Is a humidity sensor necessary for home ventilation system?

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SumBodi

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#252813 12-Jul-2019 22:54

Hi all

I have a specific question about the need for a humidity sensor for a positive pressure home ventilation system. We have had multiple quotes and while all include a temperature sensor, not all include a humidity sensor.

Energywise strongly suggest a humidity sensor (https://www.energywise.govt.nz/at-home/ventilation/home-ventilation-systems/positive-pressure-ventilation-systems/) based on a 2010 study that looked at ten draughty houses in CHCH and Wellington (http://www.beaconpathway.co.nz/further-research/article/testing_ventilation_systems).

The system we prefer based on our experience with sales and other information is from a company called SAYR. They have temperature sensors but not a humidity sensor. When the temperature of the roof space is below a certain threshhold the fan slows down significantly but does not stop completely.

It seems that different people have very different ideas about these systems. Our goal is to remove or reduce condensation, while we appreciate that we need to continue heating the house well.

The 2010 study found that humidity in attic spaces is often greater than that within the house itself. However even if this is the case, wouldn't the movement of air still contribute to reducing condensation settling on windows and sills? The study found that 7 out of 10 houses reported condensation as happening "seldom or never". But there was no before and after study of the effect of the system, so really just descriptive stuff and not experimental at all.

I have asked three of our acquaintances with ventilation systems and all three state that condensation is essentially gone, while the main issue they have is the cold air that can come through on particularly cold mornings. This personal experience suggests that there is something to be said for the simple ventilation systems, at least as far as condensation removal.

If anybody has thoughts or experiences with or without a humidity sensor, would be appreciated. Cheers

timmmay

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#2275627 13-Jul-2019 06:51

Many positive pressure systems are really dumb. The one I have just blows air in all the time, no logic to it at all. So I put it on a $10 Mitre Ten timer and it works great. In winter we have it going limited times because it does push a LOT of cold air in and the motor / air movement makes a bit of a noise you can hear sometimes.

For a half hour at around 4:30am before the heating comes on, to clear out some humidity so heating is more effective, and to freshen the air
9am to 11am to again reduce humidity and provide fresh air. This changes the air in the house a number of times.
2:30 to 3pm to do a bit more before heating it turned on
Might have it come on 7pm ish after cooking is done, but it pushes in a bunch of cold air so we don't at the moment.

I also altered it to take air from outside. The ceiling cavity of our place is very old, with lots of types of insulation, rat droppings, rat poision, massive amounts of dust, etc. The simple sock filter ended up black after a few months, and the air smelled. Fresh air from the outside is much much nicer, house smells better. Yes the ceiling cavity air is a little warmer in the middle of the day, but I didn't find any practical difference to how the house feels, given we have a couple of heat pumps.

This system helps with condensation a bit, but the key thing that reduced that by 95% was proper double glazing.

Of course, you need to remove moisture at the source as much as possible.

Back to your question though. Is a sensor necessary? No. But maybe it will help a little. But maybe you want do use a timer as well.

If you're putting this into a house you plan to stay in long term you should consider a better system with a heat exchanger. That way you're not having to heat huge amounts of cold air 24/7.

SumBodi

48 posts

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#2275643 13-Jul-2019 08:28

Thanks for your detailed reply.

It really is difficult to know what investment is best.

Our house is a 90s place with high peaked ceilings. The upstairs is 120m2 and just a small area of about 40m2 has attic space. So the actual areas where we can place vents is limited - but vents can be placed in locations to assist with the worst condensation. For this reason a balanced heat exchange system has not been recommended by any of DVS, SAYR or Smartvent. The Heating Company who only do balanced systems gave us a quote and it is nearly 4 times the cost of a simple positive pressure system, and running costs would be higher. If we are looking at spending that amount of money then we are starting to get into the territory of double glazing which seems like a better choice, but we are probably not going to stay at the house more than a few years.

Our main heating is a fireplace which is very effective. We use a ceiling mounted fan to circulate the warm air once the fireplace is up to speed. But the fireplace is only running from early/mid afternoon until late evening. We have a heat pump downstairs and will consider installing a second one if needed.

We are leaning towards just a positive pressure system. Our ceiling space is quite clean. Smartvent and DVS recommend ducting directly to outside while SAYR recommend just a simple mushroom shaped roof device to create some outside air flow into the attic, but not directly ducted, to avoid the problem of cold air at times.

To be honest it's all a little annoying because it really is just a silly way for us to deal with NZ's historically poor building codes. All this could have been avoided if all houses were double glazed/centrally heated a la Europe. But complaining doesn't help.

I know the topic is done to death on this forum, but any further advice from anybody is highly appreciated.

Once we have made our minds up and installed, I will report back to the forum our experience so other people may benefit.

timmmay

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#2275645 13-Jul-2019 08:42

We only have vents in the hallway and kitchen. If you're doing positive pressure that will be on all the time putting a vent in the bedrooms might reduce condensation, but it will also significantly increase the heating requirements.

Double glazing and central heating don't help with ventilation.

jonherries

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#2275663 13-Jul-2019 09:19

Worth noting that the balanced pressure system will cost more but it will save you in heating costs as well (ie. recycles already heated air). I think a humidity sensor would be most useful if it triggers the ventilation system, but presumably it needs to be in the right spot or you need more than one.

If you have only had one balanced pressure quote you maybe should get another? Agree double glazing is best answer but not cheap.

Jon

SumBodi

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#2276108 13-Jul-2019 23:28

timmmay:

Double glazing and central heating don't help with ventilation.

I understand that, but surely they help by improving the conditions that would lead to condensation in the first place?

bfginger

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#2278014 17-Jul-2019 00:26

I wouldn't recommend a passive roofspace ventilation system for use in winter but I think a humidity sensor could help. Humidity is not only about relative humidity but total humidity. 100% relative humidity at 10c is about 52% humidity when heated to 20c. So if the electronics can tell not only what the temperatures are but how much total water is in the air or the "dew point" it can allow them to optimise their performance better. That's the theory but passive ventilation systems are usually dumb roofspace fans which cost very little to manufacture.

A rangehood and a shower extractor fan are a must in New Zealand. If you have an enclosed bare earth underfloor it needs a vapour barrier.

SumBodi

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#2340533 19-Oct-2019 13:36

Thought I'd post back with our experience with our home ventilation system.

We went with SAYR systems. They provided an EC fan, reasonable price, answered lots of questions and weren't particularly pushy. They would also eventually install a roof kit to extract air from the outside. We didn't want to go with a full balanced system as this was going to be too expensive with unclear benefit as we only had a limited amount of attic space in an otherwise high peak ceiling area. The three ventilation companies who quoted us (SAYR, DVS and SmartVent) all had similar systems and were installing similar fans/duct sizes, and all recommended just four vents.

We think there is a difference but it is marginal, and for our space we need to keep the base fan speed at 25% or so (it is usually set to 15%) in order to see a consistent change. The system cannot cope with condensation in deep winter in our house. We think this is because (a) we live in a bushy area - although we do have plenty of sunlight as our house is in a clearing on a hill and (b) the house has high ceilings and because of limited attic space there is a limit to the number of vents that can be installed.

The condensation build up continues to be mostly in the master bedroom, and to a lesser degree the lounge. The downstairs of the house, where we spend little time, gets just about no condensation. We do have extractor fans for kitchen and bathrooms. Heating is not an issue for us when we need it, with a good fireplace in winter and a heat pump as well. We sometimes use additional fin heaters, but rarely.

The unit is really quite dumb, as posters above have mentioned. There is very limited control - the only settings are base fan speed and target temperature. The target temperature tells the unit when to ramp up based on the difference between attic temperature and temperature at the control panel. We find that in spring, temperature variations lead to significant fan ramp up which can be quite noticeable, and what we need the system to do would change depending on what time of day it is. We have had to turn it of for some periods to stop it misbehaving. In winter, when we would want it to ramp up, it has failed to do so as the winter attic temperature has never been higher than the house temperature.

SAYR have been good and in contact with us about how to get the most out of the system, although to be honest there's very little to it.

In summary: it makes a bit of difference and we don't regret getting it, it can't tell when it should ramp up automatically such that manual fiddling has become necessary, and all ventilation companies essentially do the same thing and offer the same solutions in a bit of a one-size-fits-all approach, when clearly a large space like ours with limited vent placement options, was unlikely to benefit hugely from the system.

In the future I think we wouldn't install a domestic ventilation system unless we had relatively low ceilings and were able to place vents in each living space.

Anyway I hope the above assists others who might be looking at a similar solution in the future.