Simple boiler control for off-grid?

The crux of the problem here is the burner control sensor within the boiler. It would seem that this component is too sensitive to the boiler water temperature, resulting in excessive actuation cycling.

If there is an inspection port or aperture on the boiler, you might be able to try the following suggestion:

Create a thermal shield around the burner control sensor by placing a length of ceramic tube in such a manner as to surround it. This ceramic tube would probably have to be epoxied or glued to the wall of the boiler at the burner sensor location. The opposite end of the tube would just be open, so that the tube fills with water and ultimately allows the sensor to heat up and cool down in a manner that is more 'gradual' than the original arrangement.

The length and diameter of the ceramic tube will probably be subject to experimentation. I would start off with a small diameter ceramic tube, large enough to cover the burner sensor. The longer you make the tube, the more hysteresis effect you will observe. ... Maybe a piece of vinyl pipe of some variety would serve as the ceramic tube. I would be wary of using ordinary PVC plastic pipe, since PVC is subject to embrittlement and weakening in thermally stressful conditions.

Regards, Dave Dallas, TX USA

Many thermostats use a small magnet to create hysteresis. With some, the position of the magnet -- hence the hysteresis -- is adjustable. With others, you may be able to bend the mount. Replacing the magnet with a stronger one (rare earth?) or mounting one where there was none, are other possibilities.

Jerry

Where is the primary thermostat located at. What is it measuring, firebox temperature or boiler water temp? Describe the boiler a bit more.

One possibility that might work, depending on your answer to my questions above, is to add a timer to the burner fan control so that it stays on for a time after the thermostat stops the auger. Depending on the mechanical configuration of the firebox, boiler drum and thermostat, this might continue to heat the system which will delay the thermostat from cooling. If so it will also increase overall boiler efficiency and lower your ash disposal needs.

Olive kernel may be cheap but the amount of power consumption the auger takes is proportional to heat needed, times efficiency. Changing the hysteresis of the thermostat will change the length of the on/off cycle but not the amount of on time and this is proportional to fuel moved to the furnace. Adding solar panels may be the way to go here. Not so cheap but you can't change the laws of physics. Look into some other method of moving the olive kernel other than an auger. This consumes lots of power. A hopper and conveyor would work.

Hello,

A temperature controller with adjustable hysteresis is easy to find at Omron. Choose the E5CN and you have a hysteresis that you can set at the value you want,1 or 10°C or more.

Hi,

Thanx to all who replied. I'll try and clarify a bit here...

Both stats are in a oil filled well which is immersed in water, thus measuring water temperature. Describe the boiler a bit Fairly crude, "cube ina a cube" with about 2" spacing left, back and right between them. The internal cube is the firebox, and one of its sides is the firebox door. There are several 2" pipes, left to right, at the bottom of the firebox which act as a heat exchanger as well as a "rack" to hold a log should you want to burn wood. There's also a few more pipes towards the top, again left to right, at 45 degrees to horizontal, as additional heat exchanger. Finally, at the top there's several tubes again, open to the firebox this time, front to back where the the flue gases pass through and into the back exhaust manifold.

This seems like a good idea. I can replace a plate on the door (made to take a normal oil burner) with some fireglass and play with the timer....

Alas, this is only so true Paul... There is a hopper on the system, and the auger sits under the hopper. My next move would be to replace the current auger motor with something a bit more efficient. The 4 pole motor is 180 watts; with a 50:1 reduction, I'm looking at extreme torque overkill....(I can turn the auger quite easily by hand even with a full hopper...

Solar panels have to wait....

I'm attaching a solution someone posted in another ng to control the temp with two tstats and a latching relay...I think it should work...

"You set the main thermostat to 25 degrees. You use a second thermostat,connected to the supply *after* the main stat and set to 45 degrees, to power a relay with contacts across the 25 degree stat.

Initially, both stats are closed and the 25 deg stat is shorted by the relay. The temp will rise until the 25 stat opens. No problem, the supply is maintained by the relay contacts. The temperature will rise until the 45 stat opens. This opens the relay, the 25 stat is already open - so no power then goes to the load or the 45deg stat or its relay.

Only when the temp has fallen to 25 deg will the main stat close, restarting the cycle. "

regards,

nikos

An alternative approach if you are so inclined:

Obtain a length of Pyrex glass tubing, seal one end and partially fill with a temperature sensitive opaque fluid, i.e. alcolol, oil, etc. Calibrate the tube with marks at the desired on and off temperatures. Place LED/Phototransistor pairs at each temperature setpoint, in a manner such that the opaque fluid intercepts each LED beam. Insert the tube assembly in the original thermostat well. Disconnect or remove the original burner thermostat. Depending on your choice of logic levels, a TTL AND gate and a TTL solid state relay will allow you to interface the new temperature sensor assembly to your burner and fan control.

N.B.: If you are not exchanging heat between your flue gas exhaust pipe and your return water flow, you are losing efficiency. A simple spiral of copper tubing around the exhaust flue and some appropriate insulation could significantly improve the system. This technique is always used in large scale power plant boilers.

regards, Dave

'Thou has past the double rocks and art in a foreign land' .... Medea

Commonly referred to as an economizer. Wonder why?

...

Large-scale boiler installations use forced and induced draft, and so can tolerate low flue temperatures. Cooling the exhaust makes it denser, to the chimney's efficiency as a pump declines.

...

Jerry

I had to read between the lines here to figure out your point which I think is that cooling the stack gas in a simple natural drafted firebox/stack setup by adding a heat exchanger in the stack causes the problems of decreased natural draft and clogging the stack with things like creosote. Olive kernels may not produce much condensates so the second problem may not be. The first problem is seldom a problem in this type of situation because there is almost always excess draft.

As an exercise in design possibilities this thread may be useful to us, but to the originator who wants to decrease his electrical usage from the auger without spending any money, it is useless. Changing the way the controls work will not produce much savings. Since he stated that the fuel is cheap, the only reason to increase efficiency is to lower the amount of fuel used and hence the power requirements from the auger. He needs instead to look at a motor that draws less current. The maker most likely used a motor 10 to 100 times more powerful than needed to prevent stalls. Replacing this with a smaller one and putting up with some jamming seems appropriate to me in this case. A Gerber wheel should be considered.

The situation here, as I understand it, uses a motorized fan which activates at the same time as the feed screw. In effect, this is a forced draft, and there should be sufficient draft for the boiler operation. An improvement in efficiency should occur if the return water flow can be heated by the exhaust gas.

If the system here were strictly natural draft, there might be a reduced exhaust gas velocity effect due to cooling. Whether this would be significant or insignificant would depend upon the coupling efficiency of the exhaust gas heat exchanger. In this application, I doubt whether it would be of consequence.

.... good point though.

....

Paul Montgomery made another good point about the motor. It is surely overpowered for normal operation, but using a motor that consumes less power needn't make jams more likely. The motor is very likely a shaded pole, and there's hardly a less efficient motor of commerce. Replacing it with a PM motor and a switching power supply could save substantial drain when the load is light.

Although fuel is cheap, feeding it is apparently not, Increasing the efficiency of the heating system reduces the amount of fuel to be fed.

jerry

From what I understand this guy is not on the electrical grid and is most likely getting power from solar, batteries and inverter. Switching to a DC PM motor would also eliminate the loss in the inverter.

Or, if the primary source of power from the batteries is 12VDC, just using a 12VDC motor and switching it in would do.

A good boost-buck switcher can be about 90% efficient even at quarter load and takes care of changing battery voltage. Running a 12V motor off a fully charged (13.5V) lead-acid cell wastes more power than the switcher consumes, and you still get 12V when the battery is down to 9. It's an extra that can always be added later. The major benefit is from the PM motor. With some sophistication, a brushless PM motor can combine the regulation with the drive electronics. The more I think on on, the niftier the possibilities become.

Jerry

True. I was talking about the lose from using an inverter and this does not have the efficiency of a straight switcher because of the transformer. It depends a lot on the quality of the inverter. I have a little cheap one for my car that will drain the battery in two days with no load connected.

I'm not for sure that running at 13.5 v will waste juice though. The motor runs faster which will auger faster and thus shorten the run time. I'll take your word for it though since I am too lazy right now to dig out the efficiency curves for dc motors. If there is a slight gain in power savings, I still would have to be convicted that the cost of the switcher and the increased complexity is worth it.

...

You may be right about the motor not staying on as long because it's running faster. With a brush-type motor, copper- or silver-filled brushes have slightly lower loss than pure carbon, and can probably beat the semiconductor losses in a brushless design. Squeezing every last bit of oomph out of a battery isn't something I'm accustomed to thinking about.

Jerry

I would hesitate to use metal brushes in a high-current application, though. While I've never been able to get a straight answer from an app engineer my understanding is that carbon brushes are more robust to momentary overloads, which is what you'd get with simple control of a motor.

Using a switching amplifier to drive a motor would let you bring up the motor voltage slowly, which would significantly reduce the current spike at startup over using a relay. If you didn't want to get a brushless motor and controller you could also use a correctly built switching amplifier to limit the motor current & hence be able to use metal brushes.

I'd just be crude and start with a PM brushed motor and a relay, then improve things later. This gets you going, will certainly show you if you've improved efficiency and even lets you use inexpensive automotive parts for motive power if you're feeling like a cheap skate. If it works you can eke out a bit more efficiency later.