It can be done but all I can say is be carful.

"It can be done but all I can say is be carful."

Which part worries you? Starving it for air? Or getting combustibles too close?

The part where you don't know what you're doing. Simply providing a 5" pipe is probably not going to cut it.

You would be better off addressing make up air for vent hood than worrying with this. Better still,get all the hands togeather that built the house and make sure everything was done to code. Get make up air for hood and I will assure you bath fans aren't going to over power the heater draft blower.

"The part where you don't know what you're doing. Simply providing a 5" pipe is probably not going to cut it."

If I only did those things that I already knew how to do ... I wouldn't get too far in life. For better or worse, I am a DIY kind of guy. Sometimes I screw things up and have to call in a pro to clean up my mess but not very often. And I have had the opposite experience many times.

Here's a link to my manual, #5 on page 19 says that I need 1 square inch for every 3K BTUs. My water heater is 40K btus so it needs 14 sq inches of opening for fresh air. A 5" pvc pipe has an area of 16 sq inches (if my math is correct). manual

If I am missing something, please tell me. That's why I started this thread. So that I could learn from people who know more than me.

"You would be better off addressing make up air for vent hood than worrying with this. Better still,get all the hands togeather that built the house and make sure everything was done to code."

You might think that I am nuts but the MUA solution for the 1200 cfm range hood that I prefer is an open window. It's fool proof, energy efficient (when not in use), free and easily inspected. The only problem might come at resale. I would feel the need to explain my "system" which would turn some people off.

In order to provide MUA for a 1200 cfm fan running through a 10" pipe I would need a 1200 cfm blower and a 10" pipe going in the opposite direction. And that's just not practical. If I dispense with using a fan to bring in the air I need an opening much larger than 10". That's also not practical. Hence the open window.

I like the idea of getting everyone together to make sure everything was built to code, but that's just not going to happen. This is a tract home not a custom build. If I have a specific problem, the builder will address it but they are not going to come back and look for problems after it has passed all its inspections, received a CO and had the closing. Just not going to happen.

And I can tell you that in many instances it was NOT built to code. There were things that I had to flag that were fixed. There were also things that the building inspector flagged and directed them to fix ... which they didnt. And I had to police those as well. So ... there's nobody for me to rely on but me. And that's fine.

Klem1 - if you are saying that the water heater is not likely to have a problem under these conditions then I will give serious consideration to leaving it alone.

Thanks for the responses.

Jake - I am disappointed that I didn't think of the rebates available for new units. Just today I did the paperwork for the rebate from National Grid for my furnace.

It looks like there is a $450 rebate for a tankless water heater of more than 87% efficiency. The unit you linked to is $1300 so if I did the install myself the unit would cost me $850 +/-. My tank was about $1000 new so used maybe I could get $400-$500 for it. That's pretty tempting. For a 3000 sqft home in Syracuse with 4 occupants and 4 baths how big of a unit would I need?

I know nothing about the hybrid water heaters. This would be installed in the unfinished portion of my otherwise finished basement. I have read that it makes the space cold. I wouldn't want that. Is that a real concern? It doesnt look like there is a rebate on those.

Tankless heaters work well when installed correctly, but your house may not be set up for an easy installation. For example, a tankless uses a lot of gas when running, so typically you will need to upsize the gas line to the water heater. This may impact the furnace or appliances if your primary gas lines (and service are not sized appropriately.

Tankless heaters should also be installed to facilitate regular flushing and descaling. This means a few more pipes and valves than your old water heater., which means your self-install will be more complicated.

Some folks also do not like the way a tankless operates, since it usually requires a minimum water flow before it fires. If someone just trickles the hot water out of the tap there may not be enough flow to turn on the heater, and you may never get hot water. You also hear many complaints about the "slug" of cold water before you get hot, but I'm not sure its that much worse than a standard water heater.

Bruce

Try and understand that we don't like teaching people jobs that can kill if not done right. Noone in their right mind would send an apprentice on a job like this,they sure don't like doing it long distance. There's simply jobs that lend themselves to diy and others that are better left to those that UNDERSTND. Telling you how is a long way from causing you to UNDERSTND. There's things that can go wrong that NOONE can clean up.

schreibdave:

Tankless: Bruce is correct that you must make sure you have sufficient gas service to your house for a gas tankless. In addition, you must have at least 3/4" gas line from the meter to the tankless heater (you would need to know how far you were going before you could say what size of gas line).

If you consider gas tankless, please learn more about them. I've had 2 (different houses) and I like them-- but they don't work exactly like a tank heater. I'm happy to talk more about them... but you should see if your gas service could support your furnace (60-80k BTU/h ) + a tankless water heater at 200k BTU/h + whatever other gas appliances you have.

Hybrid: They pull the heat from the surrounding air. They do make the area around them cooler (and they remove humidity also). They have to have a certain amount of space to work (to pull heat from).

Those are two choices besides the standard water heater with an outside air intake -- which is also an option.

"Here's a link to my manual, #5 on page 19 says that I need 1 square inch for every 3K BTUs. My water heater is 40K btus so it needs 14 sq inches of opening for fresh air. A 5" pvc pipe has an area of 16 sq inches (if my math is correct). manual"

And that would be super-awesome if #5 applied. Read again.

schreibdave: You asked "how big of a tankless unit do I need?"

We size tankless gas units according to the coldest inlet water temp and the amount of hot water used at the same time. So, a 200k BTU/h water 94% water heater can heat 35F water to 105F at the rate of 5.4 gallons per minute. A normal shower is 2.5 GPM so that heater could provide a little more than 2 showers at the same time in the coldest of winter. The largest standard size unit is 200k. Unless you have a small home in a warm weather climate, don't go smaller than the 200k BTU/h unit -- there's really no point.

"And that would be super-awesome if #5 applied. Read again."

I just did. And I read it a few times before I posted. The section is "Fresh Air Openings for Confined Spaces." The subsection (B) is "All Air from Outdoors."

#5 says:

"5. Alternatively a single permanent opening may
be used when communicating directly with the
outdoors, or with spaces that freely communicate
with the outdoors. The opening shall have a
minimum free area of 1 square inch per 3,000 Btu/
hr of total input rating of all equipment in enclosure
(see Figure 20)."

Maybe you are focused on "all equipment in enclosure?" The only thing in the enclosure would be the WH. It would be helpful if you gave me a hint as to why #5 doesnt apply.

Anyway, I think what I will do is ... nothing. Before I tear out a perfectly good WH that I haven't even made a mortgage payment on yet, I should probably wait and see whether I actually have a problem. I have CO detectors throughout the house so presumably I will know if there is a back draft situation.

Thanks a lot for all the replies. They were very helpful in helping me think this through.

schreibdave: Yep, that sounds reasonable.

So here's a side note on this issue for anyone interested: When replacing a gas water heater or a furnace, look for one that gets its combustion air from outside. The term is called "direct vent" -- which is different from "power vent".

When a gas appliance doesn't get its combustion air from the outside (not direct vented), it uses conditioned indoor air and then sends it out the flue. Outside air must then infiltrate the home to make up for that air -- making the house feel drafty and less energy efficient. Traditional gas water heaters have a 3" hole open to the outside at all times - in addition they have a hot water tank causing a continuous draft up the center of the tank and out of the home. It's like leaving a window open and putting a small fan in the window.

In more air tight new construction, the lack of outside air intake can be a problem and cause problems with the appliances.

I've had HVAC contractors tell me the outside air intake didn't matter - something that was specifically contradicted by the furnace maker.

I am against the naysayers, I am a fan with creativity and since you currently have a power vent WH, if you had your enclosure tight enough and installed a backdraft damper on the inlet you would keep out cold when it isnt running and essentially make it a direct vent. The inlet would be done with ductwork instead of PVC. See the diagram in that link for horizontal installation. So yes in theory your idea would work. Not like people haven't put water heaters in closets outside before,
whats the difference putting it in a sealed closet inside and supplied
MUA. Obviously would have to include a door that seals well for maintenance, which would make it more labor and make the project cost more. Another wrinkle in my brain notes that your enclosure would need to be fire rated due to it being a gas device.

All that being said, it may be just as cost effective and less labor to install the direct vent WH with the rebate instead of building this insulated/fire rated enclosure.

The difference is AIR FLOW, AIR FLOW, AIR FLOW. Creativity alone won't cut it when it isn't backed by correct air inlet/outlet design. And, NO, in theory, the OP's idea does not meet the airflow requirements specified by the water heater manufacturer. Seriously, if you can't read and understand the manual, call an expert.

As to MUA for the 1200cfm vent, you plan on keeping that window open during the winter?

Alice - Again, if you wanted to be more specific about what I am misunderstanding that would be helpful.

Vith - Thanks for the helpful reply

"As to MUA for the 1200cfm vent, you plan on keeping that window open during the winter?"

Yes. I did it in my last house for 8 years and it worked well for us. The hood is mainly on when we use the grill. The alternatives are also not great. To replace the 1200 cfm going out through a 10" pipe I would need a fan blowing in at 1200 cfm through a 10" pipe. Or I could have a much bigger passive opening to the outside and let the exhaust fan suck the air into the house. But with no fan blowing in, that pipe would have to be substantially bigger than 10" to move that much air.

Given the range of bad options I like the one that is free and simple.

You are applying the wrong method because you haven't understood what you read. Given that, it would be unethical for me to assist you from a distance because the chances of you misunderstanding what I write are just as likely as the error you have already made with the manual (which is very clear, btw). Rather, all I an do is recommend you stop trying to do this yourself and consult a local expert. That is the most helpful thing anyone can do for someone who very clearly lacks the expertise to do what they are attempting.

Alice, what I was talking about was MUA for the water heater specifically.

Obviously there needs separate MUA for the range hood because of the ultra high CFM. Best way to do that is with a separate air intake with pressure activated damper so that when the range hood turns on the MUA opens by itself when it reaches a high enough CFM, and then closes by itself when it is not needed.

As noted, not cheap but is code for high CFM range hoods. You said you have a 10" exhaust so I found you a 10" damper in that link.

If you live in a cold climate I personally would not install such a high CFM range hood. If you have a 1200sqft floor with 8 foot ceiling, you would be sucking out 1/8 of the air from that floor per minute which would then need heating again. So in 8 minutes you would have a complete air exchange of that floor with outside air. If anything install the make up air very close to the range hood so you only affect the temperature of the kitchen. It supposed to be installed close anyhow.