Gas Boiler Low Limit Setting

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Old 02-08-13, 09:36 AM
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Gas Boiler Low Limit Setting

Hello: Greetings to all.

A situation just came up. My gas boiler Olsen ODV-B 125 is not keeping sufficient heat to the second floor. It has cold cycles between firing ups. When off, water temperature is at around 90 degrees F. PSI is ok at around 12. See Link. Thermostat is mounted on the main floor hall way at 70 degrees F.

TempPSI Photo by james-1618 | Photobucket

I didn’t know about the second floor issue earlier. My son lives there. The second floor was said to be at frequent more or less 60 degrees F. He has to use baseboard heater to supplement.

I like to tweak the Honeywell aquastat (# 8 - aq02201); see link, to maintain a water temperature (low limit) at around 140 degrees F.

PipingControl Photo by james-1618 | Photobucket

It is “control L4080B” (bottom right) connected by two blue wires as appeared in wiring schematic. See link.

BoilerElectricSchematic Photo by james-1618 | Photobucket

Upon opening the aquastat, I can not find the low limit setting. I also opened the boiler to inspect the connections as per wiring diagram. See link.

AquastatOpened Photo by james-1618 | Photobucket
BoilerWiring Photo by james-1618 | Photobucket

There is no low limit provision with this aquastat nor did it mention in the boiler manual low limit information. According to the manual, this boiler is factory pre-wired for easy installation. The installer basically just needed to hook up the thermostat and hydro.

How does this boiler control the low limit setting? Could I maintain a 140 degree F between firing ups? Or is it designed to have a free fall on temperature (in my case to 80/90 degrees F)?

Thank you.
 
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Old 02-08-13, 10:11 AM
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James I feel that you are not approaching the issue correctly.

There is no provision on that boiler for 'warm start'.

Modifying to controls to cause the boiler to function as warm start would likely NOT solve the problem of the cool upstairs and would only succeed in wasting fuel.

In order to correct a problem, one must first understand the 'root cause' of the problem and work toward solving that root cause.

I know that the gauge is telling you that you have 12 PSI when the boiler is cold, but is the gauge ACCURATE? You don't know. If the gauge is reading a few PSI on the high side, you could have low pressure and this would easily be the reason for no heat on second floor.

Is there possibly an air blockage in the pipes on the second floor? This would cause low or no flow, and cold rooms.

It is highly likely that converting to a warm start boiler will do nothing but waste fuel.
 
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Old 02-08-13, 06:31 PM
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You are right. I got myself into a tunnel vision.

Once the city ploughs out the snow, I will buy a water pressure gauge to test the pressure in the system and then bleed out the rads on the second floor.

Will report back. Thank you Trooper.
 
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Old 02-09-13, 02:25 PM
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James, as long as the light at the end of the tunnel wasn't the train, then all is good!

Keep us posted...
 
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Old 02-12-13, 04:08 PM
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It is an opened sky with sun shine.

I bought a water pressure gauge (range to 60 psi, increment by 1 lb) for $9 plus adapter accessories to a total of $21 + tax. I am happy with this investment. The boiler’s psi and the gauge psi are more or less in syn at about 15. I bleed the rads. Air was trapped inside from those of the second floor.


The second floor is more reasonable now at about 66 degrees F (instead of 60) while the main floor thermostat is set to 70.

There is a 3/4 degrees difference between the floors. Is this normal? Or would it take longer (a couple days maybe) for the walls to warm up to keep in the heat? Attic is insulated but not the walls. This is an old brick 2 storey house about 80/90 years old.


I am thinking to put in an auto air bleeder valve in the summer. What specification should I watch out for? Where would be a good location for this auto-bleeder?

Thanks.
 
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Old 02-13-13, 06:51 AM
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There is a 3/4 degrees difference between the floors. Is this normal?
It certainly could be... so much depends on the construction, the amount of insulation, how many BTU worth of heat emitters (radiators, baseboards, etc) are installed.

If a home has a certain heat loss at a certain outdoor temperature, the heat emitters must be able to replace that lost heat at a rate at least equal to the rate of loss in order to maintain a temperature.

If the first floor has proportionally more heat emitter relative to it's heat loss than the second floor does, it would stand to reason that it would run a bit cooler upstairs...

I am thinking to put in an auto air bleeder valve in the summer
Might not be advisable...

What type of EXPANSION TANK is installed on your system? A large steel one in the joists above the boiler, or one that looks like a propane gas grill fuel tank? If you have the former, there should be NO auto vents anywhere on the system.

If you are thinking to install this air bleeder on the 2nd floor, understand that in certain conditions these can actually suck air INTO the system.

Depending on their location, if hidden, they can slowly leak for a LONG time before being noticed and cause all kinds of other troubles... wood rot, mold, etc...

Once the air in the system is bled out, and there are no leaks in the system, the air should STAY out...
 
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Old 02-13-13, 07:05 AM
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Wow, this system has a large steel expansion tank built in between the joists. Thanks a million. I just couldn’t help on my curiosity to ask what would be the logic behind for the different setups respective to the auto bleeder.

Hmm, as an alternative, I will print this note out for my boy. It will be educational for him as well to serve as a guideline for him to keep an eye out.

Kind regards.
 
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Old 02-13-13, 11:14 AM
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Starting with the theory of what exactly an expansion tank does:

Water can NOT be compressed, but AIR CAN.

When water is heated, it expands. From 70° to 180° water will expand a bit less than 4% in volume. If there is say 25 gallons in the system when it's cold, there will be 26 gallons when hot.

There must be some place in the heating system for this 'extra' water to go as it expands. This place in into the expansion tank. Since this tank has AIR in it, this air can be compressed and the water can flow into the tank. This principle is the same for both types of tank.

In the steel type tank, there is no 'boundary' between the water and the air. They are in direct contact. The water into the tank can and will absorb some of the air. When the system cools, this water will flow back out of the tank and into the system where the air can come out of solution when the water is reheated and form air pockets in the system.

The smaller type tanks have a rubber membrane between the air and the water, so there is no exchange of the air into the water as the tank fills and empties. The 'air scoops' on these systems DO have air vents on top of them to vent trapped air to the atmosphere, since the air inside the tank is theoretically 'sealed' in there.

Although they are rarely installed this way, the steel tanks SHOULD BE connected to the top of an 'air scoop' which would collect the air in the system and send it back to the tank where it belongs. Most of this type tank are simply tee'd into one of the pipes, without any regard to passing any air BACK to the tank.

The reason for no air vents is because you do NOT WANT to vent the air to the atmosphere as this will eventually deplete the air cushion in the expansion tank, and it will become FULL of water, with little to no air in the tank.
 
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Old 02-14-13, 06:51 AM
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Now, I understand the science. It is a very good intellectual feeling. Thank you for the great explanation. You have being the greatest help.
 
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