Water – Why do high efficiency water heaters require outside air vents and drains

water-heater

All high efficiency water heaters seem to require PVC air intake and vent; the existing water heater I have doesn't have a drain connection or intake vent; only a traditional chimney. What are these used for in high efficiency designs?

Best Answer

Low efficiency water heaters are low efficiency because their exhaust gases are very hot. Since the gases are hot, they will rise up a chimney flue with ease, but this also means you lose all that energy and warm air up your flue. Older heaters tend to be in older homes where there are enough drafts and air leakage to not worry too much about ventilation. One other thing: the hot air also carries also the moisture produced by combustion up and out the chimney.

High efficiency heaters try to reduce the temperature of the exhaust gases as much as possible in order to recover as much energy as possible. But as the exhaust gas is almost room temperature, it won't rise and thus has to be forcibly expelled. The moisture that is produced by combustion will also condense in the cooler air and will have to be drained out of a separate drain line.

High efficiency furnaces are also designed to be used in high efficiency houses that are well sealed, so you need to bring in fresh air lest you run the heater too lean and produce carbon monoxide. Another reason to bring in fresh outside (cold) air is that it is a waste to use the inside (warm) air for combustion and blow it outside the house.

Heat exchangers are used on the intake and outtake gases which further increase efficiency. This necessitates both intake and exhaust vents.

_{Thanks to commentator keshlam for additional material that was brought into the answer}

Heat Expansion

When water is heated, the pressure in a closed system increases. If the pressure increases beyond the tripping point of the T&P valve on the heater, the valve should open to release some of the pressure. This release usually involves very hot water and steam, released in a controlled manner. If you install a pressure relief valve set lower than the T&P trip value, The relief valve will open long before the T&P valve possibly releasing hot water and steam in an uncontrolled way. This could lead to injury to occupants, or damage to property.

It's possible that your area has not adopted the use of backflow prevention, so this extra pressure can simply be released back through the distribution system. In which case, you'll probably never see either relief valve ever open. If you do have backflow prevention in place, it's possible that this relief valve could open under "normal" conditions. The International Residential Code (IRC), and Uniform Plumbing Code (UPC) recommend pressure between 40 - 80 psi.

IRC 2009
P2903.3 Minimum pressure. Minimum static pressure (as determined by the local water authority) at the building entrance for either public or private water service shall be 40 psi (276 kPa).

P2903.3.1 Maximum pressure. Maximum static pressure shall be 80 psi (551 kPa). When main pressure exceeds 80 psi (551 kPa), an approved pressure-reducing valve conforming to ASSE 1003 shall be installed on the domestic water branch main or riser at the connection to the water-service pipe.

So even under "normal" conditions, your 75 psi valve could open.

Controlling Expansion in a Closed System

If backflow prevention has been used in your home, you are required by code to install a device for controlling pressure.

IRC 2009
P2903.4 Thermal expansion control. A means for controlling increased pressure caused by thermal expansion shall be installed where required in accordance with Sections P2903.4.1 and P2903.4.2.

P2903.4.1 Pressure-reducing valve. For water service system sizes up to and including 2 inches (51 mm), a device for controlling pressure shall be installed where, because of thermal expansion, the pressure on the downstream side of a pressure-reducing valve exceeds the pressure-reducing valve setting.

P2903.4.2 Backflow prevention device or check valve. Where a backflow prevention device, check valve or other device is installed on a water supply system using storage water heating equipment such that thermal expansion causes an increase in pressure, a device for controlling pressure shall be installed.

While a relief valve may fit this description, the more common method is to install an expansion tank.

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Safely Releasing Pressure

There are requirements for releasing pressure by way of a discharge pipe, which this valve may not meet.

IRC 2009
P2803.6.1 Requirements for discharge pipe. The discharge piping serving a pressure-relief valve, temperature relief valve or combination valve shall:

Not be directly connected to the drainage system.

Discharge through an air gap located in the same room as the water heater.

Not be smaller than the diameter of the outlet of the valve served and shall discharge full size to the air gap.

Serve a single relief device and shall not connect to piping serving any other relief device or equipment.

Discharge to the floor, to the pan serving the water heater or storage tank, to a waste receptor or to the outdoors.

Discharge in a manner that does not cause personal injury or structural damage.

Discharge to a termination point that is readily observable by the building occupants.

Not be trapped.

Be installed to flow by gravity.

Not terminate more than 6 inches (152 mm) above the floor or waste receptor.

Not have a threaded connection at the end of the piping.

Not have valves or tee fittings

Be constructed of those materials listed in Section P2905.5 or materials tested, rated and approved for such use in accordance with ASME A112.4.1.

Lead Safe

After doing a bit or research, I stumbled upon the NSF website which provides a lot of valuable information. It turns out, the valve mentioned in the question is indeed certified to meet ANSI/NSF 61, ANSI/NSF 61 Annex G, and California's AB 1953.

enter image description here

Which means it is safe for use with potable water (at least as far as lead is concerned). If you check the valve and/or packaging, you'll likely notice the NSF mark.

If you have any other fittings or products you'd like to check out, you can Search for NSF Certified Products.

tl;dr

This valve is not designed (or was not tested) to meet the codes and standards for a pressure relief valve on, or near a water heater. So the manufacturer was forced to mark the fitting "Not for use with water heaters".

Water – Tankless water heater requirements

The tankless water heater can be mounted with either concentric venting (preferred) or dual-pipe venting - one for intake and one for exhaust. Because of this, we would not be able to 'common vent' in with the furnace exhaust or even the chimney itself because the concentric would be pulling air down the chimney or suck in the exhaust of the furnace. We could take the exhaust pipe of the dual pipe option and add it to the chimney or furnace, if sized correctly, but regardless we'd still have to direct vent outside for the intake. So, because of this, it makes the most sense to go with the concentric venting and route it outside.

The best location that we're working with is in that old coal storage room. This provides access to 3/4" water lines and the 1" gas line that we can pull off of for the 3/4" line needed for the water heater. The only workaround now is with venting. The gas meter in my picture is actually farther down the exterior wall than is shown (the gas line does a 90 after coming through the wall) and up to where it's shown in the picture. This allows the easiest access to venting outside through this area, although we're required to keep a 3' distance from the gas meter for it's regulator. This shouldn't be a problem because we actually have to run it up the wall about 3' anyway to clear the snow level. If this isn't workable, the other option is to run the vent to the complete other side of the basement and come out of the wall there. This could come close to the maximum allowable vent span, but it's an option currently.