CINDER BLOCK WALLS FOR INCREASED THERMAL MASS

[N Clark]

hey everyone, I'm new here but a friend of mine sent me to this site and I hope I can have my question answered here

I have a Jotul wood stove on a brick hearth. I have heard that I can build additional walls around the wood stove out of cinder blocks and sand to absorb the heat and release it slowly for a more effieicent use of energy.

This would be awesome for a couple reasons:
1) It would help funnel a lot of heat up to where my fan can pull it through my ducts
2) It would help control the heat in the basement so it's not super warm down there all the time (a concern since there are bedrooms on that floor as well.

My question is this: Is that a possibility or is that a big no no? I doubt the cinder blocks would heat to a point of cracking, so the idea sounds pretty good.

Thanks for all your thoughts!

 

[begreen]

I think there is a better way to accomplish this. Running heat through ductwork is lossy and it's against code to have a return within 10 ft of the stove. But there is another method that may help. Blow cold air toward the stove. Put a table or box fan at the top of the basement stairs on the floor, pointing down toward the woodstove. Run it on low speed. It will blow the cooler air down low, toward the woodstove. The denser cool air will be replaced with lighter warm air from the stove room. That should help cool down the basement and a strong stream of hot air will flow upstairs at the top of the door opening. If you hang some toilet paper in the doorway you'll see this effect.

 

[N Clark]

I've tried that but it really doesn't work unless I have a series of 4 or 5 fans because of the way the air flow is in the home and going down stairs. Adding the duct work and in-line fan managed to move the basement from being like 110 degrees to 80-90. The real benefit of the cinder block walls is to mitigate radiation laterally in the basement and 'encourage' the flow upward where the inlet fan is.

The only success I've had with introducing cold air into the system is by opening one of the windows in the basement. That works pretty well, but is in close proximity to where people sleep so sometimes the wicked cold air is a bit much even if the overall basement temp is improved.

 

[begreen]

Have you tried reversing the fan in the system?

 

[N Clark]

No, but that would cause the same heat problem in the basement because of the lack of air flow unless I place several fans to direct it around corners and up the stairs... and that isn't practical as it would remove all of our walking space

It's a horrible house for air flow. Left to itself (just floor vents) the basement stays 110+ while the upstairs is lucky to hit 70 (even if I'm pulling cold air from outside to force all heat upwards). I'm building a shroud this season to direct more heat to the ducts out of the basement which should help some... I was intrigued by this wall idea and wanted to ensure I wasn't creating any fire-type risks by putting one in.

 

[Slow1]

All "duct work / code" issues aside (not to diminish the issue as I do believe that is rather important for good reason) - building such walls as you describe might be helpful in some regards, but the thermal mass aspect would be most helpful perhaps in terms of leveling off peaks and valleys in temperature swings - i.e. as stove / room temp falls the walls will release the energy into the room and as stove is heating these same walls will moderate the room some by absorbing the heat.

Now I imagine that if these walls were built in such a way as to help direct air flows and heat air as well near where your duct work inlet is then perhaps it could help on that front, but without lots of details of the design it is hard to speculate.

As to hazards - unless they had combustible materials then I don't see them being a fire hazard (as you describe they wouldn't) - if built too close to the stove you would perhaps be building an alcove of sorts (sans top?) and that could make the stove run a bit hotter and create risk of overheating (?). All a matter of guesswork here though.

 

[begreen]

No, but that would cause the same heat problem in the basement because of the lack of air flow unless I place several fans to direct it around corners and up the stairs... and that isn't practical as it would remove all of our walking space

It's a horrible house for air flow. Left to itself (just floor vents) the basement stays 110+ while the upstairs is lucky to hit 70 (even if I'm pulling cold air from outside to force all heat upwards). I'm building a shroud this season to direct more heat to the ducts out of the basement which should help some... I was intrigued by this wall idea and wanted to ensure I wasn't creating any fire-type risks by putting one in.

I can appreciate your dilemma. It sounds like you need a wood or pellet furnace rather than attempting code violating hacks that may or may not work.

 

[N Clark]

Yeah... increasing the # of code violations is never a good thing. At least these ones would be out in the open and can be fixed if necessary. I inherited some other ones that defy logic and I hope never turn into anything.

My dream is to have a wood furnace (indoor or outdoor) tied into one of the newer boiler systems that constantly circulate and adjust the cold/hot water based on the outside temp to produce the correct inside temp. Mix that with some of the new solar water heating panels, and I'm a happy camper! Alas, I lost my briefcase of money

 

[Mike Cook]

Sounds like what you REALLY need is to move the air in the basement to the rest of the home better. Have you thought of venting or ducting between floors that is separate from your FAU?

Some possibilities:

(broken link removed to http://www.activeforever.com/p-2129-airflow-breeze-register-booster-fan.aspx?utm_source=shp&utm_medium=csc&utm_content=A1616801&utm_campaign=occupational-health-safety&CMPID=SHP)

http://www.amazon.com/Diversitech-625-AF6-Round-Duct-Fan/sim/B001O6ABRQ/2

 

[mellow]

If you are looking to jerry rig something I would consider putting in a water heater coil connected to a heat exchanger, with a pressure relief valve of course.

(broken link removed to http://www.hilkoil.com/product.htm)

Or put soapstone around your stove, you don't mention which Jotul you have.

 

[N Clark]

I do have boosters on my vents separate from the FAU... although I wouldn't call my setup a FAU much. I just routed some ducts from above the stove to my vents and put in an inline fan. They do help a little, but not as much as I'd like. ... although as I look at the link you provided, my boosters may not have been up to snuff in terms of really moving air around.

My 8" in-line fan moves about 50 cfm, and this booster could double that pretty easy...

I have the F 500 Jotul. I hadn't thought about soap stone.