Following on from the replacement of my MHRV unit (http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17402) I am beginning to wonder if I have been vastly over ventilating the house.

With far more data available to me from the new unit and the addition of more CO² and Humidity sensors, the pattern I see suggests that I can turn the unit right down. The house is 570 m³

Firstly Humidity - I have discovered the air in the house is really dry; down to 30% humidity. I've bought a humidifier, but even in my relatively small study it is only very slowly bringing the RH up (and running 100%). To me this indicates the fabric of the house is very dry. To be fair, the outside humidity is only 41% today!

Secondly CO² - again this stays low during the day (as long as I leave the study door ajar - see other thread) and even in the evening, with 2 of us here, the large open plan room does not rise above 700ppm.

I also have an IKEA air quality sensor (2.5 PM) although I have not been collecting data long enough to examine what it might be telling me or whether I believe it!

The question really is, is the purpose of home ventilation largely to control humidity and CO² (and smells to some extent)? If so, I'm reducing my ventilation rates!

As an aside, I felt part of the issue with the old unit was that it basically ran at the same rate all the time. I am going to put in an automation that runs it at higher speeds for short periods each day just to exercise the fans.

Thoughts?

“If you can't measure it, you can't manage it.”

CO2 at 700 indicates to me very under ventilated.

With ventilation below 500 is good, above 600 poor above 800 dire

Depends on how many people/animals are in the house.

CIBSE say that 1000ppmv is well ventilated, and more is OK but indicates the room might smell sweaty.

https://www.cibsejournal.com/cpd/modules/2011-04/

Tony the lower numbers we've heard recently are from where people are trying to hyper-ventilate public spaces to control Covid, and are using CO2 as a proxy measure, then opening windows in January, IE the downside of low CO2 is just you need more ventilation hence more heat losses. Might not be an issue for Mr&Mrs Borpin.

HSE say that above 1500ppm indicates that more ventilation is needed to disperse Covid, eg in a workplace where different households meet.
https://www.hse.gov.uk/coronavirus/equipment-and-machinery/air-conditioning-and-ventilation/identifying-poorly-ventilated-areas.htm

I do remember a Mark Brinkley article years ago suggesting ventilation was over rated , again it was just looking at some basic numbers rather than coming from some trong building science research

Posted By: borpin sometime near the middle of the afternoon of 2022-03-09: “To be fair, the outside humidity is only 41% today!”

Where's that and how was it measured? Scanning around

https://www.ventusky.com/?p=52.83;-1.66;6&l=humidity

for 15:00 this afternoon finds humidities down to 50% or so in largish parts of the NE of England (around Leeds to Newcastle) with everywhere else being 60% or higher.

(PS, went to look on your website for location clues but it wasn't responding and eventually gave some sort of database error.)

Posted By: Ed DaviesPosted By: borpin sometime near the middle of the afternoon of 2022-03-09: “To be fair, the outside humidity is only 41% today!”

My weather station (although to be fair I was looking at the wrong dial). Livingston Scotland (graph last 24Hrs).

Posted By: djhThe first five years were completely different. How much buffering is there in your house?

Been built 9 years now so the building has settled down. However, I think it has got overly dry because it has been over ventilated.

Posted By: tonyCO2 at 700 indicates to me very under ventilated.

From all I have read, 700ppm is perfectly acceptable level for comfort. My study CO² meter triggers me to open the door at 750ppm.

Posted By: WillInAberdeenAny ventilator that runs at a constant flow rate (in m³/h or ACH or l/m²/h) is over ventilating for most of the time. You just need enough ventilation to get down to the desired humidity and CO2 on that day.

Ok, good to know. I can now automate this quite easily.

Posted By: Ed Davies(PS, went to look on your website for location clues but it wasn't responding and eventually gave some sort of database error.)

Ah, thanks. Not done much recently and it needs a good sort out and update of the backend. Another job for the list!

Just checked the weather at Edinburgh airport for yesterday afternoon. From 12:50 to 15:20 the temperature was 11°C and dewpoint 3°C from which I get a relative humidity of 57%. That's quite low but I'm still a bit sceptical about Borpin's readings of not a lot more than half that. It could be a matter of sensor calibration or, as WiA suggests, local effects which maybe don't reflect the actual absolute humidity.

2% of full :-)
From the datasheet that'll be around 7m^3/hr.

For interest, attached a pic of dewpoint indoor and outdoor. There's all sorts of ways of calculating dewpoint... I chose the simplest, it's good enough, I'm an engineer, not a scientist :-)

Tdp = T - (100-RH)/5

Posted By: Ed DaviesJust checked the weather at Edinburgh airport for yesterday afternoon. From 12:50 to 15:20 the temperature was 11°C and dewpoint 3°C from which I get a relative humidity of 57%. That's quite low but I'm still a bit sceptical about Borpin's readings of not a lot more than half that

Which is quite a way away, out in the open, 200M lower and close to the sea but YMMV.

Posted By: RobL2% of full :-)

I can see me getting to that. A note though, do exercise the fans at different speeds - I am sure the fact the old unit ran at a constant rate over an extended period contributed to the early failure.

Posted By: djhI don't understand the manufacturer's logic in labelling the MVHR with a meaningless value like 2%. It presumably knows the actual rate, assuming it's a constant volume system, so why wouldn't it tell you that?

I think because it depends on the static pressure of the system. I think that is related to the effort of pushing the air through and I am not sure how to measure that.

Posted By: borpin

Posted By: djhI don't understand the manufacturer's logic in labelling the MVHR with a meaningless value like 2%. It presumably knows the actual rate, assuming it's a constant volume system, so why wouldn't it tell you that?

I think because it depends on the static pressure of the system. I think that is related to the effort of pushing the air through and I am not sure how to measure that.

I'm not quite sure what you mean by 'it'. The MVHR has to measure the flowrate in order to deliver it. The static pressure might determine how much energy it has to use to do that, as does the wind strength at any particular time, but the flowrate is the actual deliverable and is what is stated to be constant. My 'standard' setting is 125 m³/hr set according to regs, and I very rarely see the actual value less than 124 or more than 127.

Posted By: WillInAberdeenI'd be less surprised if the motor speed is controlled by PWM and the "2% of full" refers to the pulse width, which is not quite the same as the fan speed being at 2% of maximum speed.

Yes I agree, I think it is a % of maximum and the maximum depends on the static pressure.

I've been investigating measuring the static pressure and it doesn't seem that tricky, just need to get a meter. It is also clear that too much static pressure can have a detrimental effect on the unit. So if your vents are too closed, you could easily end up with too much static pressure.