Feeling the Pressure from PRVs?

One thing that always surprises me in many conversations is the understanding or misunderstanding of the purpose of the pressure reducing valve (PRV) in the hydronic heating system.

The PRV is designed to reduce the incoming water pressure’what we call ‘street pressure”‘to the operating pressure required for a particular application. The PRV is normally set to 15 PSI from the manufacturer. It surprises me how many technicians feel that this should not be altered. Fifteen PSI is set by the manufacturer because at that pressure the height that the water will be lifted is sufficient to handle a three story building, or about 27 feet. This leaves about 3 PSI at the top radiator.

What do you do if you open a purge setup in the basement and no water comes out? This could be diagnosed as a blockage in the piping, like a freeze-up, but it could also be that there is simply not enough pressure to get the water to the top floor. This misdiagnosis happens mostly when the temperature outside is below freezing temperatures. It could possibly be because of the volume of work, or it could be caused by not understanding the PRV.

Many homes are heated and cooled via the air handler. A heating coil is located in the air handler cabinet and heated water from the boiler is pumped through it, and when the temperature in the coil is sufficient, the heat is distributed by the blower. Where do you think the air handler for the second or third floor is located? Yep, it’s usually located in the attic.

The attic is very possibly located higher than the 15 PSI limit. In this case you will need to raise the pressure. How much you ask? One manufacturer’s literature spells this out:

‘The regulator is set to deliver water to the boiler at approximately 15 lbs. pressure. This pressure is sufficient for a three story building. To reset the reducing valve for higher pressure (when the pressure is not sufficient to lift the water to highest radiation):

A. Calculate the number of feet from the regulator to the top of highest radiation.

B. Multiply this number by 0.43 and add 3 lbs. This will give the pressure required to raise the water to the highest radiator and keep the system under pressure.”

Since I usually don’t have a calculator in the basement with me and I am terrible at math, I use an easier method. A formula I have used for many years is to simply determine the height of the highest radiator in feet and divide it in half. For example, an air handler in the attic 30′ high, 15 PSI, radiator 40′ high, 20 PSI’simple. (If you use the manufacture’s method 40′ X .43 = 17.2 PSI + 3 PSI = 20.2 PSI.)

It’s like putting in golf; the ball will never go in the hole if you don’t hit it hard enough to reach the hole. Same thing here, you need to get the water up to the radiator if you want to heat the radiator.

Now that you know how to get the water up where you need it, you should know that not all PRVs are created equal. They all do the same job initially, but the material that the PRV is made of has a great influence on how satisfied you and your customer will be in the long run. PRVs are made of cast iron and brass. Coming new on the market are some composite materials, which still need to prove their worth. But primarily we have iron and brass.

I know you would never use iron on a feed line to a toilet or to a faucet. Think about why. In my opinion if you are going to use a PRV, make sure it is going to last.

Before we end this conversation, consider the following written instructions from these manufacturers’ instruction sheets:

‘Annual inspection of all water system safety and control valves is required and necessary. Regular inspection, testing and cleaning assures maximum life and proper product function. The strainer screen should be serviced at least twice a year. ‘ ‘ From Watts

‘Don’t think of the feed valve as a safety device. It’s not there to protect the boiler against a low water condition. A feed valve’s job is to set the initial system pressure. That’s it. For safety’s sake, you should close the supply valve to the feeder once the system pressure is established. This is important because a feed valve that’s left open can mask a system leak. System leaks that go undetected can lead to air problems and boiler corrosion problems.” ‘ From B&G

I don’t know how many of you have read this before, but following the instructions is important. Another thought, if the feed valve is to be open only when the system is being filled, why do we need one at all, unless the customer is the one going to fill the system when needed? Do you antifreeze a system and leave the feeder open? If so, even a slight leak will over time dilute the antifreeze enough to provide no protection. You will not realize this until it’s too late.