Why no T&P on boilers?

ChrisJ

We always see 15 PSIG on steam and 30 PSIG on hot water.

Why isn't there ever a T&P on a hot water boiler? Is it because 30 PSIG doesn't store enough energy for it to be a concern?

dko

Tankless water heaters also only use 150 psi relief valve as well. No temperature.

Probably due to redundancy in that they have other [electronic] safety controls pertaining to temperature? And that they are usually not for potable purposes where it will be in contact with skin - though that doesn't explain tankless water heaters.

Never really thought about it before either

WMno57

PRV vs T&P
I guess because 190F water in a HW boiler used for space heating might be a normal condition. Dairy water heaters store water at a higher temperature than home water heaters. Not sure if those have a different T&P than home water heaters.
A HW boiler in a four or five story building might have a 50 psi PRV.
Looking forward to this thread. What say the Pros?

ChrisJ

WMno57 said:

PRV vs T&P
I guess because 190F water in a HW boiler used for space heating might be a normal condition. Dairy water heaters store water at a higher temperature than home water heaters. Not sure if those have a different T&P than home water heaters.
A HW boiler in a four or five story building might have a 50 psi PRV.
Looking forward to this thread. What say the Pros?

That doesn't answer it in my opinion.
You can make a T&P pop at any temperature you want, it doesn't have to be 210F.

Unless going above atmospheric pressure makes it's function useless, I don't know.

WMno57

Hopefully the Eds will be along shortly to ED-ucate us.

ChrisJ

WMno57 said:

Hopefully the Eds will be along shortly to ED-ucate us.

The Eds?

dko

Having a hard time finding any safety relief valve that will pop at temperatures higher than 210F.

Would it not just flash to steam if it were to be released into lower pressure through the relief valve?

mattmia2

I think the question is more why is there a temp function on a water heater valve and do commercial and industrial water heaters that produce near boiling water for commercial dishwashing and processes need to have temp relief valves. I suspect that it allows less redundancy in the controls for the burner or heating elements in a water heater because the relief will blow off and the domestic water supply will carry the heat off. That is a lot less guaranteed on a boiler. Also a boiler operates at or near boiling under normal conditions so protection from boiling doesn't make sense.

ChrisJ

mattmia2 said:

I think the question is more why is there a temp function on a water heater valve and do commercial and industrial water heaters that produce near boiling water for commercial dishwashing and processes need to have temp relief valves. I suspect that it allows less redundancy in the controls for the burner or heating elements in a water heater because the relief will blow off and the domestic water supply will carry the heat off. That is a lot less guaranteed on a boiler. Also a boiler operates at or near boiling under normal conditions so protection from boiling doesn't make sense.

Please explain what you mean by "protection from boiling" ?

ChrisJ

https://youtu.be/05mtXWLdzgE

JUGHNE

ChrisJ, that is a great video, It was shown at a trade show about 40 years ago.

Great educational info that I have never forgotten. Previously I was completely unaware of the effects of superheated water.

Because of the longevity of water heater tanks in our water system, there are a few with only pressure relief valves only on some old ones. Especially electric WH's, I believe they considered the high limit CO to be the temp safety.

Most boilers have 2 temp controls, often the high limit one is required to be manual reset.

However, in the case of a run away burner, being as the system is closed, the pressure would build and the pop off would open, rinse and repeat and water feeder refill if fill valve open.

If fill valve closed, then even LWCO would not help.

Also a reason to have water fill connection near the bottom of the boiler.

PS: This statement may have been in the video or the Watts rep later reported that some plumbers asked
"why do we still have to install T&P valves; there have been very few exploding water heaters in the last few years?"

ChrisJ

JUGHNE said:

ChrisJ, that is a great video, It was shown at a trade show about 40 years ago.

Great educational info that I have never forgotten. Previously I was completely unaware of the effects of superheated water.

Because of the longevity of water heater tanks in our water system, there are a few with only pressure relief valves only on some old ones. Especially electric WH's, I believe they considered the high limit CO to be the temp safety.

Most boilers have 2 temp controls, often the high limit one is required to be manual reset.

However, in the case of a run away burner, being as the system is closed, the pressure would build and the pop off would open, rinse and repeat and water feeder refill if fill valve open.

If fill valve closed, then even LWCO would not help.

Also a reason to have water fill connection near the bottom of the boiler.

PS: This statement may have been in the video or the Watts rep later reported that some plumbers asked
"why do we still have to install T&P valves; there have been very few exploding water heaters in the last few years?"

But hot water systems have an expansion tank. Wouldn't this allow for water to become extremely hot before a 30 PSI valve would open? IE around 270F?


I sleep a whole lot better at night knowing I have a functioning T&P on my water heater.

mattmia2

ChrisJ said:

Please explain what you mean by "protection from boiling" ?

as your video illustrates as the temp increases some amount of water boils and increases the pressure which increases the boiling point. at some point this pressure exceeds what the tank can withstand and it ruptures reducing the pressure, at that point all the water in the tank flashes in to steam because it is now at atmospheric pressure. If you relieve the pressure based on temp you will relieve the pressure before the pressure begins increasing.

Since steam boilers are intended to boil, relieving pressure on temp isn't practical and would already be at the point where it is going to flash to steam. Hot water boilers may also operate above the boiling point at atmospheric pressure though that isn't common in small systems.

ChrisJ

mattmia2 said:

ChrisJ said:

Please explain what you mean by "protection from boiling" ?

as your video illustrates as the temp increases some amount of water boils and increases the pressure which increases the boiling point. at some point this pressure exceeds what the tank can withstand and it ruptures reducing the pressure, at that point all the water in the tank flashes in to steam because it is now at atmospheric pressure. If you relieve the pressure based on temp you will relieve the pressure before the pressure begins increasing.

Since steam boilers are intended to boil, relieving pressure on temp isn't practical and would already be at the point where it is going to flash to steam. Hot water boilers may also operate above the boiling point at atmospheric pressure though that isn't common in small systems.

But if you relieve a hot water boiler at say, 220F that's still way below the point which it'll likely boil in normal operation at 10 PSIG or higher. So, wouldn't that still be saver and practical?

Or is the entire point of a T&P to keep the water from flashing to steam when the tank or container fails? That would limit you to 210F. I.E. to stop you from compressing that enormous spring which will let go when the tank fails?

mattmia2

the temp function is probably to relieve it before it will flash to steam. of course that brings up questions about high altitude installation.

Jamie Hall

Protection from boiling... well,, a steam boiler is always boiling when it is running, at a temperature which is controlled by the pressure. A pressure relief valve on a steam boiler will open when the pressure reaches a safe maximum, and it should be capable, when continuously open, of keeping the p0ressuret tht level even if the burner doesn't shut off. On a not water boiler, you really don't want a valve to open on temperature, as the hot water inside may be at or very near to boiler -- but you do want a PRV to crack when, again, the pressure gets too high. You also want that valve to close promptly when the pressure drops -- which it will very quickly.

Why? Because if you remove the pressure quickly on a hot water boiler which is overtemp0eratured, the water inside will boil -- all together. This usually will result in an explosion... which you don't want. So you let the pressure off slowly with a PRV.

ChrisJ

Jamie Hall said:

Protection from boiling... well,, a steam boiler is always boiling when it is running, at a temperature which is controlled by the pressure. A pressure relief valve on a steam boiler will open when the pressure reaches a safe maximum, and it should be capable, when continuously open, of keeping the p0ressuret tht level even if the burner doesn't shut off. On a not water boiler, you really don't want a valve to open on temperature, as the hot water inside may be at or very near to boiler -- but you do want a PRV to crack when, again, the pressure gets too high. You also want that valve to close promptly when the pressure drops -- which it will very quickly.

Why? Because if you remove the pressure quickly on a hot water boiler which is overtemp0eratured, the water inside will boil -- all together. This usually will result in an explosion... which you don't want. So you let the pressure off slowly with a PRV.

If it was that simple a pressure relief is plenty of a normal water heater too.
But apparently it's not?

Matt_67

I think a T/P valve might be a good idea on a typical low pressure hot water boiler. It’s essential on a domestic water heater with their really cheaply made control components, the potential for scalding and the higher typical operating pressures.

ChrisJ

Matt_67 said:

I think a T/P valve might be a good idea on a typical low pressure hot water boiler. It’s essential on a domestic water heater with their really cheaply made control components, the potential for scalding and the higher typical operating pressures.

I don't think a T&P changes anything in regards to scalding?

hot_rod

There would need to be some listings established for manufacturers to build such a valve, a temperature and pressure boiler relief valve.
ASSE, ANSI perhaps?

mattmia2

I was hoping @hot_rod would have the answer. So far we just have it doesn't work for boilers so we don't.

Larry Weingarten

Hi, I can add that domestic water heaters live at substantially higher pressures than space heating boilers. As the Watts movie says at 50 psi, water boils at 298 F. The T&P functions by opening and letting enough cold water in to keep tank temperature under 212F, so the tank can’t explode. Probably very little of that is news to the people reading this.

At 15 psi, water must be 257F to boil, but boilers usually can take up to 50 psi, I’m guessing? So it seems in worst case that if the boiler is not shutting off, and there is no water feed, the boiler will expel all of its water and then glow red. Still, the problem of water doing a phase change, turning to steam and getting 1700 times bigger, nearly instantly doesn’t really exist in this case. So, the boiler will die an unhappy death, but not explode. Just a theory as to why only pressure relief is used 🤠

Yours, Larry

Jamie Hall

and a very good theory, indeed, @Larry Weingarten .

EdTheHeaterMan

In short: the pressures are different. the appliance that operates at the higher pressure needs more protection.

LONG WINDED ANSWER (of which i'm known for)

DHW heaters operate at higher pressures than space heating boilers. DHW are open systems. Space heating systems are closed systems. There are different things happening inside the two different appliances. High Temperature at high pressure on an open system subject to water hammer from say an open tap then closing the tap abruptly may be in an explosive condition if the tank was at 80 PSI. (125 PSI is a standard pressure for DHW relief valves).

Since a closed system has a much lower pressure relief specification, the water temperature in the closed system will not be as explosive. it is releasing at a lower pressure and then the valve will re-engage in order to prevent say 225° or higher water temperature inside the closed system reaching the lower atmospheric pressure that would cause the instant superheating of the higher temperature water at the lower pressure.

Temperature relief valves are for the most part a one and done event valve. While pressure relief valves are multiple event capable. There is a reason for the design on the closed system and another reason for the design on the open system Great Video BTW @ChrisJ. Like I'm back in training in the 1960s watching a 16 MM film with the rapid click, click, clicking of the projector.

Mr. Ed.

PS. Here is a film I also remember for the mechanic that need to learn how basic hand tools should be used
https://www.youtube.com/watch?v=sKM4qBESzrA

ChrisJ

EdTheHeaterMan said:

In short: the pressures are different. the appliance that operates at the higher pressure needs more protection.

LONG WINDED ANSWER (of which i'm known for)

DHW heaters operate at higher pressures than space heating boilers. DHW are open systems. Space heating systems are closed systems. There are different things happening inside the two different appliances. High Temperature at high pressure on an open system subject to water hammer from say an open tap then closing the tap abruptly may be in an explosive condition if the tank was at 80 PSI. (125 PSI is a standard pressure for DHW relief valves).

Since a closed system has a much lower pressure relief specification, the water temperature in the closed system will not be as explosive. it is releasing at a lower pressure and then the valve will re-engage in order to prevent say 225° or higher water temperature inside the closed system reaching the lower atmospheric pressure that would cause the instant superheating of the higher temperature water at the lower pressure.

Temperature relief valves are for the most part a one and done event valve. While pressure relief valves are multiple event capable. There is a reason for the design on the closed system and another reason for the design on the open system Great Video BTW @ChrisJ. Like I'm back in training in the 1960s watching a 16 MM film with the rapid click, click, clicking of the projector.

Mr. Ed.

PS. Here is a film I also remember for the mechanic that need to learn how basic hand tools should be used
https://www.youtube.com/watch?v=sKM4qBESzrA

If a T&P opens on temperature, it never closes again?

hot_rod

According to the info at the Watts site, the valve can reseat after a temperature relier incident. I thought they were one shot protection?

Limescale on the temperature element is one cause of the valve not releasing on temperature.

ChrisJ

It looks like a T&P uses a wax motor?

MikeAmann

Why is it that when you hire someone, you expect a craftsman to show up and what you get is PRIMITIVE PETE?

ChrisJ

MikeAmann said:

Why is it that when you hire someone, you expect a craftsman to show up and what you get is PRIMITIVE PETE?

Please explain 

hot_rod

ChrisJ said:

It looks like a T&P uses a wax motor?

Quite a few hydronic devices use a simple wax “motor”

TRV, mix valves, zone valves, freeze protection valves, relief valves are some examples

Its amazing how accurate that can be for temperature sensing and regulation

Grayfurnaceman cut one a part for a video, but seems baffled by the goop and how it works🥴

EBEBRATT-Ed

boilers have high limit controls, either a pressure control or aquastat. On a boiler the operating contrrol is the thermostat (heating only) or a low limit (with a tankless).

Most water heaters have only a thermostat and no other limit control for the most part.

ChrisJ

How many high limits do most residential boilers have?  Only one, no?  The aquastat?

The water heater also has one, the thermostat. 

EdTheHeaterMan

@ChrisJ asked: If a T&P opens on temperature, it never closes again?

That was my belief. It seem that you learn something new every day. Thanks HotRod for the info on that Watts document.

hot_rod

ChrisJ said:

How many high limits do most residential boilers have?  Only one, no?  The aquastat?

The water heater also has one, the thermostat. 

On gas fired water heaters the control for the temperature, aquastat, also has an ECO, energy cutoff. I think that may be a one time control?
On electric water heaters, the ECO is a manual reset control, the red button pops.

So you have temperature limitation, an ECO, and the relief for pressure and temperature. I suppose you could say 3 controls for over temperature.

JUGHNE

For gas WH's I believe the ECO is a single shot fuse type link in the T-couple circuit...not replaceable.
The main reasoning was to disable the gas valve from future use in case it was subject to fire.
Distort the plunger/seats etc so as to not shut off 100%.

You get a new ECO with a new valve of course.

Water heaters used to shipped without a T&P. It was up to the installer to furnish and install one.

Apparently the WH companies realized the liability coming their way and decided to install the T&P's.

I believe the change was maybe mid 1980's.

mattmia2

JUGHNE said:

Apparently the WH companies realized the liability coming their way and decided to install the T&P's.

Because people like my uncle would buy a series of adapters to put the t&p valve from the old water heater in the new water heater instead of buying a new valve.

hot_rod

Tank type water heaters are tested to 300 psi.
Use to be the relief was required to be 40% of test, so 125psi.
Somewhere along the line they increased it to 50% of test, so now most relief pressures are 150 psi, 210F.

JUGHNE

How I learned about ECO's on someone else's dime;

The farm wife who worked in town stopped and brought a new WH T-couple 3 days in a row.

I inquired about why and her husband called me that evening.

He had installed a new LP gas valve on the water heater and apparently 3 different T-couples.

The rest of the story was that he hooked the valve up and turned on the gas, he had a leak under the valve and it was burning. The only gas stop was outside at the tank...maybe 100' away.

He got it shut off and got the fitting gas tite.

But the pilot would not stay lite so tried several new T-couples.

Upon him reading the instructions over the phone, it stated the new valve had an ECO which if subjected to extreme heat would open and not reset nor could be replaced.

I did say that usually one would just get a new WH rather than replace a LP gas valve, which he paid retail for. (near the price of a new WH at that time).

But this 45 year old tank had worked so well for so long that he did not think it was time for a new one.

One has to somewhat admire his reasoning to not throw out a good tank.
He was old school in repair rather than replace, and to have a gas stop at each appliance, WH and furnace, was just wasted money when you could shut off all the gas at the tank.

These were children of the great depression and WW 2 who went thru more than we could survive today.

I have learned to admire them and not think less of them as being knuckleheaded and stubborn.

mattmia2

45 year old water heater? probably monel or copper.

JUGHNE

I am pretty certain this was a standard WH.

We are blessed with sandhill water here, his location even more than mine.

My WH storage tank was put into use in 1995 and still holding. 120 gallon solar storage tank fed by a tube in shell fed from mod con. We are on city water.

Our church has an electric from 1969.

I have seen many tanks from the 50-60's, some started to leak when we added backflow devices to the water meters. No expansion tank so the WH started to leak.
Most changed out simply do to age. The old electrics might have 1500 watt elements and were considered "fast recovery" simply because of switching to the top element as they cooled off.