I appreciate that you can get very good figures for MVHR and that it does what it says on the tin, but I am trying to avoid the cost of buying/installing and running one while having all the complications of having another mechanical object in the house to maintain etc. Passivents (with humidity/CO2 valves/sensors) combined with lockable opening windows and a couple of extractor fans in bathroom (along with opening windows) would be a much cheaper and adequate solution for me.

I am also a windows open person so will be conflicting with the system a lot of the time and am finding it a very grey area regards what to do to the system when windows are open.

Heat loss seems to be the only problem, but having a log boiler it simply means more work for me rather than money.

Am I barking up the wrong tree? Should I take the easy option and plump for the extra money and efficiency of MVHR.

Adam

From your description of how you heat and ventilate your home, neither option appears suitable for you.

Passivent would save the cost of an MVHR unit but each terminal is more expensive and my experience is that there is just as much ductwork as MVHR.

Individual room MVHR fans may be your most cost effective option.

I like the FIWI but cant help thinking it is still at the tweaking stage. I need something for life.
The amount of ducting isnt the main issue, but the cost of the units does seem a problem. I have only read the Passivent bumph and they use comparative figures suggesting (off the top of my head) that it would cost £1500 to install a passivent system as opposed to £5k for MVHR, I will look at the individual room idea, thanks for that.

Bill Dunster designs his schemes with passive ventilation.

Regards switching it off in summer, there is the issue of microbial growth in the ducting.
Bill Dunster and Kevin McCloud too.

Posted By: adwindrumthe issue of microbial growth in the ducting.
Bill Dunster and Kevin McCloud too.

wouldn't want them growing in my ducting.

Awadukt and other u/ground pre-heat air intake systems' ducting looks like ordinary uPVC drain pipes but is expensively made with silver mould inhibitors. Are these more liable to mould because u/ground/colder?

Am aiming for under 1 for airtightness. And will have 2 internal wetrooms.
Do you have any details or links for the new mechanical systems they are using?

You may well have sold me there. Thanks a lot Chris and Tom.

adwindrum,

with MVHR, you'll become less of an "open windows" person. I personally dislike overly warm/stuffy houses, but since I built my own highly insulated/airtight house, I almost never open the windows (actually regretting the extra cost of having quite a number of opening windows, having expected to be using them like in a traditional house). I now advise clients to minimise the number of openers - just one per room.

A log burner may well over heat the house, if it's of a sensible size visually (say 6 or 7 kW) but since you have free logs, why not go for a log boiler stove, and get your DHW too (DHW may well be a greater energy requirement than your heating - annually). My experience is that the house does not overheat, with the log boiler stove on, though be careful to choose one which is weighted to heat-to-water rather than heat-to-room, eg. 12kW total, with 2kW to room & 10kW to water.

And even with a well insulated house (0.12 W/m2K type level you've mentioned), you will certainly still need heating, al-be-it to a much lesser extent. The MVHR will NOT move the heat around the house adequately if the logs are the only heat source, nor can you get enough heat into the air if it's injected into the MVHR ducting.

I always design houses with the MVHR ductwork as part of the structural/layout consideration (eg. running ducts parallel to floor joist), so that installation is simplified. So £5000 sounds a bit on the high side, since parts will be about £2000 - it's not a 3 week install.

Make sure you oversize the MVHR unit, so it's not running at a high fan speed, as they become more noisy, and the energy consumption rises sharply as flow rates increase. The SAP apendix Q spec's which define MVHR units in terms of being "adequate for kitchen plus a number of wetrooms" is nonsense, so be wary of that. When you do the air flow calcs, you find that the units spec'd for "kitchen & 7 wetrooms", are only adequate for a house with kitchen and 3 wetrooms, for example.

Bottom line (in my experience), target Uvalues of 0.12W/m2K, really focus on cold bridging and air tightness, use MVHR, only employ someone to design the house who can demonstrate they have experience of "low energy houses" and who consider the house shell & heating & ventilation & hotwater as a complete, integrated package. It needn't be complicated, just well thought out.

Hope something in that helps.

adwindrum,

my house sits happily at 19oC +/-2oC, and is a very homogenous temp throughout the house. It generally doesn't need active heat input until external temps get well below 10oC. I tend to forget about heating now, as it always feels comfortable. I have to confess to occasionally lighting the log boiler stove, even when the internal temp is adequate, just to have the peripheral vision of flames and radiant heat, especially when it's cold & damp like this summer!! That's a purely personal thing.

Maybe you could install the part of the stove flue through and above the roof, plug it with insulation, and then when finances allow, add the internal flue through the house and the stove?

Can I ask what your designers estimate per square metre is for your house (ie. total build costs for everything incl carpets, but excluding loose furnitre, divided by the total habitable floor area)?

Hi adwindrum,

I have had the same thoughts, per your original post and for Joe I have found the following, as the best alternative I can find to MVHR which we are exploring.

http://www.ventive.co.uk/

Am chatting to the guys about installing the system in our new build property - applies similar principles as MVHR, just without the 'mechanical' element. The testing and efficiency calcuations look very impressive.

Hope this helps
Peter

Posted By: LondonEcoHouseI have found the following, as the best alternative I can find to MVHR which we are exploring.

"http://www.ventive.co.uk/"

I like the simplicity of the system, but I don't understand how it can work in windless conditions.

According to the brochure "Ventive drives the air in both directions (extract and supply) using two forces: Passive Stack Effect (with warmer air naturally rising due to its lower density) and wind assistance" with "Heat Transfer Efficiency (up to 97%, as tested by Imperial College London)".

If the heat recovery unit really removes the 97% of the heat in the exhaust air & transfers it to the supply air then:

a) How does the exhaust air get drawn out of the house in wind-less conditions?
b) How does the supply air get drawn in to the house in wind-less conditions?

I suspect they are being economical with the truth & that Imperial College have measured the heat exchanger efficiency in a laboratory with flow rates fixed by fans.

In any effective passive ventilation system there will always be a trade-off between using the heat to drive air movement & using the heat to warm the incoming air. MVHR overcomes this conflict by using electricity to drive the air movement, allowing maximum heat exchanger efficiency.

David

I agree that with correct detailing of ducts, etc it could work well for refurbishing leaky houses, but I don't see how it can achieve 97% heat recovery efficiency & use stack effect to drive air movement.

David

Could it be 97% of the heretical maximum.
And they do say energy with no reference to temperature.

My home made unit can go over 100% when either the unit is 'hot' and the input fan is not pumping or the outside air is hotter than the extracted air (when the sun comes out).

Posted By: davidfreeboroughcould work well for refurbishing leaky houses

I don't see that leakiness helps - to contrary, would dilute the buoyancy effect - a closed air circuit is ideal.

Posted By: davidfreeboroughI don't see how it can achieve 97% heat recovery efficiency & use stack effect to drive air movement

That's a function of how extremely low-resistance (large-area) the ducting and heat exchanger is - and I agree an existing flue half filled with another duct doesn't seem v large. Also what about leakiness, dust, toxins in the extg flue, which is used as inlet channel?