Setback thermostats and steam heat

[Deleted User]

that any fuel saved by set back is used up reheating pipes that cooled down. i think the cooler pipes would be condensing steam till they warm up again. i tell our customers not to set back their steam systems. good subject as i have nothing more than a gut feeling on this. no scientific data to support my way. i look forward to added posts on this one.

Unknown

Setback 'n steam

Mad Dog and myself were having a conversation yesterday about a vapor system with convectors in a big house that had a setback thermostat on it. Still had an old coal water tube boiler, converted to oil, in the pit.

Discussing the thermostat, what are the reasons that a setback thermostat is less desireable than a T87F round one?

My feeling is that during the long cycles coming out of setback, the boiler short cycles on pressure. The radiators get hot much farther across than the outdoor air temp would require, and overshoot a whole lot.

I think that those things generate service calls, when actually, the old system was designed to be run on coal, with a human regulating the fire size instead of a pressure controller. I think overshooting in the morning was prevented by that human not wanting to waste coal.

To take that same 80 year old system and ask it to maintain a set temperature all day and night, and the human only checks once a week, is remarkable. To correct how it acts coming out of setback is beyond my ability, without a high/low fire option on the burner.

HHhhmmmmm, now there's a thought.

Therefore, I can suggest that the temperature be left at one point. This usually makes the run cycle length match the weather, and likely as not, the pressuretrol never will shut the burner off. The room thermostat will. The burner won't short cycle.

Balance in the system is important to getting quick, balanced cycles without cold areas. Venting the system and boiler size are key. So is insulation on the piping. All the regular things. This all adds up to shorter run cycles and better fuel economy. I think so, anyway.

What other comments do you have about setting back steam systems?

Noel

John Cockerill

set backs?

BTU's are BTU's. If you interrupt the supply, you have to make them up. Set backs are alright if you are not too severe and you can anticipate the recovery time. Its like trying to save money by running a ship slowly. You cannot do it and have a good trip. The only time a setback works on a slow system is when the set back is for all day when you are at work, on vacation, and you have it come up to speed with the time it is required to meet comfort levels.
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Mad Dog

my two cents...noel's a genius and

it is true...many many service calls involve the setback t-stat as the culprit. Mad Dog

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Boilerpro

Some positive experience with steam setbacks

I believe they can be done and will save money in most cases, if the system is set up for them. The big problem is the overheat due to all that hot iron continuing to heat the spaces after the steam shuts off. There are a couple of solutions I've seen and used. One is to get rid of unecessary radiation if possible (one and two pipe)or throttle what you do have to the current heat loads (especially two pipe) so the rads no longer heat all the way across when full steam pressure is available. Another simple trick is to use two time and temp settings when coming out of setback. If you want 70F at 8:00, set the first temp at 65F at 6:00 and then the final at 70F at 8:00. The hot rads will have a chance to cool and coast the temp up to 70F between 6:00 and 8:00. Related to all this is keeping the boiler as small as possible so its takes a couple of hours to build a full head of steam....just like a coal fire slowly building. Besides, if you throtttle the steam inlets to the oversized two pipe rads, you should be able to downfire the boiler closer to you actual load since the steam never sees the full installed radiation. I also only use Honeywell Chronotherms for setback on steam and gravity hot water systems....they seem to be able to anticipate overshooting better than others.
As to savings, the heat energy released by the piping, rads and the furnishing and building materials during the cool down period is equal to the heat needed to warm them back up (good ole "conservation of energy" theory in physics). However, once we hit setback temp after cooling down the heating system runs less because there is less temperature difference between the indoors and out. To put it another way, keeping a structure at 60 when its 20 out takes the same energy as it does keeping a structure at 70 when its 30 out. A 10 F setback equals it being 10F warmer outside.
Another factor is that the only time you get peak efficiency out of a steam boiler is when the burner is running constantly, which it is much closer to doing when trying to recover space temperatures. I'm sure some boiler efficiency is lost since it is running a few degrees hotter since it is running at higher pressures, but the overall is probably a plus. If I see a boiler short cycling on the high limit control when coming out of setback, I 'd think it is very likely that boiler is too big for its job. good topic, Noel!

Boilerpro

Steamhead

My 3 cents

you're both steam geniuses! I recently resurrected an old Paragon 7-day timer/dual thermostat setup in an old factory building with one-pipe steam. When I get an idea of how well it works I'll let you all know.

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Unknown

Set Back

You said the magic word when you said Vapor System.

The pressure controller need to be a real vapor stat.
Try using a Honeywell L408A1157 0-4PSI operation
Drop back 0-16oz.

Set control at 1 PSI with adrop back at 4oz.

Put in your set back Tstat and do not drop down more than 5 degrees.

Every thing will be fine and fuel will be saved.


Jake

David Efflandt

The key is to ramp up

Setback can work IF radiators and boiler are properly sized and IF you use a thermostat that ramps up gradually, instead of all at once. But that first IF is a big one.

My 1-pipe radiators are balanced and ramp up without overshooting a Honeywell CT3500. But my boiler is oversize so it tends to frequently cycle on pressure when hot. So I just use setback in mild weather when instead of cycling every 3 hrs, it does not have to run all day (solar gain).

David Efflandt

The key is to ramp up

Setback can work IF radiators and boiler are properly sized and IF you use a thermostat that ramps up gradually, instead of all at once. But that first IF is a big one. The boiler should work most efficiently when everything is still warm.

My 1-pipe radiators are balanced and ramp up without overshooting a Honeywell CT3500. But my boiler is oversize so it tends to frequently cycle on pressure when hot. So I just use setback in mild weather when instead of cycling every 3 hrs, it does not have to run all day (solar gain).

David Efflandt

The key is to ramp up

Setback can work IF radiators and boiler are properly sized and IF you use a thermostat that ramps up gradually, instead of all at once. But that first IF is a big one. The boiler should work most efficiently when everything is still warm.

My 1-pipe radiators are balanced and ramp up without overshooting a Honeywell CT3500. But my boiler is oversize and tends to frequently cycle on pressure when hot. So I just use setback in mild weather when instead of cycling every 3 hrs, it does not have to run all day (solar gain).

David Efflandt

The key is to ramp up

Setback can work IF radiators and boiler are properly sized and IF you use a thermostat that ramps up gradually, instead of all at once. But that first IF is a big one. The boiler should work most efficiently when everything is still warm during ramp up.

My 1-pipe radiators are balanced and ramp up without overshooting a Honeywell CT3500. But my boiler is oversize and tends to frequently cycle on pressure when hot. So I just use setback in mild weather when instead of cycling every 3 hrs, it does not have to run all day (solar gain).

Unknown

way ahead of the curve on vaporstats

Did I forget to mention each control? The system never reaches 8 ounces, Jake. I set and measure steam pressure with a U-tube manometer.

My problem with setbacks in general, is that old systems with oversized radiation do overshoot. If the system is balanced for quick heating cycles (most radiator valves wide open), the long cycles tend to overshoot and can cause service calls.

The solutions suggested all seem to agree, and the answer is to ignore it.

Well, I kbnow that, already.

My reasons for staying away from them, is that they cause service calls when nothing is wrong. It's the call I'm trying to prevent. I don't reccomend setback thermostats on steam, and if you do put one in, it won't be me explaining this to you. It will be someone else, and he likely won't be paid for explaining that. That is where I'm coming from.

This works for all parties. Those people that do put them in get the calls. They have their own reasons for using them. For me, a T87F is good. If you turn it down or up, it is a concious action. If it overshoots then, the operator can tell that he or she caused it. I can live with that.

As far as one pipe systems that don't have oversized radiation, there aren't many. Steamhead built one, but the one I built had oversized radiatiors. Tough to find little tiny used radiators.

Setback thermostats do work, if you ignore the overshooting temperature. That seems to be the general conclusion.

Noel

Steamhead

Vaporstats and overshooting

If a system is overshooting a Vaporstat can help. When running ounces instead of a pound or two, you're compressing less steam into the rads. So there's less steam condensing and giving off 970 BTUs per pound of latent heat.

While this may not completely cure the problem, it will certainly help.

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Boilerpro

I believe the internal logic of Honerywell stats

is different than other brands. They do not run the system full out during recovery, so it brings the spaces up to temp more gradually. You got to remember that it takes awhile for furnishings etc. to warm up, so you don't want to heat up a space too quickly. Then all you have is 70F air, and 62 F furnishings. You also end up with huge stacking of air....probably nearly 100F at the ceiling and 55 at the floor, certainly not good for fuel efficiency.

Boilerpro

Boilerpro

Noel , have you tried a Chronotherm

I mentioned them up above. I believe thier internal logic is different, so they tend to not overshoot. I don't have a single system that overshoots with them, even gravity water conversions, but they do have new boilers sized to the heat loss.

Boilerpro

BillW@honeywell

Programmable stats

When you are using the older mechanical clock stats like the T8090 or T8095, they simply end the setback period at a certain time. The new Chronotherm 4's have what is called "Adaptive Intelligent Recovery", which is a control strategy that takes into condsideration what the temperature has been for the past three days. It automatically calculates and begins to ramp up the temperature so that it will be at the setpoint
when the system goes from the "sleep" period into "wake", or from "leave" to "return". Don't forget to set these new stats to 1 cycle per hour for most steam systems.

DanW

try an adaptive tstat

I have 2 pipe steam running with a Honeywell setback thermostat and it works great. We set back 10 degrees. Overshoot is nonexistent with their adaptive tuning technology. The only issue is that the BRs had a tendency to get ahead of the LR/DR (where the tstat is) because the radiation was redone there and is no longer as oversized as the sleeping areas which have original rads. A little throttling of the inlet valves solved that.

Beyond energy savings many people, myself included prefer sleeping in a cool house, but just aren't thrilled with having to get up to kick on the heat.