what I saw of the stuff we used. The tape used between the vapour tight boards etc was Siga, which is yellow, and the stuff used to seal the windows was white and had a very different look and feel to the airtightness tape used elsewhere. It also had a split backing, so that it could be carefully lined up on the window frames (to avoid it being stuck on too far and end up showing) and stuck down neatly, then the larger backing could be peeled off and stuck to the reveal. My concern if you have air blowing through gaps around the windows is that you may end up with condensation between the timber and window frame. That's not at all good, and really needs sorting properly, IMHO. The chances are that the silicone won't stay bonded well to the timber side, as silicone isn't really the right stuff to use for this application. An MSP sealant might have been OK, but even then I think I'd be a bit concerned about movement. The window sealing tapes are designed to seal and allow movement, when they are properly applied. I have to say that I wasn't impressed with our window fitting either, but I chose to not have them seal the windows, and said I'd do it myself. I spent a couple of days going around adjusting the fit of the frames and sealing up around them. I'm glad I did, from what I've heard of the experience of others with some window fitters.

Are there no tapes around the windows? The window tape is designed to take up a fair bit of movement and still maintain a seal all the way around, unlike some sealants that may pull away. If there is air flowing through gaps around the window frames with a timber frame then something's seriously wrong, and needs fixing, and that won't be that easy, as it will mean removing the plasterboard around the reveals and looking to see what's gone awry.

That is something I've been pondering over for a couple of years! Best place I can think of is high up in our entrance lobby, as with a lot of faffing around I think I might be able to fiddle a couple of pipes, plus a drain line, down behind the 50 x 50 battens that form the service void (unfortunately the service void battens run the wrong way...). It would look like an aircon indoor unit, but fed with chilled water. Chilled water coolers tend to be used in big offices, as they avoid having to run refrigerant lines all around the place. We had them in the last place I worked, but passive ones (no fan) that were just big flat plates in the ceiling with chilled water fed through them, like radiators mounted on their sides.

Timber movement isn't really going to have any effect with blown cellulose, as the stuff will flex with any slight movement and airtightness will be maintained. Movement is only very small, too. The only places we can see any movement is around the window frames, where I'd guess there's been around 1mm of movement in places. The tape can easily handle this sort of movement range I'm sure, as it's pretty tough stuff.

I doubt airtightness would degrade for that build up, TBH, as the blown cellulose itself provides a great deal of the airtightness sealing. The question I'd ask is whether or not the vapour tightness will last for decades, as that's dependent on how well the tapes remain sealed. I can see the tapes in our service room, as we didn't bother to board this area out, and after five years there's no indication at all that the Siga tape is degrading. I still have a part-roll left over, and that still seems to be as good as it was when new, too.

I have one of the cheapo hand augers, like this: https://www.toolstation.com/draper-fence-post-auger/p75571 Hard work, and a bit slow, but it does a reasonable job at drilling post holes. I've put in around a dozen posts with it, using this to drill the rough hole, then banging the post in and filling with a bit of postcrete. As @Ferdinand has suggested, using post saver sleeves is a good idea, as posts always seem to rot right at the junction between soil and air, and these sleeves seem to prevent that.

You could fit a duct cooler to do this. However, our experience of running an MVHR with a built in air cooling function from its own integral air-to-air heat pump has been that it's not very powerful. It will reduce the temperature a small amount, but it's not as effective as cooling the slab when it comes to keeping the house cool. I'll admit to being surprised at just how effective slab cooling is, as when I first did it I wasn't expecting it to be as good as it is. I think that fitting a separate fan cooling unit, with the core supplied by chilled water from the ASHP, might be more effective than fitting a duct cooler to the MVHR, just because a fan cooling unit will shift a lot more air, and would work with recirculated room air, which in hot weather would probably improve performance a fair bit.

The loss from flushing cold to hot through the supply pipes should be tiny. If you had, say, two 10m runs of 22mm pipe from the ASHP, then the volume would be around 0.6l, so changing that volume of water from 10°C when cooling to 50°C when heating hot water would use about 0.028 kWh, so with a heat pump COP of 3, the energy used would be around 0.00933 kWh and the cost at 15p/kWh for electricity would be about 0.14p. This ignores the fact that if the cooling was not running then the ASHP would still have to heat the water in these pipes from ambient temperature up to hot water temperature.

The unlucky person was my father in law. He'd inhaled a fibre of what was almost certainly chrysotile some time in the 1960's, he thought, when doing walk arounds pre-flight on the Comet (he was a Comet pilot at the time), which had chrysotile asbestos insulation exposed under the engine cowlings. His death was extremely unpleasant, and not something I'd wish on anyone.

I agree, I was just suggesting that the reluctance of the TF company could be because they don't want to have to faff around making changes if anything other than their specified foundation is used. My experience has been that construction companies are, in the main, reluctant to change anything, and prefer to stick with one way of doing things.

The safer option is to remove the tiles, taking the appropriate precautions and get them properly disposed of, as that removes the risk completely. Burying the tiles increases the risk, as it's not then obvious that there may be a hazardous material in the floor. Anyone could then drill through, or dig up, the floor and expose themselves to a fatal risk. It only takes a single asbestos fibre to cause a very unpleasant death.

Thanks, missed that. That increases the floor temperature, and increases the heat loss to the ground to about 750 W, from the previous estimate of about 678 W. That heat loss power is as much as we put into our house of 130m² to heat it most of the time in winter!

@Jenni The floor heat loss for a heating demand of 7 kW and the pretty poor under floor insulation level you've specified is going to be high, and waste a fair bit of the energy that you put into the UFH. I don't know your floor area, but if you are expecting, say, 30 W/m² from the UFH then you're going to waste around 16% of your heat from the UFH in just heating the soil underneath the house. If you're OK with wasting that much then that's fine, but I'd suggest that an increase in the depth of insulation will make a substantial difference to your heating bill and pay for itself pretty quickly.

I set ours up with a motorised valve in the buffer tank loop (the buffer provides hot water preheat in our system). This valve is normally open in heating mode, but closes when the ASHP is in cooling mode. There's a tank thermostat that can over-ride the cooling mode and switch the ASHP to heating mode, opening the valve to the buffer in the process, if need be, so hot water always has priority.

Sole plate interstitial condensation risk?