Just as before, when I last correspondence with you, I talked about my boiler pressure was hovering about 20-21 Psi rock solid and I was feeling good about my system. Coming back from my holidays yesterday I noticed the boiler in the 4-plex running the range of 8-9 Psi, (10 Psi when the limit switch kicks in and 19 Psi when the limit switch kicks out) I thought with the bleeding of the lines and installing a new Braukmann auto air valve that should keep the pressure steady, now I am scratching my head. My new x-pansion tank is an Amtrol Exetrol 9V and the plumber who Installed it who is by the way now on holidays till the 15th said he charged it at 20 Psi. Could I be having xpansion tank problems, a leak in the system? With the temperatures this week warming up to a balmy -8 Dec C as the daytime high and tenants not wanting high heat in their suites, and with the zone valves now restricting flow to the radiant heaters rather than full bore, will that increase pressure in the system? I have read so many times from you about shutting down the system, bleeding pressure and taking a reading of the xpansion tank I think it is almost gospel, could it be something else or should I get the plumber to take a good reading of the expansion tank and start from there. If my regulator was not leaking I would worry less about the pressure dropping past 12 Psi. 5 Psi should be max for presure differential, I think.

 

Hey Skeen!

I seem to recall that when you told us that he charged it to 20 PSI we asked you ... WHY? ... the air charge on the tank is supposed to be the same as the system pressure when the system is cold. There's no reason I can think of to charge the tank that high. No matter what size tank you have, it's ALWAYS charged only up to the cold system pressure.

There's a chance that after all the work was done with the tank change and a lot of fresh water added, that there was some air trapped in the system somewhere, and in the past few weeks that air has worked it's way out the air vent. If you are running with the feedwater valve CLOSED, when the vent removes the air, the pressure will drop somewhat. Maybe that's what's happened? dunno... but it's possible.

You said "9V" ... did you mean 90V? You got a huge floor standing tank? If the tank is that big, I can't imagine why there would be 8-9 PSI of 'swing' in the pressure...

 

Trooper, I think it is starting to make sense to me now. With no plumbing background, your analysis is making me much more informed, thanks.
My wife thinks I am getting too obsessed with this boiler and spending to much time over at my 4-plex, but answer this question please.

When you heat water the pressure is going to increase, is it very naive of me to assume(wish, hope) that when I look at the pressure gauge that it should stay a constant pressure 24/7 or is it extremely normal to have a drop in pressure with a drop in temperature and vice versa with a rise in temperature? My pump is on 24/7. I was told that 5 Psi is the number you should worry about on the low end and and add water to the system, not to fret when it is 10 psi, but if I remember you stated once 12-15 Psi should be normals. New expansion tank(Exetrol 30V), new air bleeds(Braukman), cannot seem to find a leak in any suites, just chewing on why my pressure is dropping other than having to replace the pressure gauge.
Skeenabilly

 

When water is heated and then cooled, it will expand and contract.

Water can not be compressed.

Air CAN be compressed.

When water is heated from appx 70°F to appx 180°F it will expand in volume appx 3%.

This means that if there is say 20 gallons of water in the entire system and it is heated from 70 to 180, it will expand in volume appx 0.6 gallons. While that may not sound like a lot of water, think about how much pressure that amount of expansion would create in a sealed up system of cast iron and copper if there were no place for that extra water to go. Hint: A LOT! Probably in the order of hundreds and hundreds of PSI.

Did you happen to see the episode of "Myth Buster" where they literally LAUNCHED a water heater through the building?

The expansion tank is where that expanded water is supposed to go. Since air CAN be compressed, that's exactly what happens... the heated water expands and compresses the air bubble that's in the tank. When the water cools again, it contracts and the air bubble is decompressed to take up the space. The larger the tank, the less pressure increase you will see. You will always see SOME pressure change from cold to hot, and it is completely normal.

The amount of pressure change is dependent on three things:

1. The volume of water in the system.

2. The temperature that the water is heated to.

3. The size of the expansion tank.

 

Expansion tanks are sized using the previous parameters. Each size tank will accept a different amount of expanding water.

FRESH water contains a LOT of entrained air. You can't see it, but if you were to draw a glass of water and set it on the counter for a few hours... notice those bubbles clinging to the side of the glass? Where'd they come from?

If you were able to measure the VOLUME of the water in that glass accurately, you would see that as the air comes out, the volume is decreased.

When you heat water, you further drive out the entrained air, decreasing it's volume, because hot water will not hold as much dissolved air. In a heating system, that air will be released from the system leaving a smaller volume of water in the system. It's not much, but if you run with the feedwater valve closed, it will be noticeable.

After some time though, all the air that's going to come out will come out, and the pressure should stabilize. If there happen to be pockets of air in the piping taking up space, they will slowly be re-absorbed into the oxygen starved water. Later they will come out when the water is heated again and be expelled a little at a time by the automatic air vents. The pressure will drop in the system as a result.

Taking this air out also means something else... that the pressure may swing MORE when the water is heated, because that extra air that was in the system is no longer there to be compressed and act as 'extra' expansion space.

 

All that said, I don't think it's at all abnormal for you to see an 8-9 PSI pressure swing in your system. It's a big system, and you will have a lot of water expanding and contracting. If you want less pressure swing for whatever reason, even if it's just to make you feel better about it, install a second tank. It would of course be wasted money, but you could do it.

Now... going back to YOUR tank... the one that's charged to 20 PSI... With that tank charged to 20 PSI, it means that it will not accept ANY water until the system pressure REACHES 20 PSI. The bladder in that tank is going to stay put and not move until there is MORE pressure on the water side than on the air side. Your contractor is misinformed if he thinks it will help to put more air in the tank. It's just WRONG.

The expansion tank air charge should MATCH the pressure of the system, when the system is COLD. FACT!

 

So with that print worthy explanation before me, could that also be the problem I have with the bathroom baseboard heaters, they are connected to my wild loop. They are cool compared to all the other baseboard heaters in the suites. Simply put, I have an air blockage and I must get into my tenants suites and start bleeding air. I am assuming that the air will always head to the top so better start there. With the work I had done just a little while ago, do not assume that my Braukmann will drain air out of the system in a heartbeat, it justs needs to be helped along and it will stabilize the pressure I am reading on my gauge glass once all air is gone from the pipes.

Really appreciate the time and responses you give to me, Thank You!! Could use you in Alberta for the weekend and get this fixed before stress gets to me.

 

looking at the sticker on the tank, the tank is precharged at the factory for 12 Psi and when I touch it on the top it has a hollow ring and down below a thud.

 

Let's talk about system pressure a bit.

It's generally accepted that the MINIMUM pressure that a boiler system should be run at is 12 PSI when COLD. There are a few reasons for this.

1. The pressure MUST be high enough to pressurize the entire system, top to bottom. As you go UP in height, the pressure in the system will DECREASE. In other words, if you were to install a gauge at the TOP of the system, you would see LESS pressure than you see at the BOTTOM of that system. This is because the weight of the water itself exerts pressure. Your ears pop when you swim to the bottom of a swimming pool? Yep, they do.

Water will exert appx 0.433 PSI PER FOOT OF HEIGHT.

If you had a column of water ten feet high with a pressure gauge at the bottom, you should read 4.33 PSI. If that column were 20 feet high, 8.66 PSI. etc...

Twenty feet is probably around the average height of a two story home's heating system. You need at LEAST 8.66 PSI just to get the water to the top of the system.

In addition, you need to have some 'overhead' of about 3-4 PSI on TOP OF the base number... for various reasons which I'm not going to explain just now... but that's what you need.

8.66 + 3 = 11.66 PSI ... hmmmmm... looks like 12 to me!

2. Your circulating pumps must never be allowed to 'cavitate'.

There is a PRESSURE DIFFERENCE across a running pump. If the pressure at the suction side of the pump is allowed to drop low enough, the pump will cavitate. (not going to explain cavitation, google it for more info). Depending on various factors of the installation, you may need a minimum of 5 PSI or MORE at the suction side of the pump when it is running. Maintaining the pressure in the system at 12 PSI minimum usually will guarantee that you have at least that amount.

If the pump is pumping TOWARD the expansion tank, the pump's pressure difference will SUBTRACT from the system pressure at the suction side. If the pump develops say 4 PSI of difference, that means that 12 - 4 = 8 PSI ... and that should be enough for the pump to operate properly.

You've heard of 'pumping away' ? Where the pump is installed with the expansion tank on the suction side? When pumping away from the tank, the pumps' differential pressure is ADDED TO the system pressure. The expansion tank connection is known as the POINT OF NO PRESSURE CHANGE. In this case, the pressure at the suction side of the pump will remain at whatever the tank is charged to... say 12 PSI ... no cavitation.

Can you see why you were given bad advice that it's OK to let the pressure drop to 5 PSI before adding water?

 

Don't reply yet... I ain't done...

In SOME cases, it may be OK to run less than 12 PSI in the system. One example would be a single story home with the boiler on the same level, and the pump pumping away from the tank. You MIGHT 'get away with' running as little as 5-8 PSI in such a system. But WHY? What's the point? There's no advantage to running less pressure.

BOTTOM LINE:

Replace the defective feed water regulator. Set it to at least 12 PSI, MORE if the total height of the system is greater than 20 feet.

Adjust the air charge in the expansion tank to whatever the cold system pressure is.

Grab a beer, put on the game, and enjoy life.

OK, now I'm done ...

 

You told us that the installer charged the tank to 20 PSI, so whatever the manf said it was precharged to is moot.

You can tell little to nothing by tapping on the tank.

 

I'm trying to get ahold of the plumber to answer that question about the expansion tank. In my 4-plex, from the top of the boiler to the main floor of the top suites is 10-15 feet, it is low, there are only 2 floors on this thing, it's just long. Does this help with the calculations? maybe I should brave the whiteout tonight and check again, it was around 10-11 psi when the limit switch kicked in. If I remember, the tank inlet is upstream of the Grundfos pump.
Skeena

Man you know your stuff