I have a John Wood one pipe power vented hot water heater. The model number is Jw840nvh-fv-04. Recently I've been smelling products of combustion coming out the top of the tank where the vent is

If this heater is located in a "confined space", has the "make up air" entry point(s) been restricted/blocked? In short, is there enough air present to support combustion AND to supply enough air for the power vent to move the gases at a reasonable rate to the environment?

Can you "feel" the gases coming out the vent at a reasonable thrust? Is the power vent blower motor making any noises (not rotating properly, i.e., bad bearings)? Does the vent have any debris at the end, possibly at a screen?

Is the T&P valve dripping water? Is there ANY water on the floor? Is there a thermal blanket on the heater? Is the blanket wet anywhere? Did you recently add a thermal blanket? (Some manufacturers DO NOT recommend a blanket.) Have any liquids been spilled on top of the tank? Is anything dripping on the tank?

Is the burner flame crisp, not "lazy"?

"Recently I've been smelling products of combustion coming out the top of the tank where the vent is". There is a negative pressure present at the top of the tank where the vent is - in a properly drafting vent pipe assembly. This means that vapors, odors, and moisture ARE BEING SUCKED OUT of that area under and through the hood and then into the piping. "and when blew some smoke at the vent, it drafted up through the vent no problem". Try to identify the real source of the odor.

Lastly, I think this model uses a direct connection technique from the blower/power vent termination to the piping without a hood. So if you blew smoke at the vent, and there is no hood, how would the smoke get into the vent? If the 2" vent is PVC or CPVC, and if the gases are being slowed because of any reasons mentioned, maybe the plastic piping is slowly cooking.

1) If your water heater plugs into outlet, then electronic gas controls are susceptible to power surge. and reversed outlet polarity, and undervoltage at outlet, lack of ground wire continuity between ground plug and manifold tube connected to bottom of gas control valve.

2) If sediment builds up inside tank, it will cover thermostat probe and cause probe to malfunction.
Tank can have sediment flakes build-up and still drain perfectly clean water.

3) There are 2 general types of Honeywell electronic gas controls valves installed on Whirlpool heaters.
http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html#intellivent
The gas control for atmospheric water heater (passive vent at top of heater) is modular, and is not always removed from tank when replaced.
The other gas control for power vent models is removed completely.

4) Remove thermostat from tank to reveal 3/4" opening.
Look inside tank.
To remove sediment flakes: Tape assorted plastic pipes and tubes to shop vac hose and see if flakes can be removed. And remove drain valve.
http://waterheatertimer.org/Clean-sediment-out-of-water-heater.html

5) Check that air intake is clean using feather duster or compressed air, clean combustion parts as shown in product manual. If air intake is clogged with dust, then draft vent is slowed. This causes slowed venting of hot combustion by-product that will overheat combustion chamber.
Typically this condition will trip the FV system. BUT it might also trip the gas control thermostat.
http://waterheatertimer.org/images/Flame-trap-inspection-325.jpg
Stand 3-legged water heaters on bricks so inspection and cleaning flame arrestor screen is ez.

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Used, Reliance 805 is power vent gas
10 year old water heater that is newly installed.
Unknown gas control valve.
I'm not a fan of installing old power vent water heaters.
Usually these things are so expensive that old one is only replaced when it fails.

120Volt Outlet must have correct polarity and unit must be grounded between ground prong on cord to manifold tube connected to bottom of gas control valve. Test for continuity with multimeter.

Is this an early FV model with air intake located on bottom?
If air intake is clogged with dust, then combustion chamber overheated and tripped FV system. The older heaters need replacement parts and new gas control valve, or entire heater must be replaced.

Replace gas control valve.
http://waterheatertimer.org/pdf/How-to-replace-water-heater-gas-valve.pdf

Copy following link for gas control troubleshoot by type of gas control.
http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html#intellivent

Post question on Tank forum with more information about type of air intake and gas control valve and history of water heater
http://www.thetankatwaterheaterrescue.com/forums/forum3/

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Depends on gas control valve, and if cause is overheating high-limit event or FVIR flammable vapor event.
Turn off water heater when using flammable vapors, clean burner parts to prevent overheating.

1) GSW Residential power vent gas water heater with FVIR system:
Some models have intelli-vent gas contol
Others have robershaw gas control with auto-reset

2) Parts manual shows #15 'auto-reset' high limit switch on the blower
http://www.gswpartsonline.com/model/27484
http://www.gswpartsonline.com/schematic/137/view

3) Possible manual:
http://www.gsw-wh.com/pdfs/GPD135NInstall.pdf
Shows resettable lockout page 15
Water temperature must be 120 degree F or lower for reset. Unplug power cord, and then plug back in and wait 15 minutes or 60 minutes depending on gas control valve.
Some intelli-vent controls must be replaced when 195 degree high limit event locks out the water heater.
If high-limit lockout repeats, then gas control must be replaced.
Also clean burner parts, air intake, and make sure flame is clear blue. and not yellow-or-orange.

4) Intelli-vent service manual might apply to your water heater:
http://waterheatertimer.org/pdf/Intellivent-service-manual.pdf

If FVIR flammable vapor event locked out heater, then unplug heater and plug back in.
Immediately press two temperature buttons. Wait for green light to blink. Release buttons.
Page 34 in troubleshoot chart.

5) Take a moment and rate answer:
And take advantage of fixya expert assistance live.
For a price, expert works with you while you work on water heater or any do-it-yourself project.
Fixya is always less expensive than a service call.

You're probably in for a service call and/or possibly water heater replacement if unit is more that 5-6 years old.
Your water heater plugs into outlet. Check outlet with cheap circuit analyzer to make sure the unit is receiving correct polarity without line interference.
If unit is in the attic, then add more venting to soffit and roof so pilot is not being snuffed out by hot air. If unit is in basement, then open some doors and a window and see if unit needs more air.
Most gas water heaters require yearly cleaning and service.
Clean the combustion parts and tubes.
Clean the air intake.
Have gas company check gas pressure.
Open following link for possible illustrations:
http://waterheatertimer.org/How-to-install-gas-water-heater.html#troubleshoot

The problem can be bad gas control valve, air in gas line, low gas pressure, gas shut off handle not fully parallel with gas line, dirty combustion parts, lack of combustion air, improper venting. Or combination of all things. It's an electronic gizmo, anything can be bad or shorted, and repair may not last long.

Without a trained service technician to ohm-test the parts using correct service manual, then we are guessing at the cause.

Power vent water heater implies that you have side wall venting which must be replaced with another side-wall vented unit -or- it implies rapid reheating of water.

If water heater is more than 6 years old, you might be money ahead by installing a new one from local box store. Avoid the power vent and electronics. Local box store will have passive roof-venting type water heater BUT box store does not sell side-wall power vent. If your power vent goes up through the roof, then box-store variety gas water heater will work. Total cost $350 do-it-yourself for 40 gallon gas.
Or get 40 gallon 240Volt electric for $220 and connect to circuit breaker box. Requires no venting, and is easy to repair for years to come.
Or small 120Volt electric that plugs into outlet to tide you over until gas repairs can be made.
Look for scratch-n-dent models and always offer 10% off.
http://waterheatertimer.org/How-to-install-gas-water-heater.html
Shower uses 5-9 gallons hot water.

Check your venting ..chiminy could be plugged.. w/ W/H running,,, burner on.. hold a match or lighter around vent cap at top of heater where flue pipe connects flame should bend toward cap.. check you tube there may be a vid on how to do this...

Before calling service technician, read manual for troubleshooting and yearly maintenance requirements.
Open following links to identify same-similar product and download manual.
Once you have product showing on page, click 'specs-docs' tab to see use-and-care manual.
http://www.rheem.com/product.aspx?id=C24E3F52-A6A4-4BC7-AF5B-0DF23A7A0263
http://www.rheem.com/Products/tank_water_heaters/gas_water_heaters
http://globalimageserver.com/fetchDocument.aspx?id=c0471d58-5e59-4be6-96f7-68361b8fb515
Rheem site loads slow, and some manuals never load.
Typical power vent manual:
http://waterheatertimer.org/pdf/Rheem-power-vent-manual.pdf

I believe the 21VP is older and Rheem does not show older manuals, but manuals are generally generic enough to help greatly. Year of manufacture show as second and third digit of serial number.

Lockout condition requires service technician and part replacement, or call Rheem and have them send part.
I found one link that talks about FV lockout that may or may not pertain to your issue:
http://www.diychatroom.com/f7/rheem-hot-water-heater-pilot-burner-not-staying-lit-65755/

The question is unclear.
Diminished output could mean you have less less pressure on hot side and hot water barely trickles, or it could mean you have fewer gallons of hot water available.
The type of water heater is also unclear. Do you have gas or electric, or indirect or tankless or tank or power vent, or RV heater.
The label located on side of water heater is valuable source of information.
As a general rule the serial number is not important, but the type of heater and possibly model number are important. Serial numbers are used by manufacturer to identify your product, and also contain information about age of water heater.

Causes for less hot water.
Cold incoming water, thermostat set too low.
With gas heater, location can be issue if burner not getting enough air.
With gas: poor venting, or new furnace with power vent
Sediment in water lines, or clogged pipes, or clogged shut off valve could cause problems.
Defective parts can be a suspect.

Solutions: First of all, exact information must be included because nobody can see through walls without description from person who can see.
1) Re-post question with more details.
2) Add a comment to this post and I might have some answers.
3) Take advantage of fixy phone service. For a price, fixya expert speaks to you via phone to help isolate problem
4) Post a detailed question on professional water heater forum:
http://www.thetankatwaterheaterrescue.com/forums/forum3/

make sure you have the proper piping for out side combustion air. this is a code in some areas?? this will help dispate the gas smell after the water fires up and causes the gas smell... also might check the flue make sure it is properly installed and venting properlymm

You may be smelling other products of combustion but CO2 is a color-less/smell-less gas. A water heater should be vented thru 4" vent pipe to either a chimney or a wye which joins it with another appliance and a larger(size must be calculated) connector which connects to the chimney. There can be no reductions in size as you travel toward the chimney, and all pipe must slope at least 1/4"/foot up toward the chimney. Single wall vent pipe can be used unless it is less than 6" from anything flammable in which case type "B" vent may be used with a clearance of down to 1". If all this is correct your chimney may be over-sized or obstructed. Proper venting can be very complicated and technical and takes up most of most installation manuals for any fossil fuel appliance.

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