Jeremy Harris

As long as the supply and extract ducts are a reasonably long distance apart I doubt there is much real interaction. The air velocity coming from the terminal(s) is pretty low and will slow down a great deal within a couple of metres from the terminal, so for a room with a cross section of 3.2m x 2.5m, with a single supply terminal running at 8l/s then very roughly the flow velocity a couple of metres or more across the room will probably be down to around 0.001m/s or so. This is slower than sort of typical air movement velocities associated with micro convection within a body of air.

At the building regs kitchen extract rate of 13l/s, it would take around 3.78 hours to get one air change, which is too low. I'd work on aiming to get around 1 air change every 2 to 2.5 hours; we run at around 1 air change every 2.4 hours and that seems to be OK. For a room with a volume of 177,000 litres that means an air change rate of about 20.4l/s. This is too much flow for a single extract terminal I think, especially as it needs to be quiet, so it looks as if you will need two extracts and probably two fresh air terminals at the opposite side of the room, in order to get the required performance at a low noise level (each terminal would normally be operating at around 10l/s). Doubling up semi-rigid duct runs to each terminal would probably be a good idea, the standard HB+ type 75mm duct terminal connections accept two ducts anyway, one of the duct connections is just plugged if only a single duct is used.

I think it's a good idea, too. My only concern would be how to ensure that the stainless tray is very well supported over the whole of the load-bearing area. It's quite probable that the stainless sheet will end up with some slight ripples in, as a consequence of welding the seams, and I think it will be important to ensure that whatever is underneath it is able to both bond the tray down well and take up any very slight unevenness. I doubt that there would be a problem in tiling over the tray, as long as it's very well supported so that it can't move. Not sure how best to bed the tray to the underlying floor, perhaps bed it on tile adhesive?

The choice of gas or electricity for heating and hot water has a fairly significant impact as far as SAP is concerned. Electricity is penalised relative to gas by a significant factor, so where gas is available it would pretty much always be the better choice, solely in terms of complying with Part L1a. Choosing to use electricity over gas would mean having to improve the fabric and also might mean having to install PV, just to be compliant. No reason not to use brick and block, just means a thicker wall build up than timber frame plus brick usually, to get the same performance, so slightly smaller internal space for a given footprint. Clearly the timber frame spec looks to be wrong, though.

I've just checked and I managed to get away with a two 40m rolls of 75/63mm semi-rigid duct with only about 3 or 4m leftover, for a 130m² floor area house, with pretty short duct runs. The duct cost just under £300, the terminals, restrictor rings, plenum chambers and 160mm flexible ducting to connect to the MVHR, together with the external terminals, bends etc, cost another £700.

Just done some quick estimates using some old experimental work from the 1950's. Looks like the critical Re for flow in a slot could be a bit lower than I'd assumed. I'd guessed that Re would need to be around 10,000, but it looks like I was out by a lot; the critical Re for flow in a slot of the same order of general dimensions as that under a door looks to be around 2,500 or thereabouts. For a 30mm wide door that means the flow velocity needs to be below about 1.2m/s to be reasonably confident that the flow will remain laminar. That's assuming that it's primarily the flow tripping from laminar to turbulent that is the primary cause of noise (I suspect it is, based on the hours I've spent inside wind tunnels years ago)

I think it depends on the Reynolds number of the slot, and whether the flow velocity over the length of the slot is enough to trip the airflow from laminar to turbulent (essentially whether the critical Reynolds number is exceeded). My gut feeling is that 2.5m/s is still below the velocity needed to trip the flow through a slot like this, though I could have a go at doing a bit of math to see if this is likely to be the case or not (not really done anything at such low air speeds before).

Looking at the current prices on the web, an inverter/charger with 9.6 kWh of battery storage, and a 10 year warranty on the inverter/charger and batteries, seems to be around £4k + VAT and a 4.8 kWh system around £2.7k + VAT

Regarding the amount of stored energy needed, then I've worked on the basis that I need around double the storage capacity required to handle our mean daily peak rate consumption on a cloudy day, which is roughly 4 kWh, so 8 kWh of storage seems a reasonable minimum. I've costed up the benefit from being able to use either stored excess PV generation in summer, or stored off-peak rate electricity during peak rate times in winter, to come up with the annual saving we could make from investing in battery storage. I cannot make the sums add up for the cheapest system available, over the warranty period of 10 years, even assuming that I wouldn't have received any return from investing that capital in a savings account . What swings it for me, in favour of installing battery storage, is the ability to use the UPS output from the inverter/charger in order to provide backup power during power cuts. Power cuts are fairly frequent here, and a nuisance in an all-electric house, so there is a value to us in having the ability to power some essential circuits during them, and that swings the balance in favour of battery storage.

Seems a bit pricey at £3.9k for 3 kWh (about £1,300/kWh), as you can buy a 9.6 kWh system (with a 10 year warranty) for around £4k + VAT (so around £500/kWh) or a 13.5 kWh Tesla Powerwall 2 for around £6,500 + VAT (so around £580/kWh).

I make the CSA of a 10mm gap under a 760mm wide door 7600mm², which at the maximum allowable flow velocity in terms of flow noise of around 2.5m/s, gives a flow rate of 19 l/s, which is pretty high. Background ventilation flow rates under doors are not likely to exceed around 8 to 10l/s I'd have thought.

If using 75mm semi-rigid ducting, then, as a reasonable rule-of-thumb, and if not running tens of metres in a single run, then you can flow up to about 8 l/s at virtually no duct noise as a maximum. For the boost extract rates you can comfortably double this, as some duct flow noise is usually acceptable for short periods when the MVHR is in boost mode. I was cautious, and ran double ducts to our kitchen extract to ensure that I could meet the higher BR requirement of 13 l/s and then found I had to restrict the airflow down a great deal to the point where it was clear that a single run of ducting would have been fine. In practice, it's best to design for the required whole house ventilation rate from the equation in building regs, as that tends to dominate and determine the quiet duct flow requirement. I should add that it's not a good idea to use flexible ducting for anything other than the short, vibration-isolating, connections to the MVHR unit, as flexible ducting has a higher loss than either semi-rigid or rigid ducting (the semi-rigid ducting, which comes as 75mm OD round or 51mm x 114mm oval, is almost the same loss as rigid ducting).

I did also post that this would probably be OK in a bigger room:

If it's a big enough room, then just fit the extract(s) nearest the sources of smells/water vapour and the fresh air terminal(s) as far away as possible at the "living space" end.

FWIW our house could potentially be really bad from a noise transmission perspective, as we have hard flooring everywhere and a very tall (~6m high) central hall that has doors off the lading directly to the bedrooms. In practice there's very little noise transmission at all; the TV or stereo can be on in the living room and you can't hear it at all in the bedrooms above, and that's with ~8mm gaps under all the doors.

Not at all, the airflow rate from MVHR is really low, barely perceptible if you put your hand over a terminal. It's not a good idea at all to fit fresh air supplies and extracts in the same room, as ventilation would be very uneven and heat recovery would be pretty poor, I think.

As an alternative you can fit offset ducts through connecting walls. We have ~8mm gaps under the doors and don't find that there's a sound transmission problem, though.

The normal arrangement is to install extracts in bathrooms, WCs, the kitchen, and utility room and fit fresh air supply terminals in all the living rooms and bedrooms. Air flows from room to room via larger than normal gaps left under all the doors.

Me too, I wound our ventilation rates down a fair bit after I'd had the test report accepted by building control. I'm not sure where the building regs figures came from, but they seem to demand much higher continuous rates for a house with continuous mechanical ventilation than would be the case for a house fitted with just trickle vents and extractors.

The rates are in Part F, the section dealing with continuous mechanical ventilation. There's worked examples in there to help in Appendix C.

I didn't find it hard at all to design the layout, the only slightly difficult bit was pulling the ducting through the webs of the Posi-Joists on my own; it's a job that would have been a lot easier with two people, just because the stuff tends to snag on the edges of the metal webs. The basic design rules are to locate extract ducts above sources of water vapour, and make the path length from the fresh air terminal in any room to the point where air will be extracted (typically under a door) as long as possible. I aimed to try and get the fresh air supply terminals diagonally opposite the door where air would get extracted from, within the bounds of practicality.

It's here: Heat loss calculator - Master.txt Download it, save it and then edit the file extension to change it from .txt to .xls (the forum doesn't like Excel files).

The problem with burning propane in a confined space is that incomplete combustion will generate CO, and as mentioned earlier, unlike CO2, CO is very definitely a silent killer.

Sadly it is true that, as far as planning goes, they couldn't give a stuff about sustainability in terms of low energy demand, reduced CO2 emissions etc. I had first hand experience of that when putting together our planning application.

I think the main thing is to try and ensure that the path from the fresh air supply point to the stale air exit point in any room is as long as you can practically make it. It's often the case that rooms with a fresh air supply will have a stale air exit out through a door that has a gap underneath it for this purpose, so there may be a slight benefit in having a high fresh air terminal in the ceiling diagonally opposite the door, but I'm not convinced that having it in the ceiling is an essential requirement. I tried to position all the terminals in the house so that air would have the longest path to travel across a room, but didn't achieve this on our bedrooms, just because fitting ceiling terminals in the vaulted ceilings would have been a lot of faff, plus it would have significantly increased the length of the duct runs. In practice our low-level fresh air supplies in those rooms seem to work OK.