Circulator pump undersized?

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jammy

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We have a hi-efficiency, natural gas hot water heating system. The maintenance man said that our circulator pump was undersized for the size of the boiler. He said this would shorten the life of the boiler/pump.

We don't have a problem with getting enough heat in the house. The house is almost 80 years old, so some inefficiencies are surely due to the quality of insulation, etc.

The boiler is a Crown Boiler model # FCM120NS. (see pic of label https://imgur.com/fkZP7lI)
The circulator pump is a Taco model 007-BF5-JW, 1/25 hp. (pic also attached https://imgur.com/GgLVnnP)

He recommended upgrading to the model 012- pump which is 1/8 hp.

Any learned opinions on whether this is a good recommendation, or just an upsell?

Thanks in advance for your feedback.
 

Jadnashua

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The magic to the answer is what is the outlet temperature of the boiler verses the return, plus, if the house stays warm enough. If the return temp is too low, that can damage some boilers meaning that the dwell time in the system is too long. There are limits on how fast you should pump the water, too. Without knowing some numbers, it's impossible to say if it's proper or not.
 

jammy

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The magic to the answer is what is the outlet temperature of the boiler verses the return, plus, if the house stays warm enough. If the return temp is too low, that can damage some boilers meaning that the dwell time in the system is too long. There are limits on how fast you should pump the water, too. Without knowing some numbers, it's impossible to say if it's proper or not.

Thanks, jadnashua. Are there any rough observations about in/out temp that would allow me to make an evaluation? For example, if I were to turn on all thermostats on a very cold night so the zones are demanding heat at the same time, what should the difference between in/out temp be after waiting for some time for all zones to circulate? Also, should there be a minimum differential between the boiler temp setting and the output temp during this test?
 

Dana

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Since the Crown (owned by Burnham) FCM-120 is a modulating condensing (mod-con) boiler, a low return water temp is not damaging- it is in fact GOOD for efficiency.

But if it's being pumped direct (rather than primary/secondary) the temperature difference (delta-T) between input and output may be on the high side, especially if it's a bunch of high volume high thermal mass radiators.

Most boilers don't want to see a delta-T of more than 50F from it's input to it's output. Almost all can take at least a 40F. Not sure what the max spec is on this one, but figure out a way to measure it (use meat thermometers taped to the plumbing under some pipe insulation if you need to.) All mod con boilers will work efficiently at delta-Ts between 10F & 30F . If it's a high delta-T but it keeps up with the heat load with reasonable room to room temperature differences there are plumbing tweaks (such as a system bypass) that would be more appropriate and better protection than just pumping the hell out of it with a monster pump.

While it's common to see higher power higher volume pumps like the 012 on systems with high volume cast iron radiators and oversized cast iron boiler, most of those systems are WAY over-pumped. Even though the 007 is more commonly used for low volume low mass radiation, if the radiation is keeping up with the load and the room-to-room temperature differences are reasonably bounded, it's clearly enough pump for the radiation. There is a higher efficience ECM drive version of the 007 as well as some "smart" pumps that allows you to program flows or delta-Ts there isn't a compelling reason to swap out the 007 until it actually fails, as long as it isn't stressing the boiler with a high delta-T.

Measure the delta-T and report back.

Unless this is the crummiest 80 year old house in Maryland or both old and very large, it's unlikely the FCM-120 is the right size for the heat load. If the radiation is all high volume cast-iron a 120,000 BTU cast iron boiler might have been necessary to avoid being damaged from return water that was too cool, without having to complicate the near boiler plumbing with protection schemes, but for a mod-con you want the thing to be able to modulate the firing rate up and down in response to the load, with nearly continuous very long burns during the heating season. The minimum fire input to the FCM-120 is 40,000 BTU/hr, and if condensing at 95% efficiency it's minimum output would then be 38,000 BTU/hr.

An output rate of 38,000 BTU/hr would be keeping up with the load at my house even when it's 0F outside, and that's the FCM-120's MINIMUM output! My house is no super-insulated gem, it's 94 years old, a 2x4 framed, 2400' + 1600' of conditioned basement, 1.5 story bungalow with the original single-pane windows + 1980s vintage clear glass storms for most of it. I've tightened it up a bit and improved the insulation during my tenure, but it's still a 2x4 framed house with clear-glass windows. Even when it had NO insulation in the walls and huge outdoor air leaks in the upstairs behind the knee-walls it's load was still no more than ~50-55,000 BTU/hr @ 0F, and the FCM-120 would have been more than 2x oversized. (And my 99% outside design temperature is +5F, yours is probably more like +15F or higher, maybe +10F if you're up on the Cumberland plateau.)

If you have a heating history on the place, run a fuel-use based heat load calculation on it.

With the heat load number in hand, it's also worth measuring up the radiation, to determine both the water temperature needed to heat the place, and the water temperature at which it might begin to short-cycle. A lower operating temperature is better for efficiency and lowers the delta-T, so tweaking the outdoor reset curve so that it's running it at the minimum temperature needed to still deliver the heat without short cycling boiler is going to be good for efficiency & longevity. A primer on how to go about that bit o' napkin math lives here.

With that information all on the table it'll be easier to figure out what's really called for (if anything).
 
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