Does a CO2 sensor make sense with MVHR

[le-cerveau]

I am finalizing my MVHR specification, having to self install, as there are no contractors who will fulfill my requirements (no surprise).

I will be fitting 2 x Brink Renovent Excellent 400 Plus units with Ubink ducting.  I have discovered that I can get internet connections for the MVHR units so I can do away with the 3 position switches/remote controllers and use a web interface/ app to control them.  I will be fitting RH sensors in the extract ducting to auto boost the system, but I was wondering if it is worth including CO₂ sensors also.  The theory being that I could run the system slower than the building regs extract requirement with the CO₂ ramping it up if required, the same as the RH.  The building Regs extract requirement is the key as I have a large house 439m² so this trumps any wet room extract rate.

I have the following planned extract rates:

• 0    150m³/h      Very low level for un-occupied periods.
• 1    320 m³/h     lowest keeping room extract rates at BR requirements
• 2    495 m³/h     floor area extract rate to comply BR
• 3    570 m³/h     Boosted keeping the units within PH standards

My original plan was to run the units at 1/2, depending on house occupancy levels with the RH boosting up to 3 as required, however with CO₂ sensors I should be able to just let the system do it's thing.

 

Do they work and are they worth it?  (approx. £700 extra on a £10,000 equipment spend)

 

 

[PeterW]

£10k seems a massive amount to spend on MVHR ..??! And £700 on a pair of CO2 sensors seems extraordinary as they are £15-20 at most ..! Is there an interface required that drives up the cost ..?

[le-cerveau]

1 minute ago, PeterW said:

£10k seems a massive amount to spend on MVHR ..??! And £700 on a pair of CO2 sensors seems extraordinary as they are £15-20 at most ..! Is there an interface required that drives up the cost ..?

@PeterW I am basically installing 2 systems so 2 x MVHR, 4 x manifolds, 400m of ducting and all the associated adaptors and connectors, 4 x roof vents and the associated insulated ducting, in a large house it soon adds up.

The CO₂ sensors are the Brink ones that plug directly in but they are just 0-10v proportional ones with a 24v supply.

[PeterW]

The Brink units are ~£2k each so I am still struggling to add another £6k - is that a  misquoted installed price ..??

 

BPC do a 22 room system for £4K off the shelf so that would be a benchmark for me. 

 

Titan make a 24v 0-10v proportional duct sensor for £165 as they are pretty standard -sounds like Brink just double the price of a standard HVAC component. 

 

https://www.titanproducts.com/duct-co2-sensor.html

 

 

Edited June 27, 2017 by PeterW
Missing words ..!

[le-cerveau]

It soon adds up:

£800, manifolds, connectors and bits.  £1260 for ducting, £360 for connectors, £690 for 36 ceiling adaptors (lots of runs), £400 for vent terminals, £860 for silencers and insulated 180mm duct, the list goes on.  This is a VAT inclusive price so I will be claiming back £1800 at the end.  I have a large number of ducts and terminals as I am limiting the system to 2m/s in any duct and 30m³/h in/out of any terminal, to limit any flow noise (completely over engineered).

[jack]

I have a Brink Excellent 400 Plus (without the internet module unfortunately - that became an option after I bought mine, and I don't know whether it can be retrofitted). I got the Plus model specifically so I could use CO² sensing, but in the end decided it was too complex.

From memory, the Brink CO² sensors are rebadged units that can be sourced elsewhere for half the price. 

[le-cerveau]

@jack Jack, you can retrofit the internet module, it just plugs into the service port (ebus), google Brink-Home-eModule.

 

I think I will use generic CO₂ sensors, either room or duct, or I could add them later.

[jack]

Great news, thanks!

[SteamyTea]

6 hours ago, PeterW said:

And £700 on a pair of CO2 sensors seems extraordinary as they are £15-20 at most ..!

CO sensors might be that cheap, I don't think that CO₂ ones are.  They are a different beast.

 

There is such a good correlation between CO₂ levels and humidity differences once temperature is taken into account, that I think you can rely on just RH and some simple arithmetic to control an MVHR.

 

Re the web/app control interface, how would you deal with the controls if the manufacturer stopped the cloud service?  Would you end up with either a dead system or one with just very basic functionality?

Was it Nest that pulled the plug on home monitoring and control?

[Jeremy Harris]

CO₂ sensors that are usable for an application like this start at around £60 to £80.  I bought some surplus ones for about £20, but they were a one-off, from a company that had changed the design of a commercial air monitoring unit they manufacture.

 

The only reliable type of sensor is really the Non-Dispersive Infra Red (NDIR) ones, the hot element sensors are far too non-specific for use in environmental monitoring, and very power hungry.  I have a hot element one (an MG811) that I tried at first, and it doesn't start working until the CO² level gets to around 1000ppm, so is pretty useless for room monitoring.  The Telaire NDIR ones I have (I bought a few, and have one monitoring the house 24/7) are pretty good, and give reasonably accurate readings.  They are similar to these: https://www.digikey.co.uk/product-detail/en/amphenol-advanced-sensors/T6713/235-1373-ND/5027891

Edited June 28, 2017 by JSHarris

[Stones]

Having a CO2 sensor that auto boosts the MVHR would be a nice feature to have, but, assuming you have set your ventilation rates optimally for whatever your normal household occupancy is,  would only really be of use for limited periods of time - when you have visitors / more than the normal level of occupancy.  Easy enough to boost manually in those circumstances.

 

The following sensor appears to have been designed to work with the Vent Axia Sentinal Kinetic Plus:

 

https://www.digikey.co.uk/products/en?keywords=235-1413-ND

 

and at around half the price of the Vent Axia sensor, is something I'm thinking about.

 

On 6/27/2017 at 21:49, SteamyTea said:

 

There is such a good correlation between CO₂ levels and humidity differences once temperature is taken into account, that I think you can rely on just RH and some simple arithmetic to control an MVHR.

 

 

@SteamyTea Can you expand on this ?

[SteamyTea]

21 hours ago, Stones said:

Can you expand on this ?

Our old mate DamonHD did some work on this and I looked at the data.  Seemed there was a good correlation between occupancy and RH/CO₂ at the school he monitored.

It was also possible to see increases in road traffic when the school was empty.

 

By doing co-variance statistics you can correlate for temperature differences i.e. when the house is empty and unheated, or at night.

I think I still have both Damon's and some of JSH's data and may have a look later and see what can be teased out.

 

[Jeremy Harris]

Damon borrowed my monitor to make those school measurements, and I've seen the same strong correlation between RH and CO₂ concentration in the main living areas.  The correlation falls over in bathrooms, where humidity rapidly increases at a rate higher than the increase is CO_2.  All told, I'm convinced that just using RH control would be plenty good enough, and would enable ramping up the ventilation rate when the shower of bath is in use.

 

The boost system I've installed uses RH sensing inside the extract duct plenum.  This seems to work well, but does need adjusting twice a year, as the background RH changes with the seasons.

[SteamyTea]

2 hours ago, JSHarris said:

but does need adjusting twice a year, as the background RH changes with the seasons.

That would be a reason to use AH and temperature as it could automatically adjust for that.

Or use a sensor on the input side as well and correlate the differences with internal temperature.

[Jeremy Harris]

Yes, that would work OK.  The only slight snag is that it would need a custom control box, as the cheap programmable humidistats, like the one I fitted, aren't able to do this.

 

It wouldn't be hard to make a custom unit, although it would need to derive AH from RH, as all the cheap and reliable sensors only measure RH.  Not a problem with a temperature sensor as well, though.  It would probably be good enough to just adjust the RH switching threshold with outside air temperature.

 

[SteamyTea]

The conversion is easy:

http://hyperphysics.phy-astr.gsu.edu/hbase/Kinetic/relhum.html

 

One advantage of using AH is that you end up with a measure of what the problem is i.e. how much liquid water is entering your house.

[Jeremy Harris]

Yes, but it still needs a custom widget, rather than an off-the-shelf humidistat. 

 

I have a feeling that if going for a custom humidistat approach, then the easy way to do it would be to just look at Δ_RH/Δ_t, as that should be all that's needed to trigger the MVHR to increase the ventilation rate.  Extra occupants, a shower running, or cooking, would generate a fairly steep increase in RH over a relatively short period of time, an increase that's probably significantly faster than any natural change.  Using a relatively rapid increase in RH to trigger a higher ventilation rate, and then using the slower decrease in RH to trigger a return to the background ventilation level would probably work quite well, and would only need a single RH sensor.

[SteamyTea]

Yes, I would use a simple RH 'switch' to control it as until it is up and running, and the house all settled in as it is hard to guess what is actually needed until then.

 

As a general rule it is not worth getting to hung up and very fine control with temperature and humidity levels, humans are pretty adaptable to varying conditions.

Edited July 1, 2017 by SteamyTea