Expansion tank

Hello everybody,

Can anyone tell me if my expansion tank is to small? I currently have a Amtrol-Extrol,Model #30, precharged to 12 psi, max temp 240*, max psi. is 100. My boiler holds 32 gallon of water and the loop is fairly short 15' from boiler to heat exchanger.

The problem is the system does not hold the 12 psi {cold} set at the pressure regulator. As the temp increases, so does the psi. example: ------140*-15psi.------165*-18psi-----180*-20psi-----200*-25 psi.

The bladder in the expansion tank does not seem to be damaged. If I tap on the top half of the tank it sounds solid. If I tap on the bottom half of the tank it sounds hollow.

Are these changes in pressure normal or do I need a larger expansion tank?

Thank You.

Hi, sounds like you have the same setup as me with a boiler hooked to a heat exchanger, outdoor wood boiler style. You might have too much water in the system, when the circulator is running and the system is up around 180 degrees the tank should be hot to the touch about an inch from the inlet, if it's hot halfway down then theres too much water in there and the bladder can't compensate. Also check the air in the tank with a tire pressure valve, too much air is no good, not enough air is bad also. Make sure the air vent isn't clogged also. Mine goes from 12psi at 140 to 20psi at 180 so you might be normal, my high limit is at 180 and I haven't the need to raise it yet. The factory setting was at 210 and that wasted a lot of coal for no benefit.

Mine goes from 12psi at 140 to 20psi at 180 so you might be normal, my high limit is at 180 and I haven't the need to raise it yet. The factory setting was at 210 and that wasted a lot of coal for no benefit.

You should be keeping a 50 degree spread between your low and high setting if you are talking about your efm boiler. In the warmer months, your boiler temperature may drift above 180 and neither your timer nor your thermostat will allow the stoker to start up, resulting in the fire going out. This is because the pot is still hot with burning coal after the stoker shuts down and it will continue to heat the boiler's water. With no circulator or domestic hot water use, your boiler temperature could rise 50 degrees after the stoker shuts down.

Your low limit setting, which is normally 150 for cast iron and 160 for baseboard is what controls your boiler. The boiler will maintain the low limit setting when it is resting. Upon a call for heat, the circulator will start up, cool water will enter the boiler and the stoker will start up as the boiler temperature drops below the low limit setting. The circulator will draw down the boiler temperature about 10 to 15 degrees (differential setting) below the low limit setting and then shut off while the stoker keeps fueling the fire to bring it back up to the low limit setting. This will ensure that there is hot boiler water available to heat the domestic hot water coil. Without the differential setting feature of the triple aquastat, the boiler temperature would be drawn down to a point where you wouldn't have any domestic hot water. After the room thermostat is satisfied and the boiler is at least at its low limit setting, the stoker will then shut down.

The high limit is just that...a safety limit. It won't allow the boiler to go above that setting. Leaving it at 210 will not cause any more coal use than if it was 180, but it will prevent your fire from going out.

This would be a typical example of how the aquastat works: The boiler is resting at its low limit setting of 150. The thermostat is turned up and the circulator brings cool water flowing into the boiler. The boiler temperature drops to 149 degrees and the stoker starts up and tries to keep the temperature at 150. There is too much cool water returning to the boiler and the stoker can't keep up. The boiler temperature is now at 140 and someone is taking a shower. The aquastat shuts down the circulator so that the boiler water doesn't get any lower than 140, so Mom can finish her shower. Meanwhile, the stoker is still working and the boiler temperature starts climbing again towards 150. The circulator starts again and the cycle continues until the boiler water temperature reaches a stable 150 degrees and the thermostat is satisfied. The pot is still red hot and the boiler water starts to climb, as much as 50 degrees in the Summer. That's no problem, because the high limit is set at 200. In a little while, the stoker timer will run through its 3 minute per half hour cycle and the fire won't go out. If the high limit is set too low, the timer will come on, but the stoker won't start up because the boiler water temperature is above the high limit. A half hour later the timer cycles again, but the fire is now out and raw coal is fed into the pot. This can be prevented by another aquastat which tells the stoker not to run because the outlet pipe of the stoker is too cold, indicating an outfire. But that's another story.

Thanks for the explanation. I always thought that in the summer the boiler runs off the low limit setting, during the winter it runs off the high limit, hence I set mine at 180. I just now reset it to 150/200/10 and it's working fine so I'll keep it at the factory recommended setting.

http://www.amtrol.com/pdf/extrolbrochure.pdf

Here is a good chart for sizing. kmarsh, you system should work with a #30. I have two #30 tanks because I have two boilers in one system and my pressure varies between 15 and 20#, depending on temperature.

Well I tried to reset the aquastat to the factory settings and my boiler went to 210 degrees and was sucking coal like crazy again so I had to turn it back down. Maybe my aquastat is fried or someone put the wrong control on at some point, here is what happens: Boiler is idling at ~160 degrees. The thermostat calls for heat. This turns on the circulator pump AND the stoker. The stoker keeps heating the boiler until the t-stat is satisfied or the high limit is reached at which point it shuts off. Ideally the thermostat should turn on the circulator only and the stoker should run off the low limit as you have described but it doesn't. Sorry to hijack your pressure thread

Don't forget when you check the pressure in the tank at the schrader valve you will have to release all the water pressure to 0 psi on the boiler guage . Then check you may have some air trapped in the closed loop system as well that will cause higher pressures . Bleed the system really well and check again maybe after a few days it will drop . Dave

I'm going to post info about the high/low limit setting in the efm portion of this forum, but if you're reaching high limit before the thermostat is satisfied, you are either overfiring the unit or something is impeding the extraction of heat from the radiators.

Some causes: Radiator covers, carpet too high under the baseboard, curtains covering the radiators/baseboard, or the unit is oversized for the house. Cut back on the coal and air, but keep an eye on the fire for the proper ash ring and that it isn't burning down into the pot.

Where your boiler connects to your domestic water supply, there should be a pressure reducing valve which controls how much water goes in the system and thereby the pressure. This pressure reducing valve is adjustable. Is it possible it is set too high? My wifes cousin recently installed a coal boiler and his pressure kept going over 20 psi and that was the cause.

coalkirk wrote:This pressure reducing valve is adjustable. Is it possible it is set too high? My wifes cousin recently installed a coal boiler and his pressure kept going over 20 psi and that was the cause.

Yes, most are adjustable from 10-25#. Normally they are set at about 12# and it should not vary more than a pound or two.