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OK, lot's of folks have furnaces but lots also have hot water heating systems using boilers. And boilers are a very different animal. Although they provide some of the most comfortable heat using "radiant heat" they have several uncommon parts like a tridicator (see photo), an expansion tank, an aquastat and the list goes on.

Although some boilers use ignition systems similar to a forced air gas furnace, boiler systems are more complex and have their own issues, like noisy pipes, inconsistent heat, leaks and other fun stuff to deal with in the winter.

But I've got it all explained for you in the tutorial Troubleshooting a Hot Water Boiler complete with diagrams, and troubleshooting tips.


January 20, 2008 at 11:36 am
(1) sam says:

Thanks Bob! I’ve got a system with caste iron radiators and baseboard combined. My big issue in the winter is fear of freezing if I go away and the power fails )or the oil line freezes, etc. What kind and how much and where do you add antifreeze? Is there another alternative short of compleat system drainage?

January 21, 2008 at 9:31 pm
(2) homerepair says:

Hi Sam,

Putting anti-freeze in a boiler system can be done but is not really recommended. It adversely affects the system’s efficiency and equipment lifespan and is a maintenance headache. You can read many opinions about it here: http://inspectco.com/PDFS/How%20to%20add%20Antifreeze.pdf

Would be a lot easier just to have someone check on your house periodically when you’re away.


September 26, 2008 at 11:59 am
(3) K says:

This is interesting. We have a different problem. Our furnace/boiler is of course for our heat and also for our hot water. Periodically there is no hot water (and in the winter sometimes no heat). We just flick the on/off switch and it starts up and begins creating both. Just trying to figure out the cause.

October 2, 2008 at 9:23 am
(4) Aaron says:

K, I have the EXACT same problem right now! It took 5 flicks of the switch this morning to get it to kick back on. I am trying to troubleshoot it as well…

October 5, 2008 at 5:58 pm
(5) K says:

Aaron, we had our gas company come out and they said there were loose wires in the aquastat – so for now we hope that was the problem.

flicking the switch causes vibration and the wires can slip into place –

good luck

October 13, 2008 at 9:11 am
(6) K says:

Well loose wires in the aquastat wasn’t the problem. No hot water again yesterday. Any helpful hints are appreciated. Thanks.

October 30, 2008 at 5:19 pm
(7) Joe says:

My hot water base boards are making strange noises when the heat is on. Does this mean I have air in the lines, and if so, how do I remove the air.

December 24, 2008 at 7:50 am
(8) John Cox says:

I have boiler with 3 zone manifold. When zone 1 & 2 are open and zone 2 (second foor ) closes it sends shock through out the house with pipes banging. Please advise possible solution.

January 9, 2009 at 6:26 pm
(9) Jim Davenport says:

Ok, big question…

I have no heat in my hot water system it’s a 2 zone and feeds 16 rooms on one floor. The boiler heats the water just fine but no heat in the base boards. The feed line is hot until the just before the first room. The return line is cold. I have followed all the steps in these artical but still no heat.

Here is what I have done so far.
1 – Flushed all base boards. got some air in doing so. Still no heat.
2 – Flushed the boiler, got some air in doing so, still no heat.
3 – Pump is working, can hear it. Just replaced 6 months ago.
4 – expantion drained.
5 – zone switches are working.
I still have no heat.
3 – Help freezing here….

Regards, Jim

January 21, 2009 at 7:15 pm
(10) bob says:

my boiler has had several problems. circulator pump and aquastat to name a few. now the pipes bang and the service people tell me bleed bleed bleed with no results. it helps for while then returns hope you can help

January 23, 2009 at 12:40 pm
(11) joseph says:

My boiler occassionally starts a “sighing” noise. We have a bungalo with all radiators on main floor only. You can hear this noise in the radiators (cast iron from 1950′s). I have drained excess water from the expansion tank and the sound occurs on startup. There is also a groaning sound on occassion that may last for 10 minutes and then goes away. Please help.

January 24, 2009 at 5:58 pm
(12) craig says:

just installed new hot water heater, had to replace shutoff valve also, so i turned off main water. must have gotten air in lines zone 2 makes a gurgling sound now when water flows thru. how can i get air out

January 24, 2009 at 8:26 pm
(13) homerepair says:

Here is a link on how to drain your system and recharge it.

January 27, 2009 at 11:39 am
(14) Kristin says:

My house seems to take forever to heat up. What is normal for gas hot water heat? I have a two story townhouse with only one thermostat per floor. I usually leave the top floor at 60, but I will warm up the bottom floor to 70 or 72. I usually leave it at 62 at night or during the day when I’m gone. When it gets cold out (anywhere between 20 and -30 degrees F) it will take about 5 hours or more to warm the bottom floor from 62 to 70. Sometimes it never seems to get above 68 even though the thermostat is at 75 or 80. Do I just need to keep my thermostat higher at night and while I’m at work?

February 22, 2009 at 7:55 am
(15) pete says:

when the furnace comes on i hear girgiling in the lines.with no pressure in the furnace.

February 27, 2009 at 5:29 am
(16) aaron vincent says:

I have a m-80 boiler unit and the thing shuts down on flame failure. It starts with white smoke, runs for a couple seconds and the shuts down. We cleaned the fuel system, and are setting the electrodes at 1 nickel thick, 1/2 nickel wide in front of and above the nozzle.

June 21, 2009 at 4:01 am
(17) sandi says:

hi, bob nice to know you, i have litlle problem in my boiler, my boiler type is combination fire and water tube, i using full condensat for FWB, my alkalinity and TDS, pH very low,and un stable what your suggestion, thanks

July 8, 2009 at 10:29 pm
(18) isabel couvrette says:

I live in a 5-story apartment building, It uses a boiler for hot water. I live on the 4th floor. All my neighbors seem to have hot water just fine. I have to let my water run for about an hour before it gets hot. Then other times, it gets hot right away. Do you happen to know what the problem might be? My maintenance man doesn’t seem to know what the problem is>
Thank you in advance

October 1, 2009 at 11:04 pm
(19) Jon says:

had a dripping pressure relief valve, found excess water in expansion tank, So I replaced the exspansion tank pressure relief valve and the water feed valve to cover it all, when I started to fill the unit the water blew out of thwe new pressure relief valve wanting to flood the area, checked this many times same sitaution and all with the boiler switch turned off, could the new pressure relief valve(30 psi rated) be defective?

October 25, 2009 at 11:09 pm
(20) Dennis says:

My circulating motor will run for 15 sec. and shuts down and it will kick back on and run again for 15 sec. and shut down. It will do this for about 20 to 30 min. then it will run until it reaches the thermostat temp. I have wired the motor direct and it does the same thing run 15 sec. and shut down. I have also tried a different motor and does the same thing.

November 23, 2009 at 2:27 pm
(21) Kelley says:

Sounds silly I know, BUT do boilers have filters that need changed regularly?
Single mom with no clue..


February 12, 2010 at 7:28 am
(22) richie says:

my exspantion tank has a diapham but is mounted side ways is this ok

February 12, 2010 at 9:17 am
(23) homerepair says:


Although not very common, the tank may be mounted horizontally depending on the manufacturer. For example http://www.bellgossett.com/literature/files/7516.pdf states its tank may be horizontally or vertically mounted.

February 22, 2010 at 9:35 am
(24) jamie says:

We are having a problem with our hot water boiler. It will not shut off once irt reaches the right temp., or it might not kick on when the temp. is lower than the set temp. Anyone know what this may be? They have replaced the thermostate, but that didn’t fix the problem.

March 18, 2010 at 8:05 pm
(25) Gail Myers says:

I want to heat my workshop and my basement with a Electric boiler. The shop is 495 square feet and my basement is 330 square feet. What type of boiler would you recommend. I would like a wall mount if there is one.

August 20, 2010 at 5:49 am
(26) Kevin says:

I had a problem when the hot water comes on, my boiler would make a whining noise. I phoned up my local company and they said it was due to air being trapped. They were able to fix the problem quickly. Hertfordshire

October 9, 2010 at 12:24 am
(27) chuck says:

ok, so I just bought a new house (old….. new house) and the oil – fired boiler (heat only, not 4 consumable hot water) only turns on with thermostat, has two aquastats, one for boiler temp. and one for circulator pump…. however, the boiler aquastat only works for high temp. shut-off. The pump aquastat is identical and does the same, it turns the circulator pump on when water temp. reaches about 150, as the water circulates (less than 1 min.) temp. drops and pumps shuts off, should I invest in new aquastats with both high and low cut on/off for both boiler and pump? I’m very confused but know that this system has some issues… pls send some advice

October 14, 2010 at 8:03 pm
(28) ramon bejar says:

hi, i have a boiler from one of the apartement building i rent, and everytime i turn the pilot on and put it on warm, and when the boiler tries to turn on again to start heating water it shuts off. i changed the thermostat valve but i dont kno ehat the problem might be

January 29, 2011 at 11:11 am
(29) shaw says:

I can say Bobs” web site is very good information to be shared with many issue on heating, I have good question, I have a hot water circulator pump seem to be never be able to shut off. I do not under stand fully how its true function of the pump how does it operate? When the boiler heating up the motor is running and when the flame reach it tempter the flame goes out and the pump yet is running. When I lower the thermostat down to 45 the motor does shut off some of the times, rooms” I have replace thermostat add new expectation tanks it old very good pressure.

March 6, 2011 at 11:07 pm
(30) nicholas says:

I have a natural gas boiler. The pilot is on, the pumps are running, the thermostat (heat only) has kicked on, BUT the burners don’t light up. What the heck!

November 3, 2011 at 10:55 am
(31) Tracy N. Smith says:

Hello , I am a self employed electrical contractor. But , this question regards the old American hot water boiler in my own home . It cycles on and off , and the pressure relief valve leaks water , the expansion tank is not hot to the touch and has water in the bottom. The pressure goes up to 30 and the system gets too hot . I let the air out of the lines at all radiators as always each fall . I thought that it was a thermostat problem original Minneapolis Honeywell 3 wire (red,white and blue terminals with corresponding wires) number T11B6A91A1 , but I now believe it may be my relay coil in the basement . I read only 7 volts across the red and white terminals both at the thermostat and down at the contacts of the coil . Would the low voltage ( I believe it needs 20 to 24 volts) cause the unit to cycle on and off and cause the pressure to be high and water relief valve problems? Relay is Minneapolis Honeywell no. R19A7K-3-115-60 120 volt relay which feeds the circulation pump and the immersion temp. probe and the gas valve from a junction box . Thank-you T. N. Smith Elect.

November 8, 2011 at 9:29 pm
(32) Christopher A Augustine says:

Tracy- How Hot is the temperature?Do you have one of the old style expansion tanks with no diaphram? If you do have the old style tank , try draining the old expansion tank 1st. That will probably sove your problem.

December 7, 2011 at 6:36 pm
(33) Adam says:

In a apartment building 7 floors what should the heating boiler temp be set at?

January 21, 2012 at 4:37 pm
(34) robin says:

I have an oil fired hot water boiler. When heat is on, water gets cold. Water temperature is fine if heat is not on. What could be the problem?

Thank you.

February 6, 2012 at 5:33 pm
(35) kevin says:

Hi just installed a wood fired boiler and got the temperature up to 180. The problem i`m having is a circulation problem. Do i have air in the lines if so how do i move it.


March 7, 2012 at 9:24 am
(36) Lee says:

I have a three zone hot water boiler system with baseboard registers.The upstairs zone will not turn off. I changed the batteries in the Thermostat and turned the zone off, but It is still 72 degrees in the zone.
I tried turning the power off to the boiler to try to reset the boiler but problem still exists. It seems that if I turn all of the zones down to 60 degrees the upstairs will turn off.
Any ideas?

September 16, 2012 at 7:52 pm
(37) Jerry Smith says:

Hi, My tank ran out of gas. I have had it filled after about a year and one half. Used wood all last winter. after filling tank I cannot get it to light. It is a Sentry slant fin, model S-120 EDP. I pulled a female hex pipe plug out of thegas control valve where the gas enters it, and verified that it was getting gas to that point. The electronic lighter keeps firing and it doesn’t light. I can hear the lighter clicking as it tries to light.
What can I do?

November 1, 2012 at 6:19 pm
(38) Bill says:

I have a natural gas boiler with hot water baseboard heat. I have lived with this 45 year old system for 25 years. For the first time in all those years this fall I am hearing a very loud contant flow of water though the pipes. This cant be normal if I haven’t heard it this severe before. Is there something a home repair guy can do to fix the problem or do I need to call a professional?

January 6, 2013 at 2:25 pm
(39) Larry says:

I have a 3 zone hot water heater in a 3 floor house. when the 1 floor is turned down the pipes to that zone are cool (good). when the 2 floor is turned down the pipes to that zone stay hot, WHY. when the 3 floor where my mother lives is turned up for heat the pipes in that zone only get hot when the boiler is on but as soon as the boiler cycles off they get cool until the boiler cycles back on. Is there something wrong with my boiler. thanks in advance.

January 12, 2013 at 9:24 am
(40) Michael T says:

I have hot water heat in Colorado. We’re leaving for 4 months. I’d like to turn the water to the house off and leave the furnace on at a low level. How often would the water have to be turned on so the system would be OK?

April 9, 2013 at 11:11 am
(41) Gerald says:

I have an older American standard hot water furnace that burns on propane. about 60 % of the time when it comes on it is nice and quiet. And then it comes on with a loud bang ( Like an explosion ) and the burners sorta roar. We have had the control replaced and the pressure valve on the propane tank replaced. But we are still getting the explosion sound at times. What do we need to look at for repairing it.

September 2, 2013 at 11:51 pm
(42) betty says:

i have a keystone rice coal stoker, the big old green one i have the heat turned off for the summer and sometimes heat comes through the pipes, my house gets really hot and then my air conditioner never shuts off, why is heat coming through the pipes? i thought maybe i should turn off some kind of valve, but i do not want to do any damage to the system. please help

October 6, 2013 at 8:45 pm
(43) JOE says:

Have Dunkirk hot water furnace. Pipes going to Radiators stay hot when weather gets warmer and heats house when not needed. this only happens if cold spell comes on and heat is needed and then temps warm up again and heat is not needed. Circ pump is not running when this happens. Seems like convection of heat from furnace is going on. Furnace is also used for domestic hot water.

October 27, 2013 at 6:38 pm
(44) Joe Fondacaro says:

I have a gas fird hot water heating system with 3 zones or 1 per floor.
My no heat problem started with a broken pipe. The plumber drained the system, did the repair and turned the water back on. He eliminated air 3 times and set the pressure at 15psi . We have perfect heat on the top or 3rd floor, The cast iron radiators will not get hot on the 1st or 2nf floors. The circulators run but the water is not hot if you follow the pipe up. The boiler is maintaining temp at 190 degrees.
What is the problem here???????

December 31, 2013 at 1:00 pm
(45) Jim says:

New natural gas boiler. 2 zones. With both zones calling and one shuts down there is loud water hammer. If only one zone is on and turns off it is quiet. Zone vzlves are on the supply side. What can I do to stop this hammer?

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