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1. ## Feet head resistance - 25 y/o boiler?

Hi.I currently have a 1986 Trianco oil fired boiler. I beleive the model is DB125. I've done *tons* of searching and have been unable to come up with any details on this unit other than the ID plate on the front which says it is 125000 BTU, 108000 BTU net and that it is likely a steel boiler.

I am in the planning stages for the addition of an indirect water heater. At the moment I am trying to properly size the circulator pump for the indirect zone. The problem is I need the resistance (feet of head?) for the boiler itself and honestly do not have a clue what to use for this in the calculation and doubt I am going to find anything in print for this boiler. As much as I like to do things right, I may have to just take a guess at the feet head for this boiler. I was wondeirng if anyone might know that what the typical feet of head resistance is for a 25 y/o steel boiler of this size?

Thanks very much for any insights you can provide

2. The head on a cast iron or steel boiler is typically small relative to what it is on the coil in the indirect, but as indirect heat exchangers have improved it's not always the case. But if you assumed the head on the boiler is zero and size the pump for the head on the indirect + plumbing + 25% it'll usually be good enough. If you're cutting it close you can assume the boiler's head to be something like 1-1.5 feet at ~10gpm, (in most cases it's less than 25% of the total.)

The gpm specs for getting the first-hour hot water gallons into the indirect aren't always sufficient to keep the boiler happy. On an old-skool cast iron boiler need enough pump to keep the delta-T on the boiler within spec- shoot for 25F or less unless you know for sure it can take more.

An output of 108KBTU @25F delta-T is a flow of (108000/25F=) ~4320lbs/hr, which is (4320/60=) 72lb/min, which is about (72/8.34=) 8.6gpm. If you sized the circulator to be able to deliver 11gpm+ against the head of just the indirect + plumbing you'll be good, but 10gpm might be cutting it close if it's a low-head indirect. There's little downside to erring the high side on flow- high flow=higher turbulence=better heat transfer efficiency in both the boiler and the indirect. You pay a bit more in electricity, but duty cycles are small. The bigger cost will be in the pump itself.

A primer on the subject lives here:

http://www.comfort-calc.net/circulator_sizing.html

3. Dana,

Thanks for the great info. Really appreciated. I'll need some time to process it. May come back with a follow-up question...

p.s. I had seen the primer you refer to. In fact, it is what prompted me to ask the question.

Thanks again!

-Matt

4. As long as the indirect is not a half a mile from the boiler a Taco 007 will work just fine. Do not reduce the inlet and outlet pipe size at the indirect. Your Trianco is steel. We used to sell them by the trailer load.

5. Peter: Out of curiosity, what is the typical delta-T tolerance of steel boilers vs. cast iron? (A lot of CI beasts specify 30F or 40F max operating delta- I pulled out 25F to be conservative.)

6. Depends on the manufacturer, though most steel boilers are less tolerant of large temperature differences due to thermal expansion. If the numbers don't meet up we will pipe a bypass with a ball valve on it from the feed to the return to ensure we don't get thermal shocking. It was a big problem early on with System 2000 boilers to the point that they eventually tapped the bottom for an aquastat control to keep cold water from returning.

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