Water Heater Combustion Air

[rvr6000]

Anyone familiar with the engineered sizing method for determining how much combustion air is is needed for a gas appliance?

I've been getting some quotes for a new furnace and AC unit. One of the guys that came out to give me a bid said I need to install a combustion air line for the water heater. They usually just put in an insullated 4" flexible line however the way the plumbing is in my basement I don't think there's room to get that line bent around all the pipes without having it stick half way into the room..

On the bright side, local code says you can calculate the air needs and size it accordingly. Figuring a new 60K BTU furnace can get by with about a 2" PVC line for its combustion air I'm guessing my 40K BTU water heater wouldn't need anything more than that.....and there is room to get that installed around the pipes that are overhead.

My house is 40 years old now and I've never had any issues with the water heater but I guess that just means its been working okay but is still installed wrong. Just want to bring some of these little problems up to snuff.

Thanks in advance for any help.

 

[BBQ&Love]

What kind of water heater is this? Does it have a power vent?

 

[rvr6000]

GE-Powervent / 40,000BTU - natural gas. Sorry, should have mentioned that.

 

[BBQ&Love]

Here's what you are dealing with for a 40 Gallon GE Natural gas Water Heater.

Combustion and Ventilation Air
Ventilation (ambient) air temperature
must be 100°F or less. Proper operation
of the water heater requires air for
combustion and ventilation. Provisions
for combustion and ventilation air must
comply with referenced codes and
standards.
DO NOT block or obstruct any of the
combustion air inlet openings located
around the perimeter of the water
heater. A minimum of 1” is required
between these combustion air inlet
openings and any obstruction.
NOTICE: If the water heater is installed
in an unconfined space within a building
of conventional frame, masonry or
metal construction, infiltration air is
normally adequate for proper
combustion and ventilation. If the water
heater is installed in a confined space,
provisions for combustion and
ventilation air must be made.
DO NOT obstruct or block the
Flammable Vapor Sensor.
A confined space is one having a volume of
less than 50 cubic feet per 1000 Btuh of
the aggregate input of all appliances
within that space.
The air must be supplied through two
permanent openings of equal area. One
is to be located within 12” above the floor
and the other is to be located within 12”
from the ceiling.
The minimum net free area of each opening
must not be less than one square inch per
1000 Btuh of the total input rating of all
the appliances in the enclosure (but not
less than 100 square inches), if each
opening communicates with other
unconfined areas inside the building.
Buildings of unusually tight construction
shall have the combustion and ventilation
air supplied from outdoors, or a freely
ventilated attic or crawl space.
If air is supplied from outdoors, directly or
through vertical ducts, there must be two
openings located as specified above and
each must have a minimum net free area
of not less than one square inch per
4000 Btuh of the total input rating of all
the appliances in the enclosure.
If horizontal ducts are used to
communicate with the outdoors, each
opening must have a minimum net free
area of not less than one square inch per
2000 Btuh of the total input rating of all
the appliances in the enclosure. If ducts
are used, the minimum dimensions of
rectangular air ducts shall not be less
than 3”.
NOTICE: If the duct openings which
supply combustion and ventilation air
are to be covered with a protective
screen or grill, the net free area
(openings in the material) of the
covering material must be used in
determining the size of the openings.
Protective screening for the openings
MUST NOT be smaller than 1/4”mesh to
prevent clogging by lint or other debris.

 

[rvr6000]

Yep....I've read that however I'm looking for the formula/process for calculating actual combustion air requirements. From what I have read it is called, "Engineered Sizing Method"

 

[BBQ&Love]

What code are you under.

 

[redsky49]

As quoted above by 22rifle, the code required combustion air for appliances utilizing interior combustion air are as shown:

"The air must be supplied through two
permanent openings of equal area. One
is to be located within 12” above the floor
and the other is to be located within 12”
from the ceiling.
The minimum net free area of each opening
must not be less than one square inch per
1000 Btuh of the total input rating of all
the appliances in the enclosure (but not
less than 100 square inches), if each
opening communicates with other
unconfined areas inside the building."

For higher efficiency devices that utilize outside combustion air, either through a concentric pipe or a separate inlet, the actual amount of required air will vary according to the model and manufacturer. Typically, manufacturers will not provide this information as it is considered proprietary to their designs. Just out of curiosity, I have posed the same question in the past to sales reps. Very reluctant to speak on record.

Some of the higher efficiency units are quite sophisticated in their design, starting with the Lennox "pulsed combustion" designs from the 80's.

This is also why you will be seeing fewer and fewer gravity-vented furnaces and water heaters in the future.

As an example of the efficiencies of typical gas furnaces:

Gravity vented with standing pilot - 65% effic.
Gravity vent with elec. ignition - 69% effic.
Fan-powered combustion (induced-draft) - 80% effic.
Condensing - 90% effic.

As efficiencies increase, the waste heat lost to the flue vent/exhaust gases is reduced to the point where you see now a very small flue vent size, with very low exhaust gas temperatures.

I think the next step will provide for a steady-state efficiency approaching 100%. Bear in mind that the "Utilization Efficiency" is a different test procedure, and will result in a lower number as this takes into account the start/stop inefficiencies as well as some other factors. Still, we should see commercially available residential units that advertise 96-98% efficiencies being far more affordable.

As always, offered only as opinion

 

[Jackfre]

One other consideration in your sizing that is frequently overlooked and misunderstood is the requirement for "open air". Let's say you need 40 sq in for your water heater. Assuming you could get away with a 4x10 inlet grill it would have to much restriction. Manuf of grilles provide the "open air" provided per model of grill. It is not uncommon to have a 50% reduction due to the body of the grill and the slats themselves.

One other alternative is to check our mechanical combustion air. Look at the Combustion Air Enforcer from Tjernlund.com I represent them so bias noted, but they are a very good American producer. Good folks, too!

Perhaps I didn't read this carefully enough. You water heater is not chimney vented is it? If so, you my need a chimney liner, at which point you have to look at altering your method of providing hot water just due to the costs of the liner.

 

[rvr6000]

No, my water heater is a powervent with a 2" pipe that travels about 10' before exhausting outside.

I guess my real question is how much combustion air does my water heater really need. It's kinda of a moot point now because my heating contractor is going to install the line for me when they swap out the furnace but I'm still kinda curious.

From what I have read the current standards for combustion air were more of a knee-jerk reaction due to the well sealed homes that are being built today and are way more conservative than they need to be. Like I said, if an 80,000btu furnace can draw enough combustion air in through a 2" pvc pipe then why do I need a 4" line for a 40,000btu water heater.....is there an equation that can solve that problem?

 

[rickairmedic]

RVR both 22rifle and redski gave you the formula to figure it out with . The thing they didnt explain is that your furnace is directly drawing intake air through its intake pipe and exhausting out of its exhaust pipe . The water heater on the other hand doesnt have the ability to directly draw makeup air and therefore must follow the formula further up in the thread. I honestly question a 2" flue and intake on a 80,000 BTU furnace but it sounds like your installer knows what he is doing.


I will add to this that I had an issue with an inspector last year on a furnace install .The furnace was installed to code when I left the house . The homeowner in the meantime ( before the inspector showed up ) put a bed in the room next to the furnace ( I had not installed an intake pipe ) . The inspector told me that the furnace could not draw makeup air from that room due to the bed being in the next room . We discussed the fact that by code ( due to egres ) the room could not legaly be considered a bedroom . He agrees with me but also stated that there was infact a bed in the room and I needed an intake pipe from outside . I agreed to put in the pipe and he passed the install with flying colors ( even though we both agreed that Technicaly I was right to begine with ) have to keep the inspectors happy .


Rick

 

[Jackfre]

I'm sure there are formulas for it but I'm not a mechanical engineer and don't think it worth the time to disect a code which has been in place for a long time. Your 2" pipe on the furnace is a sealed system that is fan assisted. Your water heater is not a sealed system. You are looking at combustion air only. On the water htr you must take into consideration the dilutuon air requirement for your conventional system. Dil air is needed to change the dewpoint of the flue gases and is added to that vol so you cannot compare the two as you are trying to do. Mixing DA with the exh gases allows the gases to exit the system in the gaseous state rather than condense in the unit and rust it out in a hurry.

I cover the six New England States representing over the years oil, gas, wood etc appliances. Since the early 70's we have taken the technology of the thermos bottle and applied it to home construction. The biggest problems with combustion appliances are the vent systems and improper comb air. The very best systems are sealed air systems which do not use any building air.

 

[Frank The Plumber]

Yes, the make up air may not be drawn from a bed room or room with a door. You can not put a gas fired device in a bed room closet. If you do you need to put a seal tite door and pull the make up air from a circulated internal source, a hall or open area or from outside.

There are several hot water heaters that pull make up air from outside, I believe they are mostly commercial and larger units.

This GE unit will lock out if it does not get enough combustion air, it has an exhaust flow censor and an intake air flow sensor / temperature sensor. It may damage the electronics.

www.bradfordwhite.com model # PDX1-40s6FBN

This is what you need.
I don't play with air sourcing, it can be tampered with and you can have troubles, I would put this unit in, if something goes wrong and the unit can not draw it's combustion air in through the feed, the unit will lock out protecting all within the home from it.

 

[janu52581]

I know this is an old old post, but I have a question along the same line as the original question. We are finishing our basement soon, and up until now our builder had just put in a vent on the outside of the house that allowed air to just pour in through an area in the rim joist (no pipes/ducts) and there was an orange sticker that said never to block or close off that air supply. But now that we'll be finishing the basement, that area will need to be sealed from the outside (not accessible from the inside because its strangely located behind a ceiling joist (basically can't access that hole). Plus drywall will also go up, so there will be no air from that hole. SO, now we have to install a pipe that brings in fresh outside combustion air to the water heater which will get enclosed in a fairly small closet/room that will be about 10ftx6ft. If we have a 40000 btu water heater, and assuming no air will be supplied from the rest of the basement (total basement is about 1700sqft so there will be air available with a louvered door), I'm calculating (40000/2000 ===> 20sqin of vent size needed ==> ~ 5" diameter pipe (my choices will be 4" PVC or 6" flexible ductwork.) Do we need TWO of these to supply HIGH and LOW? Or will one LOW combustion air pipe be sufficient? Also, is PVC pipe okay since it may take up less space (in an already crowded ceiling) than a flexible duct? Thank you in advance for all your help and advise.

 

[Showkey]

Old post…….see no real need for a separate make up air for heater unless the building is super tight.

Power vent water heater in the typical Midwest basement……..draws air from the basement. The home leaks enough to supply all the air thats required.

The last thing I would want is 4-5” pipe dumping ice cold air in the basement.

Maybe St Paul has some new makeup air code for a super tight super efficient LEED certification new build that they are applying to all water heater installs. ?????

 

[PoorUB]

Around here that water heater in a small room would require outside air or two grills in the wall, open to the rest of the basement. Two 10x10 grills in the wall, one high, one low.

 

[janu52581]

I’m in Michigan. And our builder told us that the fresh air was put in because our house was air tight….and to never block it (for combustion air).

We also have a couple of continuously running (on low speed) bath fans (one of main and on second floor) that supposedly help to keep air quality better in this “air tight” house by swapping out old air for new air that I’m assuming is drawn from the basement.

Since the builder had put in the fresh air initially, I felt like we had to do something to replenish that source directly into the water heater room.

As much as I don’t like the idea of cold air pouring into the basement, it is there right now and I feel like I have to relocate it when finishing the basement.

 

[danski0224]

I’m in Michigan. And our builder told us that the fresh air was put in because our house was air tight….and to never block it (for combustion air).

We also have a couple of continuously running (on low speed) bath fans (one of main and on second floor) that supposedly help to keep air quality better in this “air tight” house by swapping out old air for new air that I’m assuming is drawn from the basement.

Since the builder had put in the fresh air initially, I felt like we had to do something to replenish that source directly into the water heater room.

As much as I don’t like the idea of cold air pouring into the basement, it is there right now and I feel like I have to relocate it when finishing the basement.

If you are still breathing in your house with the doors and windows closed, it isn't anywhere near "air tight".

There are only 2 ways to properly deal with combustion air requirements.

One is to install direct vent (2 pipe, intake and exhaust) equipment.

Two is to use a mechanical forced air combustion air system and the required controls. One source is Tjerlund.

The "two openings" method doesn't really work, nor is is controlled. If you live where it gets cold, most plug the gravity/open combustion air ducts. The rest of the building will have plenty of other leaks, bringing more cold air in as heated air goes outside somewhere.

Air travels vertically more readily than horizontally.

An intermediate solution is a control damper in a gravity combustion air pipe/duct tied into the call for heat. The pipe is run close to the appliance. Damper opens with a call for heat. This is not a recognized or code compliant solution.

Damper controls are more typically seen on commercial jobs.

Honeywell has residential round fresh air intake dampers with a fairly good seal.