90% furnace requires ventilation?

[univmd]

Hi, I have a 78% furnace in zone 1 of my house right now that shares one big vertical vent with my gas water heater.

I heard that switching to a 90% furnace is not as easy as simply taking it off the big vertical vent and using two new ones. Something about the water heater being by itself with the one big vent and condensation, what is this all about?

So what is the highest efficiency furnace out there that does not require taking if off the one big vent?

--James

[coolwhip]

That would be an 80% AFUE furnace. When you orphan the water heater, you need to run a chimney liner because the water heater will not be able to heat that chimney to create a draft. At least not right away. The result is CO in the home.

[key]

From what I am able to tell by your post is that your contractor did not inform you properly.

However, if you were to upgrade to a 90+ furnace, that furnace would no longer require a chimney. It would need a 2 pipe system (plastic PVC), (one for combustion air, the other for exhaust). This is usually done through the side of your house.

NOW:

As a result you are left with an "orphaned" hot water heater, the existing flue (chimney) could and most likely be oversized.

SO DOES AN OVERSIZED CHIMNEY Have To Do With Condensation?

Simply put...when you don't have a enough heat going up the chimney, your flue gases will cool down before they reach the top and condense.

NOT a good situation...backdraft and CO

[gasguy]

Originally posted by key
From what I am able to tell by your post is that your contractor did not inform you properly.

However, if you were to upgrade to a 90+ furnace, that furnace would no longer require a chimney. It would need a 2 pipe system (plastic PVC), (one for combustion air, the other for exhaust). This is usually done through the side of your house.

NOW:

As a result you are left with an "orphaned" hot water heater, the existing flue (chimney) could and most likely be oversized.

SO DOES AN OVERSIZED CHIMNEY Have To Do With Condensation?

Simply put...when you don't have a enough heat going up the chimney, your flue gases will cool down before they reach the top and condense.

NOT a good situation...backdraft and CO


And a chimny that rots from the inside out!

[beenthere]

Even a new 80% may need a liner installed in the chimney.

[mcginkleschmidt]

Wouldn't the water heater vent be essentially orphaned during the ~6 months of the year when the furnace is not running. Is this situation during the non-heating season less than ideal and can cause chimney or vent deterioration?

Just a thought that pops into mind. Inquring minds want to know!

[key]

Originally posted by mcginkleschmidt
Wouldn't the water heater vent be essentially orphaned during the ~6 months of the year when the furnace is not running. Is this situation during the non-heating season less than ideal and can cause chimney or vent deterioration?

Just a thought that pops into mind. Inquring minds want to know!

warmer outdoor temps compensatate

[2hot2coolme]

A chimney creates draft naturally, with or without heat. Take a piece of newspaper, light it up, and stick it in the opening of a cold chimney and watch it get sucked up. (as long as the chimney isn't plugged)

The new 80%'rs run at lower temperatures at the stack so condensation can occur if the chimney is cold enough, which will create an acidic substance that will eventually erode the mortar and cause the chimney to collapse.

[TheDuke]

good post

[Midwest]

A water heater only drafts about -3 Pascals of pressure - not very darn much. It takes very little to overcome the natural draft and cause exhaust gas spillage- merely opening/closing a door can cause spillage in some instances.

You will probably need to reduce the flue size depending on GAMA standards and local codes. It's always a good practice to reduce an orphaned flue for safety, especially if it just barely meets the flue size vs. Btu limits. Often, the existing larger flue can be used as a conduit to run a smaller flue within to reach roof line.

As mentioned, in summer a flue heats up more quickly and natural draft is easier to achieve. There are also power vented water heaters, but we're talking more $. This may or may not necessarily be a DIY project due to calculations, flue charts, etc. that need to be followed, your experience and skill levels, etc. And, the helpful folks at the big box store may not be up on code requirements, either. Consider an electric water heater and remove all open draft flues and mechanical room combustion air concerns. In many cases, they work out well on energy costs, but are not intended for those homes with mega-hot water demands.

[hvaclover]

You overcome the chimney prob by going with a direct vent water heater and a direct vent 90+ furnace.

The chimney then becomes history.

[Chill]

Originally posted by Midwest

You will probably need to reduce the flue size depending on GAMA standards and local codes. It's always a good practice to reduce an orphaned flue for safety, especially if it just barely meets the flue size vs. Btu limits. Often, the existing larger flue can be used as a conduit to run a smaller flue within to reach roof line.


The GAMA tables say there is no minimum for nat draft applience? So what's that mean?

[univmd]

What is a direct vent water heater: new vent or new type of water heater? I wonder if most people in my situation stick with 80% furnace to avoid all this trouble... James:-)

[keepitsimplestupid]

A direct Vent water heater vents directly to the outside usually with a blower. Most probably draw combustion air from the outside too.

http://www.hotwater.com/lit/spec/media/res_gas/A7510.pdf

They can be power vented too like this:

http://www.hotwater.com/lit/im/media/res_gas/185363-001.pdf