Jeremy Harris

Very little. Most of the time the fan on the outside unit of ours is barely ticking over, with no noticeable breeze from it when you're more than a couple of feet away.

I'm pretty sure that @lizzie's house is pretty much all timber, with a part rendered external skin. I cut 150mm holes in our walls using a jigsaw and that made a pretty neat job. The only difficult bit will be the internal holes, as the ducts need to be well-sealed to the vapour tight board that sits 50mm back from the inner plasterboard layer. The way I'd tackle this would be to fit the template to the plasterboard layer and drill long pilot holes right through to the outside. I'd then cut an oversize cut out in the plasterboard, to expose the inner skin. After cutting the duct holes in that, right through the wall, I'd seal up around the ducts inside, through the larger cut out in the plasterboard. I'd then fill in the cut out (which will end up hidden behind the unit) with plywood, screwed to the service void battens, with some added noggins to secure it where needed.

Sadly not. For some reason I didn't opt to log energy use on the house logging system, only environmental stuff. Not sure why, as it would have been easy to do (the energy monitoring system broadcasts instantaneous power use, import or export, every 10 seconds). I do have energy metering on things like the ASHP, hot water system and car charge points, but unless I regularly read the meters and not the readings I can't give a day my day data set. TBH, energy costs are so low that it's sort of slipped off the radar as being something to spend a lot of time on.

I can't realistically see how anyone could take more than half a day to install one of the all-in-one units, like the Unico, that only need two large duct holes through the wall plus a drain hole. As for the power you will need, as we're finding that a 2.5 kW unit seems able to cool at least 150m³ in an MBC house, and bearing in mind that our house is in a very sheltered "hot spot", I would have thought that a unit of the same size should be OK for your needs. The room sizing they quote for these things is based on a typical UK house, with poor insulation, so they will cool a much larger space that is well insulated. The limitation is how far the fan in the unit will "throw" air, but we're finding that ours seems to be fairly even. Right now it's 29.2°C outside, 20.2°C upstairs in the bedroom where the air con unit is and 21.0°C downstairs in the hall (and our hall has volume of around 100m³)

We find that floor cooling, just by switching the ASHP to cooling mode and turning on the UFH, works very well where we have UFH, which is downstairs only. It is a bit slow to react, as it takes a few hours to cool the slab down, but the advantage of that is that we can pretty much get away with cooling the slab only when there's enough sunshine to make doing so free. I wish I'd fitted a couple of fan coil units to the UFH manifold, as that would have given us air cooling upstairs and saved having to fit the air conditioning unit. Retrofitting fan coil units to our system now would have been a great deal more work than fitting the stand alone air conditioning unit. It's one of those things I overlooked when modelling the house systems, as I just failed to appreciate how important hot weather cooling was going to be.

Their installation costs seem high. I spoke to a local (commercial) air conditioning chap and he reckoned that he could install and commission a typical mini split system in less than half a day (he told me he'd fitted one in two hours once). Allowing a fairly generous £300/day, that would mean installation should cost around £150 plus materials (installation materials should be no more than about £100). Appliances Direct state this: £420 for a 9,000 BTU (2.5 kW) unit and wall bracket is for a TCL (Chinese, but with a Toshiba compressor). Might be fine, but I have a suspicion that this is very much a budget unit. The well-known brand name stuff starts at around £600 for the unit, plus fitting accessories. I was quoted £1150, including VAT, to supply and install a Mitsubishi 2.5 kW unit by a local supplier. I baulked at that, as the installation cost seemed high. The Appliances Direct installation cost is even higher.

Yes, that's right. Because the exhaust air heat pump in the Genvex unit (there are other manufacturers of these) uses warmer air that's been exhausted from the house, it can deliver a higher flow temperature than an ASHP that's using cooler outside air. This means the COP is higher when delivering hot water, so saving electricity. It also means that an ASHP for heating can run at a lower flow temperature, probably no higher than around 35° to 40°C, and so also operate with a higher COP than if it were asked to deliver hot water as well. Both are pretty simple to install. A monobloc ASHP is very easy to fit, just flow and return pipes to the heating, a power cable and a control cable. Similarly, a combined MVHR and hot water unit is pretty simple, just the normal ventilation duct connections, plus cold water in, hot water out, a drain for the pressure relief valve and power and control cables.

The reality is that there is so much short-term variation in both demand and PV generation that detailed modelling to try and optimise battery size (or more probably inverter capacity) can be a bit counter productive. I started off with the premise that I would model everything in as much detail as possible during the design stage. Must have wasted hundreds of hours on it before we started the build. 99.99% of that effort was pointless, as experience has shown that all that's needed is a "big handful" estimate. Things that have thrown a spanner in my modelling have varied from the predicted thermal time constant for both the house, and the floor slab, being significantly longer in practice than theory (and estimates of material properties) would suggest. The local microclimate threw both the PHPP heating requirement predictions and my simple spreadsheet model estimates out of the window, they ended up being significantly in error. Overheating risk was massively underestimated by both PHPP and SAP, and we quickly realised that we needed active cooling and modifications to the glazing to make the house bearable in hot, sunny, weather. Overheating also caused me to rip out our hot water system, before we moved in, and replace it, as the heat loss from the thermal store was unacceptably high and led to the services room getting to around 40°C, enough to damage the oak veneer on the door. The PHPP PV prediction seems a bit less accurate than the PVGIS one, I've found. PVGIS seems to pretty closely (within a couple of %) match our actual generation, so I tend to use that as my preferred output estimating tool. I've also found that our export power is often limited on very sunny days by the local grid voltage. The inverter soft-limits to ensure that the output voltage doesn't exceed 253 VAC, and this happens surprisingly often in summer. It's a consequence of there being quite a lot of small scale PV generation in the village, together with a relatively low local demand (no industry anywhere near by).

What's your hot water demand likely to be? If it's not too demanding, then one of the combined MVHR/water heating solutions might work. The Genvex Combi 185 is a pretty good solution for a modest hot water demand: https://www.genvex.com/en/products/air-ventilation---air-heat-pump/combi-185-bp @PeterStarck has one, and has found it works well for them, I believe. It's expensive if purchased from the UK sales outlet, but can be purchased from a Danish company for a very much better price.

Not at all sure that PV makes any significant difference to the loading on a new build roof, does it? The structural loading from our 25 PV panels is less than it would have been had the roof been slate or tiles, by a fair margin, so the roof was just designed to take normal tile/slate loads and has plenty of margin given that about 3/4 of one pitch is PV. With an EV and no battery storage (yet) we export around 60% of the energy we generate over the course of a year. That's for a 130m² house that is all-electric, fitted with 6.25 kWp of PV. In summer we export a lot more than we use most days. A typical summer day generation will be around 25 to 30 kWh, of which we can use maybe 5 or 6 kWh on heating hot water and perhaps another 5 to 10 kWh on running the house cooling system and background loads. Most days we won't need much cooling, though, so the daily average house load is probably around 10 to 12 kWh or thereabouts. Car charging averages around 4 to 5 kWh/day, but only about half of this can be drawn from excess PV generation so as far as PV utilisation goes I might average around 2.5 kWh/day for car charging over the whole year. However, we quite often have to manage household/car consumption in order to match demand to the power being generated from the PV system. PV output is patchy on an hour by hour basis most days, due to variations in cloud cover. The hot water system can do this load matching automatically, as it changes the hot water system charging power on the fly, depending on the amount of excess PV generation available. It's not easy to do this automatically for other loads, and even car charging is tricky to match, due to the minimum acceptable charge power in the standard being 1.44 kW. Using a battery storage system might allow a small chunk of excess PV generation to be used, but not a massive amount. The greatest advantage that battery storage seems to give is the ability to load shift from peak rate times to off-peak rate times. I'm sizing our battery system on this basis, as that seems to be both useful in terms of cost saving, and useful in terms of reducing peak demand on the grid. It's not hard to model likely loads over time, and PV generation over time is easy to model using PVGIS, so with the two data sets it's reasonably easy to do some what-ifs and see how things might look in reality.

Not as far as I know, as this would upset the condensate drain arrangement. There's an internal drainage tray that collects the condensate with a 16mm bore pipe that runs from this to outside, along with the two refrigerant pipes. The pipes can be fed out from either end behind, or projecting down out of the base of the wall unit, as there are cut outs in the underside to allow this. The top of the indoor unit is the air intake, so needs to be kept clear from obstructions.

Our MVHR has exactly this, a built-in air-to-air heat pump that can either cool or heat the fresh air supplied to the rooms. The snag is that MVHR only moves a small amount of air, and air is a lousy way to move heat around, as it has a low heat capacity, so the best the cooled MVHR can manage is maybe a couple of hundred watts of cooling per room, even at boost flow rates. This is enough to make a useful difference if the weather is warm, but not very hot, but not enough to make a useful difference in really hot weather.

If you want to do several rooms that aren't well interconnected, then you need a multi-split. Essentially the same indoor units, but the outdoor unit has provision to attach several sets of pipework. In principle it's little different to installing a single mini-split, just a bit more pipework to run and connect up.

As a possibly useful data point, our 2.5 kW unit seems more than capable of cooling not just the ~50m³ bedroom, but also the whole of the ~100m³ entrance hall as well, if we leave the bedroom door open. We do have pretty good insulation, with a fairly long decrement delay, though, which significantly reduces the amount of heat that gets through the structure when the sun is shining on it.

Because the air con just recirculates room air, cooling it as it does so, it has no effect on ventilation, so doesn't effect the MVHR at all.

The pipes I bought came pre-insulated and made to length. Where they run outside they need additional protection. I used a "bear paw": https://www.saturnsales.co.uk/Inoac-Trunking-Outlet-Cover-NW75.html plus runs of the matching trunking: https://www.saturnsales.co.uk/Inoac-CD75-Air-Conditioning-Trunking.html You're welcome to borrow my pump and gauge set, although getting it to you might be a challenge, as the pump is filled with vacuum oil.

The way that big tube benders keep tube round whilst being bent is with a sphere on a rod that is held at the bend point, and moved out as the bend progresses.

I think you're right, that it's related to the water heating. Might be worth asking Genvex if there's any way to enable cooling, though. It may be that this is an option, perhaps like the one on our ASHP, that can be enabled, perhaps with a change to the firmware. The firmware for the Optima is on the SD card that plugs in to the underside, so it might be worth asking Genvex if this can be changed.

A mandrill is similar to a baboon: https://en.wikipedia.org/wiki/Mandrill A mandrel is a tool: https://en.wikipedia.org/wiki/Mandrel Still reckon that dent could be taken out OK with a set of long spoons. I've beaten out dents on rally car exhaust systems and bash plates a fair few times in my youth.

That's odd, I'd have though the same as you. If you go into the user menu on yours, do the two cooling options show in positions 2 and 3? On ours, the second menu item sets the hysteresis in cooling mode (defaults to 3°C) and the third menu option selects whether to turn cooling on and off automatically or not (ours is set to ON).

Yes, the Optima automatically switches between heating and cooling as needed. There's a built in 3°C minimum hysteresis in cooling mode, though, so the set point needs to be 3°C lower if you want the cooling to come on. The manual says this is for economy, as cooling costs money, but it's an annoying feature in my view, especially as we're pretty much always generating more than enough to cover the power used by the Genvex when we need cooling.

Any half decent panel beater could do it with a couple of spoons and a hammer: https://www.ebay.co.uk/sch/items/?_nkw=panel+beating+spoon&_sacat=&_ex_kw=&_mPrRngCbx=1&_udlo=&_udhi=&_sop=12&_fpos=&_fspt=1&_sadis=&LH_CAds= Not an easy skill to master, though. The hard bit is making sure that you don't stretch the metal whilst trying to get the dent out, as although you can get it back in shape with a shrinking hammer it's not easy and means more time getting the surface looking decent again.

In a post in another thread I compared some running costs for a given heating requirement using different fuels:

Might be worth a punt at that price. There isn't much in one of these things to go wrong.

I have one of those. I bought it to try and see how easy it would be to use it as a cooler - the answer is not very. I took it apart and concluded I'd need to make a new housing, as there was no easy way to make the existing housing watertight enough to allow a condensate drain to be fitted. When I get around to it the plan is to make a new housing for the heat exchanger from bonded together PVC sheet, so that it will be easy to fit a drain, with no corrosion risk (the plated steel case on the thing didn't look as if it would last long if it got wet). It's in bits in the garage, but I could dig it out and take some photos of the inside if that might be useful.