Demand Control Ventilation - Report after 6 months...

JayBee66

Demand Control Ventilation (DCV) works by controlling the flow of air into the house via humidity sensitive vents in non-wet rooms and drawing air out of the house through humidity sensitive extractor fans in wet rooms. Each vent has a humidity sensitive strip which opens and closes shutters in the vent. Supposedly more "intelligent" than ordinary "hit and miss" vents. There are effective sound baffles to reduce noise and flaps to slow air flow during high winds.

The old "hit and miss" vents allowed too much cold air into the house and had to be blocked with rags thus increasing humidity and lowering air quality.

There are at least two manufacturers of DCV. I won't mention here or in private messages which one was used. I assume they are both similar in design and performance.

Pros

• Ventilation and air quality has improved. No musty smell.
• No uncomfortable draughts from the vents.
• Reduced humidity (with caveats) throughout the house.
• No mould.

Cons

• There is no heat recovery so warm air is replaced by cold air (albeit of lower relative humidity) which needs heating either from the existing heat in the house or additional heat.
• Ireland is damp (i.e. 60%+ relative humidity) pretty much all year round. You either have to dehumidify to remove water vapour or heat the air, which doesn't remove water vapour but temporarily decreases relative humidity. We have additional dehumidifiers in the house, running off the PV, to keep humidity below 65%. Consistent humidity below 60% is just not possible in our location unless we wastefully overheat every room in the house. Mechanically reducing humidity to below 60% will see it bounce back over 65% within the hour.
• Any wind will cause the DCV vent shutters to rattle noisily throughout the night. To that end the vent in the TV room and occupied bedrooms have had rags stuffed back into them for the winter. The high winds are probably producing enough air flow with the remaining vents. However, occupied bedrooms are now dead areas with regards to air flow, which might need a dehumidifier.

Conclusions

As far as air quality (humidity and freshness) is concerned, DCV works. I have no problem with DCV during warmer months that are free of storms with high winds.

DCV combined with dehumidification has reduced house relative humidity to less than 70% (the point at which moulds grow). House humidity is generally below 65% (the point at which psocid pests (book lice, flour lice etc.) begin to reproduce).

Our system cost €2300 and was self-installed. We went with DCV as the MHRV (Mechanical Heat Recovery Ventilation) quote was €8000 installed and I didn't want any pipe work in the attic. Either I will find a work around for DCV by making bespoke baffles that reduce air flow in "noisy rooms" to a minimum that doesn't cause excessive rattling. Alternatively, some of the noisy DCV inlet vents might be replaced by Decentralised Single-Room Heat Recovery Ventilation units but only if there are no noise issues with electric fans.

loyatemu

i've been running a dehumidifier on our landing for the last few months, set at 55%. I have a bluetooth humidity monitor in one of the bedroom (the one that suffers most from mould) and it's stayed around 60% or below since I set it up. Dehumidifier cycles on and off, I only have empty it a couple of times a week, it's hardly been running at all the last few days with the cold dry weather. We do have vents above the door of each bedroom, & I try to leave the doors open during the day when the kids are out as well to allow the dry air to circulate.

Out here in the west of Ireland relative humidity bounces around the 80% mark all year. It's not uncommon for it to hit 90%. As such this sort of system would not solve many of the issues associated with house damp.

Horses for courses. We went with MVHR (self installed) but haven't occupied the house yet so can say if it will solve damp issues in our highly insulated retrofit. It's very dry at the moment.

DC999

https://www.boards.ie/discussion/2058326140/demand-control-ventilation-report-after-6-months

Appreciate the post. I'm still learning about MHRV and DCV. You've helped explain it

MicktheMan

https://www.boards.ie/discussion/2058326140/demand-control-ventilation-report-after-6-months

To my mind it makes no sense to have a centralised mech vent system AND dehumidifiers running except in exceptional circumstances. Somethings off. A centralised mechanical ventilation system, be it a cMEV, DCV or MVHR, in combination with sufficient heating should be well capable of maintaining sufficiently low RH levels in the house resulting in a dry house and good IAQ during the heating season. During the warmer months ventilation, in the sense of moisture exhaustion from the house, has no roll.

Btw, I don't believe that Ireland is any damper than any other region. It just doesn't get cold enough in the winter here to be easily able to exhaust the internally generated moisture due to the lack of a decent internal to external temperature differential. The issue isn't damp, our mild temperatures make ventilation less efficient. Anyone monitoring their internal humidity levels can attest to the drop seen last week in their humidity during the cooler spell of weather we had.

graememk

yeah the cold weather, my dehumidifer didnt run at all. The damp summer months, Id be emptying it daily! (do check the filter on them, mine was manky!)

Hard to ventilate the moisture out when its 12-14 c outside and 90+ humidity.

No experience with MVHR, Just looking to gain knowledge

With MVHR, with mild humid weather, can it keep humidity lower ? At what point can it not?

https://www.boards.ie/discussion/comment/121472248#Comment_121472248

Without some specific method of venting humidity from a house or removing it by dehumidifying, the external humidity level is the theoretical lower limit of internal humidity. That can still be very high.

Colder weather creates lower external humidity simply because cold air can carry less moisture - and this intern sets a lower limit for internal humidity.

Heating allows higher internal humidity levels because warm air can carry more moisture. If not adequately vented it will cause more condensation.

Demand ventilation primarily works to prevent wet areas leaking moisture into dry areas, which would cause higher overall internal relative humidity, by removing it at source. It also cuts down hot air leakage by only allowing ventilation appropriate to the relative humidity, ie it throttles ventilation when not needed

There are plenty of people with very warm houses who have terrible condensation problems because they have not addressed ventilation. No amount of heating will solve this if there is a ready source of moisture in the house - ie people.

Also east to west in Ireland has increasing and very high external humidity levels. Many parts of Ireland have humidity approaching 80-90% all year round - this is exceptional for Europe and the primary reason why most Irish houses have issues with damp which are relatively rare elsewhere.

listermint

OP blocking up supply vents will ensure that the DCV vents don't work effectively enough. It's not the intended operation of them. Additionally your doors will need to be undercut for flow to work effectively. Many people make this mistake and aren't looking at the entire house operation and just the specific area they've put the unit in.

Otherwise may aswell just have an ineffective bathroom fan installed.

Calahonda52

Useful Post OP, thank you

http://www.dpcalc.org/ shows the relationship between Room temp, RH and dew point.

IIRC the OP says that keeping the TRH below 70% is the key number.

so assuming a room temp of 20, and RH of 70, the dew point, which is when the condensing out starts, is 14

For folk with MVHR and high summertime humidity, often a cooling coil on the intake is required.

Having said all that, I am with Mick here, little or no discussion on managing the sources of the vapour load in the house

JayBee66

https://www.boards.ie/discussion/comment/121489661#Comment_121489661

I understand what you are saying but DCV doesn't work in occupied bedrooms because of the racket the vent makes. The iris, which opens and closes to throttle air input, is made of plastic and gusts of wind makes them hit the casing of the vent.

I have only blocked 2 of the 4 vents so there is air flow from one end of the bungalow to the other.

I am considering making a cowl to go over the external cover so that wind can't block directly into the vent but I still think that gusts of wind will still create a vacuum on one side of the house, pulling at the iris and making it rattle.

Surely, the French have wind too and know about this.

All the doors have sufficient space under them. The wet rooms have their DCV ceiling extractors and function properly.

The only negative is the noise the passive vents make.

listermint

https://www.boards.ie/discussion/comment/121489726#Comment_121489726

The occupied bedrooms are where humidity generation occurs for the bulk of 24 hour cycle.

The wet rooms are high concentration humidity spikes for short periods and are the obvious place to start.

However you've hamstrung it by not addressing humidity generators , people.

JayBee66

https://www.boards.ie/discussion/comment/121489713#Comment_121489713

65% RH is the target in our house. Moulds form at a persistent 70% RH but psocid pests breed at 65% RH so we run dehumidifiers for a few hours each day to dry the bedrooms.

There are three DCV extractors in the house; kitchen, bathroom and ensuite so all the wet rooms are catered for.

It's just your typical, badly designed and built, Irish bungalow.

150 years ago, my ancestors lived in cabins with more than the required amount of ventilation and culm burning 365 days of the year to keep the damp out. We seem to be still building houses for those requirements.

We have large downward pointing cowls on the input and output of our MVHR ducts. They seem to help.

listermint

https://www.boards.ie/discussion/comment/121489752#Comment_121489752

They also had average life expectancy of maybe 35-40