dnb

Combined MVHR, heating and cooling.

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I read UncleQ's recent thread with great interest - his problem seemed very similar to mine. Hopefully the answer is similar too.

 

I am at the building regs stage of a SIPs build house on the Isle of Wight. The SIPs supplier have made a really super 3d model of how the house fits together, and now want to make all the holes in it for ducting and pipework so they can start fabrication in their factory. (I do like all the difficult design stuff being done now while the house is virtual)

 

I have worked out from first principles what the heating/cooling loading needs to be in the worst cases as 7kW heating and 3.2kW cooling. I will get more detailed data based on modelling from the SIPs supplier very shortly but until then I need something to work with.

 

I used local weather data for the temperatures (extremes of -8degC in winter and +32degC in summer) and have made the following structural assumptions:

Wall U value 0.15 W/m^2K

Roof U value 0.11 W/m^2K

Slab U value 0.10 W/m^2K

Window U value 1.0W/m^2K

House volume 735m^3, floor area 320m^2 (including attic space "hobby room")

Window area is around 60m^2, and pretty much not facing North - so it is enough, but not excessive. (Note there are no veiws to the North anyway, only my excessively large garage!)

Aiming for air tightness of 0.6 air changes per hour. This is tough but I am told it is achievable.

 

All the above should be conservative estimates that we should be able to beat. But I haven't worried about accurate treatment of thermal bridging as yet so hopefully they balance out. Similarly I have neglected solar gain from the heating requirement. I am happy to share my very simple excel model for criticism so I can make it more realistic prior to real data arriving.

 

I would be interested to know if people think the requirements for heat to be excessive or insufficient. I'm new to these new highly efficient materials.

 

The only services on site are water and electricity, so it makes sense to use a heat pump of some kind for space heating. (Water will probably be solar thermal plus something else, and the subject of another thread)  I initially thought about air conditioning the whole house, because modern systems are in fact heat pumps and have a reasonable COP for both heat and cooling. And they are relatively cheap. I then thought that if I am putting ducts everywhere for MVHR, why can't they be combined? It seems they can, but nobody I had yet found wanted to do the designs. (This was until I found Genvex linked from this forum)

 

So the main question is - do I go for a large Genvex system - probably the HPV series 3 given my above calculations - or should I look to use something like a Mitsubishi FDUM ducted A/C system with a separate MVHR unit sharing common ducts? Both have their advantages from a system-in-use viewpoint, but I think the Genvex system is going to be far superior from a building control acceptance viewpoint because it is a single accredited package, even though it is twice the price.

 

I plan to have heated bathroom floors (2 ensuites, 1 master bathroom and one wet room down stairs) using resistive elements - I don't like having cold feet - and possibly towel rails but no other heating if I can avoid it - fewer systems in the house is better as far as I am concerned. I will not be considering a wood burner in the house - there are very good reasons but this isn't the right place for discussing them.

 

My Monte-Carlo simulation of thousands of years of heating says I can expect running costs of £350 per year on average based on a 3.8 COP and 17p per kWh, again making "safe" assumptions for efficiencies.

 

Please ask if I've neglected to include vital information - I really want to talk this through with people who have done this and have practical experiences.

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Are you considering 2 units driving air down a single set of ducts? How will you balance the flows, will this not result in the fans "fighting" each other? I assume both units will want to vary the flow e.g MVHR on boost.

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It's not that hard to get under 0.6 ACH, our build came in at 0.49 ACH on the first air test, and I've since gone around and adjusted a couple of small leaks at the doors.  We also have a Genvex Premium 1L MVHR, but we don't use it for heating as we don't much like the dry air we seem to get with it, so instead use UFH on the ground floor for both heating and cooling.  The MVHR does provide a modest amount of cooling, but wouldn't be enough on it's own.

 

We have about 9m² of south facing glazing (with a large roof overhang), 5.5m² of east facing glazing and 7.5m² of west facing glazing.  The house is 130m² and meets or exceeds the PH standard (roof 0.1 W/m².K, walls 0.12 W/m².K, floor 0.1 W/m².K and doors and windows about 0.7 W/m².K).  We run the MVHR at around 0.4 ACH, except when it's cooling where we have to boost it to about 1 ACH.

 

We had severe overheating initially, and added solar reflective film to the south glazing and also to the east.  That's significantly reduced solar gain, but it is still an issue in Spring and Autumn, when the sun is low.  We run the ASHP in reverse to cool the floor which makes a very significant difference, far more so than running the Genvex in cooling mode (and the Genvex is over-sized for our house, really).

 

I found that both my own thermal modelling and that in PHPP was seriously in error.  The heating demand is slightly lower than predicted (our peak heating requirement in really cold weather rarely gets above 1 kW in practice) and the cooling requirement is massively greater than predicted, in part because we've found that the house feels uncomfortable if it's warmer than about 23°C (unlike our old house where 24° to 25°C seemed OK).

 

Our house is all-electric, and electricity and telephone are the only services we have (borehole for water plus a treatment plant for sewage).  We have 25 PV panels built in to the roof, and they provide a great deal of our energy needs in summer (including charging my car), but we are reliant on the grid between about October and March, when PV output pretty much falls off a cliff.  Our annual electricity bill (on E7, with about 55% to 60% at the off-peak rate) is around £350 a year or so.  Heating is really sod-all, as the house rarely needs much heat, and the 6 kW to 7 kW peak output ASHP we have is a massive overkill and far too large for the heating/cooling demand.  It was the smallest unit I could find at a reasonable price, though.

 

If doing this again then I would not bother fitting the Genvex and would go for a cheaper non-active MVHR.  The Genvex cost around £3.5k or so, and we could have bought a basic MVHR for less than 1/3rd of that.  I would definitely have fitted split air-to-air aircon units, though, perhaps one at the top of the hall and maybe one or two in the bedrooms.  They would have been cheaper than the Genvex and given far greater cooling capacity.  It's really hard to get effective cooling at the sort of low air flow rates that an MVHR system works at, and having to boost the MVHR to try and improve cooling is a nuisance.

 

The other thing I'd definitely do if ever building another passive house would be to build in external shutters or blinds, to reduce solar gain.  The reflective film works, but is really a bodge and it does tint the windows a bit.  Alternatively I think I'd seriously look at SageGlass, as it apparently works very well at reducing solar gain, and even at ~£1000/m² it's probably not that much more expensive than other solutions, and has the advantage of being variable.

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8 hours ago, ragg987 said:

Are you considering 2 units driving air down a single set of ducts? How will you balance the flows, will this not result in the fans "fighting" each other? I assume both units will want to vary the flow e.g MVHR on boost.

 

I was. But I see no reason to ever turn on the fans on the MVHR unit when the AC is running (either heating or cooling) - I only want it for the heat exchanger. The AC unit fans will be significantly more powerful. "All" I need to do ;) is ensure there are dampers to ensure the AC unit draws in 90l/s or so of fresh air through the MVHR system all the time it is running. The rest of the air will be recirculated as in a normal AC unit.  I believe a sensible building management system can achieve this with the right sensors and data.

This is where the building regs people perhaps start to get unhappy.

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Just had a look at JSHarris's very useful heat loss spreadsheet so I could compare it to my own. It seems I am overestimating two parameters:

 

I assumed that the delta temperature for the slab was the same as for the outside air. This was because my understanding of soil temperature was that it was only stable at average air temperature below 1 metre in depth. Happy to be wrong on this one since it's a saving of a few watts.

 

The next one I perhaps didn't think through carefully enough. I added an allowance for uncontrolled infiltration based on total house volume and the assumed air leakage test results. This, on reflection, is a little silly because the air leakage test is carried out at half atmospheric pressure! So "real" leakage should be next to zero when the house is similar pressure to outside.

 

So if I remove the infiltration loss (by setting the AC/hr to zero) and use 8 degC for ground temperature my house model is in broad agreement with JSHarris's spreadsheet.

 

I therefore think I need only 3.5kW of heat in the worst case in January, and can therefore get away with a much smaller AC unit (or whatever scheme I settle on). This is much better! Thank you for the sheet.

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8 hours ago, dnb said:

Just had a look at JSHarris's very useful heat loss spreadsheet so I could compare it to my own. It seems I am overestimating two parameters:

 

I assumed that the delta temperature for the slab was the same as for the outside air. This was because my understanding of soil temperature was that it was only stable at average air temperature below 1 metre in depth. Happy to be wrong on this one since it's a saving of a few watts.

 

The next one I perhaps didn't think through carefully enough. I added an allowance for uncontrolled infiltration based on total house volume and the assumed air leakage test results. This, on reflection, is a little silly because the air leakage test is carried out at half atmospheric pressure! So "real" leakage should be next to zero when the house is similar pressure to outside.

 

So if I remove the infiltration loss (by setting the AC/hr to zero) and use 8 degC for ground temperature my house model is in broad agreement with JSHarris's spreadsheet.

 

I therefore think I need only 3.5kW of heat in the worst case in January, and can therefore get away with a much smaller AC unit (or whatever scheme I settle on). This is much better! Thank you for the sheet.

 

That spreadsheet only considers heat loss, and makes no attempt to model incidental heat gains, which is it's major flaw.  PHPP does a reasonable job of doing everything, but is very complex and takes a bit of time to get familiar with.  I'd strongly recommend using PHPP though, as my spreadsheet was only ever intended to be a crude "what if" tool for comparing different major parts of the construction.  I really put it together just to see if making, say, the window U value a bit better had a significant impact, so that cost/performance trades could be made with a bit of supporting data.

 

I have a sub-soil temperature probe under the slab and it barely change by half a degree from it's coldest in about late February to its warmest in late September.  In our case it sits at around 9°C, but 8°C seems to be a temperature that is quoted widely as being about the mean UK soil temperature.

 

Uncontrolled infiltration in a house that's well-sealed and fitted with MVHR is negligible, as you say.  The airtightness is noticeable when you open and close doors, as you can definitely feel the air being compressed or rarefied when a door is moved quickly.  The pressure test isn't done at a very high pressure though, it's roughly the same as the dynamic pressure from about a 20mph wind.  Mind you, a 20mph wind is pretty strong.  It's a tiny fraction of atmospheric pressure, though (which is about 101,325 Pa).

 

Your heating demand sounds about right, but will likely be a very worst case, when the house is unoccupied and has everything turned off, with zero solar gain.  In reality I find that we never need much heating at all, and certainly not as much as the spreadsheet indicates.  The reason is down to there being two of us in the house jointly contributing about 150 to 200 W, the base load from electrical appliances in the house that add another 200 W or so, plus a bit of solar gain, the heat released to the house from showers, baths and cooking, etc.  It's not hard for the incidental gains to supply all the heating needed for much of the year, and to contribute to overheating when there's a bit of solar gain.  Our house is also cut into a hillside, near the bottom of a valley, so has a surrounding microclimate that seems a fair bit warmer than the Met Office temperature data I used for this area.

 

I've not tried to properly model solar gain, but I did have a go at roughly estimating it.  I reckoned that our 9m² South facing windows (which have a fairly large overhang) could pretty easily see around 200 to 300 W/m², perhaps more on a bright, clear, day.  As supplied (before I fitted the reflective film to the outside) most of this would have ended up in the house, perhaps around 70% of it, so the heat input just from the South glazing could be around 1,300 to 1,900 W, for several hours.  This is way more than the house needs to stay at our preferred temperature, hence the overheating we've experienced.  The worst aspect in terms of overheating seems to be the glazing on the East side.  The sun rises well North of East in summer and often the early mornings are very clear here, so we have bright sun at a relatively low angle shining through the East windows for a fair time first thing in the morning.  For some reason, the West facing windows don't seem anywhere near as much of a problem, due, I think, to the air tending to be more hazy here in the afternoon.

 

Overheating seems to be a theme here for those that have built low energy homes, especially those who have incorporated large areas of glazing. 

 

 

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Your last point is exactly why I am considering AC from the outset. I too didn't bother modelling solar gains etc too much for the fabric losses.

 

I did however include them in a Monte Carlo simitation of thousands of years of randomly distributed house use. It indeed indicates that little to no heat is needed most of the time, but considerable cooling is required.

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1 hour ago, JSHarris said:

Uncontrolled infiltration in a house that's well-sealed and fitted with MVHR is negligible, as you say.  The airtightness is noticeable when you open and close doors, as you can definitely feel the air being compressed or rarefied when a door is moved quickly.  The pressure test isn't done at a very high pressure though, it's roughly the same as the dynamic pressure from about a 20mph wind.  Mind you, a 20mph wind is pretty strong.  It's a tiny fraction of atmospheric pressure, though (which is about 101,325 Pa).

 

All good stuff. To add to this, another reason that practical infiltration is typically lower than the air test is that when it's wind driven one side of the house is pumped up in pressure and one is sucked down so the effective area available for the total flow (which has to both be in and out) would be something like a quarter to a half that under test conditions.


A rule of thumb I've heard is that the practical infiltration rate is about 1/20th of the test conditions. Apart from the lack of supporting evidence for that I'd also be concerned that the heating season when infiltration really matters tends also to be the windier time of year. It seems to me that this would be more pronounced on a well insulated house where the heating season is short.

 

Also, it must matter if your house is exposed (as mine is) or well sheltered (as Jeremy's is).

 

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12 hours ago, dnb said:

I was. But I see no reason to ever turn on the fans on the MVHR unit when the AC is running (either heating or cooling) - I only want it for the heat exchanger. The AC unit fans will be significantly more powerful. "All" I need to do ;) is ensure there are dampers to ensure the AC unit draws in 90l/s or so of fresh air through the MVHR system all the time it is running. The rest of the air will be recirculated as in a normal AC unit.  I believe a sensible building management system can achieve this with the right sensors and data.

 

I don't quite follow this. Air conditioning is a closed system, so it doesn't cause "draw in" fresh air from the outside. You therefore still need to run the MVHR when you have the aircon running, otherwise you'll have no fresh air coming into the house. If you bypass the MVHR heat exchanger and draw in the air from outside for supply via the MVHR ducts, it still needs to leave somehow. Perhaps you're planning to use positive pressure provided by aircon, and to have that pressure cause air to leave via the extracts? Seems a potentially very wasteful arrangement if the MVHR heat exchanger is bypassed.

 

I also think you'd need to carefully consider how you'd provide the cooled air at enough pressure to force it through the ducts - a standard ducted aircon system relies on much larger ducts than used in a typical MVHR system.

 

Am I missing something? Perhaps a diagram would be useful.

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I think you've hit the nail on the head, @jack.  My reason for wishing I'd opted to install a split aircon system for cooling was largely driven by the fact that the MVHR just cannot ever shift very much air through the heat exchanger and always runs at a serious disadvantage, in hot weather, by drawing in hot air to cool down.  Because an aircon unit just recirculates air, as well as having a very much greater air flow rate, it is both far more effective and more efficient to run.

 

If retrofitting a split aircon wasn't such a PITA I'd already have done it.  I dearly wish I'd made provision to get the pipes and cables in where needed, right up near the apex of our entrance hall.  Being able to cool the air ~6m up, in the centre of the house, would make a very useful difference to comfort in hot weather, and the slight noise from an aircon unit up there wouldn't really be a nuisance. 

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56 minutes ago, jack said:

I don't quite follow this. Air conditioning is a closed system

I get the impression @dnb wants to create an open-loop air-con system. Thats would be re-creating the Genvex function of HPV.

 

I suspect you would only want to bypass the heat exchanger in a narrow temperature range -  e.g. between 15C and 25C.

  • the lower temperature for heating period - keep heat in the house
  • the higher temperature for cooling period - keep coolth in the house

I think there is a bit of complexity here.

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1 hour ago, ragg987 said:

I get the impression @dnb wants to create an open-loop air-con system. Thats would be re-creating the Genvex function of HPV.

 

But the genvex still operates the heat exchanger unless it's in summer bypass mode. The OP seems to be talking about bypassing the heat exchanger when the air con is operating. Generally that's exactly when you'd want the heat exchanger to be in the loop.

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I meant "open-loop" in the context that air is not re-circulated. Heat exchange and bypass is a separate, but important, point.

3 minutes ago, jack said:

Generally that's exactly when you'd want the heat exchanger to be in the loop

Agree, else you are chucking cold air out when you need to rtain it. How would the Genvex work in this case, does it engage the heat-exchanger?

 

I noticed on my MVHR that the heat-exchanger kicks in when external temperature is much higher than internal - so some reduction in heat gain on hot days.

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There are all sorts of subtle ways of implementing summer bypass modes, but in general, the heat exchanger should be in the loop at all times, except when the interior temp is too high and the exterior temp is lower than the interior temp.

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6 hours ago, ragg987 said:

I get the impression @dnb wants to create an open-loop air-con system. Thats would be re-creating the Genvex function of HPV.

 

Not quite. I want the option to mix the air (as my car can do as standard ;) )

 

Jack - I think you have misunderstood some of what I want to achieve a bit. It's most likely the lack of pictures in my explanation.

I will of course size the ducts appropriately. And there's no question of bypassing the heat exchanger unless the conditions demand summer bypass. It's the MVHR fans that will be off (or very low) if the AC is running hard - there's nothing to say the MVHR fans have to put air through its exchanger. Note that I think I have found a way around this bit of the issue - and I think the new solution is a little more elegant.

 

I have attached a picture. It shows an approximation to the ground floor of my house. The program I used to create it seems pretty good at the maths of airflow in ducts for ducted AC systems, but does not have a whole load of parts for MVHR (It's Australian, so is obviously optimised for their market) so I have "simulated" the parts I needed for trying to illustrate my scheme. There are 2 more floors of my house, but they are more of the same - introducing air and removing air - so don't add anything to the discussion of the concepts.

 

The scheme illustrated is very slightly different to the one I first described and should be easier to implement and to show it works.

There is a ducted AC system supplying air to the family room, lounge, study and hallway. It takes recirc air from the attic room as drawn, but the placement of this is open for discussion.

There is also a MVHR system. It extracts from the plant room and shower room on this floor, plus all the other bathrooms on other floors. I leave kitchen extraction out of this because my understanding is that it is best dealt with separately. The MVHR system supplies its air into the same plenum as the A/C supply, obviously with suitable non-return dampers on both pieces of equipment.

Therefore the MVHR can supply the 90l/s of "new" air, and the A/C can get on with making things the right temperature. All I need to do is make sure the ducts can cope with 90l/s plus the heating/cooling demands.  Surely this can't be too hard... ;)

 

 

AC_discussion.PNG

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90l/s is about 0.9 ACH for our house, and is about the flow rate that our MVHR runs at in cooling mode.  I can say for sure that this is not enough airflow to cool our house, with a cooled air temperature of between 10°C and 12°C..  I'd estimate that we could do with about three times this airflow rate to be effective, and our house is a fair bit smaller than yours, at 130m².

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1 hour ago, JSHarris said:

90l/s is about 0.9 ACH for our house, and is about the flow rate that our MVHR runs at in cooling mode.  I can say for sure that this is not enough airflow to cool our house, with a cooled air temperature of between 10°C and 12°C..  I'd estimate that we could do with about three times this airflow rate to be effective, and our house is a fair bit smaller than yours, at 130m².


I don't follow your point - I have described (or possibly misdescribed) a system that does broadly what you ask for.

 

The MVHR is supplying 90 l/s, without any heating or cooling (only the 10% or so loss of energy - in either direction - from the system based on MVHR efficiency) in order to meet the ventillation requirements of building regs.

The AC unit will recirculate 100s of litres of air (as much as necessary up to its capacity - say 250l/s for a 5kW system - I don't have the spec sheet of the candidate part to hand at the moment) to move the temperature appropriately.

So that's just under 3 times the airflow rate, and a lower supply temperature if need be.  An A/C unit is a bit better than "comfort cooling" tech at changing temperature, albeit with a higher cost of energy and a risk (until the temperature stabilises) of causing the feeling of drafts when the system is cooling fast.

This of course leads to a discussion about dew point and condensation when you want to have large movements of temperature... But in practice one should get the house to the correct temperature and leave it there... (Fine in theory - achieving it in practice will take good engineering)

 

What I hope to gain over the Genvex system is a decent pile of cash and full A/C (rather than "comfort cooling") at the expense of having to do some systems engineering.

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I misunderstood, and read it s being 90l/s for the cooling air flow rate, hence my observation that our experience suggests that this might be a bit low for cooling.

 

The main issue with ducted cooling at high flow rates is sizing the ducts to keep the air flow velocity below 2.5m/s.  If the velocity exceeds that then flow noise may well be high enough to cause a noise nuisance.

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No problem. I think I might be getting somewhere with the design now I've had sensible people to look at it. Does it look like it might work?

 

As for the noise, it's a good job I'm getting old and deaf! ;)  Years of hearing abuse from TVRs!

Seriously, the noise is a significant concern so I will be taking great care with duct sizing. This is where there might be a benefit in a "star" based system as I drew badly in my picture - as long as the duct lengths don't cause too much friction loss. This way, the ducts can be kept (generally) smaller because the "trunk" ducting is short and no rooms have to share. I can see a lot of maths and abuse of my Matlab licence in the next few days. Then I need to see what parts are easily available on this little island.

 

 

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9 hours ago, dnb said:

No problem. I think I might be getting somewhere with the design now I've had sensible people to look at it. Does it look like it might work?

 

As for the noise, it's a good job I'm getting old and deaf! ;)  Years of hearing abuse from TVRs!

Seriously, the noise is a significant concern so I will be taking great care with duct sizing. This is where there might be a benefit in a "star" based system as I drew badly in my picture - as long as the duct lengths don't cause too much friction loss. This way, the ducts can be kept (generally) smaller because the "trunk" ducting is short and no rooms have to share. I can see a lot of maths and abuse of my Matlab licence in the next few days. Then I need to see what parts are easily available on this little island.

 

 

 

 

I doubt that surface frictional loss will have any detectable effect on flow rate, TBH.  Bends seem to be the most significant factor, and even then all the unknown variables around the terminal and plenum end conditions, as well as turbulence around the restrictors, tends to dominate.  We're using the slim semi-rigid HB+ ducting, which is only 75mm OD, 63mm ID, and the duct losses are trivial when compared to the effect of the flow restrictors that have to be fitted in order to balance the system.

 

Balancing the system may be challenging with your proposed large duct size as it's not clear where you're going to fit the required adjustable flow restrictors for balancing.  I used the rather awkward HB+ system, that uses a range of restrictor rings fitted to each radial duct inside the plenum chambers, but the more common system is to use adjustable terminals.  There are pros and cons for both, but they both have a significant impact on your plan to switch between two very different flow rates.  Using terminal flow rate adjustment is far and away the easiest system to adjust, as it can be done from each room very simply.  The downside is that having the restrictor at the terminal may tend to slightly increase noise in the room, as the velocity is locally increased through the restricted aperture in the adjustable terminal.  In practice this doesn't seem to be an particular problem, though. 

 

The general design rules for efficient (and building regs compliant) MVHR are to design to meet the boost flow rates required for the mandatory extract rates for kitchens, bathrooms, showers, WCs etc, and to generally arrange the ducting and terminals so that as little restriction balancing as possible is needed.  This usually means more fresh air supply terminals than extract terminals (as extract rates have to be higher to meet building regs) and locating terminals so that there is the maximum opportunity for full diffusion between each room fresh air supply point and it's effective exit point (which will probably be under a door).  The same applies in reverse for extract rooms.  I arranged all our terminals so they were diagonally opposite the room extract/supply point, to try and maximise the path length, slow the velocity right down and ensure maximum diffusion.

 

In your case, with large ducts and terminals, I think you will need to find a way to fit the required adjustable restrictors to the MVHR supply and feed side for each room, and then somehow arrange to switch these out when your system switches to the recirculating cooling mode.  It can probably be done with the use of motorised valves, but might be a bit challenging in terms of maintaining access to all serviceable parts.

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A quick update:

I spoke to a local business and their initial response is that I can achieve the desired result, and if one of the major suppliers were to market a product that does this, they would sell loads of them, The sticking point is the control issue. We both believe this to be a soluble problem even if it proves a bit tricky involving some form of building management system.

We went over duct and terminal sizes and air flow requirements and found that my air flows for the heating and cooling were substantial over-estimates. This is good because it allows the ducts and terminals to be reduced in size and substantially narrows the flow difference between "just vent" and "purge with full heat".

We even found a few tweaks that should help with efficiency and comfort - more on these later if they stand up to scrutiny. They are fairly comfortable with being able to get something that doesn't need motorized dampers (the "simple usually wins" argument) but I would like to have the option for zone controls, even if they are not fitted when the detailed design proves them unnecessary. These would be located in the plant room on the ground floor and airing cupboard on the 1st floor.

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On 09/06/2019 at 14:55, JSHarris said:

We have 25 PV panels built in to the roof

That reduces the solar gain to a certain extent. More in your case as you have rooms in your roof.

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Thanks! That's really useful to see. (and it's nice to get confirmation of not being totally crazy!)

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