Boiler randomly doesnt kick on

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Dkoss08

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if anyone could give some ideas that would be great. I have a hot water boiler system that sometimes does not kick on when thermostat calls for heat. It is a 2 zone boiler system with two thermostats. The damper is in the always open position. When not running the water pressure is between 10-20psi when running it’s 30psi. I cleaned the tip off from excess carbon and the pilot lights just fine once the gas starts entering the system. I checked the voltage when the thermostat called for heat across the high voltage and it was 0 volts. When I check the B1 and B2 terminals in the aquastat and I get a reading of 20V... I thought it would be 24V. Attached is some pictures of the schematic. I am not sure if it has something to do with the gas valve coil or the block vent safety switch or possibly a wire that has a bad connection that doesn’t allow the boiler to kick on when the thermostat calls for heat. It can work for two weeks fine and then it won’t, I can always get it to start again usually by powering down the whole system and waiting a few mintues but I would like to fix the problem so that I don’t have to worry about the heater turning off and not restarting.
 

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Dana

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30 psi is the pre-set temperatur of the pressure relief valves shipped with most boilers. If it's 10 psi when tepid, 30psi after it's been firing awhile, it's highly likely that your systems expansion tank has failed, or does not have sufficient pre-charge. Most 1-2 story homes can be set up for 12 psi when tepid, and with a properly sized expansion tank won't ever go above 20 psi (or even 15 psi for low volume radiation such as fin-tube baseboard.)

The damper should not be stuck in the open position. If it's not opening & closing there is probably something messed up in the wiring, or a cold-solder on the control boards or something. As a rule flue dampers have a safety interlock with the controls to prevent the burner from firing when the damper is closed. If your damper connections are flaky, that interlock may be what's keeping the burners from firing if the controls don't sense a proven damper-open condition.
 

Dkoss08

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30 psi is the pre-set temperatur of the pressure relief valves shipped with most boilers. If it's 10 psi when tepid, 30psi after it's been firing awhile, it's highly likely that your systems expansion tank has failed, or does not have sufficient pre-charge. Most 1-2 story homes can be set up for 12 psi when tepid, and with a properly sized expansion tank won't ever go above 20 psi (or even 15 psi for low volume radiation such as fin-tube baseboard.)

The damper should not be stuck in the open position. If it's not opening & closing there is probably something messed up in the wiring, or a cold-solder on the control boards or something. As a rule flue dampers have a safety interlock with the controls to prevent the burner from firing when the damper is closed. If your damper connections are flaky, that interlock may be what's keeping the burners from firing if the controls don't sense a proven damper-open condition.

I will check out the expansion tank for this system and see if it is working or not. However when the boiler finally kicks on it will run until the temp is higher than what the thermostat calls for... it does not shut off mid heating cycle. Which made me think the pressure,etc were normal for this system. I am assuming the previous owner of the house was having trouble with it as well and a service tech switched the damper to “hold open damper” so that it would bypass and fire every time. The damper does work correctly when I turn it on automatic, however if gas doesn’t get to the pilot the damper will then close because the system is not firing which made me think it was some sort of switch. Yes I have check the vent for obstructions etc already it’s clear. I’ve had a few service guys come out and they can’t find anything wrong with it.
 
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Dkoss08

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Expansion tank is fine. Something new I found out is that I don’t have a pressure relief valve... so I will be adding if of those in. I have also turned the temp down it was set at 200 High. Hopefully this fixes or corrects the issue with the high temp and high pressure.

However still have intermittent lighting where sometimes it calls for heat and it won’t kick on and sometimes it doesn’t. I checked what I’m getting at B1 and B2 again last night when it was out and is was like 8V this time so I’m thinking it’s something in the aquastat.
 

Dana

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How do you know the expansion tank is fine?

Did you isolate it from the system and test it's pre-charge pressure?

Pressure swings of more than ~5 psi only occur in systems with a lot of water volume in the radiation if the expansion tank is properly sized, properly pre-charged and functioning. A 100' of fin-tube baseboard and associated plumbing isn't a lot of volume, and the boiler has probably less than 5 gallons of water in it. What & how much do you have for heat emitters? What boiler?
 

Dkoss08

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How do you know the expansion tank is fine?

Did you isolate it from the system and test it's pre-charge pressure?

Pressure swings of more than ~5 psi only occur in systems with a lot of water volume in the radiation if the expansion tank is properly sized, properly pre-charged and functioning. A 100' of fin-tube baseboard and associated plumbing isn't a lot of volume, and the boiler has probably less than 5 gallons of water in it. What & how much do you have for heat emitters? What boiler?

Had a friend who is more familiar with expansion tanks come test it. I’m working on the temp and pressure settings this is not my top priority right now. However, turning down the max temp did reduce pressure by about 10psi so around 25psi which is okay with me right now. Because It is negative degrees at night and my boiler doesn’t light randomly I need to get this situation fixed first. So I don’t have to keep waking up in the middle of the night to reset the system so that the house stays a decent temperature.

Just in baseboards I have approx 100’ then there is all the other piping that goes to the baseboards. Boiler is slant/fin. I pulled out the 24V wire on the ignition panel to test it and the connector came off so I put a new one on hoping that it was just a loose wire powering the ignition control. Fingers crossed.
 

Dana

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SlantFin has made dozens of different residential boiler models over the past couple of decades. They are currently selling no fewer than nine current model lines of gas fired residential boilers, with multiple sizes & variations within each line. A complete model number makes it easier to look up better, more complete documentation than just the few schematics pasted onto the sheet metal.

A peak pressure of 25 psi isn't going to stress the boiler, but if it's a baseboard-only system it means the expansion tank isn't fully working. This could either be because it has a leak in the internal and is becoming or has become waterlogged, or if there's an air leak on the air side (usually a leaky Schrader valve- the type of air valve found on tires) it's lost it's pre-charge level. A couple of quick tests:

Tap the end of the tank that is connected to the plumbing, then tap the end with the air valve. The air-valve side should have a bit of ringing, whereas the water side of the tank would be more of a thud.

If the air side rings, it's not water logged, and it can probably be pre-charged to 12-15psi. If the air side sounds as dead as the water side, press the Schrader valve. If water comes out rather than air, it means the bladder has failed and it needs to be replaced.

When pre-charging the tank with air there has to be zero (or at least low) pressure on the water side of the tank. For a system that only needs 12 psi (all one story, and most two story houses) pre-charging it to anything between 12-15 psi is fine, and will allow sufficient expansion volume to perform it's function. If the pressure gauge on the boiler more than 23' below the baseboard plumbing a the highest point in the system, your reported 10 psi is too low a pressure to have even when the boiler is stone cold.
 

Elaine Here

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Okay, not a plumber but wanted to share this very ODD cause for why our boiler started randomly not firing up when we needed it to turn on. It would sometimes but other times, for no consistent reason, it just would not. My husband took off the cover to a small box on the face of our boiler and yelled up the stairs from the basement..."you're not going to believe this one!"
....It turned out that somhow a BAT, yes a BAT!!!, had made it's way down our chimney, through the whole furnace housing and finally rested in-between the two IGNITER points where he apparently got zipped and fried! The igniter was actually able to probably arc through his body a few times but as he became more fried.... we couldn't get the furnace to work at all.... My husband cleaned him from the igniter points with a paper towel and the boiler fired up without a hitch ever since! Strange but true.... wished I'd taken a picture of it!o_O
 

jadnashua

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One spring, my a/c unit wouldn't turn on...turned out that a mouse has fried itself across the 240vac contactor...cleaned things off, all was well. The troubleshooting guides don't always come up with the right answer!
 

Dkoss08

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Thankfully there are no fried animals in my boiler system. There was a bad wire connection on my ignition control when I pulled it out the connector fell off put a new one on and it has been running fine since then. Hopefully that was the only issue and wasn’t just a temporary one. Waiting til it gets a little warmer out then I am possibly looking at replacing the aquastat because the relay is bent so when the cover goes on all the way the relay hits the cover and can’t make the connection so just leaving the cover off slightly so it has enough room to move freely.
 

Dana

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Thankfully there are no fried animals in my boiler system. There was a bad wire connection on my ignition control when I pulled it out the connector fell off put a new one on and it has been running fine since then. Hopefully that was the only issue and wasn’t just a temporary one. Waiting til it gets a little warmer out then I am possibly looking at replacing the aquastat because the relay is bent so when the cover goes on all the way the relay hits the cover and can’t make the connection so just leaving the cover off slightly so it has enough room to move freely.

If you're planning on keeping the boiler for a handful of years or more, it's probably worth installing a heat purging control (eg Intellicon HW+) when you swap out the aquastat. The vast majority of cast iron boilers in the US are somewhere between ridiculously and ludicrously oversized for the space heating load, operating at duty cycle and efficiency well below that of an AFUE test. A heat purging control buys back quite a bit of the lost "as used" AFUE efficiency by reducing the number of burn cycles as well as the average boiler temperature, but more importantly the boilers average STANDBY temperature. A boiler room in an uninsulated basement can often be the the warmest place in the house on the coldest days of the year, losing enormous amounts of heat out of the foundation.

To find out the oversizing factor of your boiler, run a fuel use based heat load calculation on the house using the boiler as the measuring instrument, and compare that to the D.O.E. output BTU on the boiler's nameplate. AFUE is tested at a 1.7x oversize factor, and if your boiler is 1.7x oversized at Green Bay's -8F outside design temperature (the 99th percentile temperature bin- 87 hours per year are colder than that in an average year), you'd be able to heat the place at -62F outside (assuming you have sufficient radiator or baseboard). A more typical oversizing factor for older boilers is 3x which is enough burner to keep the place warm at -164F, a temperature which you might not even see in Green Bay during the next ice age. ASHRAE recommends 1.4x oversizing as the best compromise in comfort & efficiency, and it would still fully cover the load at -40F during a Polar Vortex event.

A heat purge control won't turn an oversized beast into a right-sized beast, but it moves the knee in the regression curve, extending range of oversizing that still hits at least close to it's AFUE numbers.
 
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