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Direct Air vs Air from Room - for Wood stove. What's best for high humidity house?

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Krome10

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Hi all

A bit of background: I live in the Brecon Beacons surrounded by a lot of trees and a stream, all of which I presume lends to the high relative humidity (RH) in my house. It often sits in the 70% region and sometimes even creeps above 80%. In times of high RH, opening the window is counter productive and causes it to rise. We use a dehumidifier when it's at its worse.

We have a wood burner in one room, which does not have a direct (external) air feed. It's not been in long, but it does do a good job of reducing the RH. We'll soon be installing a further two wood stoves. I totally see the sense in direct air in as much as it will minimise drafts, and negate the need for a vent which would let copious amounts of cold air into the house...

But when it comes to humidity I can't get my head around it. No direct air to the stove means more air changes and air circulation in the house (I think?), which could be looked upon as a good thing. But then again, it also means more air coming from outside, where the humidity is higher, so perhaps it isn't such a good thing. And maybe that's all too basic a way too look at it anyhow.

I wondered if anyone out there has a better understanding of the science behind this and could explain which option is best and why, when it comes to humidity?

Many thanks
 

Droogs

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Watching this with interest. My thinking would be use outside air sealed from the house to prevent additional moisture and heat what's inside so that warm moist air rises and exits via high vents, but waiting to see if I'm wrong
 

Krome10

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Thanks @Droogs

That got me thinking... I guess there are three options actually. A lot of direct air stoves / kits only allow for partial external air and are not room sealed. As such, the options are:

- Room vent (or no room vent if 5kW or below) - but either way, air pulled from the house
- Wood stove with partial direct air (so pulls air from outside and inside)
- Wood stove with full direct air / fully room sealed.

Cheers
 

artie

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I can't help you about the science only my experience.

I grew up in an old house with a solid fuel stove It produced loads of heat but most of it went right up the chimney causing a vicious draught to enter under the door.
My Fathers old army coat would cure the draft, but then the fire wouldn't draw, so we all sat close to the fire and let the draught do it's work.
Fast forward 50 years and I find myself needing a heat source and since I have more wood scraps than I can get rid of I opt for woodstove and thinking back to my childhood I insist on an outside vent for it which I can open or close as the fancy takes me.

Long story short as far as heating the room, it doesn't matter where the air is sourced, my room is so small that the 5kw heater has do be closed down almost immediately to remain comfortable.

As far a RH is concerned, it is always lower when the outside vent is closed .
 

Argus

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@ Krome10

I live a bit more up country in the Ceredigion uplands and we have the same humidity 'conditions' .......... I live surrounded by trees, rivers and lots and lots of moss - plus water, in fact we make the stuff and sell it over the way.... but, I won't term humidity a problem, just yet.

Firstly, The fire. It's absolutely essential to have an adequate outside source of fresh air in a room - sealed or not - that has a combustion burner, such as an open fire or a stove. If not the oxygen in the room will become depleted...... I'll leave you to work out what happens next. A sensibly placed CO (Carbon Monoxide) detector is also a good idea.

Usually the fire will scrounge what air it can, Failing that the fire will slow down, become slow-burning, but please don't rely on noticing that...... it's trying to tell you something.
The RH (Relative Humidity) in the room reduces because the temperature rises. The actual containment of moisture in the air (usually shown as gramms per cubic metre) is the AH (Absolute Humidity) and without any condensation (moisture removal) taking place in your room, will remain at the same level.

The measured RH in a space is the proportion of moisture present in the air at a particular temperature versus the total amount that it can hold at the same temperature, shown as a percentage, therefore a change in the RH is inverse to the change in temperature.
The higher the temperature, the more moisture it is able to hold as a vapour. If there's no change in the moisture content - the AH remains the same - the proportional percentage of RH goes down.
When a portion of air has absorbed all the moisture that it can hold, the RH achieves 100%, termed the Dewpoint Temperature. It will not hold any more.
The term 'RH' is a relative measurement of something that is not static and is liable to change. The reason that we, as a species, notice changes in RH is that it affects our ability to perspire.

We are quite tolerant of a range of air conditions and the issue of humidity, in these climates, is seldom a serious problem other than when the dew-point temperature approaches our body temperature. Sometimes, the natural humidity goes up to an uncomfortable level, such as in thunderous weather, but the result for most folk is only temporary discomfort.

Next, I don't know what you are using to measure your humidity, but 'mechanical' Hygrometers tend to use a piece of hygroscopic material and are often accurate to about +/- 10%. Electronic instruments can be better, but the cheaper ones often go out of calibration. There are better, easier and more accurate ways to do it.

So, summing up, fire on, temperature goes up - humidity in the room goes down. Open the window, natural prevailing humidity and temperature equalise.

You mentioned that you don't understand humidity - I'll try to help you.

Some while ago a poster here had a problem with rust on his tools - basically the iron parts rusted in his unheated garage and the thrust of the thread got around to humidity.
A lot of the readers said that the didn't understand what happened, so I wrote a few posts about humidity understanding what happens at both ends of the scale and how to measure it accurately.

Here's a link - look for my series of posts:


Good luck.
 
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Spectric

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If you dont have good ventilation then depleting any combustion process of adequate oxygen will lead to incomplete combustion and nasty by products, they need to breath.
 

Jacob

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Do you have an actual damp problem such as excessive condensation, damp patches and mould on walls?
If not, I'd stop dehumidifying, stop worrying about it and definitely stop trying to measure it as you don't seem to understand the basics (see Argus above).
If you do have a tangible damp problem then you need to find the cause if there is one, if nothing obvious just increase ventilation - window vents, gaps under doors etc and make sure you have maximum roof insulation. If the house is warmer than the outside air then condensation from the air itself is not possible but you can still get condensation from internal sources - kitchens and bathrooms
 
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Ozi

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I can't add anything to what Argus says about humidity but I agree with what he says particularly the CO monitor. One thing that might help is fitting a fan to draw air in, sounds all wrong but bare with me on this. I had a house which developed a damp and condensation problem and was advised o fit a fan which drew air from the roof space into the house. The idea was that air in the roof was warmer than outside due to heat lost from the house and or gained from sun on the roof, that the air drawn into the roof would be the furthest from the ground and possibly dryer (not sure how true that is) and the fan would create positive pressure in the house pushing damper air out. I was desperate so checked that the roof space was dry and found it to be the driest part of the building and fitted the fan, no more damp or mold. The fan was quite expensive but came with a remote giving 100 speed settings which allowed us to pick the best for seasonal variation and turn it up when cooking or showering made the windows steamy - very pleased with the result.
 

Ozi

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Just curious, do you have a moisture meter, if so what readings do you get on wood work around the house. Floorboards, joists etc. need to be below 20% to avoid dry rot
 

julianf

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Unless you are actively running dehumidifiers in your home, the air inside will, as a generalisation, be more humid than the air outside, given it has the additional loading from its occupants.


We live in a stone house. No dpc etc. I considered room sealing the fire, but came to the conclusion that the air flow wasnt a bad thing. Sure, less efficiency, but also less mould issues.
 

Jacob

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Unless you are actively running dehumidifiers in your home, the air inside will, as a generalisation, be more humid than the air outside, given it has the additional loading from its occupants.


We live in a stone house. No dpc etc. I considered room sealing the fire, but came to the conclusion that the air flow wasnt a bad thing. Sure, less efficiency, but also less mould issues.
You've got to have air changes anyway, 5 to 15 times per hour CIBSE - Building Services Knowledge so if the presence of people is generating a humidity prob then probably need more ventilation
 

Phil Pascoe

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... all of which I presume lends to the high relative humidity (RH) in my house. It often sits in the 70% region and sometimes even creeps above 80%. ...

Many thanks
It would be great if the RH here were that low. I've just looked at the local forecast and in an hour or so's time it goes up to 98% (outdoors, so higher indoors?) - it does creep below 70% in the summer, although sometimes in the summer it's at its highest. I've never quite understood why it is a problem per se.

I have a multi fuel as the only source of heating, there is a small vent fitted in the wall opposite (which has since been enclosed by a lean to, but there is more than adequate draught.) I would imagine any certified fitter would insist on exterior draught if there were the slightest doubt about its necessity - I had a vent directly under the stove in my last house.

Last year I put in six inch insulated flexible ducting and a ceiling fan above the stove (as much as any other reason because the stove is in a small room and the room gets too hot) going to a Y and then to the kitchen (my domain) and the bedroom. This worked, but the fan - an Xpelair, was junk - so next week I'm fitting one of these in the roof -
We are luckier than some I suppose - we haven't got around to shutting external doors at night yet, let alone windows.
 

Jacob

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Our place (converted chapel) is ventilated by two "Passivent" ducts in the two upstairs bathrooms. They are as dry as a bone half hour after a steamy shower and draw air from the rest of the building continuously. Single glazing no condensation prob. Whole place otherwise highly insulated.
I highly recommend "Passivent" -cheap, needs no power and is completely automatic, opening and closing according to humidity. There's other brands too.
 

woodieallen

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I think Jacob has it nailed. Stop worrying about it unless you've got evidence of damp. I live near Scotland. We have moss on moss as it's so damp and humid. Yet the house is fine.
 

Jacob

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It would be great if the RH here were that low. I've just looked at the local forecast and in an hour or so's time it goes up to 98% (outdoors, so higher indoors?)
No - lower. The air will be warmer indoors and the RH lower
- it does creep below 70% in the summer, although sometimes in the summer it's at its highest. I've never quite understood why it is a problem per se.
It isn't a problem per se.
 

Chris70

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Krome10: one way to understand relative humidity versus absolute humidity is to consider the space (the room) as a balloon. At any given point, the room/balloon will contain a certain amount of moisture, or water vapour, which can be expressed in g/kg - grams of air per kilogram of water. If we heat the balloon it will expand, so the relative humidity will fall, but the absolute humidity will remain the same, because the amount of water (vapour) cannot escape the closed neck of the balloon.

I'm in the middle of a major refurb/extension and the architect has specified vents for a log burner being installed. Being a person who 'always feels the cold', my heart says no to ventilation, but my head says yes to ventilation. As we cannot escape science, my head wins and I'll install a vent. I read above someone mentioning a vent under the log burner, so I'm considering that.

I agree with Argus, Spectric and Jacob - you MUST have adequate ventilation :)
 

Phil Pascoe

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The vent under the stove doesn't cause draughts - the fresh air is delivered directly, not across the room. Also, air that's already been warmed isn't sucked out of the room. That's the theory, anyway. :)
 

Jacob

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....

I'm in the middle of a major refurb/extension and the architect has specified vents for a log burner being installed. ...
Building regs work to a simple spec. Once all approved you can then see how it goes. If the burner draws well the vent may be redundant and air leakage via window vents, gaps under doors and elsewhere could be enough. Certainly would be more than enough in a normal old fashioned leaky house!
The vent under the stove doesn't cause draughts - the fresh air is delivered directly, not across the room. Also, air that's already been warmed isn't sucked out of the room. That's the theory, anyway. :)
But you need 5 to 15 air changes an hour so it might as well draw from the room anyway. All the vent does is guarantee an air source as it is possible to overdo draught proofing. Which is the answer to OPs question - draw from the room, make sure the room is a bit leaky without excessive draught proofing.
 

Keith 66

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If you dont have good ventilation then depleting any combustion process of adequate oxygen will lead to incomplete combustion and nasty by products, they need to breath.
So do you! A guy i knew fitted a pot bellied stove to a small yacht he lived on, one winter the Medway patrol boat noticed his boat anchored for some days in a remote place with snow on the deck, they had a look & found him dead in his bunk, It was found that he had shut all the hatches & had died of carbon monoxide poisoning.
When we fitted our stove building regs said we must have two vents (air intakes) from underfloor near the stove. They are usually shut as they cause bad drafts but if the stove is going i open them & the cold air goes into the stove alcove & gets hot pretty quick!
 

Ged S

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We have a fully room sealed stove in our old fireplace, which is on an outside wall. It has a short length of 2” pipe drilled through the outside wall feeding directly into the stove. The only air taken from within the building is when you first light the stove (door latched slightly open for about 5 mins to draw air in while the fire roars up). once the stove door is shut, the pipe supplies all combustion air. As far as the fire itself goes, humidity is irrelevant.

PS: the stove works absolutely brilliantly: we used to have a ferocious draft pulling through the door across our sitting area to the fireplace, even though there were air vents on each side of the fire. These vents are now sealed up and we only have the standard Building Regs trickle ventilation.
 

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