What is the safety zone in PSI for boiler pressure (and temp if important) for hot water Boiler heat

Rocco

What is the "safe" operating pressure in PSI for a hot water boiler heating system, please specify high and low pressures. Also what is the "safe" range in term of Farenhite Degrees for boiler operation?

Ironman

Safety Zone...

The highest pressure the boiler should ever see is stamped on the boiler tag - Usually 30 or 50psi.



The pressure the system is designed to operate at is usually 12 - 25 psi for a two story house and 18 - 25psi for a three story.



Operating high temp should not be over 200deg, but 180 is normal max design temp. There are are alot of variables depending upon the type of system and boiler you have and many times the operating temp can be much lower.

Jean-David Beyer

You must read the manufacturer's specifications.

Most residential hot water heating systems run at 5 psi more than the pressure required to raise the water from the pressure gauge to the highest point in the system. In my Cape Cod house with the boiler on the floor of my garage, the pressure gauge reads between 12 and 15 psi (dial too small to read more accurately). The maximum pressure I am allowed it to get to is 30 psi, where the pressure relief valve will open. I believe this is typical of residential hot water boilers.



As far as temperature is concerned, my boiler will shut off if the temperature exceeds 205F (if I remember correctly) THey do not watn the watre in there to boil ever.



Of more importance is the minimum return water temperature. With a conventional boiler, you do not want condensing, so the return temperature should niormally be over 140F to prevent the boiler from rusting out and the chimney from failure. With a condensing boiler, you want the return water as cold as possible, short of freezing, to get maximum condensation and the resulting higher efficiency.

icesailor

Boiler Water Pressure

The water pressure in a hydronic boiler shouldn't be over 12# to 15#. It should have only enough pressure to raise the water a few feet over the top of the highest heat emitting unit on the system.12# is 28' above the gauge or fill valve. 15' is 34. At 12#, the boiling point at the fill valve/gauge is 246 degrees. At 28', it is 212 degrees. If the height of the water isn't high enough from lack of pressure, the suction pressure on the loop may cause the first water to boil that comes from the boiler on a warm start boiler. If you set the system pressure higher than needed, you decrease the amount of room for system expansion. If you have a one story ranch with a basement, 12# should be all you ever need. A two story Cape, shouldn't need more than 15#. A Cape is usually 34' from the ground to the peak of the roof. 

pressure

Boiler pressure too high

I have had a problem for a couple of days with my boiler.  It is leaking from the safety pop off.  I have replaced the expansion tank the safety pop off and the automatic fill valve.  The pressure runs close to 30 psi while heating.  The water is heated to 180 before the boiler shuts down until it needs to reheat the water.  After it shuts down it somehow continues heating to around 190.  If there isn't any call for heat from one of the thermostats while the water is at temperature the pressure builds until the pop off either leaks or blows like it is designed to do.  I had a contractor add a loop to my garage.  It is a unit that hangs from the ceiling with a fan behind it.  This was added in December and I haven't had a problem until now.  How do I get the pressure back in line where it is supposed to be?

Mark Eatherton

Pressure...

Does your boiler also produce your DHW? If yes, you could have a leaking heat exchanger in your DHW tank.



If no, and the system pump is pushing water towards the expansion tank connection, AND the make up is connected to the inlet side of the pump And it's a high head pump AND flow is restricted, you need to move the make up connection to the expansion tank.



To the original poster, a safety factor of 90% of rated pressure would be considered the normal maximum. So if its a 30 # valve, then 27 would be the normally acceptable maximum.



Why do you ask?



ME

pressure

Boiler pressure too high

This system has been in for 15 years.  The only thing different is the loop in the garage.  The water needs to pump higher than before because it is a ceiling unit.  Would this increase the pressure?  If the DHW had a leak would the hot water be hotter than the thermostat would call for or be rusty?What is the best way to check for a leak?  Would I shut off the valves to this zone and unhook the thermostat?  Is it possible to have too much water in the boiler?  Thus leaving too little room for expansion?  Or is the boiler always completely full because of the automatic fill valve and all the expansion done in the expansion tank?

Mark Eatherton

Needs...

The fill pressure is typically 1/2 PSI per vertical foot of system elevation above the boiler, plus 5 PSI.



A qood overnight test would be to turn off the cold water to the DHW tank, turn the aquastat on the tank down, and watch the system. If the system rises overnight then you need to start looking at the system configuraton as I pointed out earlier. Don't forget to get up early enough to turn the DHW tank back on and up so you get hot water for showering.



All expanding and contracting fluid goes to the expansion tank. It is possible that the diaphragm on the tank has failed, and you have no expansion tank, but that is rare. WHat kind of expansion tank do you have? Ceiling hung, or smaller one hanging off a pipe?



ME

icesailor

Replaced Expansion Tank:

When the expansion tank was replaced, was it replaced with the same rated tank or was it (perhaps) replaced with a smaller, cheaper one?

Asking some "Experts" might tell you need only need a smaller tank when in fact, you need a bigger one. Or at least one that is equal. Especially at on-line or big box stores.

I've seen a lot of problem systems with Expansion Tanks too small. I've never found a problem system with a expansion tank that was too big. Where the larger tank was the cause of the problem.

Jean-David Beyer

I've never found a problem system with a expansion tank that was too big.

Other than the waste of money, what is the problem with too big an expansion tank? Is it like having too much venting on a steam main?

icesailor

That;s true:

That's true.

But it didn't make the Relief valve blow or any of the other symptoms.

I would say that once cold start oil boilers came into vogue, and they still used #30 Extrol tanks on the cold start's where they always used #30's on warm starts, they should have been using #60's because of the higher water expansion rate. If that makes sense.

I went to a Greet & Eat at the supply house that Amtrol put on. They gave us this little wheel calculator. According to the calculator, almost every Extrol installed was undersized. Especially on cold start boilers. That application is a cold start application with less water content than a normal boiler.

pressure

Boiler finally working

I seem to have fixed the problem.  After replacing the expansion tank going from a 30 to a 60, the safety pop off and the automatic fill valve all to no avail, I had to try something else.  I would drain the boiler down so the pressure was around 20 and the next time it ran it would go over 30 again.  I took the automatic fill apart and adjusted it down.  I am not sure how much range there is in the adjustment, but I eventually had to go all the way down.  I drained the boiler down again and my pressure has stabilized at 25 lbs.  So in answering my previous question there is such a thing as having too much water in the boiler.  It seems that the city water pressure must have gone up above anything I had received in the past, therefore causing my leak.  I intend to buy a water pressure gauge to see what I have.  It seems I spent a lot of money for something that was out of my control.  If I have any more problems I will have to install a water regulator in front of my automatic fill valve.  I am generally lucky to have any water pressure to speak of as I am at the end of the line.

icesailor

Fix It:

There is almost nothing that I will not try to fix. I've tried to fix boiler fill valves. I've never been successful. Watts 1156F pressure reducing valves are cheap dates. If you're going to replace any gauges, start with the Tridicator gauge on the boiler. Or buy a pressure gauge and adapt it to a hose fitting and put it on the boiler somewhere. If you let water out of the boiler and it registers 8#, the fill valve should automatically fill it to 12# to 15# without any help from you. It is automatic. If it is a cast iron valve, replace it with a brass one. If you get scuzz between the seat and the washer, it will leak bye. There are some who say to never leave them on.

M Lane

Simple answer

To your question is the MAWP rating on the boiler. Maximum allowable working pressure.

Ideally someone chooses a boiler with the right MAWP for its application.

Basic design guidelines:

* Like Mark said, you need 1/2 lb. per foot of elevation water pressure in order to lift the water vertically. 5 lbs. per floor. A 10 story building needs 50 psi minimum to assure the water reaches the top floor. We might add a few pounds to make sure there's enough to add some static to the radiators so that the bleeders work better.

* Accordingly, the boiler's MAWP then must be greater than 50 psi. This is primarily reflected with the relief valve.

* The system's expansion tank must have an equal air charge to the operating pressure. Check it with a tire gauge at the schraeder valve.

* The make-up water assembly then needs a PRV that can be set at 50 psi.

* For baseboard, we usually default operating temp. (setpoint) to 180*. It can be as low as 160 or as high as 200, but your are pushing limits and risking damage an/or injury. Below 140 the boiler will start to condense, with a non-condensing boiler this will ruin the heat exchanger. Here in Denver, above 212 you get steam. But it wont be steam until it is exposed to atmosphere. Which is what happens if the relief valve lifts or something blows apart.

* One of the safeties with the boiler is the high limit switch. We normally set it 40 degrees above setpoint; or 220 with a 180 degree setpoint.



Edit- I started typing before reading all the posts. Sorry if my post seems a bit rudimentary. you didn't mention the actual problem in the OP so I just went general principles.

NassauTom

Mark Eatherton said:

Pressure...

Does your boiler also produce your DHW? If yes, you could have a leaking heat exchanger in your DHW tank.

Mark, could you explain this further? How would a leak in the DHW coil or heat exchanger cause the pressure in the boiler to rise? And, more importantly, how do you determine for sure that DHW heat exchanger is in fact leaking?

I just had a cast iron radiator shatter last night (which I isolated in triage mode) and then another radiator shattered this morning. I ran down to the basement and shut off the boiler and the circulators. I then noted the pressure and the gauge read about 40 psi !

It seems my pressure relief valve failed. (Please read that last sentence with an appropriate level of sarcasm for stating the obvious.) But, clearly, that is not the full extent of my problem. What would be causing the pressure to suddenly be so high? Would this be the result of a sudden failure of a pressure regulator or expansion tank? (There does not appear to be any problem with the expansion tank, but I am not sure I know how to tell for sure if it has failed.)

My boiler is a Weil-McLain Gold, model # P-WTGO-3, series 3 and has a MAWP of 50 psi. It supplies the DWH as well as the heat. The boiler is in the basement and this is a two-story house. So, based on the info above, I should expect the boiler to be set to operate at 12-15 psi. It was running at 28-30 psi last night right after the first radiator shattered. And, as I said, it was as about 40 psi right after I shut off the power to the boiler and circulators this morning.

NassauTom

Thank you Hatterasguy. You provide a logical step by step process. Thank you.

NassauTom

Hatterasguy, I assume that step 1A is "Drain the heating system"

NassauTom

Well, I do have isolation valves. But, unfortunately, one of them is stuck open due to corrosion. (I think I'll save the job of replacing that for the summer.) So, I did a dumb thing: I got the new relief valve ready to go then did a hot swap, removing the old one and quickly inserting the new one. And, I got water all over the burner in the process. So, now I do not want to turn the power to the burner back on until I thoroughly dry everything. I'm a bit apprehensive about opening the control box, but I should probably make sure no water got in there before I turn the electricity to the burner back on.

NassauTom

Is this the pressure reducing valve? It is downstream from the boiler fill valve and the expansion tank.

NassauTom

From the boiler, first the boiler fill input (with expansion tank), then the Amtrol air purger valve, then this valve which I assume is just an isolation valve.

NassauTom

After the isolation valve above is this "Flo Control Valve"

RobG

The first pic is an air scoop, the second is a gate valve and the third is a flow control valve.

NassauTom

Removed the shattered radiator that is not valved for isolation and temporarily capped the supply and return pipes. Replaced the pressure relief valve (which was not operating due to an enormous amount of internal corrosion). Opened the boiler fill valve, then turned the circulators and burner back on. Once the temp in the boiler came up and some of the air was purged from the radiators, the pressure came up to 25 psi and continued to climb. So, that would indicate that the pressure reducing valve is bad. The only problem is that I cannot locate the pressure reducing valve. Any thoughts?

RobG

Not all boilers were installed with a PRV. Post a photo of the boiler feed line. You may just have a fill/shutoff valve and no PRV, in which case you would pressurize the system and then close the fill valve. If that is the case you would want a PRV and backflow preventer installed. How many feet is the uppermost radiator from the boiler? That will determine what the system pressure should be.

NassauTom

First, THANK YOU SO MUCH for taking the time to help me. I turned the boiler back on and sat and watched the temp and pressure gauges. I was disturbed by what I observed. Pressure gauge was pegged at 0 until temp reached about 160. By 170 , pressure was at 13, at 175 pressure was 15, at 180 pressure was 20, at 185 pressure was 25, at 190 pressure was 30, at 195 pressure was at 34 at which point I turned the burner off. Temp continued to rise to 197 at which point pressure was 40. Pressure rose further to about 42 or 43. I then drained about 6 gallons from the system. This had negligible affect on the pressure reading on the gauge. I just ran down and checked it again: temp has dropped to 185 and pressure is now 35 psig. Obviously, there are a number of bad things going on here. Why did the boiler continue to run as the temp reached 195? Why didn't the brand new pressure relief valve set at 30 psig open at 30 psig? Pressure reached 42 or 43 on the gauge and the pressure relief valve never opened. And lastly, why didn't pressure drop as I drained 6 gallons of water from the system. The data here, I think, suggests multiple problems.

NassauTom

Checked gauges again. Temp now down to 180 and pressure down to 29 psi. It seems that pressure is varying with temperature, although there is a lag: pressure at a given temperature is lower when the boiler is heating (e.g., 20 psi at 180 F) as opposed to cooling (e.g., 29 psi at 180 F when boiler is cooling down).

Expansion tank bad? Combination of problems?

NassauTom

RobG, thank you. I will post photos. When you say "boiler feed line," do you mean the water supply that tees into the system (at the Amtrol valve)? I am assuming that you aren't referring to the system return through the circulator to the boiler.

Jamie Hall

The kind of pressure change you are seeing with temperature almost always suggests a bad expansion tank. Water logged, failed diaphragm, lost pressure through the air valve... something.

However, you also have some kind of problem which looks like an overfill -- and Hatterasguy has given some ideas as to how to track that down.

All that said, however, I am rather uneasy about this. The combination of pressures and temperatures which you appear to be getting (have you checked the gauges for accuracy?) first place should open the pressure release valve and second place should shut down the burner. Those are safety items, and -- bluntly -- if your numbers are right you are not too far off a spectacular failure. If it were mine, I'd shut it off and leave it off until I could positively determine what the problems were, and had fixed them.

BobC

I agree with Jamie that your gauge is suspect, if the pressure did rise that far the pressure safety should have gone off.

Try anther gauge so you know what your looking at and make sure that expansion tank has not failed.

Bob

NassauTom

Aha!!. The pressure tank is an Amtrol Fill-trol model 110. Pics follow. Here is the first paragraph from the instl & operating instructions taht I found on line:

"FILL-TROL® is a combination of an EXTROL® expansion tank and a specially designed automatic pressure reducing valve. Together, they manage water expansion to provide satisfactory heating system performance."

So, Hatterasguy, you are correct. I don't see where I can apply the pressure gauge to the tank/PRV to read the pressure. But, I'm thinking that I need to replace this pressure tank/PRV combo.

NassauTom

Here are pics of the water source to the heating system and the expansion tank. Water supply enters the system via the bottom of the air purge valve on the pipe that exits the boiler.

NassauTom

Jamie Hall, yes I agree, this is a safety issue. In fact, I had a cast iron radiator explode last night and another exploded this morning. Luckily, the boiler has a MAWP rating of 50 psi and I shut everything down this morning when it was in the low 40's.

NassauTom

Aha. The blue cap is threaded and twists off. Then there is also a small rubbery plug that needs to be picked out. The pressure gauge does not register any air pressure, but is does cause water to drip from the tank. I think that settles the question of whether or not the expansion tank is bad. Thank you.

Regarding the pressure and temp readings and the possibility of the gauges being bad: that is certainly possible. I will note that this all became an issue for me when two cast iron radiators shattered like over-inflated cast iron balloons. So, I tend to believe the over-pressure readings. I do, however, agree that it is very unusual for the pressure relief valve to not open at the preset pressure. Perhaps the valve was not set properly.

Another interesting piece of information is this: Earlier (not too long after I made the temp/pressure observations described above) I turned the burner back on for hot water, figuring I would only be able to leave it on for a short time before the boiler pressure went too high. It didn't. The burner shut off at about 170 F and the pressure was about 20 psi. Pressure actually dropped a bit later to about 15 psi. Boiler is currently at about 182 F and 25 psi and burner is not running. So, I would have to characterize the temp and pressure as erratic....and unreliable.

NassauTom

I would like to thank everyone who provided quality analysis and input. I am especially appreciative to Hatterasguy...you provided excellent procedures for isolating the problem, you answered all of my questions of dubious intelligence and you followed up. I am so very thankful. I am now off to find a source for the replacement expansion tank/valve. I will report back with the results...it is the absolute least I can do. Thanks again!!!!

Tom

NassauTom

Once again, thank you: Supplyhouse was a few dollars cheaper than the source I was looking at and the estimated delivery date is sooner. :-)

rick in Alaska

When you get the new tank, you need to check the air pressure in the bladder and make sure it is the same as the fill pressure you need for your system. Check it and adjust it before you install it to get the best accuracy.
With this type of setup, the fill pressure you put in to the tank will be the same as the water pressure in the system. And if you ever have to check it again, or any other expansion tank for that matter, the boiler pressure has to be at zero. And if you have to adjust it in the future by adding more air, the relief valve or some other drain valve must be open to have a place for the displaced water in the tank to go. Or you will get false readings.
Also make sure there is no air stuck in your system somewhere or the tank will still be working too hard and may still not keep the boiler pressure stable. This will show up as gurgling noises or lack of heat in a zone.
Hope this helps.
Rick

NassauTom

I am very happy to report success.

The new expansion tank with integrated pressure reducing valve arrived from supplyhouse.com very promptly and as promised. Great price and extremely fast delivery. I highly recommend them.

Replacing the existing expansion tank was a very easy job. The old expansion tank was considerably heavier than the new tank since it was full of water! :-)

Opened up the valves and the pressure reducing valve did its job filling the system with water and then shutting off at the preset pressure. Turned on the burner and the circulators and the heat is once again working properly (albeit with two fewer radiators in the loop at this time).

My next phase of this job is to remove the cracked segments from my damaged radiators and replace them with segments from another radiator that is not in use.

The only problem I have is that the pressure gauge on the boiler is now reading 0. I know there is pressure in the system based on how the air was pushed out of the radiator bleeders. I imagine that the gauge was damaged during the high pressure events that resulted in the radiators shattering. I will need to buy a new gauge and replace it. Any tricks to that or is it straightforward?

Thanks again to everyone who responded with helpful information and advise. This is a great forum because it has great people like you supporting it.

icesailor

Post photo's of the "exploded" radiators. The closest I have ever seen as far as a radiator "exploding" was when one froze up and ice blew a piece out of an end section and the piece was blown through a sheetrock wall into another room.

NassauTom

I use the term "explode" because the failures were caused by excessive pressure inside the radiators causing them to rupture. And when the ruptures occurred, each was accompanied by a loud "BANG." The first radiator that exploded in Nov had two chunks break out of a section of the radiator. The first picture shows the hole in the radiator section and the second picture shows the two pieces that broke from the radiator. The two radiators that "exploded" last week both split; the pictures of those are less dramatic. The third and forth are of one radiator split and the fifth and sixth are of the other.

nicholas bonham-carter

What a loss to lose such pretty rads!
glad you were able to get the cause under control.--NBC

icesailor

I have never in my life ever seen anything like that unless the radiators froze up.

With a dead Extrol tank, and w bad fill valve, the only way that could have happened is if someone put a plug inside the Pressure relief Valve and the boiler went from colt to hot. The only thing that can cause that kind of pressure is thermal expansion.

You can't do that same damage with a 10# top mall.