Appropriate aquastat settings for American Standard 20B-J23

ogre01

First forum post.

I have my unit serviced every season, but it's basically just a vacuuming the debris from the surface of the burners. The guy in town does not have a bunch of experience with old (ancient) boilers. This year, I wanted to understand the settings and function of the system. It is old, it is inefficient, but in pretty good shape. Replacement is in the near future when funds are saved. I serviced it this year, pulled all the burner tubes disassembled, cleaned really well, disassembled pilot assembly and fully cleaned. Have great looking flames on burner and especially at the pilot.

Attached is a sketch of the layout. All aquastats are set at 120F (I don't believe they have been changed during services, I just want to learn things this year). Everything functions, but after reading quite a bit, it seems like I'm probably short-cycling by burner (boiler runs for ~10min at a time with house warm). For an experiment, I did turn up the aquastat (L4006A) that controls boiler temp(?) and started getting some bypass at the pressure valve (boiler pressure rose to approcimately 30 psi). I did not like that, did not wait for it to equalize, and turned it back down to 120.

My questions, the aquastat at the control board has a much lower dial range, does this control the "on" temperature for the boiler? i.e. set lower than the operating temperature of the aquastat at the boiler. I need to get a better picture of the top of the aquastat to see if the model number is legible, cover is gone (was gone when I purchased the house). At the bottom of that aquastat there is a small lever with some type of range control. Is this an equivalent of "outdoor reset"? What should the main dial be set at?

On the aquastat at the boiler, I've read a bunch of temperatures a boiler should be set at, is there a minimum temp it needs to be (currently at 120). Is there a "most efficient" range for it to be set at? Currently, the house (at the thermostats on the first floor) stays at a comfortable ~62 but it will easily go to 70 (I would not want the radiators too hot to touch). If it is recommended to turn up, is there an appropriate way to control pressure outside of the pressure relief valve (Taco)? There is a (looks fairly new Extrol) expansion tank on the system.

The last aquastat, connected to the pump controls is also at 120. However, it seems the pumps will always turn on with the thermostats upstairs. Anything to modify here?

Operating pressure: the boiler is located in a 7'-4" basement, 10" floor, ~8'-6" main floor ceiling, 10" floor, 4' tall cast iron radiators on the second floor.

The circ pumps are in great shape. I dutifully add 3 drops oil each year, operation is so silent and smooth that you have to look at the coupler to see if either is running.

The various fill, check, pressure valves on the piping look newer than the boiler (unsure though), and in great shape.

I'll also take suggestions like "don't mess with it". Or "be a helpful homeowner and suggest things to the service guy…" FWIW we're actually professional acquaintances and I know him pretty well. He would gladly answer any directed questions I might be able to ask him.

Thank you in advance.

boiler sketch_447.jpg

bburd

Pictures rather than just a diagram would be helpful.

The aquastat marked "open on temperature rise" is the high limit safety control and must be set at least 20° higher than the other aquastat, "close on temperature rise", which is the low limit/circulator control to prevent circulation of heat to the house unless the boiler water is above a minimum temperature; this prevents flue gas condensation which can shorten the life of the boiler.

I'm surprised the house is getting sufficient heat. Normal settings are 120-140° circulator and 160-200° burner high limit.

These can be wired in different ways depending on your system. I assume your tap water comes from a separate water heater and not from the space heating boiler. In systems with a tankless coil in the boiler to heat tapwater, normal settings are 180° low limit and 200° high limit.

If raising the temperature caused the water pressure to hit the safe limit of 30 psi and the safety valve to discharge, your expansion tank is either faulty or undersized. They are sized to the volume of water in the system, which is large if you have an older system with steel pipes and cast-iron radiators.

EdTheHeaterMan

This sounds like you have or had a tankless coil on that old boiler for domestic hot water (DHW). First question is "Do you still use the boiler for DHW?

Back in the day, before the introduction of the triple aquastat relays like the Honeywell L8124 type controls, the function of all those parts were performed by individual controls. there is a High Limit that was generally set at 180°F and a Low Limit tht maintained a minimum boiler temperature to make sure there was enough heat in the boiler top make DHW. That Low Limit was generally set at 150° to 160°. There was also a reverse limit that would prevent the circulator from operating if the boiler water temperature dropped so much that there would not be enough heat in the boiler to make DHW. That way the circulator would not send all the hot boiler water to the radiators and the cold radiator water to the boiler when you were in the middle of a shower. No one likes to take a cold shower! That Reverse aquastat was usually set 10 degreed lower then the Low Limit. like 140° to 150°.

If your boiler no longer makes DHW, then the low limit and the reverse aquastats should have been lowered to 90° or lower. That will keep the burner off and allow the circulator to operate when ever there is a call for heat.

The problem you are experiencing with the water pressure rising to 30 PSI is that you expansion tank is waterlogged. There is not enough air cushion in the tank to accept the expansion of the boiler water as the temperature increases.

So as I see it, you have to get air in that expansion tank. Can you take a picture of your boiler with the connected pipes from floor to ceiling to include the expansion tank that is usually strapped between the ceiling joists.

Once we get the expansion tank problem resolved, we can address the boiler water temperature.

ogre01

Thank you, I appreciate the replies. It is not part of the DHW, I have a separate WH for that (I believe it's always been that way, the old water heater I replaced a couple years ago was probably from the 60s/70s). Here are what pictures I have, I'll take better ones tonight; specifically of the expansion tank label. On the picture looking at the front, there is a really old expansion tank stuffed in the joists out of sight, newer one on right. While I'm not positive, I'm fairly certain there is an air fitting at the bottom. Is it something as simple as adding pressurized air when the system is at zero pressure? (it's going to be pleasant this weekend, and I can easily shut the system down to measure/adjust). On the adequate heating part, I do keep it cooler than most, maybe the massive energy rating of the system is what lets it stay at set temps.

ogre01

As a follow up question. I do shut the system down completely from ~March/April to November/December. Is it a possibility that when it's shut down, the auto-fill (?) valve is adding water, then when I start it back up there is more volume than it expects which overwhelms the expansion tank? When I shut it down, I do not operate any of the water valves, just gas/elec. (but no leaks either).

ogre01

Ed, searching the forum now for info on the expansion tank, I came across the diagram you posted on thread #1747354. This says you can test the tank with boiler pressure less than the desired pressure or with tank off. Is testing the tank pressure attached but with system off a viable test (IIRC my system pressure does drop to 0, though that doesn't make much sense with presumably static head on the boiler tank? I would prefer not to remove, though hopefully there are isolation valves for tank removal. If removed, is there a process for bleeding the introduced air when reattached?

ogre01

Here's a better pic of the expansion tank. It has a view nipple at the bottom to indicate if it is still good or not, it is still clear (aka good). Also a better view of the front. So if the system has a functioning expansion tank, should I remove some of the water volume in the system to reduce pressure at higher temps?

@bburd, Also wanted to ask, if the aquastat on the "left" side is a high safety limit, what is used to establish the operating temp of the system. Is that in combination with the aquastat on the "control board"?

bburd

I have no idea what that third aquastat is for. You may need to get a professional in there who knows hydronic systems.

HVACNUT

The hot spot is pretty cool. It actually looks like a flame.

EdTheHeaterMan

https://forum.heatinghelp.com/discussion/comment/1824947#Comment_1824947

The concept is that you need to check the air pressure in an empty tank. If there is water in the tank when you check the air valve with a tire gauge, you will be measuring the total pressure in the system. If you have 28 PSI water pressure in the boiler, the water pressure will compress the air down to 28 PSI. But the pressure reading will not be the pre-charged pressure of an empty tank (empty meaning no water in the tank). If you get your boiler pressure to 10 PSI or lower, then the pressure in the tank can be pumped up to 12 PSI and push any water in the tank out of the tank into the system.

Simple physics tells us that 12 PSI pressure on one side of the membrane in the tank will overcome a lower pressure on the water side of the membrane, making the expansion tank empty of water.  Once you have a properly pre-charges expansion tank, then you can add water to equal 12 PSI.  Once the water pressure is at 12 PSI with a tank that is full of air, you can operate the burners and when the boiler water temperature increases, the water will expand and have a place to go in the expansion tank.  That water will compress the air a little to maybe 15 or 18 PSI.  As long as you can stay comfortably lower than the boiler's relief valve setting then you are good to go.  I personally don’t like the boiler pressure that starts at 12 PSI get any higher than 20 PSI  That leaves 10 PSI cushion for the relief valve 

EdTheHeaterMan

This aquastat looks like an old outdoor reset control that has two sensor bulbs. One of the bulbs goes outside in the north side of the building as not to get any solar radiant heat. the other bulb gets installed in the boiler water. the two temperature bulbs come up with a water temperature that is just hot enough to heat the building.

This would regulate the burner operation. There is no thermostat control of the burners, the boiler maintained a constant temperature based on the outdoor temperature. The circulator(s) would only operate on a call for heat from a thermostat in the building. The Reverse L4006B would keep the circulator from pumping if the boiler water lower than the set-point (120° in your case). The L4006A that is on the other side of the boiler should be set at a higher temperature as the High Limit. I would choose 170° or 180°, The ODR control will never let the temperature get that high.

ogre01

Again thank you all for the detailed replies. I forgot to add the info on the 3rd aquastat, it is a TA418A3X1, an antique Honeywell Outdoor reset. Does the temperature dial on this set the operating heat in the boiler (i.e. 120 will heat the tank until 120 is reached)? How does the lever below 1, 2, 3 modify the boiler temperature in relation to the outdoor temperature. Should I be looking to set the dial at a higher operating temperature?

On the expansion tank, it is a precharged unit and does not have any valve to access the bladder, only a sight-nipple at the bottom; if discolored it is bad - this one looks completely clean/clear. If it is functioning properly, is it possible the auto fill valve added more water volume than desired during system shut down time over spring/summer? I saw commentary on a different site (on a more modern boiler) where the technician reduced system pressure by bleeding a small amount (several cups) at a time at an upstairs radiator bleed valve. Is this an appropriate method to reduce system volume? As opposed to draining the system, then filling to idle pressure (~12psi).

Have a great weekend all. I'll be checking in and can sit and watch the system during any temp dial modifications.