MVHR vs DCV

Monfreid

hello,

DCV seems to be cheaper to install and run (no filter, only extracting fan running) whereas the MVHR seems to be more expensive to install and run (maintenance, running all the time, ...).
Is it too simplistic or is MVHR really more efficient and cost saving than the DCV regardless the maintenance (filter, ...)
At the moment, I'm tempted by the DCV because it's kind of the normal vent just a bit smarter whereas the other one is expensive to install and run
Please share your experience/knowledge of these systems, that will help me make a choice

Thank you

BryanF

Mvhr : new house
Dcv : existing Reno

KCross

DCV: No experience of it.

MVHR: Yearly running costs for me are about €60 for electricity. The filter costs can vary alot depending on where you buy and how often you change them. I put in a new set about once a year and just hoover/clean them in between.... its a judgement call as to when they really need a new set(€10 for a set of new filters for my Brink unit).

I havent had any other maintenance costs. 3 years installed now.

I would also add that it is useful, I think anyway, to have the ability to boost the unit if you need a quick air exchange (showers, toilets, kitchens etc). I can press a button which boosts the fan to max for 5mins and then it automatically goes back to its normal speed.

murphaph

KCross wrote: »

I would also add that it is useful, I think anyway, to have the ability to boost the unit if you need a quick air exchange (showers, toilets, kitchens etc). I can press a button which boosts the fan to max for 5mins and then it automatically goes back to its normal speed.

The "boost" button is available on the DCV system we will be installing in our new build here in Germany. I know some folks don't believe in DCV for new builds but they are much more popular than MVHR in several western European countries (especially France), even for new builds.

An advertising slogan from our chosen system (that has been verified by the Frauenhofer Institut for building sciences-a respected German institution) is "80% of the efficiency for half the cost". This claim is IMO justifiable. The difference in cost of installation, running cost and maintenance will not be recuperated in an average home IMO....but others may disagree, such is life.

The primary function of either system is to provide ventilation, not to save energy. So, with that in mind you also need to look at which system will deliver the best indoor air quality (IAQ). MVHR systems (unless you install the optional sensor units!!) do not modulate according to current IAQ. DCV systems on the other hand measure the IAQ and modulate the inlets to reduce the quantity of fresh (but cold) air entering a given room if the IAQ is better (so an unoccupied room). If a room is occupied, the inlet will detect higher moisture content and using this measurement will increase the cold, fresh air flow into the room. This all happens passively-the inlets are mechanical devices that require no power to do all this.

I personally also feel more comfortable with the simplified ductwork that we will have as we have been able to run a straight stack up from the cellar (utility room) through the downstairs WC (which back on to kitchen so we have no spur to the kitchen and then it passes up through the upstairs bathroom and on up through the attic and out the roof (these systems must vent at roof level here according to the regs). We have just one short but straight spur in the cellar from the utility room through to the guest shower room. Some day this ductwork will become clogged with dust I'm sure and will need to be rodded. It should be extremely easy (like sweeping a chimney basically). The equivalent MVHR system would have had much more complicated ductwork with many bends and inaccessible sections.

It may be possible depending on house layout to design an easy to clean MVHR system but it won't be as easy as a DCV system simply because you have ductwork to and from every room. With DCV you only have ductwork from the "dirty/wet" rooms.

The DCV system we're installing also monitors the vacuum present in the extraction duct and will reduce the fan speed if the vacuum increases (indicating all the rooms are unoccupied and the inlets are tending to closed).

Monfreid

Thanks for your input, appreciate it.

is your DCV system the one for aereco?

Thanks

murphaph

Sorry, I mentioned the name of it once before and got a friendly reminder that naming manufacturers is not allowed in this sub forum, but I won't deny that it is that manufacturer. ;-)

Builder Ben

How does a DCV system perform in a house with high (passive) levels of airtightness? I was always led to believe that if you have very high levels of airtightness you must use a mechanical ventilation system...

murphaph

DCV is a mechanical ventilation system. There's a fan that runs constantly pulling "dirty" air from wet/steamy rooms like bathrooms and kitchens and venting this bad air to the outside (without heat recovery). DCV systems perform perfectly well in highly airtight houses....ours will be (our house will be independently blower door tested by the German TÜV). Indeed, DCV probably wouldn't work very well in a not so airtight building as the special inlet elements wouldn't be regulating the incoming air-it would just come in through the leaky fabric.

The difference between DCV and MVHR is in the way the fresh air is supplied. With DCV is enters directly (so without being tempered in a heat exchanger) through special (passive) openings in the walls or windows. These openings tend to close if the air in the room has higher moisture content (an excellent indicator of occupation and hence poorer air quality) and to close if the air in the room has lower moisture content (indicating better air quality). These openings rely on a property of a (now out of patent) strip of nylon polymer, which tightens/loosens depending on air moisture levels and this tightening/loosening is used to open/close the inlet.

An MVHR system on the other hand pulls fresh cold air in at some central location and tempers this cold air in a heat exchanger using the energy in the warm dirty air pulled from the bathrooms/kitchen and then delivers this tempered air to the various "clean" rooms like living room and bedrooms.

The only disadvantage I have been able to ascertain in choosing DCV over MVHR (assuming you are happy with the cost/benefit ratios etc. as we are) is that MVHR is balanced-what goes out is pulled in and there should be no vacuum or pressure in the house. With DCV the system requires there to be a slight vacuum present (or else fresh air won't be pulled in through the vents) and this may be an issue in combination with an open fire/stove. We are installing a room sealed stove with independent air supply, so the vacuum can't cause combustion gases to be pulled into the living space instead of escaping up the chimney.

German building regs are clear on all this but building regs here don't even allow an extractor hood to vent to the outside in a house with a fireplace/stove unless a window is fitted with a contact switch to only allow the hood to be switched on when the window is open, thus preventing any vacuum forming and potentially pulling CO into the living space. I have never heard of such a regulation in Ireland for extractor hoods, though it would make sense in highly airtight buildings to consider this.

thatslife

murphaph wrote: »

DCV is a mechanical ventilation system. There's a fan that runs constantly pulling "dirty" air from wet/steamy rooms like bathrooms and kitchens and venting this bad air to the outside (without heat recovery). DCV systems perform perfectly well in highly airtight houses....ours will be (our house will be independently blower door tested by the German TÜV). Indeed, DCV probably wouldn't work very well in a not so airtight building as the special inlet elements wouldn't be regulating the incoming air-it would just come in through the leaky fabric.

So just to get this right, a fan runs constantly taking air from the wet rooms. This fan turns off when it senses a vacuum being created, right? So does each wet room have its own vacuum sensor? only 1 fan so there cant be, right? So system cant be extracting from 1 room thats in use if the other room is pulling a vacuum.

893bet

System has a humidity sensor which extracts when humidity is over a set level. Switches off when level drops. Nothing to do with vacumns.

thatslife

murphaph wrote: »

The DCV system we're installing also monitors the vacuum present in the extraction duct and will reduce the fan speed if the vacuum increases (indicating all the rooms are unoccupied and the inlets are tending to closed).

893bet wrote: »

System has a humidity sensor which extracts when humidity is over a set level. Switches off when level drops. Nothing to do with vacumns.

Murph mentions a vacuum monitor in his first post

air

The extract fan monitors the pressure drop across itself, if the drop increases it indicates a reduced flow rate (inlets closed / house unoccupied) and the fan slows to it's minimum trickle setting. Conversely it speeds up when it detects a lower pressure drop - increased flow rate (inlets opened / house occupied)

murphaph

^^ this.