Hot water heating boiler pressure & temperature setting FAQss:

Questions & answers about where and how and to what numbers the pressure & temperature are set or controlled on hydronic heating boilers.

This article series answers most questions about all types of central heating system controls in order to aid in troubleshooting, inspection, diagnosis, and repairs.

FAQs on Operating Pressures & Temperatures of Residential Hydronic (hot water) Heating Boilers

Question: effect of furnace fan limit switch not working & other worries

(June 21, 2012) abbas said:

what the problem if furnace high switch not working

(Sept 20, 2011) John said:
On my 10 year old hydronic residential boiler, I replaced the feed water regulating valve and backflow preventer, and relief valve. On a well water supply these parts were boogered up. I adjusted the new feedwater psi to 12lbs. My expansion tank had 28lbs?(don't know why) I reduced the tank pressure to 12lbs as well. I didn't notice any water coming out of the schrader valve, so I assumed the tank (bladder type) was ok.

The boiler has a tankless heater coil for domestic hot water. My perimeter heat has come on only twice so far. Since firing the boiler back up I noticed the pressure has creeped up from the cold 12psi to around 22psi. This is over two weeks now. My aquastat settings are 160 and 180 with a 15 degree differential. And these settings are working fine. My relief valve weeped prior to the adjustments with the feedwater valve and expansion tank pressure.

My concern is, at boiler shut off temp of 180 will the boiler pressure level off close to 30psi. Again, I'm at around 22psi now. The expansion tank was the only external componant not replaced. Thank You, John P

(Apr 1, 2012) Muhd Nazlan from Malaysian said:
Hallo sir..i just want ask about what type of this gauge can measure pressure and temperature.

Reply:

Abbas the problem with a Hi Limit switch that is not working is that the furnace is unsafe and can overheat, damaging or even ruining the heat exchanger.

Question: automatic water feeder not working

(Oct 13, 2012) Frank B said:
I have a residential steam heater that once automatically filled the water when it became low. A service technician changed my water level sight glass. I saw that he closed or opened several valves. Now my boiler has to be manually filled. Can I send you a picture for you to see what valve should be open or closed?

Reply:

Use the email found at our CONTACT US link at page top or bottom to send photos and we'll be glad to comment further.

And see WATER FEEDER VALVE, STEAM

Question: steam leaks from steam heating system

(Nov 28, 2012) Tonda Ladson said:
When I turn on the boiler to heat the house, steam stars to come out of everyplace and it ruins the paint and things hanging on the wall. It has not always done this. What could be the problem?

(Nov 20, 2014) Anonymous said:
steam escaping my gas steam boiler

Reply:

Look for a steam vent that is not operating properly and needs replacement. If you see steam escaping from piping or from the boiler itself that's a different matter. In that case shut off the system and ask for help from a trained steam heating service tech.

Question: boiler turns on but no hot water at faucets

(Feb 25, 2013) Ed said:
Hello, My hot water boiler kicks on fine, however I do not get hot water at the faucet when turn it on. The water comes out warm for a minute and goes cold quickly. Any advice will be a great. Thank You Ed.

Reply:

x

Question: Lochinvar model rbn 135 serial d029094 settings

(Feb 16, 2014) henry said:
lochinvar model rbn 135 serial d029094
my operating temp dial is set at 180 and my hi limit dial is set at 200
the boiler temp only gets up to 130-135 then kicks off untill it reaches 120 then kicks back on psi is steady at 20 all the time
have new circulator pump new expancion tank new auto fill valve

Reply:

Henry

Check the aquastat settings: someone may have set the LO limit too close to or even above the HI limit. If the settings are correct then I suspect a control or control sensor problem.

Question: test to see if the aquastat control has a bad sensor?

(Feb 16, 2014) Anonymous said:
is there a way to test control sensor problem

(Feb 16, 2014) henry said:
i have 24 volts at each dial when burner is firing is there any other test i can do with a meter to see if this a control sensor problem

Reply:

Interesting question, Henry, I don't know but a wiring diagram for your control might answer it. If you are talking about an aquastat on a heating boiler, typically there is a boiler temperature sensor that is inserted into a well that itself projects into the boiler water; the manufacturer recommends that the copper tubing connecting the sensor to the aquastat be bent enough to assure good pressure or contact between the sensor and the well sides, and they also may recommend use of a heat conducting paste to assure good contact; If contact is poor (or if the sensor has failed) the control may fail to respond properly to the actual boiler temperature.

So my thinking starts by observing how the boiler actually responds compared to the gauge temperature.

Unfortunately to check the sensor mount and contact you'd have to remove the whole control - a lot of trouble so not what people would try first.

That's why I stressed the importance of looking at the HI LO and DIFF settings first

Question:

(Mar 15, 2014) momma said:
Hello, My hot water boiler kicks on fine, however I do not get hot water at the faucet when turn it on. The water comes out warm for a minute and goes cold quickly. Any advice will be a great. Thank You.

Reply:

Mom,

To answer your question I'd need to understand more accurately how your building provides hot water (at sinks etc): is it by

- a separate, stand-alone water heater, and if so how is that heater powered: gas, oil, electricity?

- a tankless coil on the heating boiler itself (probably your case) ?

- an indirect water heater (tank) heated by a loop of heating water circulated from the heating boiler?

Question: boiler pressure problems, gauge readings, leaks at the relief valve: debugging & diagnosis

(Dec 15, 2014) Roger said:
My boiler pressure has been reaching about 32 and relief valve has been releasing water i replaced valve then noticed pressure was exceding valve pressure expansion tank is fill-trol 110 the air vent seems to be releasing water occasionally do you think it is the expansion tank.

Reply:

Roger

you want to see this diagnostic article

inspectapedia.com/plumbing/TP_Valve_Leaks.php

to get at the underlying cause: could be a bad expansion tank, pressure regulator, or other causes.

(Dec 16, 2014) Roger said:
turned boiler off and closed fresh water feed to expansion tank pressure was about 32 psi and stayed there for 2 hours left fresh water feed to indirect water heater open. water pressure from well does not exceed 72 psi when I turned boiler on pressure rose to 35psi and back down to 30 psi while temp rose to 175 and then shut off and cools down to 120 with 22 psi and no zones on.

Reply:

Roger pressure over 32 psi is already too high in the boiler. When the boiler is COLD the pressure would normally be around 12 psi, heating up to under 30 PSI at maximum boiler operating temp. Higher starting pressure is needed for taller buildings.

With boiler cold drop pressure to 12-18 psi. Watch to see if pressure rises while boiler remains off. I understand in cold weather when you want heat this can be difficult.

If the boiler contains a tankless coil shut water off coming into the coil to see if that is the leak source

(Dec 20, 2014) Roger said:
sorry its taken so long for me to get back i have changed the feed valve and air valve on top of separator but pressure still wants to remain around 30 my boiler is a allied mg-100 so if i can shutoff the boiler and close the return line coming into boiler i should be able to check indirect heat water tank for leaks
is that correct as the valves will release excessive heat will i have to take some pressure off the gauge first thanks for your patience.

Reply:

Roger

IF you set the boiler pressure to 12 PSI when the system is cold and leave the boiler off the pressure should not increase.

IF it does there is a leak into the boiler - sometimes from a leaky water feeder / pressure reducer valve or possibly from a tankless coil (or leaks into the boiler circuit heating an indirect-fired water heater) when the boiler has one.

The TP valve releases heat and pressure trying to avoid a dangerous BLEVE explosion.
inspectapedia.com/plumbing/BLEVE-Explosions.php

(Dec 20, 2014) Roger said:
when i drain the pressure and then open it the pressure goes to 20 psi should i have the return to the boiler open the expansion tank is new but must have 20 psi should i take some air out of it.

When I leave return open and boiler off the the pressure rises sounds like it might be the indirect fired water

Reply:

Did you tap the pressure gauge to see if it's sticking?

(Dec 21, 2014) Roger said:
No tapping on gauge doesn't do anything I shut off boiler about an hour ago and no movement still at 20 psi where it goes when i cut in the feed valve after draining

Reply:

(Dec 21, 2014) (mod) said:

Roger if the gauge pressure does not fall when water is removed from the boiler, it's probably stuck or debris clogged and needs replacement. Try measuring boiler water pressure at a boiler drain using an independent gauge.

Watch out for scalding burning hazards.

(Dec 22, 2014) Roger said:

gauge goes to zero when I drain boiler doesn't go up to 20 until i cut water back in

boiler just shut off at 175 degrees with 30 psi boiler cools down to 120 and now pressure is 27 boiler has been off for an hour and pressure is staying at about 27 i have replaced the expansion tank, feed valve, and air valve when i put new expansion tank in i didn't check air pressure as it was supposed to be preset amtrol filltrol but when i cut in water gauge went to 20 psi it seems that should have been 12 can't figure why psi only drops 2 or 3 psi after boiler cools down

Reply:

Roger you'll only see 12 PSI when the boiler is *cold* and your system, if your home is 3 or more floors tall, may actually need a higher starting pressure. So when the boiler is off for an hour its temperature is still pretty hot. Still I'd think pressure would drop more than 2-3 psi.

If the relief valve is not spilling (which would be a second indicator of high pressure or temperature) I'd move on to make an independent measure of the actual boiler pressure, taking care not to get scalded.

At inspectapedia.com/water/Water_Pressure_Measure.php we show how you can buy or make an inexpensive independent water pressure test gauge that would work on the boiler drain - provided your boiler drain can be opened and shut without leaking.

(Dec 22, 2014) Roger said:

the relief valve is spilling the house is a ranch with boiler in basement it really seems that the gauge is accurate the spillage doesn't occur until gauge reaches 30 which is valve setting

Can i close the return from indirect water heater and close zone valve to water heater to check for leakage from water heater into boiler circuit and turn boiler on it would be isolating the water heater from boiler

Reply:

Roger,

You should indeed be able to isolate the indirect water heater from the boiler - as the water heater too has its own pressure relief valve.

Question: monitor boiler pressures when water supply is shut off - to check for a boiler leak

(Dec 23, 2014) dean said:
Can the "cold" pressure go to 0 if the water intake has been shut off (i.e. new water can not be introduced). Trying to determine if we have a leak, so we have turned the water intake off to ensure no new water can be supplied to the system. Cold pressure starts at 12, but after 4 to 5 days it goes to 0. The boiler does run during this time. When it is running, the pressure is either 15 or 19 depending on pump speed:
- pressure is 19 when pump set to medium position (temp rises to 150 before boiler shuts off due to temperature setting on boiler)
- pressure is at 15 when pump set to high (water temp only rises to 108 in this case).

But after 4 to 5 days, the cold pressure goes to 0. I this normal? Can it be caused by something other than a leak?

Reply:

Dean

Cold pressure in a hot water heating boiler (hydronic) won't normally fall to zero unless there is a leak. Typically it'd be around 12 psi. A steam boiler pressure will drop to zero when cold.

I'd be looking for a leak.

Question: gurgling sounds in hot water heating baseboards

(2 days ago) roger said:
i get gurgling in my water baseboard units, when i bleed and fill system the relief valve leaks water starting problem over again what is happening?

Reply: check for and purge air in the hot water heating boiler & piping system

Roger

Usually gurgling means there's air in the system that has not been purged.

See inspectapedia.com/heat/Air_Bound_Heating_System.php for help with getting air out of the piping, baseboards, radiators. The bleeding you've done may not have been effective.

If the relief valve is leaking the system may be operating at too high a temperature or pressure or might be (more rarely) suffering from a water hammer problem as a circulator pump starts and stops. Or of course the TP valve itself may need replacement.

Start by purging air, then

Continue by checking for a water-logged expansion tank on your boiler.

Reader comments:

Mike Burke said:
Thank you for your detailed, informative articles. Your information has been very helpful to me in understanding, monitoring, and maintaining my hot water heating system.

Reply:

Thanks for the nice note, Mike. We work hard on this material to make it accurate, useful, unbiased, so I'm naturally elated when a reader finds it actually helpful. We also welcome content suggestions, questions, criticism. Working together makes us smarter.

Comment:

(Jan 8, 2015) Mike Burke said:
Thank you for your detailed, informative articles. Your information has been very helpful to me in understanding, monitoring, and maintaining my hot water heating system.

Reply:

Thanks for the nice note, Mike. We work hard on this material to make it accurate, useful, unbiased, so I'm naturally elated when a reader finds it actually helpful. We also welcome content suggestions, questions, criticism. Working together makes us smarter.

Question: water feeder feeds water to the tank after every cycle

(Jan 9, 2015) Joseph said:
My water feeder feeds water to the tank after every cycle at more than 3/4 of tank and I get a lot of noise ion some of the radiators ,Is there a problem with the feeder?

Reply:

Check for a leak in the hot water heating piping or if you are discussing steam, check for steam vents that are not closing down even when the radiators are hot.

Question: fed up with heating boiler troubles

(Jan 11, 2015) Roger C.S. said:
I have had it up to my ears with this boiler system! I have a 20+ year old Utica Boiler on a two story house. My pressure relief valve was leaking so i replaced it.....no improvement...I was instructed to look for moisture in the expansion tank. I pushed the air valve open on the expansion tank and and light (droplets immediately came out) amount of water came out, so i replaced it....no improvement. I moved on the the pressure reducing valve and replaced it and some of the surrounding piping (someone had used black pipe along with copper!!) all copper and brass now....no improvement.

The operating pressure is right around 30psi according the gauge. The temperature is around 180 degrees. The house is on a well and minerals in the water is common problem with everything else the water touches.
The next thing is the circulation pump right? It is the Taco 007-F5 and it appears to have been replaced at least once. However, I've not noticed the upper levels not heating correctly.
BTW this website is awesome your write ups are the best I've found thankyou!

Reply:

Roger

I understand the frustration but it sounds to me as if you need a competent onsite heating repair technician - the repairs you tried are incorrect even though in the right direction. Therefore it's no surprise that they were not successful.

1. "moisture" in the expansion tank is misleading. A waterlogged expansion tank - full of water and not enough air, can cause TP valve leaks. The place to check that is not by pressing an air valve.

2. Replacing the pressure reducer valve might work if the valve were defective and feeding high water pressure but replacing it without diagnosing that can be barking up the wrong tree.

3. The operating pressure at 30 PSI is too high and would be expected to make the TP valve empty. Moving on to the circulator pump is not a coherent step. You need to find why the pressure is too high. For example if your boiler has a tankless coil it could be leaking water at house pressure into the boiler.

Diagnostic suggestions for leaky TP valves are at inspectapedia.com/plumbing/TP_Valve_Leaks.php

Diagnostic suggestions that include a tankless coil leaking into a boiler and how to detect that are at inspectapedia.com/plumbing/Tankless_Coil_Leaks.htm

4. "not heating correctly" at upper levels is a bit vague but if you mean radiators or baseboards are not getting hot the system may be air-bound, possibly a separate problem

5. Back to the circulator: it's a bit obscure but conditions that cause water hammer in any water system including a heating boiler, can cause TP valve occasional leaks. That could be (but is uncommon) a sudden stop of water running anywhere including a circulator.
See inspectapedia.com/plumbing/Water_Hammer_Noise.php

Let us know what you find as that will assist other readers.

Question: OK to shut off water to the boiler while on vacation?

(Feb 24, 2015) mike said:
while on vacation I was going to shut the main water valve off. will this be safe to do with my boiler in operation

Reply:

Yes and no, Mike.

IF IF IF there is no boiler leak anywhere then the boiler can run ok for a while with no guarantee of make-up water.

Watch out: But should something go wrong and the boiler lose water, a melt down, or blow up could ensue. I'd prefer to shut off water to other parts of the house but leave water to the boiler on, even if that means making some plumbing changes.

Question: eaks at my circulator pump flanges due to poor choice of metals?

2015 - Valerie asked: are leaks at my circulator pump flanges due to poor choice of metals by the manufacturer?

Reply:

Valerie,

We discuss leaks at circulator pumps and flanges in detail at

inspectapedia.com/heat/Circulator-Pump-Leaks.php

It would be most unusual for a plumbing valve failure to be Dir to the manufacturer's choice of metals. Typically a valve body is grass with stainless steel shaft. Dezincification can explain leaks at brass fittings on some systems, depending on water chemistry, but not at the flanges of a circulator.

Question: not enough hot water for showers when the heat is on

(Apr 28, 2015) Patty said:
What would be the reason for only getting hot water long enough for a shower when the heat is turned up?

Reply:

Patty the answer would depend on how your hot water is being made. If for example your hot water is made by a tankless coil on a heating boiler, residential boilers with tankless coils do not provide endless hot water.

Further if the aquastat controls HI LO and DIFF are not properly set that'd make matters worse.

See inspectapedia.com/heat/Aquastat_Settings.php

Question: banging pipes after a popped PRV

(Dec 3, 2015) Jeff B said:
Have a 25 year old hot water boiler in basement of 2 story house. 150K BTU unit. Aquastat is Honeywell L8148A. Recently had issues with unit "popping" the PRV. Technician replaced a number of parts, and all was working fine, until recently when knocking in expansion tank was noticed.

This was followed by banging in water pipes and again "popping" of the PRV. technician found the Automatic filling valve at 0 #s and the high limit on the aquastat above 210 degrees (don't ask me how either got set that way), but that's what he told me. He reset autofill at 7#s and high limit on aquastat at 150 degrees.

I now have the boiler cycling between 140 and 150 degrees, no hot water making it to the upper floor, minimal heat to the main floor and limited hot water (I've a 41 gallon hot water maker fed by the boiler). From what I'm reading above, seems I should take the incoming water to a minimum of 12#s and move the high limit to 180 degrees?

Reply:

Jeff:

I suspect debris clogging; search InspectApedia.com for "BANGING PIPES & RADIATORS" to read details.

Question: what temp should a steam heating system be set at

(Dec 23, 2015) Dexter said:
At what temp should a steam heating system be set at had a pipe replaced an now it seems like the water takes longer to get hot thinking the repairman turned the temp down an now seems like the water is taking longer to shower

Reply:

Dexter:

Steam is made by boiling water - so at sea level that's 212 degF or 100C. It's not adjustable.

What is adjustable is the room thermostat or individual radiator thermostats, or steam system pressure - none of those have to do with the time for hot water to arrive at a shower: that's hot water, not steam, and is a different system.

If your hot water is made by a tankless coil mounted in the water reservoir of your steam boiler the coil may be clogged.

Question: hot water boiler is cycling on and off.

(Dec 31, 2015) Mike said:
My hot water boiler is cycling on and off. My pressure reads 18 psi and my water temperature is 120 when it goes off. My high limit is set to 180. What would cause this to happen?

Reply:

Mike I don't have enough detail to diagnose your heating system; I'd ask for help from a heating service tech. She may find a problem with the burner (fuel is not stated) or a control.

Questions:

(Jan 10, 2016) kam said:
My boiler's temperature is running at 2.2 bars. I have been told to keep it at around 1.6 bars. I don't know how to adjust it
Could you explain and advise me what to do please. Many thanks.

(Jan 20, 2016) Jim Finizio said:
At what temperature should the loop circulator be set at in a residential oil fired hot water mono flow heating system?

Reply:

Jim I don't understand the question; if you mean heating zone or hot water baseboard or radiators piped in a loop, the circulator is controlled by a relay that is in turn controlled by the room thermostat not by a local temperature setting.

Question: how fast I can get 1 bar of pressure in my water heating tank ?

(Oct 9, 2016) Poopathi said:

I have 10 liter capacity tank heating system..

So , I top the water until 8 liters
Below I have 3 kw heAter
Start the heater until 1 bar
Top area 2 liter volume I have
My question is how fast I can get 1 bar?

Reply:

Poopathi,

I don't fully understand your water heating system, but from just your note I don't think the pressure you seek is reasonable. If you have a closed water tank in which you plan to heat water, and if you do not fill the tank completely, then heating water in the tank will produce only a very small pressure increase in the tank, almost below what you can measure, UNTIL you boil the water, producing steam that in turn raises pressure in the area of air above the water. This is not practical nor is it safe for a water heating system - too hot, too dangerous.

Also your question omits some necessary data: the temperature of the incoming water, the starting pressure inside the heater (assumed 0 or 1 Atmosphere at sea level about 14.86 psi but reading "0" on a typical pressure gauge)

The article above describes pressure increasing in a fully enclosed and totally filled tank of water that is then heated.

For a FULL and ENCLOSED water tank, here is the formula describing electrical energy in watts, time, and water temperature rise:

Rated Hot Water Heat Output in Gallons per Hour = (KW x 3413) / (Temperature increase x 8.25)
or
Rated Hot Water Heat Output in GPH = (KW x 414) / (Temperature increase)

A 3 KW heater element has a rather modest heat output rate. If we have a single 3 KW heating element the calculation is

3 KW x 3413(constant) = 10,239

Assume: incoming water temperature of 55 degF.

Assume we want (because we're crazy and don't understand the danger) to boil our water: we need to raise its temperature to 212 degF.

212-55 = 157 degF; - necessary temperature rise
157 x 8.25 = 1,295.25

Now we divide:

10,239 / 1,295 = 7.9 GPH - we will require one hour to increase the temperature of 8 gallons (30 Litres) of water to boiling.

Next we calculate how much time will be required to heat up 8 litres of water to boiling, assuming sea level and the incoming temperature I gave.

8 Litres = 2.11 gallons

2/8 = 1/4, so we need about 1/4 of an hour or 15 minutes to boil 2 litres of our 55 deg water using a 3 KW electric heater.

Now we have JUST STARTED boiling the water. How long will it take for steam pressure above the water in the tank to increase pressure in the open 2 litre space to 1 bar?

1 Bar of pressure = 14.5 psi

Starting absolute pressure above the water = 1 bar at sea level
Starting gauge pressure above the water in the tank = 0 gauge pressure

To get to 1 bar of gauge pressure in the enclosed tank you will need to produce enough saturated steam in the 2 litre volume to increase the pressure to that level. (13 cu.ft. per pound) See Keenan and Keyes, THERMODYNAMIC PROPERTIES OF STEAM, by permission of John Wiley & Sons, Inc.)

Question: boiler is exceeding 30 psi

Michael asked a question that I accidentally deleted. Something like: "Why is my water heater pressure going over 30 psi - I start it at 19 psi cold"

Reply:

I'm confused between heating system pressures and water heater pressures in your discussion, Michael.

If your boiler pressure is at 19 PSI when the heating system is cold, and heat is turned on and pressure gets to or over 30 psi then you may need to lower the operating temperature of the system for safe operation.

But first I would look at the expansion tank. If your expansion tank is too small or if it waterlogged that'd cause spillage at the TPR valve.

The domestic hot water system (DHW) is operating at a completely different pressure range and uses a different TPR valve accordingly. For water heaters, some useful diagnostics on pressure and TPR spillage are at inspectapedia.com/plumbing/Water-Heater-Relief-Valve-Repair.php

A water heater going to 182 deg F is unsafe unless you're also using an automatic mixing valve/ anti-scald valve to protect from scalding at the plumbing fixtures.

Question: why does the gauge on my steam boiler go all the way up to 18 psi

(Nov 29, 2016) joe said:
my pressure switch is set at 2 but the psi gauge goes all the way up to 18 psi - my pressure switch is set at 2 but the psi gauge goes all the way up to 18 psi is this normal

Reply:

Joe: on a normal residential steam boiler your operating pressure will be between 0.2 psi and 0.5 psi.

See STEAM BOILER PRESSURE for details.

...

Continue reading at PRESSURE & TEMPERATURE SETTINGS, CONTROLS or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Recommended Articles

• STEAM BOILER TEMPERATURE
• AQUASTAT CONTROL FUNCTIONS
• EXPANSION TANKS, BOILERS
• GAUGES on HEATING EQUIPMENT
• PRESSURE GAUGE, BOILER
• PRESSURE CONTROL, STEAM BOILERS
• RELIEF VALVES - TP VALVES
• RELIEF VALVE, TP VALVE, STEAM BOILER
• STEAM BOILER PRESSURE
• STEAM PRESSURE GAUGE
• WATER FEEDER VALVES, HYDRONIC BOILER

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PRESSURE & TEMPERATURE SETTING, CONTROL FAQs at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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INDEX to RELATED ARTICLES: ARTICLE INDEX to HEATING BOILERS

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