Some prices and data:

Vent Axia FH
Total £1868
Airflow 300 M3h/r @100Pa
Efficiency 90%
Includes Radial ducting kit (plastic), 5 supply valves, 5 extraction valves, 3 rolls radial ducting, 2 distribution boxes
Design included (£100 deposit)
Silencers not included (£26 each)
Delivery included
Commissioning not included


Duco-Box Energy 325 1Z
Total £2556
Airflow 350 M3h/r @100Pa
Efficiency 91%
Includes Radial ducting kit (plastic), 5 supply valves, 5 extraction valves, 3 rolls radial ducting, 2 distribution boxes
Design included (£100 deposit)
Silencers not included (£26 each)
Delivery included
Commissioning not included


Zehnder Comfoair Q350
Total £4600 - 5100
Airflow 350 M3h/r @100Pa
Efficiency 96%
Includes rigid metal ducting
Design included
Silencers included
Delivery included
Commissioning is £800

Thanks. The efficiency figures came from the quote for the first two and for the Zehnder it is what I could find on their website.

Interesting to know that you are happy with the plastic ducting. Some are adamant that metal ducting is the only way.

I find it baffling that the market is prepared to pay such preposterous prices for mhrv units - essentially two fans in a box - just compare those prices against the prices of more complex appliances such washing machines, or heat pumps...! Though the market is small in the UK, it's large in other places, so that's no excuse.

Not got there yet, but I am looking at using a number of smaller mhrvs such as HR100R or equivalent (£250 each). These would be distributed around the house, each serving one wet room and one dry. The main saving for us would be to avoid threading a network of ducts through the Victorian house. The claimed heat recovery efficiency is 20% poorer than the units you mentioned, but over an appliance's lifetime that doesn't justify the exorbitant price of fitting the larger units afaics.

Posted By: modernvictorianDoes anyone have any real-life experiences they could share?

I designed, installed and commissioned my own MVHR system. I chose a serial rather than radial system design for my Genvex compact unit. This compact unit is a combined MVHR, EASHP and DHW tank so not directly comparable to yours. I used rigid plastic ducting with an attenuator on the supply circuit and the system is silent and is run on the low setting all the time as I have found humidity levels acceptable without using boost. My unit is PHI certified. A consideration that not everybody realises is that the supply air will be at lower than room temperature by a couple of degrees. This can be offset by having a unit with an EASHP, hot water or resistance electric post heater. Another consideration is that building regs. ventilation rates are higher than PH rates and we run our system at PH rates.

Vent Axia/Semi rigid here. As above, semi rigid seems better in every way - same principle as manifold plumbing where there are few joints and those there are can be accessed.

No pre/post heater and have never noticed air being cool. We're in SE though - might be different if you live somewhere colder.

Unit price is unbelievable - materials cost of the units looks so low. If they became a mass market appliance they'd cost about £200....

OK, I'm now leaning towards the cheaper options so its good to know that the Vent Axia/semi rigid option is working for you Simon. I imagine the installation would be a lot simpler in my victorian house compared withe the large metal ducts. I am thinking that as long as the ducting is installed well and performs well then the unit could be changed in the future if there were any issues or if better technology became available.

I still have a slight concern about the longevity of the semi-rigid ducting because of comments like this:

"MVHR ductwork is embedded into the fabric of the building and would be very disruptive to
have to replace, so it is worth investing in a high quality system from the outset. We specialize
in, and normally only offer, galvanised steel spiral wound rigid ducting for our systems,
designed to last the lifetime of the building."

And this article describing the downsides of the cheaper units (note written by a supplier):

https://www.heatspaceandlight.com/how-the-cost-of-an-mvhr-relates-to-quality-and-performance/

Posted By: bhommelswould make sense to use rigid pipes (if they weren't so expensive)

I understood every turn/junction in a rigid pipe adds resistance (needed to calculate this for the large ridid external ducting). Semi rigid thus much more efficient for a given diameter.

Cant see any reason semi-rigid wouldn't' last life of building as well.

I built my own system a Titon unit and 200 x 60 rectangular and 150 and 125 round Domus rigid ducting. Only flexible is connection from the unit to the rigid duct. That is quiet except on boost. One thing I was not happy with, was the plastic air valves I originally acquired, so bought air valves from lindab. These are of a metal construction and fit into a galvanised socket bonded to the ceiling. It is then a simple matter to remove and replace the valve with a single turn without disturbing the connection to the ducting. This is of great assistance when you want to lock off the position of the valve once balanced. Don't forget that in the kitchen to fit a fire air valve that shuts off the intake when it gets too hot.

http://www.lindab.com/uk/pro/pages/search.aspx?cat=pro&k=air valves

@djh - what sort of qualifications should you look for in a professional? Is this a mechanical engineering type discipline? Most retailers and manufacturers have their own design teams, some offering free designs.

To the OP - have you considered a decent dc-MEV? You might find the costs a step down from the Rolls-Royce PH certified MVHR units, and depending on your permeability levels it might make a sensible trade-off. Look at Aereco who seem a decent outfit.

I've taken a look at dc-MEV recommended above but I'm still leaning towards a 'not so Rolls Royce' MVHR unit. The costs (for the cheaper quote) are reasonable compared to other systems.

The system will be designed by the supplier (after a deposit is paid), I also have no idea of their qualifications for designing this successfully. The more expensive suppliers think they will do a better job on this, which could be true, but in a fairly standard house layout such as mine surely there are only a limited number of options for laying out the ducting etc.

in answer to the original question, I would guess it is always worth paying more for higher efficieny in the heat exchanger, because the efficiency is cumulative over time.

i.e. imagine you begin with 100% heat, no other system inputs or outputs and presume a perfect 1 ach.

Over 24 hours a 96% heat recovery would give 39% of the original heat

100
96
92.16
88.47
84.93
81.54
78.28
75.14
72.14
69.25
66.48
63.82
61.27
58.82
56.47
54.21
52.04
49.96
47.96
46.04
44.2
42.43
40.73
39.11

a 90% would give only 9% of the original heat

100
90
81
72.9
65.61
59.05
53.14
47.83
43.05
38.74
34.87
31.38
28.24
25.42
22.88
20.59
18.53
16.68
15.01
13.51
12.16
10.94
9.85
8.86

With 2 ach those same figures would be achieved in 12 hours, and so on.

Therefore my assumption has been that it isn't correct to say that a 96% is only 6% better than a 90% heat exchanger. Please correct me if I've missed something?

Posted By: Silkyin answer to the original question, I would guess it is always worth paying more for higher efficieny in the heat exchanger, because the efficiency is cumulative over time.

i.e. imagine you begin with 100% heat, no other system inputs or outputs and presume a perfect 1 ach.

Over 24 hours a 96% heat recovery would give 39% of the original heat

a 90% would give only 9% of the original heat

Posted By: WillInAberdeenBut when I looked at it last, it still wasn't worth paying £1000s more for a more efficient unit, it didn't pay back in the likely lifetime of the appliance (or its owner!).

And the heat loss due to a less efficient heat exchanger should also be put against the heat loss of no MVHR and sufficient 'natural' ventilation to keep the mould at bay.

Posted By: WillInAberdeenYour numbers illustrate how important it is not to over-ventilate. There's regular stories of people installing big expensive mhrvs to meet some arbitrary number of AC/h, then running them at 20% of capacity, or (worse) over-ventilating to excessively low humidity. The more you ventilate, the more heat is lost. A 90% efficient mhrv running at 5 times too high flow rate, loses more heat than a 70% efficient mhrv running at the required flow rate.

Couple of thoughts.

Running a bigger MHRV unit more slowly can mean it is quieter.
However, running it too slowly may not clear the moist air sufficiently from the fans and it then suffers reliability problems.
The only way to get the 'right level' of ventilation, is to measure the humidity and CO2 levels and ventilate accordingly.

YMMV.

Semi-rigid duct is usually plastic. Metal duct is usually rigid. Flexible metal ducts are normally bad news for smooth airflow. Did you have some specific product in mind?

Posted By: modernvictorianSome prices and data:

Vent Axia FH
....

Buy the Vent Axia. Run it at half what the design says, then tweak it according to a humidity/co2 monitor

Have a look for second hand units on eBay etc- they do come up


I loosely kept to the designs and specs when fitting mine; some air flows are different but the general notion of "bigger room , more flow" is kept to. Big rooms (over 15sqm) have an extract and a supply. Rooms furthest from the manifold (not quite centrally located) have two pipes to one vent, otherwise it's one. I also swapped one of the supply to be an extract to even out the flow rates a bit. All in all, physics/the universe doesn't care much for this meddling and everything will even out - it's not the case that if you fail to get it to within 1% of spec then it won't work - the biggest step for an amateur installer is ensuring to get the supply and extract the right way round :)

All semi rigid ducting - easy to fit (but mine was a new build) and can't see why it wouldn't last - a) it's made of the same stuff that your water supply pipe is made of, b) it's buried inside something that stops UV light getting to it, like your water supply pipe is,, c) it encounters no mechanical stress, and d) it's made of the same stuff that will be polluting our oceans and landfill sites for the next billion years.. I find the notion that someone would try to make out that twinwall plastic pipe won't last the life of the building laughable to say the least

Even when my McGraw wasn't balanced it still had its profound effect (and tbh balancing it made no discernible difference but it was probably reasonably well the way there to start with), namely that the house doesn't smell of anything - sure, you let the steamer boil dry and the burnt veg smell lingers for a few days but the thing I notice now more than anything else when I come home from being anywhere that doesn't have an night is how much my house doesn't have a smell -@ no more forty bedrooms" the mm said, and the boss agrees. She says she now wouldn't consider living anywhere without one, and if I built another house I'd fit one just for that reason alone

BPC Ventilation by any chance?