t-stat choice & reason

Jimbo_5

I have a Weil Mc CGi3 that fires up for 10 minutes, causing low supply & return temps. Having been told by Honeywell that the t-stat automatically cycles the unit 3 times per hour. Is this a good thing, or a bad thing? Or am I netter off with a simple yoyo since they don't use the programable feature anyway. Or is it a benefit to fire the spark-start boiler in a dank basement regularly despite the 10 minute running with 118° supply and 103° return? I am very curious which is the better choice.

NYC007

What application is this being used for ? Residential Home ?

Jimbo_5

Residential. Do you think there is an advantage to using the digital which cycles the boiler rather than a simple yoyo?

Jimbo_5

Let me try it this way, does anyone use simple t-stats on new gas, hot water installs? Or is the digital, programmable today's standard. I am thinking of replacing the digital in favor of the round yo-yo to stop the hourly cycling low temp water and condensating. Suggestion????? Ideas?????

FranklinD

I used a Honeywell T87 basic round set for 2 cycles per hour for a season. It operated the exact same way as my basic Honeywell digital, so I went back to the digital (also set for 2 CPH).

The thermostat really doesn't have much of an impact on your water temp. That's just a result of being able to heat your place with low water temps. I heat my house, at zero degrees outside, with 125* water to maintain 67* inside. I have big old cast iron radiators.

My boiler can take return water safely down to 110*, but I will be installing a boiler bypass this spring since my water temps in the 'shoulder seasons' rarely top 100*.

Edited to add: fewer/longer cycles WILL raise your water temp, but will probably cause larger temp swings in your home.

Jimbo_5

Thanks, but I'm still twisted about what's happening here. Sitting in the house I can hear when the boiler fires. So I go down to the basement with my digital thermometer in hand. By the time I start checking supply & return temps it barely reaches 118° supply and 105° return before the unit shuts off. This continues for much of the day. In fact, I didn't hear the boile fire and get up to it 180° setpoint at all. Very strange. Something does not seen right. The OAT was mid-20s and the t-stat was set at 65° in a 120 year old house with no wall insulation, good storm windows, unheated basement. Ideas???

Gordy

Bottom line your over radiated for the tstat setting of 67*. That's not a bad thing

The present tstat is not in an area influenced by other sources of heat is it?.

Jimbo_5

Thanks. No the t-stat is not. It's sort of funny, but I don't remember it running like this a few years ago. No changes were made. I'll have to camp out in the basement and monitor the cycling, temps of S&R, and see what's what. Thanks again.

icesailor

Set the CPH setting to "2".

Again, Is the circulator running and does it continue to run after the burner shuts off? What do you have the High Limit control on the boiler set at? If it is set to 180 degrees, and the burner is stopping ay 118 degrees, AND the circulator is too, you probably have a bad high limit thermostat. Especially if you have an electronic thermostat and the CPH is set for 2, and the circulator stops when the burner stops. Unless you have some complicated wiring scheme that some Sparky took liberties with. And we know how that can work out.

Do you have improperly wired zone valves?

SWEI

FranklinD said:

I heat my house, at zero degrees outside, with 125* water to maintain 67* inside. I have big old cast iron radiators.

My boiler can take return water safely down to 110*, but I will be installing a boiler bypass this spring since my water temps in the 'shoulder seasons' rarely top 100*.

Why not a thermic boiler protection valve -- or better yet, a motorized valve with ODR control?

FranklinD

SWEI: I'm still in the process of figuring out what I want to do in regards to boiler protection. Haven't decided yet. I'll start a new thread when the time comes... -10 is too cold for me to start cutting pipes.

I just wanted to relate my situation to the OP's...the boiler can satisfy the thermostat without ever hitting the high limit. That's how our old boiler installation was too (in fact, that's how most boilers are in my area. No bypasses or resets or anything, just a new cast iron boiler direct piped to a 100 year old system of cast iron radiators).

For example, if I wanted my boiler to hit the high limit, with it direct piped, and the circulator running, I'd have to run a 25* setback on my thermostat. Turn it down to 50 at night and 75 in the morning. Then during that one loooong morning cycle, I just *might* hit 180 degree water and cycle the burner on hi limit. Maybe.

icesailor

So set the high limit to 160 degrees and let the burner cycle off. As long as the circulator is running, it is heating the building.

Its a gas boiler. Condensation isn't considered an issue with gas boilers until below 120 degrees. Or so I understand. If it doesn't reach set point on the thermostat when it is cold, turn up the high limit to 170 degrees.

IMO, boiler by-passes can be over rated. Look at the Google Earth link I posted. There's a big man made bypass made. Look how much Mother Nature liked it. She ignored it. So they gave up on it. Water goes where it wants to go.

If in your heart of hearts you want a boiler by-pass, put in a 4 way mixer. That is just another kind of bypass.

Gordy

Does everyone understand that the high limit is just that a high limit? It means you don't want the boiler to go above that for what ever reason.

Just because it's set to 180 does not mean it will get to 180. Hitting the high limit will depend on the length of the heat call, and or the heat loss, and or the emitter sizing compared to the heat loss.

If it does not hit 180 so be it. It can if it needs to. The boiler will shut down when the tstat reaches set point. At that point the boiler could be anywhere between 180, and low limit, or below depending on where the boiler is at in its own cycle point.

Your not saving any money by lowering the high limit from 180 to 170 to 160. The boiler is firing full input until it stops firing. When it reaches high limit setting. So that being said whether you burn a long one never maybe hitting 180, or you do several short burns bouncing off a lower high limit setting your probably still burning the same amount of gas if not more because the flame can never get stabilized with short burns 10 min is usually the time it takes to get an efficient burn cycle going.

Seems like everyone is diddling with their hi limit thinking it's saving them money. The only emitter high limit needs to be watched is with radiant panels, with a dumb mixing valve. The higher the limit is set the higher the supply temp can get, and that could mean trouble for ceiling radiant inbedded in plaster, or concrete, or hardwood etc.

icesailor

I used to explain "High Limits" to people like this.

There is this high limit, see. And it has a setting that it doesn't allow the system to get higher than that.

You have this thermostat. That tells the boiler to run until told to stop, or get so high and hot, that the boiler explodes. Before that happens, the high limit tells the burner to stop.

So, it does.

As it was once explained to me,

The power comes from the electrical panel. It goes into the boiler wiring. Each safety control is a switch. Each and every switch along the way MUST be closed for the current to travel to the next one. If the next switch is open, it can't go farther. The last rung on the ladder is the High Limit. The last ladder on the rung is the burner. As long as that is closed, the burner can run. If it is "Open", the burner can't run. If the burner is running, and you open (turn down) the high limit, and the burner doesn't stop, or the flame doesn't stop, something is wrong and needs immediate attention.

I saw old gravity systems that were converted to being pumped. On warm days, the thermostat opened long before the temperature came even close to High Limit Land.

icesailor

@Gordy:

I thought of it like this.

I boil eggs to make egg salad.

I put 14 eggs in a pot and cover the eggs with cold water, put it on the stove, cover the pot, and turn the burner on high. Once I see the pot starting to boil vigorously, I turn the burner way down so as not to splash steam and water over the stove. I boil them for one minute and turn the burner off. I let them sit for 10+ minutes, cooling down. Then I run the put under cold water . I peel them as fast as I can while hot to warm. The shells come off easily. If I let them cool, the shells don't come off easily. I get perfect eggs.

Unless I out the eggs in a pressure cooker, I won't get the egg water any hotter than 212 degrees. The eggs will cook just as well and hard at 190 degrees. It just takes longer.

Just like heating a building.