Has anyone had any experience with MHRV systems,

We will require one in each of the passivhaus(ish) semis we are building next year, two issues that I don't understand are:

1. The need for a cooker extract hood - possible problems with grease depoits, rate of extract etc
2. Woodburning stove
2a - how do I provide the incoming air without compromising airtightness?
2b - can MHRV distribute heat from the room with the stove (that's probably too hot)

J

Tony's 1 is right, I think. Get a straighforward extractor with as short and straight a pipe run as possible, and only use it when you're frying.

Regarding air to feed a wood burner - some stoves can accept a ducted air feed. The Clearview people told me that their stoves can do that. I would have thought some other UK stoves could as well... just need to start asking the manufacturers.

Air flow with MHRV and stove - It is possible to have more air flow into the room with the stove to help move the heat from the stove around the house better, however, I'm not sure if this is actually necessary. Might depend on how often you expect to use the stove.

no 20 !!!

Fostertom, straight mixing is far from the best possible result. As you say, it would produce warmed supply air at 15 °C which would correspond to an efficiency of 50%. Practical systems are more likely to have efficiencies around 80%.

http://en.wikipedia.org/wiki/Countercurrent_exchange

Fostertom

I always thought the same way as you, however I recently discovered something different.

Consider the heat exchanger to be a simple tube split in half down its length. the cold air gets passed in one and the hot the other.

So say intake is 10, extract is 20, becuase they are fed in at opposite ends, there is always a temperature differential along the whole length. At the 'cold' end, the intake is at 10, and say the extract is at 12 (12 because its already been through the exchanger). At the other end (hot end) the intake air has now been warmed to say 18, and the extract has just entered so it is at 20.

As such there is always a temperature differential, but the actualy temperatures rise. I don't know exact efficiencies, but they are able to exceed a 50% efficieny (cold air raised to 15!)

Timber

Anyway, back 'on message' as those dastardly marketing types say.

Tony,

Re extract... thanks for bringing me back to the real world but would still be interested if any manufacturers offer such an option.

Re passiv haus, some heating is still potentially required in coldest of winter days and to reassure those of lesser faith, (and it looks nice..! hell why not, do you see me wearing a hair shirt?) Also idea would be to get a something with back boiler to thermal store to indirect DHW sort of arrangement.

Skywalker,

will look at your Xbox but so far have looked at Genvex and Paul from the green building store.

J

Well the efficiency was very much related to the topic because if you could conserve a lot of the heat in the exchanger then it could be worthwhile to use an extract vent in the room with the stove to circulate the heat via the heat exchanger. So if efficiency means what you say it means, then this is quite viable with exchangers that have good efficiency ratings.

I was referring to the wood burning stove, not the cooker hood!

Regavent, Biggleswade, Bedfordshire do a British built, high efficiency MVHR; DC motors etc. When I discussed the feasibility of connecting to cooker hood they suggested obtaining a hood with no motor/fan and installing the damper valve option in line with the cooker hood with a booster switch for kitchen extraction when required. Does anyone else find cookerhoods on high speed unbearably noisy? The 'atmosphere' in our kitchen becomes quite fraught when it is on and the radio inaudible!

Stephendv, my (lack of) Dutch was not up to reading your reference to Brink climate Systems. I could not find a picture on their website that resembled the RegaVent unit that I saw at the Homebuilding show at the NEC earlier this year. RegaVent definitely gave me the impression that they built the units; perhaps they were being economical with the truth and were referring to the ducts, dampers and their noise level reduction ducting.Incidentally,when I questioned them about the "90%" efficiency, they said that in the British climate 75% would be normal; 90% only applies in more severe continental type winters. The British built claim has more appeal now that exchange rates have moved against us; the continental kit seemed pricey enough a few months ago.I will make further enquiries.

The Brink page displays in English for me.. odd. Anyway, there are other UK resellers of the Renovent HR, see: http://cart.vacuumsdirect.co.uk/index.php?p=catalog&parent=35&pg=1
Have you received a quote from Regavent yet? I'm going to be in the market for an HR unit next year so would be interested in price comparison.
See my post of 1st August on: http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=2243 for prices of other MVHR systems.

Stephendv, had an e-mail from Linda at Regavent, today. no updated price, yet, as I was too vague about the exact nature of the quote I was given; apparently a "budget price without a design". Have asked them for the same again to see how prices have moved. Hopefully not a lot as Linda re-assures me that they do make the units themselves using German fan units from ebm papst. She says "we have no link at all to the Brink system...your informant is probably mixing us up with a company called Ubbink" - incidentally also at the same show at the NEC.

Hi Julian, just the base units as systems vary in layout,duct length, bends, junctions and number of vents etc.
By the way, can anyone tell me how efficiently the units with DC motors convert to DC from the AC mains? I sem to remember seeing a figure of 33% less power consumption for a DC fan cf. an AC fan; but how much is lost in the conversion?

Hi James,

1. I plan to build to PH standard next year and MVHR is part of the spec. I've recently contacted Itho, based on their BRE listing under passive house ventilation systems. They can supply cooker hoods designed for MVHR systems and which include downdraught hoods which, would seem to offer a solution to the otherwise limited air flows such systems would have.

I don't know how effective such units are at containing smells but I'd appreciate any comment from those that know as it is something that concerns me.

2.a Wood burning stoves to PH standard are available, consult BRE web site for options. They are in effect balanced flue by design, drawing combustion air from outside therefore not affecting leak tighness other than where the flue penetrates the building fabric.

The PH consultant who reviewed my design has suggested installing one in my house, as a back up, to the main heat source which would be an ASHP, possibly a compact unit incorporated into the MVHR, during the heating season and Solar Thermal for the other 6mths. I have reservations, I think that the heat input to the room from a WBS is grossly excessive in a passive house, unless most of the heat is distributed away from the source such as a back boiler type or ducted hot air type were to be used.

2.b. I guess an MVHR could play some part in distributing the point source heat of a WBS but this strikes me as probably inefficient and less effective than providing the right amount of heat input in each room to balance losses.

regards
Ian