Digital Thermostats and Steam Heat

Jim Murphy

Hi all,

Hoping for some help. 3 weeks ago we had a new replacement Utica steam (one-pipe system) boiler professionally installed. I noticed some short-cycling and surging so I skimmed and blew down the boiler after about a week of steaming.

It continued to short-cycle and surge so I went back to the manual and saw that it said that the thermostat anticipator setting might be the problem, so I cranked it up to 1.0 - the highest setting (the Utica installation manual recommends starting at 1.2 for steam). This seems to have solved the short-cycling problem.

I'm considering changing to a setback thermostat (Honeywell CT3500 has been recommended) but I came across this on Utica's website -

"Q: My new steam boiler cycles very frequently, what could be wrong?
A: A new steam boiler could be short cycling due to a number of reason. The first may be something as simple as having installed a room thermostat that has an adjustable heat anticipator setting with in it. This setting needs to be at 1.2 for the longest cycle setting as possible allowed by the thermostat. The best thermostat for this is the standard Honeywell T87F round design unit that has this internal setting on the unit. The new digital thermostats of today are timed and many do not offer the option to give you a set point to satisfy the system requirements."

Now I'm wondering if I should switch to the digital setback thermostat.

Can anyone help....

Jim

Firedragon_2

Try this link:

http://content.honeywell.com/yourhome/ptc-thermostats/Therm_Choose.htm

Steamhead

I've had good results

with the Honeywell T8600D digital programmable unit. This has a different program for every day of the week, and doesn't even need batteries. You can set the number of cycles per hour from 1 to 9- try 3 for a steam system. I like this unit so well, I put one in my own house.

If your system is slow to respond to a call for heat, the type of thermostat won't make a difference. In this case, you may have dirty water, bad, missing or wrong main vents, uninsulated pipes or some other problem. Fix the problem and watch your fuel consumption drop.

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Mad Dog

The T-87 is Idiot -proof.......................................

..thats' why I like it...set it and leave it. Mad Dog

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Unknown

I'd get the system in shape first

by setting the day and night setting to the same number for a while. Once you determine that the system is balanced, not overheating, and without any "wild" rooms that are cold or hot, only then put a setback program into it.

It'll make it easier to tell which problems are system problems and which are thermostat problems.

Noel

Boilerpro_3

The Wisdom of Noel strikes again.....

and I'd pay attention to it. Setbacks can make alot of things happen to a steam system that's not set up right and are often masked when temperatures remain the same all the time. Get the system in proper tune and then look to setback.

I believe that anticipators of some sort are definitely needed on all steam systems even without the great big old boilers. This is because the radiation is often grossly oversized for the load, sometimes from when the system was first sized or because of improvments like insulation, storm windows, etc. On these systems, if the thermostat only shut offs on setpoint, you tend to get lots of overheating because the radiators stay hot and continue to heat the space long after the boiler shuts off.

When using setbacks, this is like trying to stop a freight train moving at 60 miles per hour. You've got full steam, the rads are piping hot, and then you reach your warm up set point and need to hit the brakes. The system often will eventually coast for a couple of miles to a stop at about 8 degrees above your setting.

I have found that this is not a problem when using the Honeywell Chromotherm stats. Thier internal logic cycles the system on and off when coming out of setback to bring the space more gently up to temperature, so it prevents overshoot. Most stats just run the system "full steam" until the setpoint is reached, cauing overshoot in high mass hot water and steam systems.

Often this also results in "cold 70" because the air may be at temp, but everything you touch is still at 62.... creating quite a chill. If you went to a typical elementary school, remember what it felt like in class on Monday mornings after a cold weekend, the air was stifling, but the room was cold (very low radiant temp, with overheated air).

Boilerpro

David Efflandt

All digitals are not alike

My old home came with a RobertShaw digital that had no anticipator setting (just gas/electric), and recovery was immediate from setback, so the best you could set it was +-1 (2 degree spread) and it would overshoot 1-2 beyond that.

Replacement Honeywell CT3500 works great. It has steam/vapor setting (max 1 cycle/hr, but mine never needs to cycle that often). It attempts to control temp within 1 degree, comparing wall and air temp with electronic anticipation, so it shuts off before setpoint, and rarely overshoots by 1 during sudden mild weather. And it has Smart recovery to ramp up gradually and recover from setback at set time.

I actually use setback most during mild weather to eliminate boiler cycles, since it takes less increase in run time to raise temperature than running a cold boiler extra cycles. But since my boiler is oversize and would pressure cycle too often during extended run time, I only use a little setback when cold enough to keep boiler warm, just enough to have radiators throwing heat when I get up in the morning.

I just wish I had gotten CT3600 which can log run time so I could tell how effective setback was. Or the Honeywell guy here said the 8600 models have the best learning logic.

BillW@honeywell

When someone asks which Chronotherm is best for steam...

I always recommend the T8602D2018. It's a battery powered, 7 day, 4 period/day digital stat with "Adaptive Intelligent Recovery" as mentioned above. Set it to 1 cycle/hour(CPH) when you install it, since it comes factory pre-set a 6cph. The CT retail stats DO NOT have the Adaptive Intelligent recovery option, so be aware of that. I prefer a battery-powered stat on steam systems, because some shut down completely during the off cycle, and can cause the power-stealing or hardwired stats to blank out. The T8602 uses regular AA batteries, and gives at least 6 weeks warning before batteries wear out. If the warning is ignored, the worst you'll have to do is reset the day & clock, but the program stays regardless. Good luck on your project!

kk_2

Uh, Bill... according to your own document...

http://content.honeywell.com/yourhome/ptc-thermostats/CT3600.htm

...the CT3600 *does* have the Adaptive Intelligent Recovery. I have one of those, and I remember the manual saying you can either set if for conventional recovery, or Adaptive Intelligent Recovery. What gives?

(I don't know about the other CTxxxx series controls).

-kk

Jim Murphy

I was away from the computer yesterday, but was shocked this morning to see the unbelievable reponse to my question. Thanks all!

Hard to decide (especially when the Honeywell rep makes a recommendation) but I think I'll go with the 3600 and follow Noel's advice and let it run without setback for a while. I'll let you know how it turns out.

What do you guys think about my surging problem? I skimmed the boiler for about 3 hours and followed the Lobenstal (sp?) (found in Lost Art) method and boiled the skimmed boiler water on the stove which eventually produced no foam. Fired her up and she still surged. The water still appears clean 2 weeks later and I still have surging and water coming down from the top of the sight glass.

I should mention that there is a bullheaded tee at the top of the riser from the header, where the main takes off in two directions. I knew it was wrong as I watched the guy put it in, but the "wiggle words" accompanying the manufacturer's piping diagram coupled with a weak moment on my part conspired to allow me to let him put it in. I did say something, but he assured me it wouldn't be a problem, and I let it go. (I had already made him use 2 risers from the boiler tappings because the manufacturer's installation instructions said to, when he was only planning on one)

Also, the wet return is pretty plugged-up. I'm almost positive that if I bring up the surging he'll bring up the wet return and point to that as the source of the problem, not the bullheaded tee.

This guy was the low bidder but really did seem conscientous and did the right thing in lots of other ways. Gave me a skim port, used black pipe on my insistence despite the fact that he usually uses copper, used good valves, etc.

What should I do next? Clean again? With chemicals? Fix the bullhead first? Is he right - fix the Return first? Other ideas?

Thanks in advance for any and all responses,
Jim

Unknown

Can you show us a picture?

Noel

Jim Murphy

No pics until tonight

I'm at work today. Will try to take a photo and post tonight.

JM_2

For what it is worth

I use a depot cheapo LUX 500 digital thermstat in each of the 5 rental apts i own. two are forced hot water (oil), and three are steam (gas). They all work fine. on my own steam unit i set it to 58deg. all night, a few hrs before i get up it goes to 65deg then back down again all day. no problems at all. maybee it is so cheap it does not have a heat anticipator.

scrook_2

Hot Rad's and Temp overshoot...

If the steam system has good fast vents on the mains and good s-l-o-w vents on the oversize radiators, shouldn't the radiators only heat part way across (in all but the coldest of days) when the t-stat is satisfied, thereby reducing/eliminating overshoot? When a setback occurs the boiler will go off immediately, if on, and the temp will coast to the lower setpoint, and when coming out of setback, the slow vents should limit the rate the rads heat so the rate of temperature rise is reasonable, again allowing a minimal overshoot, right?

With fast rad vents and oversize rad's, I'll grant you it becomes like stopping a freight train -- she'll overshoot.

In any case (hot water or steam) a system with high radiation mass and oversized radiators is going to be prone to overshoot, so slow vents or lower water temp's are going to be desirable, and some overshoot on return from setback will be inevitable w/o a more sophisticated control system than an on-off t-stat (w/ or w/o intelligent recovery -- which just selects the correct time to start rewarming from setback so as to reach the desired temp at a predetermined time)?

Agree the system should be running smoothly before setback is invoked, and if electronic t-stats are used vs the classic round T-87, that cycle time should be set to 1/hr, or at most 3/hr for steam, and the vents limit the radiation response to that needed for steady heat.

As for setback, I like it for, if nothing else, that that I find it much more comfortable to sleep w/ it cooler than I find comfortable when up and about.

BillW@honeywell

A retraction...

The CT 3600 does have a form of Adaptive Intelligent Recovery, after you called this to my attention, I checked it, and you are correct. This is only true of the CT3600, none of the other retail models have it.

Jason Lambrou

No main vents

I just found out that my mother's house doesn't have any Main Vents-she has a gas fired steam boiler about five years old.There are about 20 houses in this street all built in the 30s and looks like they all never had main vents.
A few lucky ones had them installed when they changed boilers.My question is can I install a vent on the wet pipe
right below the main steam elbow?

Steamhead

Better location

is to drill and tap a hole in the last tee where a radiator line takes off. If you put it in the drop pipe at the end of the main, water may rise and flood the vent.

How long are the mains, and what pipe size?

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