Thermostat Question

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Old 10-23-06, 07:15 PM
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Thermostat Question

This is my first winter in my house. When I turn the thermostat, which looks like it could be the original thermostat on the 60 year old house (says Detroit), to a warmer temperature the thermostat triggers the boiler and the circulator pump. The circulator continues to run and the house warms up just fine. What I have noticed though is that the circulator pump and boiler will continue to run and the temperature will go above what I set it to on the thermostat. Since it is early in the season, I have not had the heat on for a consistent period of time, but the circulator pump has not turned off when the temp has reached or passed the set temp. For example, if I set the temp to 65, the unit will keep heating and the temp in the room will reach something above 65. However, if I manually adjust the thermostat down a degree or two, then the unit will turn off. If I then turn the thermostat bac to 65, the unit will not turn on since the current temp is above 65. I would have to turn it to something above the current temp for it turn on. From my limited experience in the house, it appears that the thermostat will not turn the unit off automatically. Has anyone encountered a similar problem? Wouldn't this problem be isolated to the thermostat rather than the boiler or the cirulcator pump? Ideally, I would like to get a new thermostat that can be programmed. As always, any general thought or insight is appreciated.
 
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Old 10-23-06, 09:17 PM
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Thermostat Operation

jjobev:

You have to avoid thinking of a thermostat for heating equipment as the same as a volume control on a radio or television set; the T-stat is basically a low-voltage switch that closes when the room or house gets cold, & is known as a "call for heat".

When the switch closes it "pulls in" or closes small electomagnetic switches on the boiler control (aquastat); if there is already hot water in the boiler, the circulator will be activated; if the boiler water is cold, the burner will also be activated to heat the water.

This process will continue to circulate hot water thru the convectors until the T-stat is satisfied & it opens, but the cold water that was in the radiators or baseboard will mix with the warm water in the system & this will keep the boiler firing until the "high limit' of the aquastat (usually 180 degrees) shuts off the burner (but not the circulator).

With all this temp variation of the boiler water, T-stats are designed to have a temperature "overshoot" and temp "undershoot" of ~3 to 5 degrees F., called a differential; so a T-stat setting of say, 70 degrees, will approximate that setting, give or take approx. 5 degrees.

Some thermostats have an adjustable heat anticipator that can be adjusted to lengthen or shorten the heat cycles; if you take the cover off your T-stat, you may be able to see it; a small screwdriver or pencil tip can make the adjustment; Google the name & model of your T-stat to see if there is further info.

But also look at other things; if your room is very drafty with loose-fitting windows, no storms, no insulation inside the walls or ceilings, tighten up the building; also make sure there is no air in your convectors; use bleed screws on the convectors to vent air out of the system.

You'll be amazed at how your boiler no longer cycles on & off so much when you add more insulation, caulk & seal every window, add insulation to the attic & exterior walls, etc.; you'll also save a lot more on fuel expenses; when a building is insulated & tight, it has a lot more "thermal mass"; it holds the heat for a very long time, & thus the boiler doesn't have to come as many times as before.

Heating plants are not designed to have very short on-off cycles; it wastes fuel & is hard on the equipment; if the rest of the equipment is as old as the T-stat seems to be, have it checked out by a contractor to see how efficient it is burning the fuel; if it is less than 70% efficient, consider buying a new boiler.
 

Last edited by jack horner; 10-23-06 at 09:34 PM.
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Old 10-24-06, 05:08 AM
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Thermostat Question

Thanks for the helpful information. I guess I need to just let it run for a prolonged period of time and get a better feel for how my system operates and what the usual overshoot is. If the unit does turn off if I lower the thermostat, then I would imagine that it will eventually turn off when it reaches its overshoot temperature.

You are correct in the assumption that the unit is old, but it is not the original heating unit for the house. We had a professional service the boiler this year and while he agreed it is old (about 25 years), he didn't recommend proactively replacing the unit. This is my first house and I grew up with a heat pump, so this is all new to me. I am not used to the heating unit continually running for long periods of time, but based on what you said about not having short on/off cycles, maybe things are working fine.
 
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Old 10-25-06, 03:48 PM
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Changing out the T-stat to a programable one, is still a good idea.

The new ones have a temp range adjust (as described above) I would not recomend settings any closer than 3 deg.F. otherwise you get the formentioned "short cycle".
Also they have a switch inside labled for- Gas-elec or oil-
Set this to the correct source/fuel type.
 
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Old 10-25-06, 04:54 PM
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Anticipator

Your hydronic system will continue to dump heat into the room after the stat is satisfied. Hot water remaining in the radiation system will continue to warm the room--even if the circulator has stopped. This is especially true if you have cast iron radiators or CI baseboards, they really hold some BTUs!!

Most older mechanical stats have a heat anticipitor to correct the problem. The anticipator is basically a small electric resistance heater that warms the stat while heat is being called for, and fakes the stat into thinking the room has reached temperature before it has. Try adjusting the anticipator setting.
 
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Old 10-25-06, 05:17 PM
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Wink

On that anticipator setting. If its there just push it down one lower # most of the time push it to the right.
The thermostat triggers the boiler and the circulator pump. Might check that out. Most of the time The tstat just turns the pumps on for that zone. The boiler run on its own aquastat pre set on at 160o and off at 180o.


ED
 
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