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jmon 12-01-2012 08:03 PM

hi temp boiler cutoff
I notice my hi temp boiler cutoff is not working correctly on my hot water boiler (see pics 14,15,16). When I have it at 185 it shuts off at just over 200. yikes! water is boiling and pipes banking. Now I have it set at 175 and bolier shuts off at about 182 or so. No pipes banking, psi normal. I had a simalar problem with this on my hot water heater. Same thing, hot water heater went over the cutoff temp. I had to replace the thermo control unit, it was so loaded with all kinds of sediment. No wonder it couldn,t detect the correct temp. and yes, I do have hard water here. Can the same thing be going on with the boiler? Boiler is over 26 years old, and these have never been replaced. If you look at the pics you will see two sets of heavy gauge cooper wire going from boiler control box into the heat exchanger. one is the hi temp cutoff and one is the lo temp cutoff. I would just like to replace the whole thing like I did with the hot water heater. I wonder how hard it would be to just replace both? and how would you disconnect them from the heat exchanger? They have some kind of clamp going over the heavy gauge cooper wire which is connected threw the socket bolt where it connects to the heat exchanger. I can buy brand new hi and lo temp cutoff for about 175.00 dollars at pexsupply. Or is this a job better suited for a professional hvac technician? Any suggestions would be greatly appreciated. Thanks in advance.

Pics here.

rootboy 12-01-2012 10:39 PM

There are some things that you should go to a pro for, this might be one of them.

However, you could take the old unit off and replace it with a new one and you should be good to go.

Which one is it? The one on the left, or the right? The one on the right looks like it is rusted in. Hit it with penetrating oil daily for a few days (weeks?) before trying to remove it. If you decide to go with a pro, then do the same for him. You don't want him breaking off the nut and having to drill it out.

First go to the auto store and buy yourself a deep socket the same size as the gland nut (what your tube goes into). Introduce it to a grinder (better yet a Dremel) and cut a slot from the end that goes onto the nut up to where the copper tube will clear the nut when the socket is put on the gland nut (take a look at an O2 socket for a car to get an idea of what I'm talking about).

Open up your boiler (after it has cooled down) and remove the switch from its bracket and disconnect any wires connected to it (remember where these go).

Carefully bring out the switch making sure not to crimp the copper *tube* (it's not a copper wire, it's a tube, and if you crimp it by bending it too tightly you will ruin it). Having an assistant hold the old unit clear and having him/her rotate it as you loosen up the nut (bolt, whatever) will keep you from ruining the tube. Getting the old one out without destroying it will be good practice for putting in the new one without ruining it. :)

Put in the new one, using teflon tape *only* if it is recommended (I don't see why not), otherwise use pipe dope.

Mount the new switch where the old one was and reconnect your wires.

You did take this opportunity to flush out your boiler?

beenthere 12-02-2012 04:45 AM

Wells may or may not be corroded. might just be the control is going out. Those are insertion probes. Remove the clamp that keeps them in and they should pull out somewhat easily. That boiler is very sensitive to wter flow, if too low it will over shoot set temp and it will make lots of noise.

Canucker 12-02-2012 05:46 AM

beenthere makes a good point. You did get your zone pumps running recently, does it overshoot when only 1 zone is calling for heat? Its possible you may have to change the piping to a primary/secondary layout to rectify it.

old_squid 12-02-2012 09:44 AM

Your pictures and description didn't give an indication of brand or model#, but the unit looks like a Laars, Peerless or Raypak fin tube type boiler. This is what they called a high efficient unit when it was made. When was the last time it was serviced and cleaned? The arrangement inside the boiler just inside of where the pipes connect is tubes with fins on them spaced very close together. The heat from the burners passes through these fins and the heat is transferred to the water.

What may be happening is that over time scale and rust has fallen on top of these fins (happens from the outside ones first and works it's way to the middle usually) or the passages for the flue gasses are becoming blocked. The sensors you're talking about are on the outside edges and the thermometer is in the middle. If blockage is the issue then the outside edge sensors are going to be less accurate at "seeing" the average temperature across the heat exchanger than the thermometer.

If these type of boilers are not cleaned regularly the blockages will cause the center fins to deteriorate and the efficiency of the unit will drop, sometimes drastically. I'd suggest you have a service company come and do a cleaning and to check over the entire unit.

jmon 12-02-2012 10:12 AM

You all make very good points. Thanks for the great info.

I never had any problems with this until recently. It started doing this right after i put in two new honey well 4110D thermostats. one for each zone. When I replaced the circulators the banging and humming stopped. I don't know if that would effect it any. I know the circulators i replaced where old, one was bad - but i figured I would just go ahead and replace both of them while i was there. But it appears it is overshooting cutoff again. that's when i hear the banging and loud humming as the water is close to boiling. As we all know steam and water don't mix.

Another interesting thing is when zones first call for heat, i sit there and watch the gauges. It shuts off at the cutoff setting like it's supposed to. Everything is working fine. Its when the system has been on for a long time, really cold nights etc. something goes out of wack. If i hear the loud bang and humming I know it's overshooting cutoff. I throw the kill switch and let boiler cool off. So the cutoff switch does appear to be working - at times, something is making it overshoot the cutoff setting? right now I have the hi cutoff set at 175 and lo cutoff set at 150. haven't heard any banging or humming as of yet. But now we have a warm up. Have to wait for a really cold night to test it out.

The thermostat settings came standard with (AIR) adaptive intelligent recovery on. I turned that off. standard with gas furnace less than 90% efficient. selected and set to 5 cph. I tried that first, still overshoots. I change that setting to hot water and furnaces with 90% efficient or more. I called honeywell they told me to set it for hot water and set cph to 3, because that's what i have. I have a boiler not furnace. I'm heating water not air. i could try putting the old lux ones back on and see if that stops it. i don't see how that would matter though.

Maybe you are right here, boiler is not getting enough water? When I drained it to replace the circulators, I shut off my isolation valves and only about a gallon of water came out. I found that kind of strange - seems like that exchager should hold at least 2 or more gallons.

Could it be the water pressure valve where the water is coming into the boiler? That has never been replaced. maybe that is getting clogged with sediment? thanks.

jmon 12-02-2012 10:31 AM


Boiler is a lochinvar, mini fin, RBN135, 135,000 btu's, 1987, high efficient (for the time) with cast iron heat exchanger.

Boiler was serviced in october. I watched the technician, he cleaned all fins and took apart the entire firing mechinism. He replaced ingnition cable, ignition/pilot and burners. Cleaned fins with a wire brush, air compressor and vaccuum. he tested everything and everything worked fine. He said heat exchanger looks great and in good shape - he had me look under there to see it. He said over time eventually they leak and when that happens it's time for a new boiler. He said he seen some of these old cast iron boilers last as long as 40 , 50 years. He also said, they are not very efficient for todays standards - but they can last a long time if serviced properly each year which I have done annually by a certified hvac technician. I have to say he did a great job - boiler looks great.

ben's plumbing 12-02-2012 12:51 PM

you could also install a strap on aquastat to control high limit...a very easy project fo a diy.....ben sr

beenthere 12-02-2012 02:32 PM

Does it do it when both zones are calling for heat, or just when only one zone is calling for heat.

jmon 12-02-2012 02:44 PM

mainly when the furthest zone from the bolier calls for heat (zone 2). Although the other one (zone 1) does it as well. not as much. when both zones running at the same time it doesn't appear to happen as much. I will have to observe this the next cold spell we get. I will write down when it overshoots cutoff. thanks

jmon 12-02-2012 02:50 PM

ben's plumbing, thanks. How do I install strap accross aquastat. Can you buy that at pexsupply? Where can you purchase one? thanks

ben's plumbing 12-02-2012 03:25 PM


Originally Posted by jmon (Post 1065053)
ben's plumbing, thanks. How do I install strap accross aquastat. Can you buy that at pexsupply? Where can you purchase one? thanks

ask the counter salesman if they have aquastats...they should....we strap it over the feed line comes with a strap....strap it on pipe.. set to high limit temp...job done.. sometimes gauge is temp rise on boiler gauge when it reaches 180...adjust aquastat to that setting....ben sr:thumbsup:

Canucker 12-02-2012 06:09 PM


Originally Posted by jmon (Post 1065051)
mainly when the furthest zone from the bolier calls for heat (zone 2). Although the other one (zone 1) does it as well. not as much. when both zones running at the same time it doesn't appear to happen as much. I will have to observe this the next cold spell we get. I will write down when it overshoots cutoff. thanks

Jmon, it sounds like you have too little flow through your heat exchanger. Were your replacement pumps multispeed? You probably have some repiping work in your future, from the sounds of it.

jmon 12-02-2012 06:23 PM

the new circulator pumps I installed were taco 007-f5-ifc. the old ones were taco 007-f5. The difference is the ifc (some kind of check valve that only allows water to flow one way). As far as multispeed - i don't know. The old 007 did not have this feature, and water was able to pass both ways and also pass even when the pump was off. The newer ifc ones do not allow this. Basically the hot water stays in the pipe longer for more even heat throughout the house. They seem to work very well - It does keep the house more even with heat - no cold spots in the house as there was with the old 007 pumps. thanks. maybe some plumbing work maybe in my future. i'll keep you posted let you know how things are working. Right now we are in a warm spell. got to wait till that temp stays below freezing all day and at night it gets down right cold, like we had a week ago. bottom line, if you look at both pumps stats the old 007, pump more than the new 007 w/ifc. But the new 007 w/ ifc seems to keep the house warmer.

I tend to agree with you that there may not be enough water getting into the heat exchanger. I have looked at the exchanger. by the size of it - it's not very big, maybe holds a gallon to gallon and half of water i'm just guesting. like I said when I drained it to replaced the circulators, I closed my isolation valves, I drained out maybe a gallon if that.

beenthere 12-02-2012 06:26 PM

Those IFC circs also don't move as much water as the old non IFC circs did.

Your system probably already has flow checks on it, and doesn't need IFC cirs.

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