... have you/they checked the filters, any clogging of the HX?
45% does seem a bit low, but then that may be due to many factors?

Which mfr/model is it?

Good luck

Thanks for your answers
The filters were cleaned before the engineers tested the system and they inspected the unit and only found that one fan was operating slightly faster than the other.
The temperatures were effectively tested at the heat exchanger as they were taken at the spigots off the unit itself.The manufacturers accept that it is at only 45% but don't want to do anything about it.
I feel they have broken the terms off the contract by not achieving 70% as claimed and am looking for some way of backing this up rather than just my opinion

Ask for a refund, Trading standards, demand withdrawal of product, write shed load of reviews - this isn't like the mpg for a car, even a 10% error would be cause for concern (for me). There is no-one who would buy a MVHR unit with 45% HR.

I'd agree: money back or trading standards, their choice.

They've inspected it and said it's OK so further speculation is a bit of a waste of time. Still, there's no possibility that some sort of summer bypass is open or partially open, is there?

I generally get about a 1 deg difference in inside/supply temps. Helios EC500 Pro.

My problem is that I cannot find anyone to service the thing. One of the fans has a slight whine which I suspect is some dust that needs blowing out.

Supplier, Allergy Plus have ceased trading - what a surprise. http://companycheck.co.uk/company/04203741 Watch out for Nev Boon - I find these folk keep turning up under different guises (similar issues with the roof supplier).

Doing efficiency seriously should take the flow rates and humidities of the flows into account but to get a good approximation, if the flows are reasonably well balanced, just measuring the temperatures of the outdoor air, clean air being supplied to the rooms and dirty air being extracted from the rooms should be enough. You don't need the temperature of the exhaust air being chucked outside.

http://edavies.me.uk/2014/03/mhrv-oops/#exchg-param

Patent issues...........maybe???

100 m² house, 2.4 m room height, 0.4 AC/h, 12 °C inside to outside temperature difference [¹], 1.28 kg/m³, 1 kJ/(kg·K): each percentage point of efficiency is worth 4.1 W (or 0.36 kWh/(m²·y)). That's £4.67/year at 13p/kWh for direct electric heating, less for almost any other sort of heating.

While 45% efficiency is seriously disappointing and needs tracking down and dealing with, chasing down the last few percentage points of efficiency in the exchanger is probably not the best use of time and money.


[¹] Year round average for Wick, Highland - 4374 HDD to 20 °C.

Another example of MVHR heat recovery efficiency (a Sentinel Kinetic B). Just looking at temperature, I'm using the SAP method to calculate efficiency: (supply-intake)/(extract-intake). The sensors are Sheepwalk's 'DS18B20 in a stainless steel tube' pushed into holes drilled in the ducting. The system takes pre-warmed air from a solar gain space (yes, I realise that isn't universally accepted as a useful thing to do). Ducting is to manifolds and then semi-rigid pipe (i.e. it’s easy to balance). I wasn't sure what to do when the unit's summer bypass operates as the calculated efficiency goes well over 100% (this is most days at the moment, but probably not today - location is N53 deg.). For now, I plumped for recording the efficiency as 100% if intake T is greater than extract. Perhaps it would be better to log it 'as is' and eventually do calcs for power gained (eventually, a RPi will control fan speed and so I can then log flow rate too).

The fluctuations at midday yesterday are because intake temp was close to triggering summer bypass. I'm assuming the efficiency gets messy when all the temps are very close - at the moment it's 18.9 intake, 19.8 extract, 19.8 supply and 19.3 exhaust (all being logged).

As the graph shows, the unit mostly runs at low 80s efficiency. However, at this time of year (unless we have a cold period) the intake is always in double figures (range is 24.8 to 14.1 over the last couple of days). I'm interested to see how the unit copes with large differences (e.g intake down to 5), but unfortunately that's unlikely to happen until next winter.

Any suggestions for improvements welcome and is the efficiency as expected?

Yes, it's a recent installation (Thursday afternoon). As you say, good to store the data for later use. Agreed re discounting data where T's are close (currently, I 18.8, S 20.1, Ext 20.2).

Re using the SAP method, it seems more suited to what the effect of the MVHR is for the house, whereas the PH method is oriented to the unit's performance. As you might expect, it's also complicated and could be expressed as ...

((extract-exhaust)+(power/mass flow x SHC)/extract-intake) + 0.08 x humidity recovery

Having said that, it does seem more rigorous (whereas SAP is perhaps more helpful for unit makers) and with no vent-axia units in the PH component database it perhaps shouldn't be surprising that the advertised 'up to 95%' claim for the Sentinel would seem to have to be taken literally, i.e. 95% might be a sweet spot it occasionally reaches.

At some point, I hope to do some combining of T data and the efficiency calc (in graphs).

Point taken about balanced flows - I've only recently got the system running and, for the time being, haven't done any more than turn it up to No. 11 and feel with my hand the draft/ suction on all the valves. Obviously, I need to do more and will - however, the house form is mostly a 15m square with the unit in the centre and with supply and extract via semi-flex manifolds (i.e. it's been designed to be 'balanced' from the off).

With humidity, this is attenuated somewhat by taking intake from a solar gain space - glazed roof, masonry walls on three sides, 300mm concrete floor. While very much acknowledging that pre-heating is a little contentious, this conversation has brought the potential humidity benefit to light.

I haven't methodically calibrated the T sensors, other than when setting things up and reading the network ID on the bench, I've done that in batches of 3 and always seen almost identical readings. But it's a good point and something to work on. I have 28 in use in 4 networks - one 11 sensor network has been running continuously since March 2013. Some of the sensors are inside, some in outbuildings (housed in camdenboss potting boxes where I haven't yet bothered to screw the lid on) and, so far, it's never missed a beat. As such, I'd thought they were pretty reliable things. Also, I'm wary that the low cost of these things (including the RPi) guarantees a bad press from 'professionals in the field'. As you say, though, it would be prudent to have some kind of calibrating and checking procedure.

Details about the PH method can be had from www.passivehouse.com - go to the 'component database', select any MVHR unit and there's a ‘download certificate’ link (the certificates detail the formulas used).

Edit- just seen that not all the PH certificates include '+ 0.08 x humidity recovery' in the 'efficiency criterion' formula. This one does ...

https://database.passivehouse.com/Media/certified_components/zehnder-group-nederland-bv_comfoair160-erv-comfod160-erv-comfod150-erv_0325vs03_en.pdf

Posted By: John WalshI'm using the SAP method to calculate efficiency: (supply-intake)/(extract-intake)

I'm going to have a go at this. I'm able to collect the data from the Helios sensors although the outside temp on that is recorded at 13 right now and my weather station (not a million miles apart, shows 11.

On Bypass temperature, I run mine much lower in winter (so never bypasses) and much higher in summer (so bypass kicks in earlier). I cannot quite get my head around the Helios logic, but it does what I want it to do so even on a warmish winter day I never bypass and even on a relatively cool summer day it does bypass, as this then helps to cool the house. YMMV.

The other thing for me would be that when the heater kicks in, that will distort the figures.

Borpin: re the 2 deg difference between your weather station and the unit's sensor (presuming you do mean that you can read the unit's sensors? If yes, 'envy emoticon' - vent axia don't allow that kind of thing), I often see the same kind of difference between the solar gain T and the sensor I've installed close to the unit. On another thread, djh talks about heat from his post-heater dissipating before it reaches outlets, that you do get a lot of T leakage from ducts (see http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14971&page=1#Item_0 ). Anyway, be good to hear how your unit's efficiency compares with the manufacturer’s claims.

On heat loss from ducts, Tony, the unit is within the heated envelope and so I can't see a reason to insulate the extract (as you say) or the supply. Of course, the exhaust will need insulating given it'll (hopefully - HR efficiency permitting) be down to around 10 or lower in mid-winter (the ducting is already insulated where it passes through the loft space). The intake I'm less sure about - if it picks up some heat from the house along the way to the unit then is that a problem?

There are 2 intake routes. One from a solar gain space which will be used between around early October to say early June. Given logging from sensors in the space over the past 4 years, the temp will not go below around 5 - presuming that the air extraction rate won't be high enough to reduce the temp. The second route is from an east-facing open porch, which will be used to bring in cooler air in the summer. With this second intake route, insulation doesn't seem necessary - if there's temperature gain over the duct run then that's fine, that's what we want to happen. Hope that makes sense.

Posted By: John WalshThe intake I'm less sure about - if it picks up some heat from the house along the way to the unit then is that a problem?

Yes, it does matter because, if your heat exchanger is 80% efficient, then in effect 80% of the heat from the house which gets into the intake will be chucked out of the exhaust. In other words, the exhaust air will be slightly warmer than it otherwise needs to be and so will carry more energy out of the house.

Consider the extreme case where the intake duct warms the intake air up to indoor temperatures before it reaches the exchanger. Then no heat would be exchanged and you'd have 100% ventilation heat losses (± humidity effects).

Posted By: John WalshThe latter has shown to be contentious on here perhaps due to an intuitive expectation that if we extract, say, 150m3 of air from a 20m3 space then we'll very quickly run out of solar warmed air.

Previous discussions of this sort which I remember have been about heating of loft spaces where it seems there's been a confusion between the temperature reached in the loft vs the rate at which heat is getting in.

For a glazed solar heat collector it's just a matter of the area of the glazing and the amount of sunlight getting through it against the heat capacity of the air passed through. The internal volume is a secondary matter, at most.