Demand Control Ventilation V MHRV?

Ned78

Hi there,

I am looking for advice regards Demand Control Ventilation System versus an MHRV regular system. will be building a 1700 sq.ft bungalow with attic conversation next February/March. As far as I can tell the DCV seems the best option,
1. unit and installation seems cheaper at this stage
2. cheaper to run
3. only vents bedrooms/sitting room when needed

Has anyone here any advice please.

Thanks in advance

Condenser

Theres no cross over between these two systems. If you're building an air tight house use HRV, if not use DCV. DCV is not compatible with air tightness and HRV is not compatible with houses which are not to a large degree air tight.

Ned78

Thanks for the reply

I am hoping to get as air tight as possible within my budget, DCV was recommended to me by a renewables company, they do not sell or install same, I presume a DCV is not suited to airtightness is due demand control vents venting directly to the outside in the bedrooms/sitting room, thereby losing heated air, is this the case?

Thanks again.

Condenser

Ned78 wrote: »

Thanks for the reply

I am hoping to get as air tight as possible within my budget, DCV was recommended to me by a renewables company, they do not sell or install same, I presume a DCV is not suited to airtightness is due demand control vents venting directly to the outside in the bedrooms/sitting room, thereby losing heated air, is this the case?

Thanks again.

Yes exactly. Theres no point in going to all the bother of air tightness then putting holes in nearly every wall in the house. DCV is a great alternative to wall vents or window vents as it gives a decent degree of ventilation control but offers no heat recovery aspect at all.

simon334

Hi all

Recent post deleted by mod (my fault did not read the rules) apologies again.

Just a quick note to clarify a common misconception.

Demand Controlled Ventilation (DCV) is a method of control of ventilation. To be exact.

" a system that provides automatic regulation of the ventilation system by sensing the indoor air quality (IAQ) and determining the required air change rate. It does this without user intervention (automatically) optimizing air flow rates to reduce ventilated heat loss and primary energy consumption without compromising IAQ. "

This principle is commonly applied to in whole or part to Centralized mechanical extract system (CMEV), Passive Stack (PSV) and Mechanical Ventilation with heat recovery (MVHR).

The most common of the above would be CMEV, and this system would typically comprise of a central fan extracting through wet rooms and then ventilation opening directly to outside to supply fresh air.

It is however a mistake to simply think that one system MVHR is more suitable than another DCV or the other way round because of airtighness. Both systems perform very well in air tight buildings.

There is a small difference in the heating season in ventilated heat loss between the two systems, obviously because one is recovering heat, but remember that the heating season only makes up half the year. Also while it is the heating season DCV is matching air change rates and also fan consumption to demand minimizing the effect of no heat exchange.

The end result in a temperate climate like Ireland is very small. Both systems are very good at controlling indoor air quality in an efficient manner. They simply do it in very different ways.

A simple explanation from an energy point of view is this.

MVHR is a bit like inventing an energy efficient light bulb and then insisting that all the lights are left on regardless of weather it is day or night. DCV is the invention of the light switch.


Hope this helps.

Many regards

MicktheMan

simon334 wrote: »

Hi all

Recent post deleted by mod (my fault did not read the rules) apologies again.

Just a quick note to clarify a common misconception.

Demand Controlled Ventilation (DCV) is a method of control of ventilation. To be exact.

" a system that provides automatic regulation of the ventilation system by sensing the indoor air quality (IAQ) and determining the required air change rate. It does this without user intervention (automatically) optimizing air flow rates to reduce ventilated heat loss and primary energy consumption without compromising IAQ. "

This principle is commonly applied to in whole or part to Centralized mechanical extract system (CMEV), Passive Stack (PSV) and Mechanical Ventilation with heat recovery (MVHR).

The most common of the above would be CMEV, and this system would typically comprise of a central fan extracting through wet rooms and then ventilation opening directly to outside to supply fresh air.

It is however a mistake to simply think that one system MVHR is more suitable than another DCV or the other way round because of airtighness. Both systems perform very well in air tight buildings.

There is a small difference in the heating season in ventilated heat loss between the two systems, obviously because one is recovering heat, but remember that the heating season only makes up half the year. Also while it is the heating season DCV is matching air change rates and also fan consumption to demand minimizing the effect of no heat exchange.

So, what's stopping the mvhr user switching off their system during the non-heating season and using their windows?

simon334

MicktheMan wrote: »

So, what's stopping the mvhr user switching off their system during the non-heating season and using their windows?

No nothing at all.

The only danger of this is in summer is the main visual indicator of increased RH is gone (condensation on cold surfaces).

Increased, CO2, VOC's etc are much harder if not impossible to assess. This only really leaves our ability to detect stuffy room conditions. Anybody that has recently been or has a teenage boys knows that they can get pretty used to poor indoor air quality if they spend enough time in it!!

In more airtight buildings unless many windows can be left open all the time, how can you be sure the building is getting enough air exchange. If leaving the windows open is not a security issue anyway.

Most HRV companies would not recommend switching the system off for this reason, it is why they invented a summer bypass. Generally when you design a mechanical system of any sort to deliver controlled air exchange it is that system that has to deliver it at all times of the year.

Many regards

brophis

Don't know much about DCV as I went for MHRV myself. Does DCV use filters to filter the air coming in? I think this is a great feature of MHRV.

simon334

brophis wrote: »

Don't know much about DCV as I went for MHRV myself. Does DCV use filters to filter the air coming in? I think this is a great feature of MHRV.

Some do most don't.

It can be a good feature depending on your point of view. If your a diligent home owner willing to change the filter regularly and you have the right combination of filter then yes you can argue it has a benefit. If your a social landlord hoping your tenants will do the same then it can be a very big liability!

My personal opinion is unless your in a high pollution area, the likes of which we don't really have in Ireland, then whats wrong with fresh out door air any way.

My understanding is the filters are there mainly for mechanical filtration to protect build up of dust and particles in the ex-changer and incoming fresh air ducts.

At the AIVC conference last year in Brussels there was a lot of focus on air quality through ducting systems and performance fall off of machines through poor filter maintenance. (in short change your filters regularly)

I lived with a heat recovery system for 5 years and it is surprising how quickly they became very bad looking. Good system though!

Many regards

sinnerboy

Condenser wrote: »

DCV is not compatible with air tightness

That is simply not true. DCV will work irrespective of of air tightness.

MVHR requires good airtightness to work efficiently.

One thing I think must be clarified with respect to MVHR and DCV. The primary function of both systems is not to save energy but to provide fresh air. Both do save energy but that is a side issue.

Condenser

sinnerboy wrote: »

That is simply not true. DCV will work irrespective of of air tightness.

MVHR requires good airtightness to work efficiently.

One thing I think must be clarified with respect to MVHR and DCV. The primary function of both systems is not to save energy but to provide fresh air. Both do save energy but that is a side issue.

My point is not that it won't work, its that your house cannot be defined as air tight if you have DCV.
I don't see the point in making your house airtight and then putting 10 or so 4" holes with baffles in the wall which in themselves are not airtight. If you go to the extent and the amount of work required to achieve airtightness then I think doing the above is ridiculous.
I have no problem with DCV, great product but as with all things its about application.

simon334

Condenser wrote: »

My point is not that it won't work, its that your house cannot be defined as air tight if you have DCV.
I don't see the point in making your house airtight and then putting 10 or so 4" holes with baffles in the wall which in themselves are not airtight. If you go to the extent and the amount of work required to achieve airtightness then I think doing the above is ridiculous.
I have no problem with DCV, great product but as with all things its about application.

Be careful not to get intended and unintended ventilation mixed up.

A hole in the fabric of the building for the purpose of ventilation is not a leak but intended or purpose ventilation, this is why it is blocked up when an air tightness test is carried out.

It does seem counter intuitive to add holes to an air tight building, but as we remove unintended ventilation (Leaks) then unless there is a continuous mechanical supply and extract to all rooms then holes are critical to ensure the building and its occupants are protected. (this counter intuitive element is your point i think.)

However if those holes in the fabric are controlled as part of a DCV system then the overall energy loss will be very similar over twelve months to an MVHR system. (think about it this way, is your building air tight with MVHR if its blowing air in all the time?)

The point with ventilation as pointed out in part by sinnerboy is two fold
1. to secure good indoor air quality through adequate air exchange to meet demand and.
2. to limit ventilation heat loss where possible.

Both DCV and MVHR do the same but in different ways.

Many regards

sinnerboy

Condenser wrote: »

your house cannot be defined as air tight .

Please find me a link - anywhere - that defines what an air tight house is. Building Regulations impose a minimum level of permeability of the building fabric.

Even a passive house is not airtight it just has a low air permeability rate ( expressed in terms of air changes per hour ) .

So to have a healthy indoor environment must connect our indoor air with external air. Swap it over. It's called ventilation and we have to do it by law.

With all respect simon334 both MVHR ( aka HRV ) and DCV must first perform this function. I would list it as follows

1. ventilate
2. ventilate
3. ventilate
4. ventilate
5. ventilate
6. ventilate
7. ventilate
8. ventilate
9. ventilate
10. Limit heat loss whilst doing items 1-9.

Sorry to be pedantic but that's the reality as I see it.

sinnerboy

I have moved this thread our from Renewable Energies as it does not belong there.

simon334

sinnerboy wrote: »

Please find me a link - anywhere - that defines what an air tight house is. Building Regulations impose a minimum level of permeability of the building fabric.

Even a passive house is not airtight it just has a low air permeability rate ( expressed in terms of air changes per hour ) .

So to have a healthy indoor environment must connect our indoor air with external air. Swap it over. It's called ventilation and we have to do it by law.

With all respect simon334 both MVHR ( aka HRV ) and DCV must first perform this function. I would list it as follows

1. ventilate
2. ventilate
3. ventilate
4. ventilate
5. ventilate
6. ventilate
7. ventilate
8. ventilate
9. ventilate
10. Limit heat loss whilst doing items 1-9.

Sorry to be pedantic but that's the reality as I see it.

HI Sinnerboy

Actually could not agree more. Kind of what I was making in points one and two. perhaps I should have added more points!!

Kind regards