Home>Discussions>INSULATION & HVAC>How many pressure release valves needed?
5 posts / 0 new
Last post
How many pressure release valves needed?

I have an oil burning furnace with hot water baseboards. How many pressure release valves do I need? On the first floor there is one in each room on the side where the pipes run back under the floor. On the second floor, there are only two. One is very low to the floor in a back closet, the other is in a closet but this one...the pipes do an upside down U shape (4 - 90 degree turns) and there is a release valve at the top, about 12" off the floor. Interestingly, there is no release valve at the furnace or in the basement. I am getting air in the pipes. Could all of these valves be the issue? Do I really have to bleed each of them? Uggg!!!

Re: How many pressure release valves needed?

Are you sure these are all pressure relief valves or are just bleeder valves?

Pressure relief valves are larger, have threaded outlets usually 3/4" in diameter. You'll see one on the side of your hot water heater. These work automatically to relieve any excessive pressure and protect equipment. When they release, they blow scalding water out through a pipe to someplace safe.

Bleeder valves are usually much smaller, have no automatic feature and require a small key to open and close to let the air out each winter that accumulated over the summer as the water sat dormant in the pipes. You may also have an air scoop in line near the boiler to scoop out the air from the line as it passes by the scoop. But if the air is trapped on higher floors, the bleeder valves do their thing.

There are also adjustable air vents which are more passive to use.

Re: How many pressure release valves needed?

If you have a tank water heater, you already have a pressure relief valve. Most plumbers prefer an additional valve right where the main enters the house.

Re: How many pressure release valves needed?


You have a BOILER there, not a furnace; and we're dealing with AIR VENTS, not pressure relief valves, on your piping as a means to eliminate the air out of the hydronic (hot water) heating system; a pressure relief valve is something quite different.

Check the valve images listed in Houston Remodeler's excellent post to familiarize yourself with hydronic air vents.

So how many AIR VENTS are needed on a hot water heating system would depend on how large the piping/baseboard system is; ideally, there should be an air vent on each RETURN pipe, as it leaves the living space & returns back under the floor to the next baseboard element/main pipe---however even a few air vents on each floor can often be enough to eliminate all the air that accumulates in the system, unless something else is going on, like a small piping leak somewhere in the system, or boiler water pressure that is below 12 psi, etc.

Always concentrate on the HIGHEST POINT IN THE PIPING SYSTEM---which in your case would be the 2nd floor, especially where you have the "upside-down U-shaped fixture"---since air is lighter than the hot water running thru the pipes, it will ALWAYS accumulate in the highest point in the system---so always bleed the U-shaped fixture first & often.

First check the gauge on the face of the boiler in the boiler room---you should have a reading of at least 12 psi for your water pressure; this will assure that the boiler water will have enough pressure to get up to the 2nd floor baseboards---if you open the bleed valves on the 2nd floor & get nothing out (no air/no water) then you will have to slightly increase the boiler water pressure to at least 12-15 psi.

The bleed valves should look something like: http://www.powertechindustrial.com/files/product-hydronics-specialties-hydronic-specialties6.jpg---there is almost always a little slot at the top so a flat-head screwdriver can be inserted to open/close the bleed valve--the little spout at the side is where the air/water comes out--as you've no doubt already learned, the valve is opened until all the air exits, and water starts to come out; as noted, if NOTHING comes out, see the previous paragraph.

Quite often (almost always) there is a major air eliminator at the near-boiler supply piping, called an air purger, or air scoop, like this: http://inspectapedia.com/heat/Air_Bleed_Valve153-DFs.jpg. Check the MAIN SUPPLY PIPE near the boiler & see if you can find such a fixture---the bottom of this cast iron fixture has metal baffles inside that agitate the hot water being pumped from the boiler to separate the air, which rises to the little brass air vent on top & is expelled from the system---these purger vents generally work automatically, but you can twist-turn the top finger valve to see if any air or water comes out to assure that the valve is working---the twist-turn cap should be closed only finger-tight so that no water escapes from the valve.

If you continue to have problems with air in the system, it may be caused by several conditions: 1) if there has been recent work done on the boiler/piping system, NEW WATER has probably been added to the system; NEW WATER (from the house fresh water supply) ALWAYS has entrained air included in any added water to the boiler system---as the boiler water heats up during normal operation, this entrained air is released from the boiler water to create a temporary AIR PROBLEM----the air vents, especially at the top of the system (and the AIR PURGER, if present), then have to go to work to eliminate the air in the system. 2) check the piping system for any staining (especially light green stains) anywhere on the boiler piping, the near-boiler piping, the baseboard piping, and any distribution piping going to and from the baseboards---quite often, a solder joint in the piping develops a leak & sucks air into the system when the boiler pump goes on to pump the hot water thru the baseboards; if there are any water leaks anywhere in the piping, the loss of water will cause the AUTOMATIC REFILL VALVE near the boiler to inject more NEW WATER into the system with the attendant problem entrained air also being injected in. 3) if you have the newer-type of EXPANSION TANK http://inspectapedia.com/heat/ExpansionTank012DJFs.jpg on your system, sometimes the neoprene bladder inside the tank fails and springs a leak & adds air to the system---this will often also be accompanied by an increase in system water pressure, which will open the PRESSURE RELIEF VALVE (B&G Relief Valve in diagram) http://inspectapedia.com/heat/BnGInstallation.jpg at 30 psi to dump some water onto the cellar floor. 4) as noted above, your boiler water pressure has to be at least 12 psi in order for the HW to get up to the 2nd floor baseboards.

Re: How many pressure release valves needed?

Thank you Pelton - This information will come in extremely handy and I appreciate your time.

Sponsored Stories

TV Listings

Find TV listings for This Old House and Ask This Old House in your area.