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

[carbs for arbs]

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

[carbs for arbs]

Also, I was just thinking... I guess there are actually 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

[openspaceman]

I live in a fairly basic draughty house but do have modern double glazing that seems to seal well but some of the windows have vents and bedroom windows generally have a top pane at least slightly ajar all the time. I have a nominally 4-5kW stove drawing air from the room.

 

Air changes per hour differ between rooms but ~ 7 changes per hour recommended for a living room. This little room is about 3m by 4m by 2.4m so needs about 200m3 of air per hour.

 

To get 4kW of heat, excluding flue gas losses I need to burn about a kilo of 20% mc wwb wood per hour. The stoichiometric air required (to react perfectly with that amount of fuel) is 6kg or about 10m3 of air from the room, in practice most stoves will supply 150% to 200% stoichiometric air, say 20m3/hour.

[daveatdave]

10 hours ago, carbs for arbs said:

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

it must be nearly raining indoors with that humidity  have you tried water seal on the outside walls 

[openspaceman]

27 minutes ago, daveatdave said:

it must be nearly raining indoors with that humidity  have you tried water seal on the outside walls 

72% RH in my hall now and it has been a dry warm day after previous rain., so a fair amount of moisture in the air increasing RH as it cools to 9C outside.

 

Once the stove is lit it will hover around 40% RH  when the temperature differential gets above 10C

[daveatdave]

7 minutes ago, openspaceman said:

72% RH in my hall now and it has been a dry warm day after previous rain., so a fair amount of moisture in the air increasing RH as it cools to 9C outside.

 

Once the stove is lit it will hover around 40% RH  when the temperature differential gets above 10C

is your house a solid wall construction if so has it been repointed with sand and cement rather than lime morter 

[openspaceman]

8 minutes ago, daveatdave said:

is your house a solid wall construction

Yes

8 minutes ago, daveatdave said:

 

if so has it been repointed with sand and cement rather than lime morter 

It looks like it has never been repointed on the exposed brickwork  but the hall is a single storey extension from 1959, still solid 9" but rendered.

[Sutton]

Granddad's pre WW1 terrace in Bolton had no cavity walls, no DPC (not slate anyways) and poor attic insulation when I knew it in the 1970s. If you'd put an old dry brick from the same period in a bucket of water on a summers day, it'd fizz like an Alcaceltzer! ie highly absorbant. Modern building materials are about impermeable containment. Older houses need to breathe. In your vent choice, you need to consider more than humidity and warm air rising. By that, I mean if you select the most efficient, most promoted & poss the most costly, it might not be the best. Give us more info

[Woodworks]

We get similar humidity up here on the west of Dartmoor. Only thing that really works is the dehumidifier when we are living in the clouds for weeks on end. Its worst when it's relatively warm and damp as the warm air carries more moisture. 

 

Our stove does have a closed combustion system and suspect humidity would be lower if it didn't but rules required that or a bloody great hole in the wall which somewhat undermines all insulation efforts.

[openspaceman]

1 hour ago, Woodworks said:

We get similar humidity up here on the west of Dartmoor. Only thing that really works is the dehumidifier when we are living in the clouds for weeks on end. Its worst when it's relatively warm and damp as the warm air carries more moisture. 

 

Our stove does have a closed combustion system and suspect humidity would be lower if it didn't but rules required that or a bloody great hole in the wall which somewhat undermines all insulation efforts.

Yes, my home is by no means damp, I live in one of the driest regions in UK, going room sealed is probably the only way around the regulations with a bigger stove but drawing air from the room and a whole house heat exchanger makes more sense to me.

 

The solid wall brick built with lime mortar eveolved over a long time, the walls need to breathe both sides so I am loathe to attempt insulating them, inside or out.

 

I do wonder what effect wall hangings have as you see them in colder places of eastern europe.

 

Modern build methods, done correctly which is the exception, may be the way to go to satisfy people sitting on the M25 but knocking down a 160 year old house and starting again is not an option.