Looking on the web I have seen mention of MVHR with humidity recovery. The Paul santos F 370 DC is one example and several products from Zehnder Comfosystems (eg. ComfoAir 350) offer it as an option. The Paul documentation refers to a "salt ion membrane moisture-heat exchanger" and Zehnder Comfosystems refers to an "enthalpy heat exchanger" and goes on to state "The problem of too dry air in buildings with ventilation systems arises particularly in winter".

Is low humidity a problem with MVHR in the UK? Or is this option targeted at places with cold, dry, alpine winters?

I'm wondering what features a MVHR should have for my house renovation in Cambridge.

I don't think humidity recovery is an issue for these isles . Rh levels are always high .

For the heat recovery efficiency - consult here to select an appropriate model
http://www.sap-appendixq.org.uk/search.jsp

Fire safety
1.Insist that the fan motor is linked to your fire alarm or smoke detector installation to power down in the event of fire
2. Insist that all ceiling inlet outlets are fitted with fusible link dampers - intumescent collars do not work

It is best that your building achieves at least a reasonable level of air tightness - at least Q50 ~ 5 . In a leaky building a heat recovery installation - won't .

Take a look at this guidance, emanating from the Canada Mortgage and Housing Corporation recently, on what you need to do to keep your MVHR (which they refer to as HRV) systems tickety-boo.

MAINTAINING YOUR HEAT RECOVERY VENTILATOR (HRV)

Your heat recovery ventilator (HRV) can help make your house a clean, healthy living environment, while keeping fuel bills down. But your HRV can't do all this without your help.
It only takes seven simple steps to keep your HRV happy…

The Seven Steps to a Happy HRV

First turn off the HRV and unplug it.

• Step 1: Clean or Replace Air Filters
Dirty or clogged filters can lower ventilation efficiency. Try to clean your filters at least every two months. Filters in most new HRVs can be easily removed, cleaned with a vacuum cleaner, then washed with mild soap and water before being replaced. Older units have replaceable filters. If your HRV is easily accessible, this is a 5 minute job.

• Step 2: Check Outdoor Intake and Exhaust Hoods
Remove leaves, waste paper or other obstructions that may be blocking the outside vents of your HRV. Without this vital airflow, your HRV won't function properly. During winter, clear any snow or frost buildup blocking outside vents.

• Step 3: Inspect the Condensate Drain
Check to see if your HRV has a condensate drain, a pipe or plastic tube coming out of the bottom. If it does, slowly pour about two litres of warm, clean water in the drain inside the HRV to make sure it's flowing freely. If there's a backup, clean the drain.

• Step 4: Clean the Heat Exchange Core
Check your HRV owner's manual for instructions on cleaning the heat exchange core. Vacuuming the core and washing it with soap and water will reduce dust which can build up inside the core.

• Step 5: Clean Grilles and Inspect the Ductwork
Once a year, check the ductwork leading to and from your HRV. Remove and inspect the grilles covering the duct ends, then vacuum inside the ducts. If a more thorough cleaning is required, call your service technician.

• Step 6: Service the Fans
Remove the dirt that has accumulated on the blades by gently brushing them. Most new HRVs are designed to run continuously without lubrication, but older models require a few drops of proper motor lubricating oil in a designated oil intake. Check your manual for complete instructions.

• Step 7: Arrange for Annual Servicing
Your HRV should be serviced annually. If you are not comfortable doing it yourself, contact a technician accredited by the Heating, Refrigerating and Air Conditioning Institute of Canada. Make sure the technician you call has been trained by the manufacturer of your HRV.

Check Your HRV Balance: the Garbage Bag Test
HRVs need to be balanced, with the fresh air flow matching the exhaust flow. If you do not know if the HRV was balanced when installed or if you have changed or added HRV ducts, you may want to check the balance with the following simple procedure. This rough test will take about 10 minutes.

Use a large plastic leaf collection bag, typically 1.2m (48 in.) long. Untwist a wire coat hanger. Tape the wire to the mouth of the bag to keep it open. You now have a garbage bag flow tester. Go outside to where your HRV ducts exit the foundation.

• Step 1:
Crush the bag flat and hold the opening tightly over the exhaust hood. The air flowing out of the hood will inflate the bag. Time the inflation. If the bag inflates in eight seconds or more, go to Step 2. If the bag inflates in less than eight seconds, turn the HRV to a lower speed, and repeat the test. Then go to Step 2.

• Step 2:
Swing the bag to inflate it and hold the opening against the wall around the HRV supply hood. The air going into the HRV will now deflate the bag. Time the deflation. If your HRV is balanced, air going into the HRV will balance the air coming out of the HRV. The inflation and deflation times should be roughly equal. If you find that the bag inflates twice as fast as it deflates, for instance, your HRV is unbalanced. If you can see no problem with the filters that would cause such an imbalance, you should call a service person to test and adjust your HRV.

Please don't ignore your HRV. Just a little bit of your time is all it takes to keep it running smoothly.

• April or May
— Turn dehumidistat (the adjustable control on many HRVs which activates the HRV according to relative humidity) to HIGH setting or to OFF.

• September or October
— Clean core
— Check fans
— Check condensate drain
— Check grilles and ducts in house
— Reset humidistat (40%–80%)

As well as comfortable RH, another thing that humidity recovery is doing is recovering the latent heat of evaporation from the outgoing air, as well as the 'sensible' heat ('sensible' means you can sense it). If your outgoing air contains a lot of evaporated water e.g. from bathroom, laundry or kitchen, then indoor heat was used up in evaporating it, resulting in lowered indoor temp, which you don't notice because the heating system kicked in to make up the loss. Or else the evaporation was 'powered' by the heat put into the evaporating water by the hot water system. Humidity recovery recovers that heat and puts it back into the incoming air. Without humidity recovery it's just discharged. Humidity recovery only happens when the evaporated water recondenses within the MHRV box - so there will be condensate discharge. The question is whether there's enough indoor evaporation happening, to be worth recovering by spending extra on pukka equipment. In some parts of the world such as Canada much indoor humidity occurs because of (continental) climatic factors, which can then be worth recovering - prob not that in UK. Can anyone explain the last bit better?

<blockquote><cite>Posted By: CWatters</cite>There are foam filters on our ceiling extract vents. </blockquote>

Don't want to alarm you CW but look here

http://www.youtube.com/watch?gl=GB&hl=en-GB&v=jQV_MJwYQ3k&feature=related

How do you expect these foam ( really - foam ? ) filters do behave in case of fire - after 90 seconds

Please read through this thread too

http://www.boards.ie/vbulletin/showthread.php?t=2055385732&highlight=fire