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

[Oxbow16]

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

[Oxbow16]

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

[ProDave]

I assume this is an old house so not in any way built as "air tight" and it doesn't have mvhr or anything like that?

 

So a simple air from room stove would do it but provide an air intake ducted to behind the stove so it does not draw cold air in and across the floor to get to the stove.

[TonyT]

Isn’t an external air kit only required if the stove is above 5kW?

[ProDave]

2 minutes ago, TonyT said:

Isn’t an external air kit only required if the stove is above 5kW?

An external air kit is available for the little 3kW burley stove and I have one on my 5kW Mendip stove.

[SteamyTea]

58 minutes ago, Oxbow16 said:

 It often sits in the 70% region and sometimes even creeps above 80%.

What temperature is your room?

 

I have had a quick look at your weather, and a station near me that has similar readings. (Aber Village 87% RH 15.5°C, Mount Hawk 86% RH 16.2°C)

My house is currently at 75% RH and 21.7°C

 

Feels fine.

[Cpd]

I would not put a dedicated air duct in for sub 5kw stove as in an old house you will have plenty of leakage already and sticking a bloody great hole in the wall behind the stove will just make the house even colder during the times the stove is not burning. In my 130 year old farm house I find that when the 5kw stove is on in winter the RH really drops in that room considerably  and is very pleasant, I have just added 70mm of internal wall insulation this year so it will be interesting to see how it all performs. Unless you have a cheep supply of wood running 3 stoves will be an eye watering cost as seasoned wood is not cheap, also your going to be forever taking ash out etc etc. What insulation do you have  and how good is it ? I have done the insulation in my house as I want to decommission 2 fire places as it’s just a bloody pain and often as not electric fires get used instead which cost a fortune to run. 

[Thedreamer]

Our stove which is just under 5kw and has a dedicated air feed under our suspended timber floor.

 

This prevents uncontrolled cold air coming into the room and this approach can only be a good thing for ventilating the joists. 

 

Our house has a good level of airtightness/insulation.

 

 

 

 

[Iceverge]

The relative humidity (RH) of air changes drastically with temperature.  If you bring "damp" air into a house with RH 80% at 10deg and heat it up to 20deg the RH drops to less than 50%. Likewise if you take "dry" interior air and cool it down you'll have very high relative humidity. 

 

It's very unlikely the total water content per m3 in the outdoor air is ever above the indoor water content per m3. ( Unless you are in the tropics) . Simply put, when outdoor air is heated to indoor temperatures, it is always dryer than indoor air. 

 

First sort any bulk water issues like leaks, blocked gutters, cracks in exterior render, external soil higher than the interior floors etc. 

 

Assuming you've done this you need to draw as much external air through the house as possible, heat it up to let it absorb moisture and then send it out again. 

 

There are many ways to achieve this. On our old rental  cottage we used to leave the kitchen extractor running all day when we were out, light a stove every night and run a dehumidifier 24/7. We were careful to let the bathroom window wide open any time there was steam in the air. Same with the kitchen. Only closing it when any condensation had disappeared from the mirror. 

 

As to installing more stoves I probably wouldn't. Running one hard and leaving internal doors open will work better. Consider installing a constant extract fan to suck air from the dampest corner of the house constantly. 

 

Through the wall stove ducts, in my opinion, are only for houses with exceptional airtightness levels in conjunction with stoves manafactured to extremely tight tolerances. 

 

 

 

Edited October 9, 2021 by Iceverge

[Oxbow16]

Fantastic responses - thank you all very much!  A bit tied up right now, but will respond more fully soon.  Until then, thanks again