1. In which area you live and ambient temperatures you usually experience.
Southern NH
2. House style and construction details.
1600 SF two floors, two heating zones, third zone for Indirect HW tank
3. Make, model and age of equipment related to the problem.
Thermo Dynamics S-100, Beckett AFG F-3, Tjernlund HST-1, Top Performer 35 gallon HW tank, HW 845A circulator relays X3, Taco Circulators X3, HW 8124 Aquastat, HW 7284 digital primary. Boiler and burner installed 1991, some part replaced on burner since then including motor and primary twice
4. Fuel type.
Oil
5. Water temperature and pressures of boiler systems.
Low Limit 160, High Limit 180, Diff 10, 20 psi pressure
6. What type of zoning do you have with your boiler system.
3 zones, 2 for heating, 1 for HW
7. Thermostat type.
2 digital programmable

8. The problem
Unreliable operation. Over the years I have had unreliable firing of the boiler causing the primary to lock out. The unit is cleaned and tuned annually and even the HVAC techs have trouble getting it to work. The latest cleaning they could not get it to stay fired, the burner motor would turn on and off rapidly, which is what has commonly happened in the past. The HVAC owner came out later and replaced the primary with a new Honeywell digital model which displays the cad cell reading. This worked, but the cad cell reading is 2100 ohms. My concern is that this high cad cell reading has the primary lockout on the edge, and as soon as the boiler gets a little dirty or out of tune, I will get a lockout, probably when I am away and my wife will be stranded.
The flame seems fine, and there is no smoking. I suspect that the large static plate that Thermodynamics specifies for this boiler (3 1/2 inch) blocks out too much of the light from the flame. The standard spec for the AFG static plate is much smaller.
Question: Does this seem like a reasonable conclusion? And does moving to a slightly smaller diameter static plate pose any problem? The 3 3/8 plate has a lot more open space just above the electrodes to allow light to pass through to the cad cell.
Any advice would be appreciated.
Mike

 

MM,
Does it go off on reset while there is a fire. If the cad cell doesn't see the flame it will shut down, but if there's no flame to see that's a different issue.
If you are firing between .75-1.25gph and have an F3 head you should have a 2 3/4 U static head according to Beckett specs but I do see where TD wants a 3 1/2" disk.
Has your pump ever been changed and not adjusted.
I found your boiler manual and it specifies a .75 80°B nozzle @ 140psi pump pressure which will give you a larger and possibly a brighter fire.
You might want to check your pump pressure for proper operation.

Good Luck

 

Thanks for the response. In the past, the unit has locked out with a flame present. It seems like the flame is just not quite bright enough, cad cell sitting at about 2100 ohms with burner running right after having been tuned and serviced. It is a new cad cell and a new primary. I'll check the pressure. How does static plate diameter affect the burner? I would like to test a smaller static plate but don't want to harm the system by doing so. Attached a photo of the current static plate showing the tight clearance around the air tube.

 

I've always been told that cad cell resistance should be below 1500 ohms for reliable operation. Do you have access to combustion test equipment? If so, try the smaller static plate. If cad cell resistance drops and you can get good combustion test numbers, I'd run it like that. Steve

 

MM,
It has to do with the air delivery and combustion. As I said for an F3 head Beckett calls for a 2 3/4" disk.
I don't know why Thermo wants the 3 1/2". You can try the smaller one, it won't do any damage besides effecting the combustion.
As was mentioned you should use instruments to set it up.

As far as light around the disk, the cell sees the light down the tube. That's why it has to be positioned right. There are openings in the disk so the eye has a clear shot at the flame.

Looking at your plate I don't see any openings. You have a place that looks like it should be a hole but it's not punched out.
Check the disk for openings. It may be defective.

 

Thanks for the responses. I was waiting for the weather to warm up before trying some changes. I tried a couple of smaller static plates and it didn't change the resistance. I then checked the oil pressure and found out it was set to 100 psi. I changed it to 140 psi and resistance dropped to ~1500. I am going to get another tech in to retest combustion as I don't have access to test equipment. Hopefully when combustion is set properly at the correct oil pressure the unit will operate more reliably. Not sure how previous techs missed setting oil pressure to correct value, is this commonly tested when a boiler is serviced?

 

The power vent is likely the source of your problem. Why anyone in his right mind would install a power vent on an oil fired piece of equipment is beyond me.

 

I think the builder was looking for the least expensive solution when the home was built. A lot of spec homes around here have them. Would love to get rid of it but chimney quotes were out of my budget. Any thoughts on other options?

 

Make sure the tech checks draft at the breech and overfire. One of the big problems with power venting is too much draft which will pull the flame away from the retention head.