[anon125]

here is the gas furnace problem.
we had 2 noisy bathroom fans using 4 inch flex ducting.
the quiet fans use 6 inch ducting.
we had them fans and ducting replaced.
then as were concerned about dampness in the crawl the electricians said they would put another 110 CFM in the crawl.

the only air intake in the house is the 4inch duct to the furnace.

The crawl fan was not installed properly. (too near furnace etc)

we ended up with the chimney fumes coming into the house!

the gas fella says -
1700 sq ft house with only a 4 inch intake, the 3 110cfm fans were too much. so they got intake wherever they could - like from the chimney!

how do we figure out just what size of fan to put in the crawl?
thanks all
PS we are hoping there is a chart somewhere online.

 

[anon125]

Just realized there are 2 4 inch inlets. one for the furnace and another for the furnace burners.

 

[TheEplumber]

Your only intake is a 4" - Is this your combustion air or your furnace vent?

 

[joecaption]

What's a bathroom vent have to do with a furnace?
Should be totaly different vents.

 

[Bud9051]

Most but not all houses will have enough leaks to provide the necessary makeup air for a couple of exhaust fans. But apparently yours does not and that isn't the worst case. Add a kitchen fan and run the dryer and you will need more air.

If your furnace is drawing its combustion air from outside it sounds like a newer unit or the installer suspected there would be an issue.

What is your climate zone, warm, cold or inbetween?
When was the house built?
How is your hot water heated and exhaust vented?

Bud

 

[anon125]

Your only intake is a 4" - Is this your combustion air or your furnace vent?

one of each. thanks

 

[anon125]

What's a bathroom vent have to do with a furnace?
Should be totaly different vents.

well they are all linked by incoming and outgoing air.
see other posts

 

[anon125]

Most but not all houses will have enough leaks to provide the necessary makeup air for a couple of exhaust fans. But apparently yours does not and that isn't the worst case. Add a kitchen fan and run the dryer and you will need more air.

If your furnace is drawing its combustion air from outside it sounds like a newer unit or the installer suspected there would be an issue.

What is your climate zone, warm, cold or inbetween?
When was the house built?
How is your hot water heated and exhaust vented?

Bud

Bud...13 year old bungalow. inbetween climate. today is sunny.
hot water is heated by gas. it has its own vents.
thanks

 

[Bud9051]

Our terminology is a bit different so to clarify, a furnace or water heater can be a naturally drafted appliance taking its combustion air from inside and venting it to the outside via a natural draft. Or they can be sealed combustion where they draw in outside air for combustion and then vent it back out. The sealed combustion appliances will often have PVC pipes for the air flow rather than metal.

If the appliances are sealed combustion, you should not be having a problem with backdrafting due to exhaust fans.

Are the water heater and furnace located in the crawlspace and if so I would assume the crawlspace is somewhat closed off from the upstairs. Adding a 110 cfm fan down there might be too much for the amount of air intake.

I'll wait for clarification on sealed vs natural draft for both appliances.

Bud

 

[TheEplumber]

one of each. thanks

So, your combustion air comes from the crawl space? Is that why the crawler's exhaust fan is affecting your vent?
If so, can you get combustion air from another source such as outside or the attic?

 

[anon125]

there are 3 vents for incoming air.
to the furnace
to the furnace burner
to the gas water heater.
apparently some folks have a ventilation certificate.
this might interest you...
thanks
Make-Up Air Fans: What you need to know
Homeowners, builders and equipment suppliers should be aware there is a premium to pay for use of high cfm range hood exhaust fans. This is because large fans can create unsafe conditions due to the fact they can depressurize your home. Depressurization can cause chimneys and vents to reverse and extinguish pilot lights or cause carbon monoxide from smoldering fireplaces to enter the home. This is very dangerous. In order to protect the public from depressurization, contractors are required to install make-up air fans for large (in proportion to your home) exhaust fans. Also due to the volume of cold outside air these make-up fans introduce into your home, the outside air must be tempered. This tempering is usually done with an in-duct heater. So installing a very large kitchen range hood fan can increase the costs of installations from $1000 to $5000 dollars depending on your home. And your electricity bill will also increase to run the duct heater.

 

[Bret86844]

You currently have 330 CFM being exhausted from your house. The 4" inlet vent for your furnace does not count. That is sized for your furnace and I would not assume it provides any other makeup air.

It sounds like your problem really started only after the basement exhaust fan was added. So you need 110 CFM of makeup air to the basement.

 

[Bud9051]

"there are 3 vents for incoming air.
to the furnace
to the furnace burner
to the gas water heater."
Most all energy auditors are well past a simple "ventilation certificate". We cover CAZ worst case pressure testing (Combustion Appliance Zone) very thoroughly and we have the test equipment for that.

Of those three air sources, how are they connected to those appliances. Sealed combustion is more than just a vent dumping air next to an appliance. Yours may very well be as needed, but again our terminology is different.

At this point it would be nice to see pictures of those ducts.

The fact that you smell exhaust fumes says that chimney is backdrafting and I suspect the conditions are not worst case. Your quote mentions a large range hood. Do you have one?

Bud

 

[supers05]

I believe he has a natural draft appliance, or at least had one, before a replacement. I too wouldn't include the 3 vents in the calculation, since they are sized to the appliances that they serve. For the 3 exhaust fans, you will need at least a 6" metal intake, if not 8". If your appliances are also in the crawl space, then I'd probably add a make up air fan, after measuring air leakage of the crawl space. (it'll probably be between 150-300cfm fan)

PS. On a side note, make sure any water pipes down there are well insulated. Exhaust fans can take a fairly insulated crawl space from above freezing to below when in some locations and situations.

Cheers!

 

[anon125]

the ducts are just typical insulated 4 inch vents. (shown on another post)
also we would like to change our range hood only now we know not to exceed its cfm!
the 2 replacement fans are now connected to 6inch flexible insulated tubes
thanks all

 

[Bud9051]

I would look at the other pot if you would provide a link.
Basically, we are not communicating. Having 4" ducts from outside providing fresh air to the inside is next to worthless. The objective is to prevent the space from being too negative and causing a backdraft. But it requires a lot of negative pressure to bring in the fresh air.

Bottom line is you have qa can of worms and it will not get sorted out until you get some onsite testing and advice from someone who knows how to resolve the issue.

I wish you luck,
Bud

 

[anon125]

thanks Bud