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18 posts

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  # 1951811 5-Feb-2018 09:45
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Aredwood:
deanos: Not quite true.  If you have a positive pressure system the intake in the roof would generally be dryer during the daytime.  If you have a balanced system (heat exchanger), the temperature differencial between inside and out has a dehumidifying effect at the core


How does the roof space remove moisture from the air? I get that if you heat air up, the relative humidity goes down, due to warm air being able to hold more moisture than cold air. So if there is warmth in the roof space, that can have a drying effect.

And how does the heat exchanger remove humidity? Apart from warming the incoming air?

 

Your roof space is generally designed to have a bit of airflow so when it heats up the moisture can evapourate. Good reason not to fill the attic up with junk

 

As for heat exchangers, when you heat cool moist inflow air it will condensate at the core, lowering the humidity. Many models therefore have a condensate drain.




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  # 1951823 5-Feb-2018 10:04
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itlearner: as the south side 2 bedrooms ard ready damp. 1 of them even more worse as nobody liveing in, at the moment (summer time) has a mini-dehumififier on 24/7, every few days got half of liter water, very frustrated.

 

Well, the first thing you need to fix is to stop moisture getting into rooms. Check the insulation, under the house, moisture barrier under the floor. If you have space - put some plastic on the ground under the house to stop moisture getting into the house through the floor. In your case you are treating symptoms, not the cause of the problem.

 

How does the roof space remove moisture from the air? I get that if you heat air up, the relative humidity goes down, due to warm air being able to hold more moisture than cold air. So if there is warmth in the roof space, that can have a drying effect.

 

And how does the heat exchanger remove humidity? Apart from warming the incoming air?

 

Roof space is warmer during the day (unless it's raining all day long). So, the relative humidity is lower. Right now it rains, outside temperature is 20.7C and the roof space temp is 27C.
The point is to change the air as there are lots of things producing and holding the moisture inside the house.

 

deanos: As for heat exchangers, when you heat cool moist inflow air it will condensate at the core, lowering the humidity. Many models therefore have a condensate drain.

 

When you heat cool moist air - nothing happens as when you increase temperature, the air can hold more moisture.
When you cool warm moist air, then relative humidiy goes up. When it reaches 100% it starts to condensate as the air can't hold moisture any more (this temperature is called dew point).


 
 
 
 


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  # 1951847 5-Feb-2018 10:40
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Starina:

 

deanos: As for heat exchangers, when you heat cool moist inflow air it will condensate at the core, lowering the humidity. Many models therefore have a condensate drain.

 

When you heat cool moist air - nothing happens as when you increase temperature, the air can hold more moisture.
When you cool warm moist air, then relative humidiy goes up. When it reaches 100% it starts to condensate as the air can't hold moisture any more (this temperature is called dew point).

 

 

Bugger me you're right! Net result however is the same, dryer air in, moist out. 


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  # 1951867 5-Feb-2018 10:54
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The house has insulation on the roof and under the floor. It's old brick and tile, the roof has been repainted, has heatpump in living room.  The heatpump couldn't dry the air from the bedrooms. Gas heater is in the living room too but hardly use as it creates h2o and co2, more moisture. As I mentioned previously, 2 south side bedrooms are very damp, can see the mold on the furniture very couple of months...... Hope it will be changed after installing a ventilation system... @deanos, dryer air in, most out. YES, that's why want to get fresh air from outside when is not raining, and circulate the air between living room and bedrooms when is raining. Any suggestions please.


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  # 1951885 5-Feb-2018 11:15
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itlearner:

 

dryer air in, most out. YES, that's why want to get fresh air from outside when is not raining, and circulate the air between living room and bedrooms when is raining. Any suggestions please.

 

 

If you get a heat exchanger it doesn't matter if it's raining or hot (as long as it has a core bypass function) it will bring in fresh air all year round.

 

What it comes down to is your budget.  Smartvent do have options but read my limitations mentioned earlier


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  # 1951976 5-Feb-2018 13:10
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@deanos, have a look all the Smartvent systems, yes, it's possible to achieve what requested but in the same time is a bit waste of materials/money. Don't want ceiling air, use outside air (Summer feature - extra), heat transfer damper (extra), but don't want these two extras run at the same time, not sure they could run separately respectfully. And the price is not cheap, SmartVent Synergy Evolve System - 4 Vents + Heat Transfer Damper Kit + Summer Upgrade Kit is more than $4500, then plus installation.  Have requested some companies for quoting, will see what comes up.


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  # 1952243 6-Feb-2018 00:29

deanos:

 

Your roof space is generally designed to have a bit of airflow so when it heats up the moisture can evapourate. Good reason not to fill the attic up with junk

 

As for heat exchangers, when you heat cool moist inflow air it will condensate at the core, lowering the humidity. Many models therefore have a condensate drain.

 

 

 

 

You still haven't explained how a roof space removes moisture from the air. Imagine the outside air is 50%RH. If you heat that air, the RH goes down, but if you cool that air back to the same starting temp, the RH increases back to the original value.

 

Here is a humidity calculator.

 

https://www.rotronic.com/en/humidity_measurement-feuchtemessung-mesure_de_l_humidite/humidity-calculator-feuchterechner-mr

 

Set it to calculate the specific humidity, then vary the temperature input value. You will see that the relative humidity value changes with temp, even though you are not changing how many grams of moisture are in each KG of air. You will see that the dew point stays the same, which further confirms that the actual amount of moisture in the air is not changing.






 
 
 
 


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  # 1952249 6-Feb-2018 01:09

itlearner:

 

The house has insulation on the roof and under the floor. It's old brick and tile, the roof has been repainted, has heatpump in living room.  The heatpump couldn't dry the air from the bedrooms. Gas heater is in the living room too but hardly use as it creates h2o and co2, more moisture. As I mentioned previously, 2 south side bedrooms are very damp, can see the mold on the furniture very couple of months...... Hope it will be changed after installing a ventilation system... @deanos, dryer air in, most out. YES, that's why want to get fresh air from outside when is not raining, and circulate the air between living room and bedrooms when is raining. Any suggestions please.

 

 

 

 

@itlearner Heatpumps only dehumidify when they are set to cooling mode. The only way a heatpump can remove moisture on heating mode, is to heat the room, temporally switch to cooling, then switch back to heating etc. This uses lots of power, so a heatpump will only do that if the room is very damp.

 

Also have a play with the above humidity calculator I linked to. Again set to specific humidity, and enter a Q value of say 10g/Kg.

 

At 27deg, you get 45% RH, nice and dry.

 

at 20deg, now up to 69%RH, beginning to get damp.

 

at 17deg, up to 83%RH, well into the danger zone of mould being able to grow.

 

at 15deg, now at 95%RH, now at extreme humidity levels as seen in tropical countries.

 

And 14.21 is the dew point, where water will start condensing on everything. Usually the windows first, as they are typically the coldest thing in a room.

 

In all of the above examples, the absolute amount of moisture in the air is exactly the same.

 

Short answer, either get more dehumidifiers, set them to 50%RH, and leave them running all the time. Otherwise invest in better heating and during winter, maintain an average indoor whole house air temp of approx 7deg warmer than outside. The dew point temp is important, as if your windows get colder than the dew point temp, that is when they will start "crying".

 

Now you can see why my original question on how damp it is outside is so important. Note also that even quite new houses in NZ are often quite "leaky" (not very airtight). So if you maintain that 7deg or so higher indoor average air temp, air leakage rates will often give you plenty of air exchange with outside to keep the air "fresh".

 

Ventilation systems that take air from the roof space, are mainly intended to recover free heat from the sun. And the drying action is then mainly from helping to increase the average indoor air temp. Unless the moisture problem is actually from moisture sources inside the house. In which case extractor fans or just opening the windows would fix it.








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  # 1955572 12-Feb-2018 10:44
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itlearner:

 

As I mentioned previously, 2 south side bedrooms are very damp, can see the mold on the furniture very couple of months...... Hope it will be changed after installing a ventilation system... @deanos, dryer air in, most out. YES, that's why want to get fresh air from outside when is not raining, and circulate the air between living room and bedrooms when is raining. Any suggestions please.

 

 

The first thing you need to figure out it why these rooms are damp. If no one lives in the room then there shouldn't be much moisture there. Any chance that you have a stream running under the floor of these two bedrooms? :) And apart from the insulation - check the moisture barrier (or install one). It's just a plastic on the ground. Easy to install - really helps. 

 

Aredwood:

 

You still haven't explained how a roof space removes moisture from the air. Imagine the outside air is 50%RH. If you heat that air, the RH goes down, but if you cool that air back to the same starting temp, the RH increases back to the original value.

 

 

You are absolutely correct. The roof space doesn't remove moisture from the air. The amount of water in the air is the same in the roof space and outside.

 

 

 

Aredwood:

 

In all of the above examples, the absolute amount of moisture in the air is exactly the same.

 

Short answer, either get more dehumidifiers, set them to 50%RH, and leave them running all the time. Otherwise invest in better heating and during winter, maintain an average indoor whole house air temp of approx 7deg warmer than outside. The dew point temp is important, as if your windows get colder than the dew point temp, that is when they will start "crying".

 

 

3 deg during the winter night. Not sure that 10 degrees in the house is a comfort temperature. And how are you going to cool the house while you asleep without ventilation? Where is the magic 7 degrees came from?

 

Aredwood:

 

Now you can see why my original question on how damp it is outside is so important. Note also that even quite new houses in NZ are often quite "leaky" (not very airtight). So if you maintain that 7deg or so higher indoor average air temp, air leakage rates will often give you plenty of air exchange with outside to keep the air "fresh".

 

Ventilation systems that take air from the roof space, are mainly intended to recover free heat from the sun. And the drying action is then mainly from helping to increase the average indoor air temp. Unless the moisture problem is actually from moisture sources inside the house. In which case extractor fans or just opening the windows would fix it.

 

 

If anyone lives in a house - they are the major sources of moisture. People breath, cook, eat hot meal in the dining room (do you have extractor fan in the dining room?), drink hot tea/coffee. Everything increases amount of water in the house.

 

Moreover, the absolute amount of water in the air usually higher during the day and lower in the night. It means that if you keep your windows opened during the day and close overnight - you house (air, carpets, furniture, walls, wardrobes, clothes - basically everything) keeps amount of water that WAS in the air during the day. When outside air cools, then not only relative humidity but also an absolute amount of moisture decreases - the air in you house is much more moist then outside. Please, have a look at the first picture. This is the data from my sensors from Jun 2017. 

 

 

During the day dew point is almost 12 degrees, during the night - 6.5. It means that absolute amount of water in the air decreases.

 

You can see on the chart that the dew point in the master bedroom decreases even though there were 3 people in the room (this number is not on the chart, just believe me :)

 

Difference in 2 degrees between master bedroom's dew point and attic space temperature (which is the same during the night as outside air temperature) doesn't matter. The result was just 1 or 2 cm of fog on the window in the bottom. 

 

In the bedroom we use an oil heater. The temperature is always around 20 degrees during the night there.

 

Here is the data for the same period from the rooms where noone was during the night.

 

 

You can see that dew point in these rooms is very close to the attic one.

 

So, the main point of ventilation is to blow much dries air from the outside (or from the roof space - in this case during the winter you will also get free heat). If it's a positive pressure (like mine) then old air will be pushed out of the house. If it's a balanced one - old air will be sucked outside. Doesn't matter.

 

To sum up: lots of things produce moisture in the house and even the air during the day which is stuck in the house keeps more moisture than the air during the night. So, you need to have a constant air coming into your house which will be much drier. Then this air will be heated to the comfort temperature, but the amount of water will be the same as in the outside air, which means that the dew point will be the same as outside. In this case you won't get crying windows (damp rooms, mold, etc.)

 

 

 

 


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  # 1955657 12-Feb-2018 12:42
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There is normally some second hand units going on Trade Me.  That's were i found a replacement fan unit for our DVS system.


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  # 1955954 12-Feb-2018 18:59
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@Starina - Slightly off topic, but what sensors / back end are you using to collect all that data?



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  # 1955958 12-Feb-2018 19:11
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Ge0rge: @Starina - Slightly off topic, but what sensors / back end are you using to collect all that data?

 

@Ge0rge: Sensors in most cases are BME280. They are connected to ESP8266.

 

Back end - custom hand made :) I couldn't find a solution which suited me when I started everything, so had to do it myself. I think that something like OpenHub will be enough for simple tasks.


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  # 1956090 13-Feb-2018 00:55

Starina:

 

Aredwood:

 

In all of the above examples, the absolute amount of moisture in the air is exactly the same.

 

Short answer, either get more dehumidifiers, set them to 50%RH, and leave them running all the time. Otherwise invest in better heating and during winter, maintain an average indoor whole house air temp of approx 7deg warmer than outside. The dew point temp is important, as if your windows get colder than the dew point temp, that is when they will start "crying".

 

 

3 deg during the winter night. Not sure that 10 degrees in the house is a comfort temperature. And how are you going to cool the house while you asleep without ventilation? Where is the magic 7 degrees came from?

 

 

 

 

@Starina Not sure if you are referring to winter or summer with your above comment. My comments are in relation to winter. There are plenty of times during winter when it rains for days on end, with little or no sun. And of course at night, when things cool down enough for dew to start appearing. These are times where outdoor humidity is (or could be considered to be) at 100%. So the only practical means of humidity control are: Dehumidifying and maintaining a higher average indoor temp than outside. The 7deg figure I mentioned earlier is both an average (over a whole 24 hour period), and a "rule of thumb". You would have to be living somewhere pretty cold (deep south of NZ, in an inland area maybe?) for 3deg to be an average temp over a whole day/night.

 

 

 

As for right now (summer with high humidity) it is currently 25deg in my room, And although I don't have a humidity sensor in there, Weather websites currently estimate the outdoor humidity to be 90% 23deg outdoor temp, 22deg dew point. I have had a fan running for over 4 hours now - bringing fresh air in from outside and to try and cool the room. (room gets lots of afternoon sun, and easily gets over 35deg during the day) More ventilation (AKA a bigger fan) isn't going to be able to lower the temp or humidity by any useful amount tonight.






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  # 2000076 20-Apr-2018 23:46
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has anyone DIY'd their kitchen extractor units?

 

I was looking at using scheweigen rangehood (90cm, 1600cm3/s) with remote motor (roof mount), but I'm a little worried about the roof mount because 1. we have a twin layer roof (a metal roof then tile roof ontop of the metal roof), it is really windy where we live and I don't know how well the external motor will cope with the high wind and will it transfer wind noise down to the rangehood?  The cost is about $1650 + installation.

 

Looking on the internet you can buy lots of inline motors, then I could just have an eave mounted exhaust instead.  The residential kitchen extractors with remote motors tend to all be roof mounted motors.

 

Examples of inline motors (nz)

 

- http://simx.co.nz/categories/extraction-fans/in-line-fans
- http://hausgroup.co.nz/tech_motor.html
- http://venttech.co.nz/fans/inlin-fans/

 

Inline silencers

 

- http://venttech.co.nz/silencers/

 

And rangehoods for remote motors

 

- http://hausgroup.co.nz/tech_hood.html

 

 

 

 





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