Aquastat setting question

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Old 12-14-08, 04:38 PM
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Aquastat setting question

I am new to boiler heating. I have an older oil fired boiler and baseboard heating my 2 storey 1120 foot cabin.

The boiler is set to start up at 155 degrees and shut down at 180 degrees. The return side would come back at about 135 degrees. The system is running at about 25 psi. It takes about 6 minutes to heat to 180 and about the same amount of idle time to reach 155; therefore it would do roughly five cycles per hour.

This weekend, a couple hours before leaving, I turned the heat down in all the zones and watched the temperatures. After it cooled for awhile, I forced a call for heat, let the boiler run up to near 180, and then turned down the thermostat. The temperature on the supply side continued to rise to near 230, until I forced a call for heat and it cooled quickly.

If the primary loop pump is running the supply temperature doesn't get above the aquastat shutoff temperature of 180. I was thinking that if the primary loop pump as wired to run continuously, rather than just when there is a call for heat, I should be able to turn up the aquastat setting so that it shuts off at 195 or 200. Any suggestions would be appreciated.

Mike
 
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Old 12-14-08, 08:30 PM
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Originally Posted by Yamahammer View Post
I was thinking that if the primary loop pump as wired to run continuously, rather than just when there is a call for heat, I should be able to turn up the aquastat setting so that it shuts off at 195 or 200. Any suggestions would be appreciated.
Mike
What would be the purpose of doing that? The boiler would operate less efficiently.
Doug
 
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Old 12-14-08, 09:23 PM
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Aquastat

Originally Posted by gilmorrie View Post
What would be the purpose of doing that? The boiler would operate less efficiently.
Doug

When the pump is not running and no zones are calling for heat the supply temperature continues to rise, reaching a temperature of about 230 degrees. If the pump ran continously the water would circulate within the primary loop and not over heat. Other than using a little electricity I didn't think there would be any problems. Am I wrong?

Mike
 
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Old 12-14-08, 09:57 PM
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The fact that the temp rises after the boiler shuts down is called 'heat soak'. There's heat trapped in the metal mass of the boiler and when the circ stops it transfers to the water. This is the same reason that many 4 wheel vehicles have fans that run minutes AFTER the engine is shut off. There's really no harm in it as long as the PRESSURE stays under control (i.e. under 27 PSI)

Is there some reason that you want to turn the temp up on the boiler? Unless you NEED the extra temp to heat the home, it's a pure waste of energy.

With your system pressure approaching 25 PSI, it's a good bet that your expansion tank may need service. If it's a bladder type, it may need an air charge, if it's the compression type, it may be waterlogged and need draining.

With the boiler cold, pressure should be between 12-15 PSI. When HOT, maybe 8-10 PSI more, MAX ...
 
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Old 12-15-08, 10:26 AM
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aquastat

Thanks for the information. As I stated, I am new to boilers. Now everytime I see rads in other homes I feel the fins. Often I can't hold my fingers on them for very long. On my system the fins are warm, but not terribly hot. This past weekend it was -32 outside and with the windchill it felt like -45. I do have a lot of windows, but with the thermostat set to 68, the best I could get was 65 degrees. I thought that it may be possible that the supply was not hot enough.

What is the best way to test and recharge the expansion tank. It does have a bladder. I have checked the pressure while it is connected to the system and it reads the same pressure as the system. I expect that I should bypass and isolate the expansion tank, but will that give me an accurate reading on the tank?

Originally Posted by NJ Trooper View Post
The fact that the temp rises after the boiler shuts down is called 'heat soak'. There's heat trapped in the metal mass of the boiler and when the circ stops it transfers to the water. This is the same reason that many 4 wheel vehicles have fans that run minutes AFTER the engine is shut off. There's really no harm in it as long as the PRESSURE stays under control (i.e. under 27 PSI)

Is there some reason that you want to turn the temp up on the boiler? Unless you NEED the extra temp to heat the home, it's a pure waste of energy.

With your system pressure approaching 25 PSI, it's a good bet that your expansion tank may need service. If it's a bladder type, it may need an air charge, if it's the compression type, it may be waterlogged and need draining.

With the boiler cold, pressure should be between 12-15 PSI. When HOT, maybe 8-10 PSI more, MAX ...
 
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Old 12-15-08, 10:57 AM
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Originally Posted by Yamahammer View Post
What is the best way to test and recharge the expansion tank. It does have a bladder. I have checked the pressure while it is connected to the system and it reads the same pressure as the system. I expect that I should bypass and isolate the expansion tank, but will that give me an accurate reading on the tank?
In normal operation, the expansion tank pressure will read the same pressure on either side of the bladder whether it's properly charged with air or not.

You'd have to drain the the water out of the tank, and then fill the air side to, say 12 psi. If there is an isolation valve and a drain valve behind it, then it's pretty simple. Otherwise, you may have to drain/depressurize the whole system.

Where do you live? It's very possible that your system just wasn't originally sized for the temperature conditions that you cited.
Doug
 
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Old 12-15-08, 11:29 AM
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This past weekend it was -32 outside and with the windchill it felt like -45. I do have a lot of windows, but with the thermostat set to 68, the best I could get was 65 degrees. I thought that it may be possible that the supply was not hot enough.
OK... this is the 'why' we were looking for.

Doug's probly right... MINUS 32 F ? I highly doubt any of those other homes radiators you been feelin' up felt very warm during that time! wow... dats cold man. In a situation such as this, you would be justified to temporarily increase the setting to 190-195 ... when the cold snap ends, set it back ...

I bet the boiler was pretty much firing non-stop, correct ? Struggling to even get to high limit maybe. Read the nameplate on the boiler and tell us how many BTU it is ...

Before you do this... is there Glycol Antifreeze in your system? If there is... don't drain it... only do this if there's plain water in your system.

Turn off boiler and allow to cool to 100F or less...

Close manual valve on water feed line to boiler...

Drain only enough water from the boiler to drop the gauge on the boiler to zero PSI ...

check extank pressure ... if below 12 PSI, add air with bicycle pump, or small air compressor (or big air compressor if you have one), until it's between 12 and 15 ...

re-check boiler gauge ...

if it has come off zero, let a little more water out, and re-check tank...

if it's dropped, add a little more air...

repeat until boiler is at zero, and tank is 12-15...

re-open manual feedwater valve and allow boiler to pressurize to the regulating valve setpoint (should be between 12-15...

give it time... be patient... some valves take a while to stabilize).

Turn power back on to boiler.
 
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Old 12-15-08, 11:35 AM
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Yama, you said:

The boiler is set to start up at 155 degrees and shut down at 180 degrees. The return side would come back at about 135 degrees.
Can we clarify this a bit?

What's important here is the DIFFERENCE between the supply and the return pipes ... so, I'd like to know this:

How hot is the supply side when you are measuring the 135 at the return? Is it 180? or 155? This is important because if you are measuring 135 with a 180 supply, that 45 difference is a bit extreme. You might could have an issue with flow ...

Did you mention what kind of heat emitters are installed? fin-tube baseboard? cast iron radiators? cast iron baseboard?


Can we see pics?
 
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Old 12-15-08, 12:00 PM
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How hot is the supply side when you are measuring the 135 at the return? Is it 180? or 155? This is important because if you are measuring 135 with a 180 supply, that 45 difference is a bit extreme. You might could have an issue with flow ...

Did you mention what kind of heat emitters are installed? fin-tube baseboard? cast iron radiators? cast iron baseboard?
As it only takes about six minutes to raise the temperature from 155 to 180 it is hard to tell with certainty, but the return typically is in the 130 to 140 range. I do have 3/4 inch Slantfin rads.


I bet the boiler was pretty much firing non-stop, correct ?
Yes it would fire about every six minutes.

Thanks for the tip on the expansion tank. I do have gylcol in the system, but I do have a gylco feeder, so I can catch what I drain and put back into the feeder.

The boiler is at the lake so I couldn't post pictures until later.

Thanks guys.
 
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Old 12-15-08, 12:03 PM
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Originally Posted by NJ Trooper View Post
I bet the boiler was pretty much firing non-stop, correct? Struggling to even get to high limit maybe.
If that is (was) the case, then jacking up the aquatstat limit would do no good. If the burner is firing continuously, it is maxed out.

On the other hand, if the burner is cycling under such severe weather conditions, then the limiting factor is the radiators/convectors - in which case, increasing the water temp could help.
Doug
 

Last edited by gilmorrie; 12-15-08 at 01:54 PM.
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