Burner didn't shut off at limit

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RinCT

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Newbie here. I just noticed my Oil Fired Burnham Boiler wasnt shutting off when heat runs. I turned Tstat down so it stopped. Went to check temp and it was almost 200 with pressure rose to 20psi. Normally at 15.

I have Hi/Lo set at 180/150 and diff at 12.

Maybe I never noticed before but why wouldnt the boiler cycle off at 180?
It does with showers or nothing running. I have a wood stove so maybe I just never noticed?

Thanks guys.
 

RinCT

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With a pressure setting of 20 psi, this sounds like a heating boiler, which would have nothing to do with the showers running.

Its an oil fired burner for heat and hot water. Hydro air system.
All is fine when showers are running. But only when heat runs it gets up there. Wonder why it would go over limit like that only with heat. Odd

Edit: forgot to mention I have forced air with the oil boiler. Sorry.
Thanks for replying.

Let me try something...
 
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Dana

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Crud on the sensor maybe.

More likely, lack of (thermal heat transfer compound) crud on the sensor, assuming simple aquastat controls. Most cast iron boilers have sensor wells to accommodate a range of aquastat sensor sizes, which will often need some purpose-formulated thermal grease to reliably sense the boiler's temperature if the aquastat's bulb doesn't otherwise have good contact with the well.

When the showers are running and the heat isn't there is no flow (other than convection) on the heating water side of the tankless coil. Flow & pump vibration can affect the quality of thermal contact the aquastat sensor gets in the well too.

For high water volume systems it's not unusual to see a swing from 15psi at idle to 20psi @ 200F. For a hydro-air system the water volumes are pretty small, so a swing of 5 psi over a 50F rise in temp may be an indication of an expansion tank that is improperly charged.

Since Burnham has only produced 500 different oil fired boilers over the past 100 years it might be useful to know the model number, and the model numbers of the aquastats or other controls on the boiler. The model number of the hydro-air handler doesn't hurt either. This is sort of like asking why your Ford car overheats sometimes, and oh yeah it's being used to haul stuff with a trailer. The answers will be different if it's a '20s vintage Model A vs. '60s vintage F100 vs. '9os vintage 5 liter Mustang, and the heft of the trailer it's hauling can matter too.
 

RinCT

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More likely, lack of (thermal heat transfer compound) crud on the sensor, assuming simple aquastat controls. Most cast iron boilers have sensor wells to accommodate a range of aquastat sensor sizes, which will often need some purpose-formulated thermal grease to reliably sense the boiler's temperature if the aquastat's bulb doesn't otherwise have good contact with the well.

When the showers are running and the heat isn't there is no flow (other than convection) on the heating water side of the tankless coil. Flow & pump vibration can affect the quality of thermal contact the aquastat sensor gets in the well too.

For high water volume systems it's not unusual to see a swing from 15psi at idle to 20psi @ 200F. For a hydro-air system the water volumes are pretty small, so a swing of 5 psi over a 50F rise in temp may be an indication of an expansion tank that is improperly charged.

Since Burnham has only produced 500 different oil fired boilers over the past 100 years it might be useful to know the model number, and the model numbers of the aquastats or other controls on the boiler. The model number of the hydro-air handler doesn't hurt either. This is sort of like asking why your Ford car overheats sometimes, and oh yeah it's being used to haul stuff with a trailer. The answers will be different if it's a '20s vintage Model A vs. '60s vintage F100 vs. '9os vintage 5 liter Mustang, and the heft of the trailer it's hauling can matter too.

Wow, great info. Thanks!

So here's a couple pics and more info. Not sure if it helps, like you said, its tough to do over the phone or internet without being here. Thanks for taking the time to post!

Boiler: Burnham PV83 WL
Pump: Beckett AFG 70MBASN
Air Handler: First Company 36HBXB-HW
1 Level Ranch home. 2 zones.

(Ignore the hose, I had to drain it to change the HyVent)
(Ignore the unattached tube its for future baseboard heat in basement).

Expansion tank is old but I only use this thing for showers and dishwasher and sometimes for heat when its below 15° outside. Pressure said 13 in the tank.

bU7wEQKh.jpg
t4aCS0Ph.jpg
 

Dana

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If the air pressure reads 13 when the system pressure is north of 15 it means there is not much expansion space for the water as the temperature changes. Pump it up to 20psi- the system pressure will rise with it. Then (without getting scalded) bleed the system down to 13-15 psi, and re-measure the air pressure. It will probably have dropped. Repeat a few times until pumping the tank to 20 psi doesn't change the system pressure, then back off the air pressure to the system pressure + 1 psi. Then you'll know the full volume of the tank is available for water expansion.

Even at the highest blower speed the 3- ton 36HBXB-HW only delivers ~60,000 BTU/hr at an entering water temp (EWT) of 180F. With the 1.05 gph nozzle it was probably in the burner the PV83 is giving it about twice that (see Table 1.B in the manual), so during long calls for heat on the air handler zone it should be hitting the high limit, cycling on & off. (That's not a great way to run the boiler from an efficiency point of view, but that's the behavior you should expect.) Burner techs usually leave a tag with information about combustion efficiency, nozzle size (or part number) when the tune up an oil-burner- see if you can verify the nozzle.

Depending on what other radiation (type & amount) there is on the other zone it may be worth down-firing the boiler with a smaller nozzle to limit the number of burn cycles bumping up net "as-used" efficiency a bit. It looks like a 0.75gph nozzle was standard for the "WM" version of the _V83.(Table 1.B in the manual.)

Unless your house is enormous and uninsulated it probably doesn't need anywhere near 3 tons of cooling or 120KBTU/hr of boiler. A typical reasonably tight reasonably insulated 2000' rancher in CT would have a design cooling load between 1-1.5 tons, and a design heat load under 40K (for the whole house, not just one zone). It's common to see big (even ridiculous) oversize factors in houses, but it's not great for either efficiency or comfort (often the opposite.) It's not cost effective to swap out a fully functioning HVAC system, but if you're ever in the position of needing to replace piece it's worth taking the time to figure out how to right-size it. At the current cost of #2 oil and electricity in CT a right-sized modulating ducted cold-climate mini-split heat pump would be both more comfortable and have a lower operating cost.
 

RinCT

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If the air pressure reads 13 when the system pressure is north of 15 it means there is not much expansion space for the water as the temperature changes. Pump it up to 20psi- the system pressure will rise with it. Then (without getting scalded) bleed the system down to 13-15 psi, and re-measure the air pressure. It will probably have dropped. Repeat a few times until pumping the tank to 20 psi doesn't change the system pressure, then back off the air pressure to the system pressure + 1 psi. Then you'll know the full volume of the tank is available for water expansion.

Even at the highest blower speed the 3- ton 36HBXB-HW only delivers ~60,000 BTU/hr at an entering water temp (EWT) of 180F. With the 1.05 gph nozzle it was probably in the burner the PV83 is giving it about twice that (see Table 1.B in the manual), so during long calls for heat on the air handler zone it should be hitting the high limit, cycling on & off. (That's not a great way to run the boiler from an efficiency point of view, but that's the behavior you should expect.) Burner techs usually leave a tag with information about combustion efficiency, nozzle size (or part number) when the tune up an oil-burner- see if you can verify the nozzle.

Depending on what other radiation (type & amount) there is on the other zone it may be worth down-firing the boiler with a smaller nozzle to limit the number of burn cycles bumping up net "as-used" efficiency a bit. It looks like a 0.75gph nozzle was standard for the "WM" version of the _V83.(Table 1.B in the manual.)

Unless your house is enormous and uninsulated it probably doesn't need anywhere near 3 tons of cooling or 120KBTU/hr of boiler. A typical reasonably tight reasonably insulated 2000' rancher in CT would have a design cooling load between 1-1.5 tons, and a design heat load under 40K (for the whole house, not just one zone). It's common to see big (even ridiculous) oversize factors in houses, but it's not great for either efficiency or comfort (often the opposite.) It's not cost effective to swap out a fully functioning HVAC system, but if you're ever in the position of needing to replace piece it's worth taking the time to figure out how to right-size it. At the current cost of #2 oil and electricity in CT a right-sized modulating ducted cold-climate mini-split heat pump would be both more comfortable and have a lower operating cost.

Wow. Thank you so much Dana for the info and taking the time into looking at it.. I want to try 2 things then will post back when I do.
1. Turn heat on, lower hi limit down and see if it cycles off before it gets to 190°
2. What you said about pumping into the expansion tank.

I have been using a 75/60° nozzle in there since the plumber installed it in 2007. (Yes, I know it was used, I asked for it)
House is 1 level and not enormous. Only 1500 sq ft.

(BTW - water was dark with some crud coming out from where the hose is attached to in the picture. Probably different issue)
 

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Most cast iron boilers accumulate black iron oxide sludge over a few years of service. Flushing the boiler at the boiler drain and completely re-filling every year or two keeps it from becoming a problem. When there's too much it can affect pumps, valves, vents etc. Magnetic filters can do a good job of clearing some of it.

The heat load @ 0F outdoors/68F indoors of a 1500' 2x4 framed insulated rancher with clear glass (not low-E) replacement windows or storms, over a full basement with NO foundation insulation will usually come in around 30,000 BTU/hr, give or take 3K. If the basement walls are insulated to current IRC code minimums it can even be under 20,000 BTU/hr. An oil boiler idling at 150F to be able to deliver the domestic hot water has high jacket losses, so it's probably worth your while to insulate the basement walls, even if you're never going to turn it into living space. If you have 2' of above-grade exposure on a poured concrete foundation the basement wall losses could be as much a 1/4 of the total annual heat load, which can be cut by an order of magnitude by bringing it up to current code. There are several threads on this site over the past handful of years on this site on how to insulate basement walls on the cheap without creating mold farms or fire hazards.
 

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Update:
I ran the heat, lowered the HiLimit down and the boiler cycled off before it went over 170. So we know that's all good.

I did what Dana said and added some pressure to the tank then bled system down, and WhaLa!! Pressure now staying at 10-11psi which was about where it was before I replaced the HyVent.

I will also drain the boiler down and flush it soon and thinking I should change that 12yr old tank while I'm at it... Thanks for the tips!

Thanks all for your help. No wonder why terrylove forums are #1 with Google Searches. God bless you guys.
 

Dana

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Measure the burn times when it's cycling. Ideally the minimum burn time would be 10+ minutes (for efficiency's sake) but even 5+ is OK. If it's much shorter than that it starts to eat into the as-used efficiency pretty badly, even if it's steady-state combustion efficiency is still well north of 80%.

Running the system at 10 psi is bit on the low side, low enough to risk pulling air into the system when the pumps are running on some systems. (With your pump & vent configuration I'm not too concerned.) A 12 psi system pressure is more usual. There are other nits to pick with the system plumbing (such as the placement of the expansion tank relative to the pump) none of which are worth changing as long as the system is otherwise working.
 

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Measure the burn times when it's cycling. Ideally the minimum burn time would be 10+ minutes (for efficiency's sake) but even 5+ is OK. If it's much shorter than that it starts to eat into the as-used efficiency pretty badly, even if it's steady-state combustion efficiency is still well north of 80%.

Running the system at 10 psi is bit on the low side, low enough to risk pulling air into the system when the pumps are running on some systems. (With your pump & vent configuration I'm not too concerned.) A 12 psi system pressure is more usual. There are other nits to pick with the system plumbing (such as the placement of the expansion tank relative to the pump) none of which are worth changing as long as the system is otherwise working.

5-10 minutes?? When there's no call for hot water The thing cycles every 2 hours for 2-3 minutes each time.

Temp drops about 20 degrees every hour. It is winter though but the basement stays 55-65° year round. In warmer months I lower the low limit so it doesn't cycle so often.

Luckily I use a wood stove for primary heat.

I bet you draft is the next thing you'll mention. :) (I haven't checked it in years)
 

Dana

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5-10 minutes?? When there's no call for hot water The thing cycles every 2 hours for 2-3 minutes each time.

Temp drops about 20 degrees every hour.

There isn't much you can do about temperature-maintenance burns when there are no direct loads, but they're very infrequent. It's when the air handler is running is when you would (hopefully) see 5+ minute burn cycles. But it's probably so ridiculously oversized for the heat load of that zone it probably satisfies the thermostat in under 10 minutes (?).

Under current IRC code the heating system plumbing would all need to be insulated to R3 minimum, both supplies & returns, along with any near-boiler plumbing. The plumbing is just a weirdly shaped convector/radiator dumping heat into the lossy basement. That vintage boiler probably has at least R2 in the jacket, so a goodly-fraction of the standby losses are from the plumbing, even when it's not pumping. It's hard to estimate without more information, but you may be able to cut the standby loss of ~20F/hr down to ~15F/hr without a big spend. Use pre-split 1" wall thickness fiberglass pipe insulation from an online supplier or local plumbing supply house, not the overpriced half-inch wall stuff at box store. Grainger carries the right stuff, but usually more expensive than most online suppliers or plumbing supply stores that cater to the trades. The cheap foamy R3 stuff used on potable plumbing isn't fully rated for the higher temps of your heating system plumbing.

It is winter though but the basement stays 55-65° year round. In warmer months I lower the low limit so it doesn't cycle so often.

Raising the high limit to the maximum or lowering the low limit will deliver longer burns. It's really the difference in high to low temp that determines the minimum burn time, whether it's a temperature maintenance cycle or serving a zone. For best as-used efficiency, max out the temperature difference to whatever the controls will allow, making maximal use of the thermal mass of the water + cast iron in the boiler.


Luckily I use a wood stove for primary heat.

Actually the primary heat keeping the basement so toasty is the standby & distribution loss of the boiler, not the wood-burner upstairs. That's why it's still totally worth it to air seal and insulate those lossy foundation walls. That's heat you ARE paying for.

With insulated walls the basement might idle along at 65-70F in winter with a lossy less insulated boiler like that. With the lower difference in boiler to basement temp comes a double-digit percentage savings in oil use.

With the lower difference in basement to upstairs temp comes a (smaller) decrease in wood use.


I bet you draft is the next thing you'll mention. :) (I haven't checked it in years)

Nah- I'll let you be the one to mention it. I'm not worried about it- are you? As long as the barometric damper isn't jammed and it seems to be doing something when it fires up, assume it's doing roughly the right thing. It's primary purpose is to dilute the exhaust with much drier room air, lowering the dew point of the gases going up the chimney so that it doesn't condense acidic liquids onto the susceptible masonry (or if the chimney has a stainless liner, dribble condensate back down to rust out the non-stainless venting between the boiler & chimney).
 
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