Modernized my controls, adding new features and adding data logger

I have added a Dwyer Instrument temperature switch to my 1989 Essex boiler. The switch is used to turn the boiler off when the fuel is gone. I found that my Honeywell dual set point thermostat was ready for replacing.

I had wanted to add circulator pump start temperature on this thermostat and it would have worked but my clumsy work overstressed the contacts. Since the Dwyer Instruments worked so well in the first application and it was less expensive, I decided to switch to one of Dwyer Instruments dual set point thermal controllers. It was easy to install, just hook up power, move the switch wires from the Honeywell, and connect the temperature probe. Then using the front panel keys I could program the set points. There is a bunch of functionality, but in my case I just want to get it into service.

Here is a picture of the new installation, the old Honeywell part is sitting on the top, the two Dwyer Instruments switches are on the front panel, as you see they also display the current temperature.


If you are thinking about replacing your older controller parts, or adding a shutdown controller for when the fuel is gone, then here is an option to consider.

New features and data logger on next post.

I have created a new schematic and then removed all of the obsolete parts. The picture also show the Control by Web data logger connected to a wireless router, I can either log on to that router or load the web page from any Internet connection (the data logger has security). Note how empty box looks now.

The new features are:
1) automatic shut down when out of fuel, using a K type thermocouple in the firebox (I tried flue gas which works too but I like the fire box measured temperature change better).
2) selectable circulator control
3) fan/pump run indicators
4) one button start, master power off switch

Here is the Essex boiler new schematic:

Yes, Scott that is the thermocouple version. Nofoss has a good point, when you put the probe in the chamber you want to get it close but not in the secondary burn. I used the sight tube and then move the probe in from there, at 3 inches in it isn't even up to the temps found in the flue. The probes I am using has a max temp spec of 2050F, which is right at the theoretical burn temp., it is not necessary to continuously measure that high. I purchased mine from Grainger, and went with 18" (I knew 3" was way to short, I wanted it to be too long).

The challenge with the flue measurement is that if you have the boiler at 180F then the flue temp will hold there even with the fuel gone (it resists going down any further), if your boiler is at a lower temp and it runs out of fuel it will drop to that temp and then hold. It is important to have a low set point so that when the high limit stops the burning process the flue gas temp will be above the fuel shutdown set point. I wanted a measurement that was less sensitive to the boiler water temp. When the probe is in the fire box it will have a lot larger swing and the temperature drop is slowed down by the refractory. All of this makes it easier to pick a temperature set point that will shut the boiler down when the fuel is gone.

With all of that said, a 180F flue gas set point (putting the probe in the flue) works pretty good. In that case I use 2 degrees of hysteresis (temperature must drop by before restart), which equates to about 10 minutes.

The above schemo should be applicable to any boiler.

I do use an "Off delay" timer for start.

You would want a duty cycle type of timer (I don't know the real name). It would closed for a program period every so many minutes. I did remove one of these because the device had a fixed duty cycle of 15 minutes on and 180 minutes off. It was used to keep the fire alive in the case where the machine idled.

The thermocouple switch should handle this job; it should allow the boiler to stay on (idle) as long as there is fuel. In my configuration the circulator continues to run after the hi limit switch is reach (I also added a switch so that I could chose to let the circulator run full time even when the safety SW is open to help cool down the boiler or let the TC switch shut it down).

A quick search found this, go down to programmable relay:
http://www.nteinc.com/relay_web/relay.html

here is a Dayton one, a little overkill you can go to Grainger.com and use a time delay relay search:
http://www.grainger.com/Grainger/items/6A855?Pid=search

you will find relays as low a $13, the lower cost ones tend to have low contact current specs, but would be OK if you were just closing another relay.

I watched my boiler run out of fuel tonight to see the difference between shutting down with a flue gas measurement and the fire box measurement.

The flue gas measurement is much more touchy, I normally set it at 180F and that works, but the boiler will hold the temp near there and that is why it is more difficult to use.

The fire box temperature (which is not in the turbulent gas) is a lot more forgiving. I set the shut down temperature switch at 200F the probe measured between 300-500F when it has fuel. When the fuel was gone the temp migrated down to 200F, in about 20-30 minutes. That is plenty of time for the ciculator to move the last bit of heat out of the boiler.

It should also be plenty of time for the case when the boiler needs to idle. During idling the drop in temperature should be even slower because the pyrolysis process (chemical decomposition in the absence of air) will continue to release heat energy. I will measure that tomorrow.

Scavenging heat is a good move. I removed the glass window (about 2" around) in the rear of the boiler and mounted my probe on a steel replacement part. I only need to put the probe in about 6" so a 12" would have been a better choice than the 18" I purchased. Yes, I did not want to drill an errant hole.

I think my boiler hold about 50 gallons and after 24 hours it is down 50F. Where my storage, which is ten time bigger is only down about 15F (at the top) while it has supported the house load for almost 20 hours. I was thinking about scavenging the boiler heat for the DHW. I haven't decided what clever way to do this; I am currently thinking about using a zone valve to modify the path of the DHW floor heat loops (yes I have two small loops that heat the kitchen, bathroom, and laundry room floors). I kept the older system because it allows for backup heat. So now I am trying to trick it into dual function, floor heat and DHW (I do have 100 gallons of DHW storage to use).

JR, I did modify my method based on you idea. The control schematic I came up with has two modes: 1) off with fuel, 2) off with boiler temperature. I flipped the circulator to the switch so now it shuts off with the boiler temperature.

That gave me about an hour of circulator run time after the fuel was gone to collect more heat for the storage tank.

You timer idea would work automatically if you added it to the "fuel is gone" switch. Set the timer for one hour; in that time the boiler temperature would be lower than the storage temperature.