Boller and Circulating Pump Recommendations

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wjcandee

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A client recently purchased a home with a 24-zone hydronic heat system. Furnace has 20 old-timey Bell & Gossett circulating pumps of varying age running off a 175 KBtu Weil-McLain rear-vented oil-fired boiler that looks to be 25-ish years old. (This boiler is for heat only; domestic hot water is provided by a smaller boiler with a huge indirect tank.)

The client has two questions, based upon a recent experience where a circulating pump seized, water failed to circulate in a zone, the heating pipe froze during a deep-freeze here, and water poured out. He wants no more issues with the system, so the highest-level of reliability is the primary criterion.

Regardless of whether it seems sensible financially to replace 20-odd circulation pumps, would it materially-improve reliability to replace the B&Gs with a more modern, maintenance-free design? (About four of the b&gs appear to have crapped out already over the years and been replaced with another smaller design.) If so, what's the recommended style and manufacturer? Is there a downside to doing so, other than the cost?

Second, while I understand that a Buderus condensing boiler might raise efficiency substantially in the system (provided that it can be redesigned to bring the return water in at a low enough temp), is there a boiler that would be significantly-more-efficient than the 30-year-old one that nevertheless retains the present no-muss-no-fuss, highly-reliable nature of the present boiler? In other words, with reliability and minimal-maintenance (i.e. a couple of visits a year by the oil dealer) as the highest priorities, and return-on-investment being a lower priority, is there a recommended style and brand of boiler with a relatively-efficient design that is uncomplicated and rock-solid reliable?

Also, I see online where they seem almost to be giving away some "obsolete" Trane triple-pass-boilers. Anybody know why the prices are so low on these?
 
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Tom Sawyer

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Any if the newer cartridge circulators would be reliable. Several boilers would,be more efficient also. Any two or three pass design is going to operate more efficiently.
 

Dana

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Any high-mass boiler serving 24 zones is going to have short-cycling issues on zone calls, and nothing will run very efficiently when micro-zoned to the Nth degree like that unless you first size the boiler to the actual heat load, and slave it to a buffer tank, with the micro-zones pulling from the tank. The buffer tank should have sufficient thermal mass to deliver at least a 5 minute minimum burn time at some reasonable temperature differential, which is a pretty big tank for a 175K boiler at a 20F differential. At 80% efficiency that's 140,000 BTU/hr of output, or (divide by 60 minutes/hr) 2333 BTU/minute, or 11,667 BTU over 5 minutes. The amount of water it takes to raise 20F degrees with 11,667 BTU of heat is 11,667/20F= 583lbs of water. At 8.34lbs/gallon, that's a 70 gallon buffer tank. If the true heat load of the place is only half that and you can use an ~83,000BTU/hr oil boiler, you can get there with a ~35 gallon buffer tank.

This is not a cheap solution, nor is it something well suited to design-by-web-forum, but if efficiency and boiler longevity matters, it's at least a solution. Or maybe there already is a buffer tank in the system? Un-buffered the current boiler as-operated is delivering less than 70% efficiency, even if it's steady-state efficiency when tuned up is north of 80% (which it might still be.) It might even be as low as 50%.

Micro-zoning a 24 zone system with individual pumps is also a bit silly. If you're going to replace them all it's probably cheaper to buy one really expensive and smart ECM-drive pump, and zone it with 24 zone valves.

If the pumps are all 25 years old they're pretty much at end of life. Cartridge pumps will go longer, replacing the cartidges rather than the whole pump as they crap out in 20-25 years, but cartridges don't last forever either.

Whether pumps or zone valves, any system with 24 zones is going to have failures- which will occur in proportion to the total number of zones.
 

wjcandee

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Thank you, Dana and Tom, for providing such very-helpful information. This has really helped immeasurably!! Thanks again!!

One observation is that the boiler really doesn't short-cycle in cold weather. Rather, you will get overlapping zones calling for heat such that the actual on-off cycle of the boiler will be pretty long. One zone may be happy and stop calling, but in the meantime another has decided that it wants heat, so while pumps may turn on and off, at least one zone is calling for heat and so the boiler keeps rolling. I actually was concerned that the damn thing seems to go on for TOO long, but of course that's not correct: if 10 zones were one zone, it would still take a significant period of time for the boiler to provide enough energy into the system to raise the temperature overall in those zones, including whichever one had the thermostat. Given how large this house is, I actually was surprised at the BTU rating of the boiler; this one almost certainly replaced what must have been a larger boiler before the house was renovated.
 
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Dana

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Short cycling of micro-zoned systems is far more common during the lighter loads of the shoulder seasons than during the cold weather, where overlapping zone calls are effectively guaranteed.

Out of curiosity, how large IS that house?
 

Dana

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A homey weekend getaway spot, mayhaps? ;-)

It sounds like the designer did a decent job of right-sizing the boiler, anyway.
 

wjcandee

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A homey weekend getaway spot, mayhaps? ;-)

It sounds like the designer did a decent job of right-sizing the boiler, anyway.

Yep. Gorgeous place.

On the boiler size, I was thinking the same thing -- a lot smaller than I would have expected a home of that vintage to have. Undoubtedly replaced when major renovations were done, and every other aspect of the renovation is quality without being over the top. The interior is flawless and pristine, and the new homeowner appreciates modern equipment, thus his questions about the boiler. And the place is tight; not a draft anywhere. From the discussion here, I am inclined to shy away from a condensing boiler unless the system is revised along the lines of your suggestions, but an intermediate and reliable approach might be to replace with a 3-pass boiler, if I understand correctly.
 

Dana

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With 24 zones and a boiler output of ~150,000 BTU/hr you're looking at an average zone size of no more than 6250BTU/hr, which is WAY below the min-fire output of any modulating condensing boiler with that much output, which means except for the 6-8 coolest mid-winter weeks it's going to be cycling on/off a lot. With a buffer tank between the boiler & zones you could still do it, but that makes it a more complex system, with a bit more design work & tweaking to get there.

Since the previous boiler replacement designer did at least some of the design math, unless you're going to sign off on a whole new set of calculations, it's probably safer & simpler to just drop in a boiler with comparable BTU output and call it a day. If the boiler doesn't come with heat-purging controls, it's worth installing an after-market control.
 
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