Central Boiler cycle questions

Bob Lentz · November 20, 2022, 05:51:31 PM

So, i have a 750HD - something i noticed was that when we don't have huge heat load, the cycle from low water temp to set point can be rather long but the max temp in the reaction chamber gets to around 600 to 700 degrees - no higher. It never has to go into high mode with light load.

Does this seem "normal"? Today over an 7.75 hour time span, we went through 3 complete cycles.

Cycle 1 - 720am to 12:22pm - 156 water to 185. 5 hour burn, max react temp 1584

Cycle 2 - 1:15pm to 1:40pm - 174 water to 186 35 minute cycle, max react temp 559

Cycle 3 - 2:34pm to 3:11pm - 174 water to 185, 35 minute cycle, max react temp 511

My firestar controller setting for idle pulse temp is 200 degrees - maybe i should raise this? The reaction chamber is getting too cool?

Thoughts?

Thanks!

PoginyHill · November 21, 2022, 07:34:53 AM

The reaction temp wouldn't be impacted by your pulse settings. That is only to keep the fire alive. In my experience, low reaction temps can be caused by 1) wood that is not seasoned well, 2) coals/ash have plugged much of the clearance between the upper and lower chambers, or 3) not enough time during the cycle to get a decent fire going in the upper chamber before the setpoint is reached. If you have little or no load, then #3 is the likely reason. You can try spreading the difference between setpoints (longer fire time) to see if that makes a difference.

Bob Lentz · November 21, 2022, 12:58:12 PM

We did a complete clean, firebox air chambers, tubes and back side heat exchanger so i don't think we are plugged.

I agree with you that it's most likely load related. How would you extend the fire-time? larger differential between set-point?

PoginyHill · November 21, 2022, 02:29:41 PM

The plugging I referred to is the area on either side of the "charge tube" that separates the upper chamber (where the wood is) and the lower chamber (reaction chamber). Not the heat exchanger or anywhere else.

Yes, increasing the difference between the lower and upper setpoint will increase burn time during fire and lengthen the time between firing. I think the default is 10deg. To test out the theory, I'd go to 20 or more degrees. Then set to whatever you feel works best for your conditions overall. I normally use 10, but have used 15 before.

Bob Lentz · November 25, 2022, 11:34:03 AM

i think if i made any changes to differential i would do:
15 degrees
raise target to 190 from 185
that way the low point in water temp would still be 175

I find that if water temps get into the 160's, that it takes a lot of fire at high burn to bring water back up to set point during super cold months and high demand.