Monitoring and controlling MVHR with a Raspberry Pi

[PeterW]

Got a Shelly on some outside lights which has impressed me so far. See they have now brought out a Shelly “shield” for a DHt22 which makes me think that replacing the dumb / binary type relay in the current setup may be easier and will allow fairly simple adjustment of the RH set point unless I can see a simple way of programming a Shelly to spot a rate of rise in RH (without resorting to Tasmota etc) 

[MJNewton]

3 minutes ago, joe90 said:

back during that heatwave in the spring. (RH just dropped to 60%)

 

20% RH is pretty exceptional but, granted, I suppose it can happen. With it being seasonal however it shouldn't affect rate-of-rise detection, or was/is the issue that you've only got absolute RH detection available?

[SteamyTea]

I shall have to come over and see if that logger is still l logging, that had a couple of the better DHT22 on it. Though you do then your MVHR off in the summer.

 

[joe90]

1 hour ago, MJNewton said:

With it being seasonal however it shouldn't affect rate-of-rise detection, or was/is the issue that you've only got absolute RH detection available?

yes mine is “measured RH” not rate-of-rise which work much better.

 

53 minutes ago, SteamyTea said:

I shall have to come over and see if that logger is still l logging, that had a couple of the better DHT22 on it. Though you do then your MVHR off in the summer.

 

 

more than welcome mate (unless we are locked down ?‍♂️). I do tend to switch it off in summer as lots of windows are open so no point in it running IMO.

[SteamyTea]

Would a simple logger with inside and outside temp/RH sensors be useful.

Once the scale of the problem us known, it is fairly easy to design a switching system to cope with it.

Though I think @MJNewton has the right idea, rate of change.

 

[joe90]

4 minutes ago, SteamyTea said:

Though I think @MJNewton has the right idea, rate of change.

I tend to agree, just don’t know how to achieve it. (Over to you boffins).

[SteamyTea]

6 minutes ago, joe90 said:

I tend to agree, just don’t know how to achieve it. (Over to you boffins).

Sample every 5 minutes, if second value is equal to previous value +5%, then switch on.

If next value is previous value -5%, switch off.

 

(Switch on and off could be a change in fan speed)

 

May need a bit more polishing i.e. if switched on, delay next reading for 15 minutes.

Edited October 31, 2020 by SteamyTea

[joe90]

10 minutes ago, SteamyTea said:

Sample every 5 minutes,

That seems a bit long to me!, might be better if 30sec so if having a shower (bathrooms tend to be small) fans clear the air before a build up occurs, then off based on drop of RH. ???

[SteamyTea]

6 minutes ago, joe90 said:

That seems a bit long to me!, might be better if 30sec so if having a shower (bathrooms tend to be small) fans clear the air before a build up occurs, then off based on drop of RH. ???

To a certain extent, the timings can be set with trial and error.

 

Do you now how your MVHR fan speed is controlled?

[joe90]

Simple 1 to 4 controller on wall panel, I connected the RH monitor to 4 (boost), as you know I am a Luddite but the tech is here,

 

http://www.mitsubishitech.co.uk/Data/Lossnay/LGH-RS/2013/LGH-50RSDC-E1/LGH-50RSDC-E1_IM.pdf
 

I,m off to service my tractor..........?‍?

Edited October 31, 2020 by joe90

[MJNewton]

1 hour ago, joe90 said:

That seems a bit long to me!, might be better if 30sec so if having a shower (bathrooms tend to be small) fans clear the air before a build up occurs, then off based on drop of RH. ???

 

You could do that, but bear in mind that the longer the sample period the bigger the jump is expected to be and so it makes it easier to detect and easier to avoid false positives. If your window is small the threshold would likely have to reduce and you may find it triggers when it shouldn't.

 

As @SteamyTeasaid trial and error will guide it well, and to be honest I only went with 5 minutes because that's how regularly the script wakes up to take temperature measurements for plotting so I just piggybacked on the back that. That said, I do find a shower gives a 5-10% rise in that timeframe and so it stands out from the noise (natural variations) quite well. 

 

 

[Gone West]

4 hours ago, joe90 said:

you don’t live in Devon next to the Atlantic, just looking at my RH reading in the conservatory (during storm Doris or whatever) and it is 80%.

I'm only three miles from the English Channel and it's been raining now for a couple of hours and it's 96% outside and it's gone up 2% inside to 52%. The inside RH always seems to track the outside value but it's never extreme inside.

[joe90]

I might hang my conservatory temp/Rh thingy in the bathroom whilst I have a shower later, be interested to see what it does.

 

 

 

 

[SteamyTea]

Here is today's outside data.  I think my sensors has got flooded.

 

[Gone West]

24 minutes ago, joe90 said:

I might hang my conservatory temp/Rh thingy in the bathroom whilst I have a shower later, be interested to see what it does.

I've found in the past that type of hygrometer gives readings that vary quite a lot depending on the type of surface they're on. If it's on an absorbent surface the readings are lower.

[Levo]

On 31/10/2020 at 15:22, PeterStarck said:

I'm only three miles from the English Channel and it's been raining now for a couple of hours and it's 96% outside and it's gone up 2% inside to 52%. The inside RH always seems to track the outside value but it's never extreme inside.

I am in the process of choosing my MVHR unit and one area I have been struggling with is whether I need an Enthalpy heat exchanger. (Though I have pretty much made up my mind that for where I live, an enthalpy exhnager will likely be more detrimental than useful as for temps above 10C it may end up increasing the RHI inside the house)

 If I got my calculations right, ~11C increase halves the RHI. Basically if it is 10C 100% RHI outside, when you pump it in at 21C, it will be at 50% RHI. And if it is 0C 90% outside, it will turn into ~25% inside. Doesn't matter whether you use the MVHR or open a window. As with open window, there will be a temporary temp drop inside but the Central heating (and the fabric of the house) will restore it to the regular temp soon after you close the window.

The question I have is what happens on summer days. Basically if you need to keep inside, say 11C below the outside temp, e.g. 35C outside 24C inside, then even if you use  the heat exchanger to reduce the temp, you are still blowing in air that is double the outside RHI. Even if it is as low as 50% outside, that still means 100% RHI (or more) in the supply air. This means condensation may form on the incoming air path.

Connected question; if you use the heat exchanger in the summer, what happens to the condensation draining as I think it will collect on the wrong side?? Does it mean you are stuck with Summer bypass even on really hot days?

My conclusions are that in southern England, Enthalpy exchanger may help both in the extreme winter (less than 5C with less than 80% humidity) or extreme summer (30C+ with 70% humidity outside. But we only get a few of those days?

[joe90]

My MVHR has an Enthalpy heat exchanger (didn’t know this when I bought it off Ebay!!!), which means it has no condensate drain (well mine does not and at least one other on this forum as well)

 

3 minutes ago, Levo said:

for temps above 10C it may end up increasing the RHI inside the house

Interesting as my RH does seem high compared to others here so I will follow this thread closely.

[Levo]

As I said in my previous message, I am looking for an MVHR unit. Needs to be able to supply ~130m3/h nominally so looking for a unit rated at ~300m3/h to make sure it is quiet.

I want to go British made, so had the Ventaxia Advance SX (1- F7 filter, 2- Constant Flow and 3- Enthalpy option) as the one I was focusing on.

Thanks to this thread I have learned about Titon which, on paper, seems to be comparable to Ventaxia. Thanks MJewton?.

Base on my three criteria above HRV 2Q seems to be a good fit (or HRV 3Q but probably a bit noisier).

Has anyone got any comments/recommendation about these two? Support, noise, longevity etc. Anything that could help me will be welcome.

[Gone West]

@Levo TBH I never considered an enthalpy heat exchanger as an option.

[ProDave]

@joe90  If you wanted to control the Mitsubishi fan speed from something like a Raspbery Pi or Arduino then you would need four small relays each driven from a digital output and they would close the contact on one of the 4 available speed inputs.

[SteamyTea]

4 minutes ago, ProDave said:

@joe90  If you wanted to control the Mitsubishi fan speed from something like a Raspbery Pi or Arduino then you would need four small relays each driven from a digital output and they would close the contact on one of the 4 available speed inputs.

That should be easy enough to set up.

Is the current very low, and at what voltage is needed?

[ProDave]

2 minutes ago, SteamyTea said:

That should be easy enough to set up.

Is the current very low, and at what voltage is needed?

Yes low voltage presumably DC but I have never measured what it is.

 

I use a mains voltage timer as a timed boost and that just drives a relay to close the chosen boost speed contact.

 

another "feature" is you can close more than one of the speed inputs at a time and it will go to the fastest speed chosen.  Hence my normal speed input is always selected and my chosen boost speed input is closed as well when required.

[SteamyTea]

So these relays are just closing and opening a circuit, they are not actually carrying the motor current.

Shall have to see what small SSD there are. I think I have a mechanical relay 'hat' for a RPi somewhere.

[SteamyTea]

Maybe something like this 15 quid special

 

https://www.switchelectronics.co.uk/5v-8-channel-solid-state-relay-board

[joe90]

Sounds good guys, I would only need one relay as boost to level 4, not bothered about 2 or 3.