Ventilation: Vent Axia Sentinel Kinetic - summer bypass info

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- CommentAuthorcjard
- CommentTimeMay 4th 2017 edited
The commissioning manual didn't make some things particularly clear about my Vent Axia Sentinel Kinetic Plus B, with regard to the options on the summer bypass mode screen. I called vent Axia technical and got some answers, thought I'd post them up for the benefit of the wider internet's knowledge:

On models with a summer bypass fitted, the mode screen may have up to 4 choices:

Off
Normal
Evening Purge
Night time Purge

The unit has a clock, and it knows the day and time. I rang vent Axia tech because my unit was in Evening Purge mode, and had started to run a purge at 14:15..

Turns out, the evening and night time purge modes would perhaps be better named as "time limit purge" and "continuous purge" as they can happen at any time. The causes and effects are:

Off - the summer bypass will never open

Normal - the summer bypass will open if the extracted stale air is warmer than the set Indoor Temperature and the intake fresh air is above the set Outdoor Temperature, the fan speed remains unchanged

Evening Purge - the summer bypass will open if the extracted stale air is warmer than the set Indoor Temperature and the intake fresh air is above the set Outdoor Temperature, the fan speed switches to boost for a maximum of 5 hours and a minute timer counts down on the display. The boost deactivates if the timer expires or if the summer bypass closes (because the temperature conditions no longer apply).

Night time purge - the summer bypass will open if the extracted stale air is warmer than the set Indoor Temperature and the intake fresh air is above the set Outdoor Temperature, the fan speed switches to boost. The boost deactivates if the summer bypass closes (because the temperature conditions no longer apply).

You can check the temperature conditions the unit is currently measuring by holding UP and BOOST for 5 seconds to enter the diagnostic menu. Screens 1 and 2 have the temperature readings from the extract and intake sensors respectively

They weren't clear on what the reset condition is to allow the unit to enter another evening purge, but there must be one otherwise the unit would enter another cycle immediately after the current one ends, which would make it effectively night time purge. Perhaps the clock is used in this regard, in that an evening purge can only occur once per day, but the "evening" and "night time" labels have no condition related to the current time, as their names may imply
- CommentAuthorEd Davies
- CommentTimeMay 4th 2017
That looks like a very useful post, thanks. I'm glad you added the last paragraph as I was just about to ask that. Odd though that summer bypass closes at the end of the evening-purge 5 hours even if the temperature conditions continue to apply; why not just turn off the boost?
- CommentAuthorAMG
- CommentTimeMay 4th 2017
Thanks very helpful. On a separate note but related to the Kinetic B Plus - - I also did not realise at the time the the humidity sensor in the unit only triggers boost if the average of the whole house is is above the set humidity point. When we have a shower, the extract humid air will not trigger the boost because the humidity of the rest of the house is lower and the average falls below the threshold point. We got round this by installing our own humidity sensor in the bathroom that triggers the boost on the system.
Ayaz
- CommentAuthorSimon Still
- CommentTimeMay 4th 2017
<blockquote><cite>Posted By: cjard</cite>Turns out, the evening and night time purge modes would perhaps be better named as "time limit purge" and "continuous purge" as they can happen at any time.</blockquote>

Ok. So, for clarity - there's no way of controlling when these happen? Our boost is quite audible so having this run in the evening before we go to bed would be useful but having it run all night wouldn't.,

Also, boost is obviously using more power. In theory the 'nighttime' could result in the unit running on boost for days at a time in summer - even if the house was unoccupied.

<blockquote><cite>Posted By: AMG</cite>humidity sensor in the unit only triggers boost if the average of the whole house is is above the set humidity point</blockquote>

that's all it can do as it is taking air after the manifold. I thought it did some magic based on rate of rise so that it could detect something like a shower. I have humidistats in the bathrooms but before they were wired up the unit did seem to switch into boost mode based on the internal stat.
- CommentAuthordjh
- CommentTimeMay 4th 2017
Posted By: cjardTurns out, the evening and night time purge modes would perhaps be better named as "time limit purge" and "continuous purge" as they can happen at any time.

I don't understand why evening purge would ever come on, since night-time purge comes on at the same time and stays on longer?

Or is it the case that you can choose whether or not evening purge and/or night-time purge are active? But in which case, I would want to be able to stop an evening purge when I went to bed.
- CommentAuthorEd Davies
- CommentTimeMay 4th 2017 edited
Posted By: djhOr is it the case that you can choose whether or not evening purge and/or night-time purge are active?
Yes, I think you choose exactly one of the four modes CJard lists: off/normal/evening/night.

But in which case, I would want to be able to stop an evening purge when I went to bed.
http://www.dealec.co.uk/acatalog/pdf/ventaxia/vent-axia-sentinel-kinetic-selector.pdf says:

In Evening and Night Time Purge modes, the user can turn off the boost function by pressing the Boost button.
That reference also says:

Evening Purge mode: The fans run on Boost speed until the internal temperature falls below the set 'Indoor' temperature. If, after five hours the internal temperature is still above the set 'Indoor' temperature, the unit will switch down to normal speed for the remainder of the 'bypass open' period.
That answers the question about when it'll next do a boost: after it's dropped out of the bypass temperature conditions for a while. It also sort-of implies that the bypass remains active at the end of the 5 hours, it's just the boost that turns off.
- CommentAuthordjh
- CommentTimeMay 4th 2017
It's also interesting that there seems to be no hysteresis on the temperature setting.
- CommentAuthorcjard
- CommentTimeMay 5th 2017 edited
Posted By: Ed DaviesThat looks like a very useful post, thanks. I'm glad you added the last paragraph as I was just about to ask that. Odd though that summer bypass closes at the end of the evening-purge 5 hours even if the temperature conditions continue to apply; why not just turn off the boost?

I've edited my original post though I haven't had direct confirmation off VA that this is the case, but it does seem most logical and it also seems to be what my unit does

Posted By: djhIt's also interesting that there seems to be no hysteresis on the temperature setting.

I don't understand why evening purge would ever come on, since night-time purge comes on at the same time and stays on longer?

Or is it the case that you can choose whether or not evening purge and/or night-time purge are active? But in which case, I would want to be able to stop an evening purge when I went to bed.

Probably not required; the rate of change of the house will likely be quite slow (it's not an effective cooling mechanism) and there's another mechanism that prevents a purge starting right after one has finished

You choose a behavioural mode for summer bypass: off, bypass only, bypass + timelimited boost, bypass + infinite boost. THese are not 4 behaviours that may come on and off as and when independently; I already mentioned that they don't take any notice of the clock for start purposes (unless, I suspect, time purge is active, in which case theres something that prevents another 5h purge starting soon after one stops) so the "evening" and "night time" are poorly named - they can happen at any time of day

To turn a boost off when going to bed, one presses the boost button to drop to normal bypass mode this once (deactivate boost for this bypass session)
- CommentAuthorcjard
- CommentTimeMay 5th 2017
Posted By: Simon StillOk. So, for clarity - there's no way of controlling when these happen? Our boost is quite audible so having this run in the evening before we go to bed would be useful but having it run all night wouldn't.,

Also, boost is obviously using more power. In theory the 'nighttime' could result in the unit running on boost for days at a time in summer - even if the house was unoccupied.

Posted By: AMGhumidity sensor in the unit only triggers boost if the average of the whole house is is above the set humidity point

that's all it can do as it is taking air after the manifold. I thought it did some magic based on rate of rise so that it could detect something like a shower. I have humidistats in the bathrooms but before they were wired up the unit did seem to switch into boost mode based on the internal stat.

True, you can only set a timer for when a boost will happen - that's just a straight boost nothing to do with summer bypass
True, night time looks like it could boost forever in an overheated unoccupied house

As a separate note, I've never felt like it was a great idea to boost ventilate an entire house just because of one brief shower; I'm sure that the 24h effect of normal ventilation would adequately deal with any built up moisture, even if it's buffered in the walls. I don't plan to stress too much about boosting one bathroom.. What we really need though is intelligent valves on every duct, that call for airflow from the unit, much like thermostats call for heat and control UFH zone valves; forget faffing with a manometer, just set each butterfly valve/mass airflow/humidi meter based intelli-controller at the manifold end with what airflow the design sheet says it should have and then forget it. Then you could have your shower, and the exhaust valve (and its paired supply) could open up more to boost that one room..
Or maybe that's like fuel injection in a world where a carburettor does just fine and doesn't warrant anything more
- CommentAuthordjh
- CommentTimeMay 5th 2017
Posted By: cjard
Posted By: djhIt's also interesting that there seems to be no hysteresis on the temperature setting.

Probably not required; the rate of change of the house will likely be quite slow (it's not an effective cooling mechanism) and there's another mechanism that prevents a purge starting right after one has finished

I beg to differ, at least for my MVHR. It's a very effective cooling mechanism but only when it works for a long time, which the hysteresis allows. Last year it managed the temperature in the house very well from the beginning of spring to the end of autumn, except for a few days in summer when we opened windows to assist with cooling.
- CommentAuthordjh
- CommentTimeMay 6th 2017
Posted By: cjardAs a separate note, I've never felt like it was a great idea to boost ventilate an entire house just because of one brief shower; I'm sure that the 24h effect of normal ventilation would adequately deal with any built up moisture, even if it's buffered in the walls. I don't plan to stress too much about boosting one bathroom..

Agreed, although I have some permeable walls as well as the ceiling in every room.

What we really need though is intelligent valves on every duct, that call for airflow from the unit, much like thermostats call for heat and control UFH zone valves; forget faffing with a manometer, just set each butterfly valve/mass airflow/humidi meter based intelli-controller at the manifold end with what airflow the design sheet says it should have and then forget it. Then you could have your shower, and the exhaust valve (and its paired supply) could open up more to boost that one room..
Or maybe that's like fuel injection in a world where a carburettor does just fine and doesn't warrant anything more

You can buy a system like that (I forget what it's called, I'm afraid). But I suspect it's a cost question. I'm not sure injection vs carbs is a great analogy. Carbs were much more fiddly to set up and keep set up, whereas injection just worked in my experience. Presumably injection these days doesn't have a great price premium either.
- CommentAuthorlineweight
- CommentTimeMay 6th 2017 edited
I happened to be looking at Brink systems earlier today and they have a 'zoned' option (2 zones only) using a valve between ducting runs, you can see it around 30 secs from the end of this video

https://m.youtube.com/watch?time_continue=188&ebc=ANyPxKrySQpASYEE-N33yYYvxKF8C0k7NKbSOuDnOkEYlkOyNO0R0JIgK6K6v2CxYasQpIbj1iPpKhPLBoL-JlvPLf86L1dRGA&v=7sBKMoCEfuY
- CommentAuthordjh
- CommentTimeMay 7th 2017
Interesting video and interesting to see that they're moving towards zone control and demand control.

One thing puzzled me. He stressed the balanced flow at all times from the flow-controlled fans. I agree that it's useful and wasn't surprised that he mentioned wind pressure changes. But I don't understand the bit about internal temperature changes. If the internal temperature increases, the air inside will try to expand and increase pressure instead, so I'd have thought you actually wanted to temporarily increase the outflow at that time, and similarly to slightly increase the inflow when the internal temperature drops. Perhaps the volumes are sufficiently small that the excess or shortfall is made up through slight imbalances, but then why mention it as a feature at all?
- CommentAuthorEd Davies
- CommentTimeMay 7th 2017
Expansion/contraction of the inside air with temperature change is too small to be worth considering. He doesn't make it clear but I suspect he means compensation for stack effect.
- CommentAuthorcjard
- CommentTimeMay 8th 2017
For me the analogy works; carburettors are incredibly cheap, and work well enough that certain contexts (like a lawnmower) will probably never have fuel injection - a system that will always be more complex than "when this aperture is sucking harder and that aperture is less blocked, more fuel is allowed to be sucked in". The apparent simplicity of fuel injection for the user is only so because the computer is doing an immense amount of fiddling inside the black box. As a whole process, the complexity of a "fuel injected MVHR" may always exceed necessity.. it's like in the other thread where we're really getting deep into the nuts and bolts of having an intelligent computer program altering the MVHR paramters continuously based on a plethora of current and future inputs - I get the feeling that no one in business is doing it because it just won't deliver a leaps and bounds difference over standard performance to make he cost and complexity increase viable
- CommentAuthorEd Davies
- CommentTimeMay 8th 2017 edited
Posted By: cjardI get the feeling that no one in business is doing it because it just won't deliver a leaps and bounds difference over standard performance to make he cost and complexity increase viable
Probably yes, but only if you're looking at the MHRV in isolation.

My interest is that it should be considered in conjunction with the performance of the house overall. E.g., for my off-grid setup they'll be a distinct advantage in running the MVHR during the daytime when it can come directly off the PV rather than cycling the energy into the batteries then out again during the night. Except if I have a wind turbine and it's breezy over night. Or if it's mild and dry overnight but going to be cold and wet in the morning. COâ‚‚ matters in the short term but generally the limiting factor is humidity but that's longer term if there's any buffering in the house so it's not just the RH in the house now that matters but also moisture content of the walls, etc.

If it's good drying weather it's worth running the house at low RH to dry out the walls. Other times when the walls are dry you can tolerate a higher RH because you know the vapour will be absorbed by the walls releasing a small, but not completely negligible on PH scales, amount of heat as it does so.

Ditto running the pump to refill the cold water tank in the loft. And deciding whether to run the solar thermal pump quickly for low-temperature water for the thermal store or slowly for DHW (which depends on the state of the battery (in the house and the EV?) as to whether PV energy will be diverted to DHW). And so on.

In general, there's a sort multiplicative effect of small optimizations. If the MVHR and other kit can respond to grid conditions as well as their immediate requirements then we can burn less fossil fuel and need fewer power stations.

Having a small computer sitting there just to run the MHRV is probably a net loss. If it's controlling other things as well then begins to make more sense.
- CommentAuthorSimon Still
- CommentTimeMay 8th 2017
<blockquote><cite>Posted By: cjard</cite>To turn a boost off when going to bed, one presses the boost button to drop to normal bypass mode this once (deactivate boost for this bypass session)</blockquote>

Which requires access to the machine or a remote control - I don't have either. Mine is linked to my home automation via the simple contacts which lets me boost (on humidity/cooker hood/toilet occupation), low (empty house), or off (fire). This might be a usecase for proper 'smart' link.

<blockquote><cite>Posted By: cjard</cite>As a separate note, I've never felt like it was a great idea to boost ventilate an entire house just because of one brief shower; I'm sure that the 24h effect of normal ventilation would adequately deal with any built up moisture, even if it's buffered in the walls</blockquote>

Additional energy/heat loss is pretty minimal though isn't it? the heat exchanger is c90% efficient. I suspect there would never be any payback on the increased complexity of making it smarter (the off grid example here is an exception). My instinct is that the boost on showering is useful - the room gets dry quickly which then means the towels dry quicker.
- CommentAuthorEd Davies
- CommentTimeMay 8th 2017
Earlier I wrote: â€œâ€¦the vapour will be absorbed by the walls releasing a small, but not completely negligible on PH scales, amount of heat as it does so.â€

A repeat version of previous calculations I've done:

Suppose you have 200 mÂ² of exposed timber 18 mm thick of density 600 kg/mÂ³ and its water content increases, by condensation at room temperature, from 12% to 18%. The latent heat of vaporization of water is around 2450 kJ/kg at 20Â°C [Â¹].

Mass of water: 200 * 0.018 * 600 * (18-12)/100 = 129.6â€¯kg
Energy released: 129.6 * 2450 = 317.52 MJ

That's 88 kWh which is nearly the PH annual heating allowance for 1/10th of my finished floor area.

Released over three months: 317.52e6 / (86400 * 3 * 30.5) = 40.16â€¦W.

I'm seriously thinking about large boxes of cat litter in the loft to increase this capacity. Dry the house out in the summer to leave potential for absorbing water in the winter to release heat. Of course, that's all predicated on managing the average humidity levels appropriately over the winter.

[Â¹] https://edavies.me.uk/2014/03/mhrv-req/#specific-latent-heat
https://en.wikipedia.org/wiki/Latent_heat#Latent_heat_for_condensation_of_water
- CommentAuthordjh
- CommentTimeMay 8th 2017 edited
Posted By: Ed Daviesfrom 12% to 18%

That's a huge jump, very unlikely IMHO. Pure guess but I'd start with a 1% change as more likely (say 11% to 12%). Is there a sorption curve for cat litter? I'd expect it to need very high RH to reach 18% MC.

Dry the house out in the summer to leave potential for absorbing water in the winter to release heat.

It doesn't work like that in our house. Other than to say that the RH doesn't vary much, though, I can't suggest exactly how it does work. I feel its low 40% in winter and higher 40% in summer but I may be imagining that. Perhaps you could achieve something using an active system that heated the cat litter in summer to dry it out?
- CommentAuthorWillInAberdeen
- CommentTimeMay 8th 2017
Don't MHRV houses dry out in the winter, when there's a bigger contrast between the water content of ambient and exhaust air? In the summer you lose that effect and the fabric soaks up humidity again.

(Well ... except in north Scotland.)
- CommentAuthorWillInAberdeen
- CommentTimeMay 8th 2017 edited
Speaking Thermodynamics - the latent heat of adsorption is not the same as latent heat of condensation. The 'adsorped' state in not energetically equivalent to the 'condensed' state, otherwise you'd experience condensation on all kinds of hard surfaces as easily and at the same time as you get adsorption into wood/ plaster etc.

A visualisation is: water 'prefers' to be adsorped in wood rather than be condensed in a puddle. 'Adsorped' is a lower energy state than 'condensed', so is thermodynamically more likely. You have to add more heat to desorb a kg of water vapour than you would to make it evaporate.

You might like this: http://www.sciencedirect.com/science/article/pii/S1876610215014605
- CommentAuthorEd Davies
- CommentTimeMay 8th 2017
Posted By: djhThat's a huge jump, very unlikely IMHO
Neil May's Breathability In Buildings document [Â¹] gives equilibrium moisture contents for wood at different RH's and temperatures. 21â€¯Â°C, RH 30% 6.2%; RH 70% 13.1%. So my numbers look a bit high but the right general sort of range, much more than 1% change though 6%, I accept, is probably a bit ambitious.

Posted By: djhIt doesn't work like that in our house.
That's probably because you're not running your MHRV specifically with the intention of keeping the RH low in late summer/early autumn.

Posted By: djhPerhaps you could achieve something using an active system that heated the cat litter in summer to dry it out?
Yes, that's what I was wondering about - using my solar warm air panels on the gable ends for that when they're not needed for space heating.

Posted By: WillInAberdeenSpeaking Thermodynamics - the latent heat of adsorption is not the same as latent heat of condensation.
Good point. Because it's in a lower energy state when adsorbed it'll presumably be a bit higher, which is good (at least until you want to dry it out again).

[Â¹] Needs a web search to find - NBT don't host it anymore but it's still around on other sites.
- CommentAuthorWillInAberdeen
- CommentTimeMay 8th 2017 edited
<blockquote><cite>Posted By: me</cite>
You might like this:<a href="http://www.sciencedirect.com/science/article/pii/S1876610215014605
/blockquote>

"An Open Sorption Heat Storage Concept and Materials for Building Heat Supply"
"The potential of an open sorption storage process for space heating and hot water was evaluated using sorptive material....sufficient high temperature shifts for room heating applications were achieved."
- CommentAuthordjh
- CommentTimeMay 8th 2017
Posted By: Ed DaviesNeil May's Breathability In Buildings document [Â¹] gives equilibrium moisture contents for wood at different RH's and temperatures. 21â€¯Â°C, RH 30% 6.2%; RH 70% 13.1%.

You may be right - I have a copy but aren't going to look it up. My RH never gets anywhere like as low as 30%. The only time I've measured that was at work when people were complaining of dry eyes and suchlike.

That's probably because you're not running your MHRV specifically with the intention of keeping the RH low in late summer/early autumn.

No, I don't think so. I agree that I'm not doing that but I think the reason is more to do with some as yet unexplained feature of my construction that stabilises the RH. It doesn't track external RH as well as some people would like to believe. In practice my MVHR rate will be at the upper end of its range in late summer because the summer bypass will be open and I'll be using it to cool the house. Early autumn is a movable feast.

Posted By: WillInAberdeenYou might like this: http://www.sciencedirect.com/science/article/pii/S1876610215014605

Interesting paper thanks. The need for very high temperatures to dry the material may be a problem?
- CommentAuthorcjard
- CommentTimeMay 10th 2017
Posted By: Ed Davies
I'm seriously thinking about large boxes of cat litter in the loft to increase this capacity. Dry the house out in the summer to leave potential for absorbing water in the winter to release heat.

Do you bring your boxes of cat litter downstairs then?
- CommentAuthorEd Davies
- CommentTimeMay 10th 2017
Loft will be internal to the conditioned space.

But, yes, could bring them down and put them in an external solar drying box in the summer. Or in an enclosure with a dehumidifier running off excess summer PV to inter-seasonally store energy in the form of adsorption potential. The point about needing high temperatures to dry out some of this stuff is a bit of a blow, though.

But it's all mostly about buffering humidity. Any long-term heat gain is a tiny, by-the-way, bonus. Still, it might be something which is fun to play with sometime.
- CommentAuthorcjard
- CommentTimeMay 10th 2017
I'm not certain if it's the weather, or whether my recent activation of the mvhr is a/the factor, but the RH in my house in winter time was 50-60, now it's 35-45.. I've reduced the fan speed of the mvhr (from 30% to 20%) to see if this makes a difference. I don't really want it to keep dropping.. I'll be boiling the kettle with the lid off!
- CommentAuthordjh
- CommentTimeMay 10th 2017
Posted By: Ed DaviesBut it's all mostly about buffering humidity. Any long-term heat gain is a tiny, by-the-way, bonus.

I'd suggest cellulose (warmcel or straw) as better options for humidity buffering than cat litter, since they also double up as insulation. You certainly don't want straw directly in the conditioned space though and probably neither of the others either, unless you like sharing the space with insects.
- CommentAuthordjh
- CommentTimeMay 10th 2017
Posted By: cjardI'm not certain if it's the weather, or whether my recent activation of the mvhr is a/the factor, but the RH in my house in winter time was 50-60, now it's 35-45.. I've reduced the fan speed of the mvhr (from 30% to 20%) to see if this makes a difference. I don't really want it to keep dropping.. I'll be boiling the kettle with the lid off!

I'd guess the MVHR definitely reduces the RH below unventilated winter readings. Theory says that with copious ventilation the internal RH should track the external RH after adjustment for temperature. Any deviation is either internal generation (from your kettle) or buffering.
- CommentAuthorcjard
- CommentTimeMay 11th 2017
As an aside, I'm not sure how effective summer bypass is for "providing energy free cooling" quote unquote the manual

Finding the house at an average 24, and the outside world helpfully at about 10 last night I reconfigured my unit to do continuous purge rather than 5 hour purge. It's been on boost all night and my heatmiser stat history shows its manage to achieve a 0.6 degree reduction in temperature. For 6.5 hours extra electricity and noise from boosting, I'm not sure it was worth it over opening a window for purge vent. I've only just set the stats up and started logging temps but I'll see if I can get some reasonable comparisons of night purge mode vs plain summer bypass from a cooling effectiveness vs electricity bought. Must find that plug in power monitor..

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Ventilation: Vent Axia Sentinel Kinetic - summer bypass info