Weil-McLain WGO-5 breaks all the rules

jesmed1

Thank you all for your professional advice as I continue trying to understand the heating system in our 4-unit condo building. The building is a 100-year-old 4-unit building in the Boston area, and is heated by two identical Weil-McLain WGO-5's, each of which heat half the building. (Domestic hot water from a separate gas heater, so boilers start cold.) Total building heat load on a zero-degree design day is about 88,000 BTU/hr. So each boiler needs only to supply 44,000 BTU/hr max.

Today I ran one 45-minute test with both the supply and return pipe surface temperatures instrumented identically using three different methods: (1) mechanical surface-mount dial thermometers designed for measuring pipe surface temps, (2) digital temp data loggers with probes foil-taped onto the pipes, and (3) a thermal imager pointing at areas of the return pipes wrapped with black tape. All three methods gave results within +/- 5 degrees of each other, which gave me good confidence in their accuracy. Moreover, despite the slight difference in absolute temperature readings, all three methods gave the same delta T of 12 degrees consistently throughout the test.

So here's the raw data, for one WGO-5 boiler, which has been slightly downfired. The BTU input rate reflects a 1.0 gph nozzle running at 140 psi, for an input of 1.18 gph.

BTU input rate: 165,000 BTU/hr input
Efficiency: 85%
Gross output: 140,000 BTU/hr

delta T: 12 degrees F, consistent throughout the run time
Run time: 45 minutes
Starting water temp: 68 degrees
Ending water temp: 140 degrees (from boiler temp gauge)

Circulator: Taco 007
Implied flow rate (140,000 BTU/hr /60 min/hr /8.33 lbs/gal / 12 deg delta T) = 23 gpm

So now my questions:

(1) Since the boiler starts cold, and typically runs for 40-45 minutes, the water temp rarely exceeds 140 degrees. And with a delta T of 12, that means the return temp rarely exceeds 128 deg. Doesn't that mean this boiler (and its twin) are condensing virtually all the time? I've been warned repeatedly to avoid condensing in boilers, and yet both these 25+ year old boilers apparently have been condensing their entire lifetimes. They're still in good condition. Also, our terra-cotta-lined chimney was just inspected and is in good condition. Why are these boilers in such good shape after 25+ years of condensing, and does that mean condensing isn't as bad for certain boilers (specifically ours) as some people say?

(2) I went into this test expecting to find a delta T of around 20 degrees, since that seems to be the rule-of-thumb. I measured an actual delta T of 12 degrees concistently throughout the test using 3 different pairs of instruments, so there's no question in my mind that the 12 degree delta T is correct. But when I do the math, that gives me an implied flow rate of 23 gpm. When I look at the Taco 007 head curve, 23 gpm is the max output of the pump at zero head. How is that possible? This is a large building with quite a lot of 2-inch steel/iron piping in the basement running every which way to feed 4 different units, so there's hundreds of feet of 2-inch pipe. (They adapt down to 1-1/4" copper at the boiler.) So is it credible that so much large-diameter pipe, with only short runs of 1-1/4" copper at the boilers, has zero head? (For the Taco 007, I'm using curve #5 in the graph below:)

https://www.nationalpumpsupply.com/content/pdf/taco-00-series-cartridge-circulating-pump-curves.pdf

(3) Doesn't 23 gpm break the rule-of-thumb that the 1-1/4 copper pipes at the supply and return should only see 14 gpm max, to avoid noise? Maybe the 1-1/4 runs are short enough that there's no noise despite that rule violation.

(4) Should we switch to a lower-flow circulator like the Taco 008 (15 gpm at zero head) to reduce flow rate and increase delta T?

Thanks for all the professional advice. It seems the more I learn, the less I understand!

captainco

What you have learned is the mechanical world and the hearsay or opinion world don't always agree. A water heater always has water cooler than 128 degrees but it don;t condense

jesmed1

captainco said:

What you have learned is the mechanical world and the hearsay or opinion world don't always agree.

LOL. And sometimes the hearsay/opinion comes from the mechanical world, which makes it harder to know which is which.

fentonc

Do you have data from a combustion analyzer? The higher the level of excess air, the lower the condensation temperature: https://www.thermalsolutions.com/wp-content/uploads/2013/04/High-Turndown-Condensing-Boilers.pdf - I was wonder the same thing (my boiler is just oversized, so it cycles a lot from a cold start and never gets hot for very long), and it turns out the condensation temp for mine is probably closer to 116F.

HVACNUT

It was more of a concern before Ultra Low Sulfer Diesel.
As it gets colder, hopefully the Delta T will widen and the return temperature will rise. But if you're trying to boil a large pot of water, on simmer, without a cover, it might not happen. That's where better controls come into play.

jesmed1

fentonc said:

Do you have data from a combustion analyzer? The higher the level of excess air, the lower the condensation temperature: https://www.thermalsolutions.com/wp-content/uploads/2013/04/High-Turndown-Condensing-Boilers.pdf - I was wonder the same thing (my boiler is just oversized, so it cycles a lot from a cold start and never gets hot for very long), and it turns out the condensation temp for mine is probably closer to 116F.

@fentonc

I don't have the full printout, just what the tech wrote on the ticket:

Gross stack temp: 420
Net stack temp: 350
12% CO2
0% CO
0 Smoke
85% efficiency

You might be interested in this short article by Ron Beck, a consultant with US Boiler. The article is about outdoor reset, which doesn't apply to me with cold start boilers that can't be modulated and that never get hot, but he does say some interesting things about cast iron radiators. For example that, contrary to belief, cast iron radiation these days is mostly low-temperature heating, not high-temperature as in the past. Modern homes with cast iron radiators may have been built 100 years ago when they had leaky windows and poor or no insulation, so they had massive amounts of radiation. And now that those homes have been modernized, the massive amount of radiation will satisfy thermostats with much lower water temps. Which is exactly my situation.

He says he's often asked what water temp cast iron radiators require, and his reply is "it depends." Houses with upgraded windows and insulation can heat with very low water temps in their cast iron radiators. He says in those cases he likes to see low boiler temperature, and minimum water temperature.

In our case, we have so much radiation that we could probably heat the entire building with 120 degree water. The most efficient setup for us would probably be a large buffer tank heated by one boiler instead of two, but that would involve a lot of replumbing that would be too expensive for our condo members to go for.

https://www.usboiler.net/outdoor-reset-doesnt-work.html

I also found this old comment that's relevant to the discussion about condensing. The writer says that, in his opinion, many boilers in his state (NJ) probably have a seasonal average of only around 140 degrees or less. He also says about condensing that "boiler water temp has little to do with stack temps and condensing." He says he's seen boilers with water temps at 200 and stack temps at 140, and also seen water temps at 140 and stack temps at 300+ (that would be our boiler). So if he's right, you can't just look at boiler water temp to judge whether the flue gases are condensing or not. And your statement that condensation temperature depends to some extent on flue gas composition would seem to agree with his point.

https://forum.heatinghelp.com/discussion/comment/838116#Comment_838116

jesmed1

HVACNUT said:

It was more of a concern before Ultra Low Sulfer Diesel.
As it gets colder, hopefully the Delta T will widen and the return temperature will rise. But if you're trying to boil a large pot of water, on simmer, without a cover, it might not happen. That's where better controls come into play.

Thanks. But I don't expect our delta T will widen, because our boilers never run longer than my 45-minute test in which the delta T stayed steady at 12 deg. And then the boiler stays off for 3-4 hours while the water cools back to 68 degrees. So they're always starting cold. Like your uncovered pot analogy, we have so much radiation that it just cools the water too fast.

PC7060

So each Boiler has a net output of 140 KBTU and the building requires about 90,000 BTU?

Are the boilers collocated? If so, have you considered configuring the system to run just one boiler and the other one as a failover unit?

pedmec

Depends on where you are measuring with your instruments. You should have a bypass set-up near your boiler. When efficiency standards were introduced all cast iron boilers with cast iron radiators were recommended to have a system bypass installed. So if you measuring before the bypass you will not be measuring the correct temperature returning from the system. Typically to prevent condensation a bypass is installed between the supply and return (system bypass) piping to allow for warmer supply water leaving to blend in with return water thereby increasing the boiler water temperature.

jesmed1

PC7060 said:

So each Boiler has a net output of 140 KBTU and the building requires about 90,000 BTU?

Are the boilers collocated? If so, have you considered configuring the system to run just one boiler and the other one as a failover unit?

Correct. Yes, boilers are side-by-side, each heating one half of the building. Yes, that's a good idea about running both loops off one boiler. @EdTheHeaterMan designed a primary/seconday loop system for us to do just that, but the cost of repiping and the fact that doing so would cool the one boiler even further (since now it has twice the flowrate of relatively cold water going through it) forced us to abandon the idea.

jesmed1

pedmec said:

Depends on where you are measuring with your instruments. You should have a bypass set-up near your boiler. When efficiency standards were introduced all cast iron boilers with cast iron radiators were recommended to have a system bypass installed. So if you measuring before the bypass you will not be measuring the correct temperature returning from the system. Typically to prevent condensation a bypass is installed between the supply and return (system bypass) piping to allow for warmer supply water leaving to blend in with return water thereby increasing the boiler water temperature.

Yes, @EdTheHeaterMan already pointed that out too. But the installers ignored Weil-McLain's instructions and did not plumb a bypass in. So we've been running them 25+ years at low water temps with no bypass, and apparently no ill effects that we can see.

So this may be a case in which low water temps did not, in fact, cause flue gas condensation, or if it did, the effect was negligible.

See this comment from @Ken_8 that I posted upthread about misconceptions about condensation and the temps at which it happens:

https://forum.heatinghelp.com/discussion/comment/838116#Comment_838116

As I said, I don't know whether or not he's right, but our situation with our boilers seems to jibe with what he says.

Steamhead

@RonBeck is one of the best.