Vaporstat causing boiler to cycle too much?

mathewb001

Good Morning,

I am hoping a kind soul on this forum can help me. I own a 6 family building built in the 1930s with single pipe steam. Yesterday, I noticed that my Honeywell L404F-1367 pressuretrol failed to cut out on high pressure causing my backup manual reset pressuretrol to trigger at 7 psi. I checked to make sure the pigtails were clear and there was a good pathway for the steam. I have been successful running the system at 2 psi but some upstairs radiators would hiss unpleasantly. I want to try and run my boiler at lower pressure so I replaced the pressuretrol with a Honeywell L408J-1009 Vaporstat.

I set the cut out pressure to the maximum of 7 KPa (~1 psi ) and set the differential to 6.5 KPa ( 0.07 psi ). With these settings, when there is a sustained call for heat from the thermostat, the boiler fires for about 3 minutes 30 seconds and cuts out for 1 minute 6 seconds. Is this too fast a cycle rate? All the radiators I have checked so far seem to get good heat even at these pressures. Should I be worried at this rapid duty cycle? Will this damage the igniters or other parts? I am not a profession HVAC guy, just a small property owner that has been involuntarily educated on my steam system over the years.

Thank you all kindly for your help.

Regards,

Bipin

KC_Jones

That is an expected result. If you were previously running at 2 PSI and lowered the cutout then it will cycle more. Is it a problem? Probably not.

Now, for me, this is indicative of either an oversized boiler, not enough venting, or combination of the 2. Many of us on here couldn't possibly get to the pressures you talk about and with proper boiler sizing and venting, it really shouldn't.

My opinion is different from some, but the reality is the pressurtrol and vaporstat are safety devices meant to keep the boiler from overpressure. Due to the prevalence of over sized boilers the industry started viewing them as operating controls, and situation I just can't agree with.

My $0.02 worth.

Jamie Hall

Except that i view the vapourstat as a control, and the manual pressuretrol as the safety, I agree with @KC_Jones here. On both sets of comments. The cycle timing you mention -- two thirds on, one third off -- does suggest that the boiler is oversized, but not outrageously so. That is -- assuming that the cycling begins after most of the radiators are getting at least warm.

ethicalpaul

I'll pick cycling over "burning a bunch of fuel for no reason" any day. Consult with a real steam contractor to see if you can do anything to better match your boiler's steam production to your radiation's capacity to condense it

mathewb001

Thank you @KC_Jones , @Jamie Hall . I am a bit confused. This is a 3 - story 6 family building. From @DanHolohan 's talks I thought no more that 2 psi would be necessary for my situation? When you say most folks here couldn't possibly get to the pressures I talk about, are most people higher or lower? My major concern is if the rapid cycling rate can cause any damage to other parts of the boiler? Indeed I don'e even know if this is a faster cycling rate. Is this normal behavior? Here is further results:

Thermostat is in the basement and set to 65.

1. Thermostat calls for heat.
2. Boiler turns on for 22 minutes 37 seconds.
3. Cuts out for 1 minute after high pressure trip
4. Cuts in for 4 minutes after low pressure trip.
5. Cuts out for 1 minute after high pressure trip
6. Cuts in for 3 minutes 22 seconds after high pressure trip
7. Cuts out after 51 seconds on Thermostat set point achieved.

Thermostat does not trigger again for 16 minutes 22 seconds.

1. Thermostat calls for heat.
2. Boiler turns on for 17 minutes 58 seconds
3. Cuts out for 1 minute after high pressure trip
4. Boiler cuts in for 2 minutes 35 seconds after low pressure trip
5. Boiler cuts out for 1 minute after high pressure trip
6. Boiler cuts in for 1 minutes 17 seconds after low pressure trip
7. Thermostat stops calling for heat.

So the boiler turned on or off 11 times in about 70 minutes. Is that normal behavior?

Thanks again kindly for your all help.

Jamie Hall

Looks pretty normal to me. The idea is to keep the pressure down, if possible. The boiler in the big place I care for is on a vapour steam system, and the vapourstat is set to cut out at a bit over 6 ounces. It doesn't reach that on most runs maintaining heat, but it will if it runs longer bringing the place up from a setback.

EBEBRATT-Ed

@mathewb001

What kind of thermostat do you have? Can you increase the cycles/hour on the stat?

KC_Jones

I never see more than about 1-2 ounces of pressure, that’s what I mean about not getting to those pressures.

ethicalpaul

My old boiler would cycle at approximately that rate before I started fixing things. I made sure the main venting was good and I downfired the boiler by turning down the gas pressure a little--but you can't just mess with it. You have to have a manometer to ensure you don't go too low, and you should have a combustion analysis done if you or someone else does this. In fact, since this is a multi-family installation, you shouldn't even be touching anything, so have a steam professional look into that possibility.

Don't use a setback temperature. Keep it steady and you'll have shorter calls for heat with fewer cycles. This is related to what @EBEBRATT-Ed asked above about if you could set it for more calls per heat per hour. Somewhat paradoxically, if you set your thermostat to do more calls per heat per hour, then it has to recover less to satisfy the thermostat each time, resulting in shorter calls for heat and fewer cycles per call for heat.

Is it possible that some residents have turned off their radiators, effectively making your boiler oversized? (Many boilers are installed oversized so your condition is by no means rare--installers over the decades often want to go to the next size up because they are cowards)

But like I said before, shorter cycles are better than longer ones that are doing nothing more than building pressure needlessly.

You're not going to hurt the boiler with that cycling. The gas valve does have a limited number of cycles in it before it fails, but it's rated for a few hundred thousand cycles and it only costs a few hundred bucks.

jhewings

You could try a higher cut-out, say 1.7psi, and a higher differential, say 1 psi. Also, some people here use a timer to keep the boiler off for a minimum time between firings. I picked up a 10 minutes delay on break timer for this purpose but didn't try it yet because my boiler usually shuts off by the Tstat at about 0.6psi