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venting old HVAC equipment in a sealed crawl space

ohscrifle's picture

venting old HVAC equipment in a sealed crawl space (post #192681)

ohscrifle on Sat, 08/21/2010 - 21:03 (updated 08/22/10 - 08:18) 
in

First post.  Just bought a slightly used house.  Atlanta, GA. Built 1986. 2 story wood frame over crawl space. Currently vented crawl space is ~48" high, dry, gravel over dirt, has a bone dry sump crock with working pump. 19% [!] moisture content in [edit: non insulated] floor framing.

My intent [edit: in the crawl space] is to seal the perimeter vents, insulate perimeter walls/rim & lay down a sealed 15mil reinforced poly VB turned up the cast in place walls. GA requires a termite inspection gap, so I'll leave sill plate visible. My concern is the resident gas fired furnace & AC system in the crawl space.  HVAC equipment is old & inefficient but not going to be replaced at least until 1 yr home warranty expires. 

Now: my question:  When I seal the crawl space, I need to provide combustion air to the HVAC unit. I have read about a nice damper in a small duct solution, but I'm not clear where small duct must terminate (at the end that doesn't go outside).  Is it appropriate to duct into return air plenum or just "near" the unit?

 

 

 

   

-A +A

I'm not aware of (post #192681, reply #1 of 29)

by davidmeiland in reply to ohscrifle [original] on Sat, 08/21/2010 - 21:45

any way that you can provide combustion air to an open combustion appliance other than by providing free air to the space that it's installed in. If you seal the crawl space vents you will be starving the furnace, and unless the furnace manufacturer or an approved third party makes some sort of retrofit kit that allows you to connect a pipe to the unit and run it to the exterior, you're out of luck. There is no way that ducting into the return air plenum does anything at all for combustion air.

The other issue you would have is free air for the chimney to draft.

Much better to just replace the unit with a sealed-combustion appliance. I would go ahead and install the ground cover now, but abandon plans to insulate and seal until you have a new furnace. The project can't be phased the way you propose.

Thanks Dave.  I'm afraid my (post #192681, reply #2 of 29)

by ohscrifle in reply to davidmeiland [original] on Sun, 08/22/2010 - 06:55

Thanks Dave.  I'm afraid my "ducting to the return air plenum" suggestion has distracted from my other idea which still seems plausible in my mind.  A new sealed HVAC system is definitely going to be plan A, but budget & long "to do list" makes me want to continue to consider a plan B.  That said..

  

Is there an accepted way to calculate the amount of 'outside' air necessary for combustion & chimney draft in a gas appliance?  From the manufacturer plate, I can determine the crawl space HVAC unit's sizing info - I'm just not sure how to translate that into an amount of required fresh air in the room.  Assuming the volume of outside air required is small (meaning far less than the "free air" currently flooding my crawl space with moisture) it still seems feasible to seal/insulate the crawl space, and provide a small outside air duct with (*without?) damper - terminated near the combustion box.  My goal would be for the small duct to supply necessary minimal "fresh" air to the system so the unit isn't starved; yet allow the crawl space to be conditioned and dehumidified (via leaks from supply and return ducts or through floor above).

 

* this (or similar) is the product I plan to use for the dryer makeup air - perhaps one of these on the supply duct would limit humid air from freely flowing in...  http://www.rewci.com/noname6.html

 

 

Found a product that proposes (post #192681, reply #3 of 29)

by ohscrifle in reply to ohscrifle [original] on Sun, 08/22/2010 - 13:31

Found a product that proposes to deliver only enough air for combustion of specific size furnaces. Seems this would cut most of the humidity from the crawl space, since it only allow a small amount of outside air in, only while the furnace runs.

  http://www.tjernlund.com/combustionairintake.htm

  Anyone think this idea has legs in the context of my original post?

Uh, how do you figure that (post #192681, reply #4 of 29)

by DanH in reply to ohscrifle [original] on Sun, 08/22/2010 - 13:43

Uh, how do you figure that closing off the crawl will keep the humidity out??


We are like tenant farmers chopping down the fence around our house for fuel when we should be using Nature's inexhaustible sources of energy -- sun, wind and tide. ... I'd put my money on the sun and solar energy. What a source of power! I hope we don't have to wait until oil and coal run out before we tackle that. --Thomas Edison

My understanding (post #192681, reply #5 of 29)

by davidmeiland in reply to DanH [original] on Sun, 08/22/2010 - 15:09

is that folks in the hot, humid south with vented crawls and vented attics do get a fair amount of humidity in them. He mentioned having an A/C unit in the crawl, which I bet means ductwork down there too. Unless it is well-sealed and well-insulated he probably will be better off with a sealed crawl.... but I agree with what I think you're probably saying, i.e. install the ground cover now and see if that doesn't help. Ultimately I think he does want a new furnace and a sealed crawl. Supplying combustion air thru a duct seems only marginally better from a humidity standpoint, and possibly a lot worse from a safety standpoint.

All I know is I have 19% (post #192681, reply #6 of 29)

by ohscrifle in reply to davidmeiland [original] on Sun, 08/22/2010 - 15:48

All I know is I have 19% moisture in my floor framing & I'd like to see it dry.  

The 25 year old furnace will be replaced with a two pipe system.. when it dies.  Meantime, I'll wrap the ground & monitor progress.

I think 19% is probably (post #192681, reply #13 of 29)

by DanH in reply to ohscrifle [original] on Mon, 08/23/2010 - 14:19

I think 19% is probably pretty good for that situation.


We are like tenant farmers chopping down the fence around our house for fuel when we should be using Nature's inexhaustible sources of energy -- sun, wind and tide. ... I'd put my money on the sun and solar energy. What a source of power! I hope we don't have to wait until oil and coal run out before we tackle that. --Thomas Edison

Dan, After it's sealed & (post #192681, reply #7 of 29)

by ohscrifle in reply to DanH [original] on Sun, 08/22/2010 - 15:57

Dan,

After it's sealed & perimeter insulated, it'll become a conditioned space [by duct leakage or possibly by adding a small supply & return if leakage isn't enough]. 

I'm trying to determine if drawing a small amount of outside/combustion air in (to keep the old AHU drafting properly) will kill the concept.. ie. I'll obviously be trying to condition greater atlanta, but hopefully only in very small amounts..  

Likelihood is I'll just seal the floor & leave the vents open until I upgrade to a new 90% plus two-pipe furnace - then seal vents & insulate perimeter.

Suggestions/discussion wanted.

My point is that most of the (post #192681, reply #8 of 29)

by DanH in reply to ohscrifle [original] on Sun, 08/22/2010 - 17:52

My point is that most of the moisture down there is coming out of the soil, not from the air.


We are like tenant farmers chopping down the fence around our house for fuel when we should be using Nature's inexhaustible sources of energy -- sun, wind and tide. ... I'd put my money on the sun and solar energy. What a source of power! I hope we don't have to wait until oil and coal run out before we tackle that. --Thomas Edison

That sounds the most (post #192681, reply #10 of 29)

by rdesigns in reply to ohscrifle [original] on Mon, 08/23/2010 - 07:17

That sounds the most reasonable to me for now. It's likely that the bare dirt is a major source of moisture, and sealing it with a vapor barrier will probably reduce the humidity signifcantly.

Clewless raises an important point: Is your system designed to have the crawlspace functioning as a return air plenum? Or is the return air ducted from the living space?

It could be that, even with a ducted return system, a lot of air from the crawlspace is getting sucked into leaky return ducts, especially if the returns are undersized. If they are, (and this is a common problem in forced-air systems) the entire system is out of balance, which means that the crawlspace will be de-pressurized and sucking in a lot of outside, humid air, and the living space will be correspondingly pressurized, with conditioned air being pushed out of the house envelope and into building cavities like walls where, in wintertime, the warm, humid conditioned air will condense when it meets cold surfaces toward the outside of the wall cavity.

You can easily test for this possiblity of imbalance. With the furnace blower running, (but not firing) and all doors and windows closed, along with the crawlspace vents, open the crawspace access hatch just slightly and see if the smoke from a blown-out match gets sucked down into the crawlspace. If it does, it means you need more return air from the living space, and that you have leaky return ducts in the crawl.

If these problems exist, they need to be corrected, even if you install a high efficiency furnace and convert the crawl to a conditioned space.

RE: the combustion air (post #192681, reply #9 of 29)

by Clewless1 in reply to ohscrifle [original] on Mon, 08/23/2010 - 05:35

RE: the combustion air question. Yes, you can install a motorized damper for combustion air. It must be interlocked w/ the furnace. I assume you are sharing return air plenum (i.e. the crawlspace itself) w/ the combustion air for the furnace. Check w/ your building official. Normal combustion air in a mechanical room must be supplied BOTH w/in 12" of the ceiling and 12" of the floor ... so you may have to split your combustion duct after the damper ... but the BO should tell you this.

Another thought ... if the crawl is the return air plenum, you may not need outside combustion air as you were planning. The combustion air comes from the house. Most/many installations with furnaces that have free access to house air may not need combustion air as with small mechanical rooms (read 'in confined spaces').

There ain't NO free lunch. Not no how, not no where!

REALLY?!? (post #192681, reply #15 of 29)

by Tim in reply to Clewless1 [original] on Tue, 08/24/2010 - 05:25

"I assume you are sharing return air plenum (i.e. the crawlspace itself) w/ the combustion air for the furnace."

Why would you assume this? He describes a ventilated "outside" i.e. outside of the building insulated envelope crawlspace. Probably under a pier and beam type of construction.

I would assume that such an installation would fall in the category of "No one can be that ignert, can they", which I am frquently proven wrong. They can be and are, and are allowed to buy enough tools to be really dangerous.

Someone was talking here (post #192681, reply #12 of 29)

by DanH in reply to Tim [original] on Mon, 08/23/2010 - 14:17

Someone was talking here maybe two years ago about a small house in the SF bay area where there was no return air plenum -- "return" air was drawn from an unsealed crawl.  This made minimal sense in that area (only made some sense because of the mild climate), but would make no sense if the structure has central air in a warm, humid climate where moisture removal is a major issue.   (But one might see this in the humid south if the AC is window units, I suppose.)

But, given the variations, one should never assume anything.


We are like tenant farmers chopping down the fence around our house for fuel when we should be using Nature's inexhaustible sources of energy -- sun, wind and tide. ... I'd put my money on the sun and solar energy. What a source of power! I hope we don't have to wait until oil and coal run out before we tackle that. --Thomas Edison

There are correct ways (post #192681, reply #14 of 29)

by Tim in reply to DanH [original] on Tue, 08/24/2010 - 05:23

to do things and there are incorrect ways to do things. In most situations, there are areas of "gray". Regardless of how many times I am proven wrong, I still assume that the most stupid things are not regularly done.

In the south, in older (post #192681, reply #16 of 29)

by DanH in reply to Tim [original] on Tue, 08/24/2010 - 05:33

In the south, in older housing, a furnace is an upgrade from space heaters.  It might be used maybe 30 days a year, mostly in the mornings to "take the chill off".  Efficiency isn't a big issue.


We are like tenant farmers chopping down the fence around our house for fuel when we should be using Nature's inexhaustible sources of energy -- sun, wind and tide. ... I'd put my money on the sun and solar energy. What a source of power! I hope we don't have to wait until oil and coal run out before we tackle that. --Thomas Edison

eh ... right. I got a bit (post #192681, reply #20 of 29)

by Clewless1 in reply to Tim [original] on Wed, 08/25/2010 - 05:59

eh ... right. I got a bit confused in the discussion, there, I guess. Happens often in this medium of dialogue.

and right ... it is ducted as the OP pointed out to me.

There ain't NO free lunch. Not no how, not no where!

return side is ducted. (post #192681, reply #17 of 29)

by ohscrifle in reply to Clewless1 [original] on Tue, 08/24/2010 - 17:49

return side is ducted.

Then you will be largely (post #192681, reply #19 of 29)

by Clewless1 in reply to ohscrifle [original] on Wed, 08/25/2010 - 05:57

Then you will be largely moving toward having an unconditioned space in the crawl ... which as I think you mentioned would tend to defeat the purpose of sealing/insulating the crawlspace walls.

There ain't NO free lunch. Not no how, not no where!

First, (post #192681, reply #11 of 29)

by Tim in reply to ohscrifle [original] on Mon, 08/23/2010 - 07:48

the crawl space is NOT used as a return plenum. The return is ducted directly from the occupied space to the unit. If not, get a professional HVAC company to fix it. This would be a major FU and the entire installation should be reviewed for similar hackery.

Second, how big a volume is the crawl space and how big (gas input rating) is the furnace? How old and inefficient? IF the space is classified and "confined" i.e. less than 50 cu. ft per 1000 BTU/hr of gas input, then you should provide combustion air to the space. Combustion and and return air are completely separate and should stay that way.

The unit takes combustion from the space surrounding it, unless it is "direct vented", which I would think its not, being "old and inefficient". This is where the combustion air needs to be provided. Most Codes will require 1 sq.in. of opening in the combustion air supply duct for every 1000 btu/hr of appliance gas input capacity. A simple 24V spring return motorized damper could be used, tied into the start-up gas valve safety sequence by a knowledgeable individual. Unless you do an exception job of sealing this space and it is truly "confined" it, for all parctical purposes MAY not be necessary.

I recommend you find out which codes apply in your area and follw their direction. My experience is with the IMC, and the latest version on my shelf is 2003.

Crawlspace is about 4,400 cu (post #192681, reply #18 of 29)

by ohscrifle in reply to Tim [original] on Tue, 08/24/2010 - 18:08

Crawlspace is about 4,400 cu ft & furnace plate states 60,000 btu - this unit serves only the first of two floors.  The inspector I hired said it's ~60% efficient.  This is a 2500 SF colonial (1250 sf/floor) & there's not a heckuva long heating season.  Cooling wise, the electric bills average $89/mo* according to the utility & it cooled surprisingly well (subjective) when we inspected on a 97 degree day last month.. hence the reason I'm not running out to replace it.  

The fan device I linked above is intended to be tied to the 24v control circuit so it operates with the unit - I'm leaning away from that concept though since a bird nest could conceivably kill my family [if the CO detector fails].  The house IS probably leaky enough so that it won't starve.. and I'll have a makeup air vent in my laundry room.  Many layers to this onion.

*Seller is only there on weekends & I don't know his setback habits, so this number will certainly go up... but it's all relative

Crawlspace is about 4,400 cu (post #192681, reply #21 of 29)

by rdesigns in reply to ohscrifle [original] on Wed, 08/25/2010 - 07:19

Crawlspace is about 4,400 cu ft & furnace plate states 60,000 btu

Those numbers indicate that you meet the 50 cu. ft.per 1000 BTU requirement in the code. By code, you would be able to install up to 88k BTU without an addtional source of outside air for combustion.

So, you could close the crawlspace vents and still have enough combustion air for the old furnace due to infiltration that results from construction leaks.

When you get to the point of insulating and sealing the crawl, however, you will likely seal off many of the air leaks if you seal the rim joists and sill plates.

But, at the same time, you will be converting the crawl to a conditioned space, so you could simply cut in a small register (about 25 sq, in.) in the supply duct in the crawl that will allow air from the main living space to enter the crawl. The reason this will work is that you will be using the duct system to "tie" the large volume of air from the rest of the house to the crawl, and the rest of the (leaky) house will have even more volume of air than the un-sealed crawl now has, which will be a more-than-adequate source of combustion air.

In the cooling season, close the register in the crawl so that you won't be dumping expensive air into the crawl when no combustion air is needed.

That's really interesting (post #192681, reply #24 of 29)

by ohscrifle in reply to rdesigns [original] on Wed, 08/25/2010 - 17:48

That's really interesting regarding the volume rule & it appears to validate my hunch.

Can you tell me what code you are citing?

thx

All 3 of the (post #192681, reply #26 of 29)

by rdesigns in reply to ohscrifle [original] on Thu, 08/26/2010 - 07:27

All 3 of the nationally-recognized codes (Uniform Mechanical Code, International Fuel Gas Code, and NFPA 54) all have that same formula that one of your other posters also referred to:  50 cu. ft. per 1000 BTU's of fuel gas input. This formula assumes that you don't have "unusually tight construction", which your house probably does not, given its age.

I quoted the definition of "confined space" (post #192681, reply #27 of 29)

by Tim in reply to ohscrifle [original] on Thu, 08/26/2010 - 09:33

from the International Mechanical Code (IMC) 2003, Chapter 2.

IMC Chapter 7 covers the other aspects of Combustion Air requirements.

Section 702.3 states: "The room or space containing fuel-burning appliances shall be an unconfined space as defined in section 202."

That reference is valid, as (post #192681, reply #28 of 29)

by rdesigns in reply to Tim [original] on Thu, 08/26/2010 - 14:18

That reference is valid, as far as the definition of confined space is concerned, but as you probably know this already know, the IMC does not cover fuel gas appliances--they are covered by the IFGC. The IMC combustion air provisions are for oil-fired equipment.

I understand (post #192681, reply #29 of 29)

by Tim in reply to rdesigns [original] on Fri, 08/27/2010 - 06:28

the IMC states that it is for "other than gas-fired" appliances. The requirements are, for this example, identical. For design purposes, I DO grab the yellowish book instead of the purple-ish one. For talking purposes, usually not.

With a heckuva (post #192681, reply #22 of 29)

by Tim in reply to ohscrifle [original] on Wed, 08/25/2010 - 08:30

long heating season, a high efficiency furn would pay for it self quickly, but if the crawls space will fall below freezing, the best you can do for gas efficiency is the 80% versions. 90% plus units condense products of combustion in the secondary heat exchanger and do not do will in freezing conditions.

90+ systems will handle (post #192681, reply #23 of 29)

by DanH in reply to Tim [original] on Wed, 08/25/2010 - 16:34

90+ systems will handle freezing temps if equipped with heating tape.


We are like tenant farmers chopping down the fence around our house for fuel when we should be using Nature's inexhaustible sources of energy -- sun, wind and tide. ... I'd put my money on the sun and solar energy. What a source of power! I hope we don't have to wait until oil and coal run out before we tackle that. --Thomas Edison

great dialogue guys. thanks (post #192681, reply #25 of 29)

by ohscrifle in reply to DanH [original] on Wed, 08/25/2010 - 17:49

great dialogue guys. thanks