dMEV and heating

[MarkyG82]

I'm coming back round the houses (pun intended) with the whole ventilation thing. 

BC have commented that we need extract from the ground floor WC (not external walls) and the upstairs wet rooms need new ventilation. 

My plan all along was to retrofit MVHR.  I try to put my money where my mouth is and do things right for all (PV, EV, etc.) and I thought MVHR was the right thing as it would rescue some of the heat lost while still offering some nice fresh air.  I'm now considering dMEV as it's dramatically cheaper and easier to install.  Then combine it with a positive pressure inlet it basically creates a modular MVHR without the R.

 

Long way of asking: how does a dMEV property cope with heat loss through the ventilation?  I don't want to go running the heating only to have it all sucked out by the vents.  We also sleep year round with the windows open but was hoping the MVHR would put a stop to that with the fresh supply.

 

Thinking out loud moment: with the relatively low cost of dMEV, could I install that then swap out to MVHR later if it's not enough?

[JohnMo]

9 minutes ago, MarkyG82 said:

I'm now considering dMEV as it's dramatically cheaper and easier to install.  Then combine it with a positive pressure inlet it basically creates a modular MVHR without the R.

No, do one or the other not both.

 

Wouldn't bother with PIV.

 

dMEV would be my choice or MEV. But you want to make sure it extracts the minimum amount of air - based on internal conditions. You need the following for it perform as required.

 

1. All internal doors need to be undercut (around 10mm clearance to floor covering). This allows cross ventilation.

2. You need inlets in dry rooms, trickle vents or otherwise - but you need these to automatic open and closed based humidity

3.  Your dMEV or MEV unit needs to automatically boost based on humidity. Greenwood CV2GIP are silent, draw next to no power, can by connected and controlled by a 3 core cable only.

 

You can get heat recovery bathroom fans, but they just ventilate the room they are in. You also get reversing fans with heat recovery but start to get expensive, as you need them in every room.

 

Heat losses is minimised by only ventilating at minimal rates as required.

[ProDave]

Well ventilation running 24/7 WILL suck heat out of the house.  That is the whole point of mvhr, that most of the otherwise lost heat, is recovered.

[Alan Ambrose]

Does the Greenwood CV2GIP unit close off the airflow somehow when it's not running, or is that not important?

[JohnMo]

28 minutes ago, Alan Ambrose said:

Does the Greenwood CV2GIP unit close off the airflow somehow when it's not running, or is that not important?

dMEV units run all the time at a low rate.

[JohnMo]

39 minutes ago, ProDave said:

Well ventilation running 24/7 WILL suck heat out of the house

You would be surprised how little a condition based system does. 

 

trickle rate for bathrooms/WC/utility is 5l/sec (21.6m3/h) and kitchen at 8l/s (28.8m3/h). Boost is only in the rooms requiring it

 

So house 3 bathroom, utility and kitchen, trickle rate 115.2m3/h

 

Ventilation heat loss = Q_v = 0.33 × Air volume x ΔT 

0.33 * 115.2 * 24 = 0.9kW at -3 OAT

 

90% efficiency MVHR at Scottish Building regs rates would, be 0.23kW at -3 OAT.  So 670W difference

 

dMEV advantages - No filters, heat exchangers and duct to pump through (supply and extract, only extract fans not supply fans. No filters to replace.

 

Cheap to install, I paid £35 for my fan, used in my summer house.

 

 

 

 

[MarkyG82]

Thanks all.

@JohnMo by don't do both I'm assuming you mean don't do PIV and dMEV together. A couple of articles I read suggested doing them together. I guess it's a way of controlling what air is coming in and gives the option for filter and heat?

I trust your input though. dMEV units are inexpensive to fit so I think that's the way we'll do rather than MVHR.

[OwenF]

FWIW I was swinging between PIV, PIV&MEV or just MEV.

 

I’m now settled on just MEV (probably centralised - Vent Axia multivent) for wet rooms. Plus a ‘dumb’ extract for hob.

 

I like the idea of humidity controlled trickle inlets but 1) they are difficult to source, especially not in window vent form, 2) my house is not airtight (only partial downstairs with good levels of airtightness)

 

Therefore I’m going to bank on enough background intake and the MEV controlling low-rate/boosted extract based on humidity 

[JohnMo]

15 minutes ago, MarkyG82 said:

don't do PIV and dMEV together

Yes choose one system or the other.

 

Not all dMEV units are equal. The Greenwood mentioned above was my second make, the first was quite noisy. Always seem to be plenty of Greenwood units on eBay for good prices.

 

2 minutes ago, OwenF said:

now settled on just MEV

I looked when I was designing my house quite liked the Duco system

 

https://www.duco.eu/uk-ie/products/mechanical-ventilation

[MarkyG82]

The building work is done and final touches qre coming to an end (for the new space at least).  I thought I'd come back to this for an update and hopefully extract (pun?) some more info from you lot.

 

We now have an Envirovent dmev unit with switched boost in the downstairs WC. Function wise it's really good. Has a mesh filter that so far I've needed to clean once a fortnight. Hoping that will calm down (or could we run it without?). One thing I've noticed is that it's noisy vibrating through the house. I plan to take a look and see if I can fit some sort of gasket between it and the ceiling to isolate it.

 

I was planning on getting the same unit for the ensuite but if the noise is standard for that type (surface mount) I have now been looking at central MeV units to service both upstairs wet rooms. It seems a single unit is similar in price to two dmev units and has the desired benefit of remote location for sound isolation.

 

Am I barking up the right tree? The units I'm looking at (vent axia and Envirovent) both have humidity sensing boost and constant trickle. This should give us the fit and forget function we desire along with the performance. When I first entered this project I didn't appreciate the variety of equipment and this type had passed me by until now. It seems like the perfect solution to my needs.

 

To sum:

- Envirovent dmev noisy. Can it be improved?

- Is a single MEV better than two dmev units for noise and performance?

[JohnMo]

13 hours ago, MarkyG82 said:

Envirovent dmev

That was my first unit and then moved to Greenwood and you cannot tell it's running.

 

I would just do two dMEV units, as recommended in March. 

On 19/03/2024 at 18:03, JohnMo said:

Greenwood CV2GIP

 

13 hours ago, MarkyG82 said:

Envirovent dmev noisy. Can it be improved?

Yes - See above. My wife hated the first unit we installed before the noise.

 

[LowCarbonLiving]

Hi All,

 

I'm completely new to the forums so a warm hello to all. I work in the Retrofit Industry, where MVHR is the theoretical gold standard. I say theoretical because the majority of retrofit homes will not reach the air tightness required to make MVHR worthwhile. You would need to carry out a very deep retrofit to EnerPHit standards for MVHR to work poperly. What we tend to routinely instead recommend to our clients is some form of passive and mechanical extract ventilation combination (window trickle vents + continuous running extract fans + 10mm door undercuts) . 

I've invented a system that uses these principles, but with the addition of a demand driven component (Co2 and Humidity sensors), and plan to use this in the house I am in the process of purchasing (1950's semi-detached house up in Bedfordshire). The system is on my website if anybody should be curious.

[Dave Jones]

On 19/03/2024 at 19:08, JohnMo said:

You would be surprised how little a condition based system does. 

 

trickle rate for bathrooms/WC/utility is 5l/sec (21.6m3/h) and kitchen at 8l/s (28.8m3/h). Boost is only in the rooms requiring it

 

So house 3 bathroom, utility and kitchen, trickle rate 115.2m3/h

 

Ventilation heat loss = Q_v = 0.33 × Air volume x ΔT 

0.33 * 115.2 * 24 = 0.9kW at -3 OAT

 

90% efficiency MVHR at Scottish Building regs rates would, be 0.23kW at -3 OAT.  So 670W difference

 

dMEV advantages - No filters, heat exchangers and duct to pump through (supply and extract, only extract fans not supply fans. No filters to replace.

 

Cheap to install, I paid £35 for my fan, used in my summer house.

 

 

 

 

 

interesting. 

 

But how are rooms without a dmev unit in, bedrooms, studies, living rooms etc supplied with fresh air ?

 

Really needs to be a low cost midpoint between full blown MVHR and through the wall fans with trickle vents.

[Redbeard]

Via the trickle vents and undercuts referred to. The dMEV units are the extract/'pull' which pull fresh air in via the trickle vents.

[JohnMo]

18 minutes ago, Dave Jones said:

But how are rooms without a dmev unit in, bedrooms, studies, living rooms etc supplied with fresh air

dMEV and MEV use the principle of cross ventilation, if you do not provide either trickle or other vents in dry rooms, you may as well not bother, because it does not work, many studies have shown this. For most efficient operating you need moisture activated vents or auto vents, also limit householder intervention or closing off vents. Both of these can close down when that room doesn't need ventilation - such as a bedroom in the day or living room over night.

[Dave Jones]

13 minutes ago, JohnMo said:

dMEV and MEV use the principle of cross ventilation, if you do not provide either trickle or other vents in dry rooms, you may as well not bother, because it does not work, many studies have shown this. For most efficient operating you need moisture activated vents or auto vents, also limit householder intervention or closing off vents. Both of these can close down when that room doesn't need ventilation - such as a bedroom in the day or living room over night.

 

be interesting to see if a combination of trickle vents and one of the single room type MVHR units, this one does 50m2, would be a comprimise on heat recovery vs full mvhr

 

https://www.bpcventilation.com/bsk-zephyr-single-room-heat-recovery-unit

[JohnMo]

13 minutes ago, Dave Jones said:

 

be interesting to see if a combination of trickle vents and one of the single room type MVHR units, this one does 50m2, would be a comprimise on heat recovery vs full mvhr

 

https://www.bpcventilation.com/bsk-zephyr-single-room-heat-recovery-unit

Trouble is it wouldn't provide ventilation anywhere in the house except the room it's installed within. Basically it will supply and extract equal amounts. Leaky house and MVHR is a waste of time and money. Very leaky house (open fire places etc) doesn't really need additional ventilation. MEV and dMEV  are for the middle ground leaky houses, not enough leaks to give adequate ventilation, so give slow steady background ventilation, add 

 

10 hours ago, LowCarbonLiving said:

demand driven component (Co2 and Humidity sensors)

then the system becomes very efficient, so no-one home in the day the system almost shutdown, and can actually on average, performance just about as well as MVHR. 

[LowCarbonLiving]

Totally agree with the comments above. I do retrofit plans as a full time job. Your energy rating will drop (by a full 10 points) if you have a MVHR system in your property. This is controversial, and looks to change in the revised rating system(definitely should not reduce the rating of a Passivhaus). However, I think the assumption that an MVHR system inappropriately installed will simply make a house more draughty, with very little heat recovery benefits, is correct.

The middle ground is to have passive ventilation features, with a designed ventilation path utilising mechanical extract fans (look at a floorplan of the property, work out how the air will move inside the house based on where trickle vents and fans are located). I've designed and manufactured such a system which I plan to install in the house I'm buying. As my system is CO2 and Humidity sensor driven, it's fairly agnostic of your actual air tightness level. The system will simply extract as actually required to keep occupants and the building healthy (Wifi networked extract fans being controlled by an air quality monitor reading CO2 and humidity). It's also fairly low cost and DIY friendly. You can simply swap existing extract fans with my system, and add fans as required by occupancy levels(rough guide, 1 extract fan per adult).

The single room Heat Recovery Ventilation systems work out as very expensive, and you would need one for every room, and the same counterproductive pitfalls of a whole house MVHR system apply (very high air tightness required).

Edited August 24 by LowCarbonLiving