Boiler bypass question

FranklinD · June 2016

So this is something I've been pondering for awhile now.

I have a burnham esc4 boiler, and a 100 year old house with 435 EDR of cast iron radiators, and a heat loss of 60k btu or so at a design temp of -17*f. It does get that cold here during the winter, sometimes for a week or two straight, and the boiler has no issues keeping up. I suspect it's very oversized even for that temp, as at -20*f my supply temp barely hits 150*.

Looking back I wish I would've gone with a TT or even the WM Eco, but it is what it is and this boiler will have to do for at least 5-6 more years before I can justify a replacement (to my wife -- I'd do it now, but...you know...ha).

The installation was a nightmare, despite doing everything possible to guarantee the opposite. I ended up recouping some of the labor cost. Last fall, prior to heating season number 2, I piped in a boiler bypass since, for the majority of the season, my water temps didn't exceed 120* and I don't want it to condense and die an early death. This allowed me to run the system at 110-120, and the boiler at 140-150.

My question is this: in reading some of Dan's books, he talks about how modern, low water content cast iron boilers are shown piped with a bypass so that the majority of the water 'bypasses' the boiler and goes back to the system, therefore the boiler heats a smaller amount of water with each pass and "can heat up to high limit and shut off, thus saving fuel".

My bypass is plumbed exactly as Burnham shows in their I&O manual, and as Dan's illustration shows as well. My question is how to adjust the valves. Currently my boiler never hits the high limit, unless I adjust it all the way down to 140. Even if I do that, it will short cycle as the boiler temp will drop quickly from high limit to the differential setting and refire.

Reading about the boilers in the Levittown homes, where the heating water was restricted by a 1/8" bushing, makes me wonder. Should I adjust the valves so that there is THAT little flow through the boiler? Right now it seems like maybe half and half, flow-wise. I get about a 30-35* rise through the boiler. Still heats the house at about the same rate as when it was direct-piped with no bypass. Should I adjust it so 90% goes around the boiler, so that the boiler fires up to its limit, and stays there for at least 15 minutes or so? Like more of a trickle of heat out to the system (as long as it's enough to heat the house)?

Probably a dumb question but I've been wondering it for awhile since my boiler never hits the high limit even with the bypass. This boiler is supposed to be okay with 110* return and 130* supply, but during the shoulder seasons I barely even see 85* return and 100* supply. I do have a bumblebee circ and have tried a little bit of every setting over the last 2 years...except for set point, as the lowest it will go is 140 and I don't want to pump a big glut of cold water back to the boiler.

Looking around, I would LOVE a Taco mixing valve with built-in ODR and boiler protection, or even a Tekmar. The problem is, of course, that I don't want to spend a lot of $ on this stuff when I hope to replace the boiler with a modcon in 5-6-7 years. Probably ten years to be realistic. I just don't know.

Primary secondary would be an option but I'm not sure how to apply it to this situation.

I just plugged the intake and exhaust outside for the summer, and opened up the sealed air box so that it can be exposed to dehumidified basement air for the summer instead of rusting away in the humid outside air. In doing so I took some pics up into the sections and they look the same as last spring, no obvious buildup in the pins or material flaking off into the pan underneath, also no obvious signs of condensate puddling in the bottom. My flue length is VERY short, about 4 feet up and 5 feet out.

So. My apologies for the long and boring narrative. If you wonderful folks have any suggestions regarding the boiler bypass situation, and how to adjust the valves for proper temps and fuel savings, I'd be very appreciative. I'm just a lowly Fire Dept mechanic...I know much about big 1600gpm fire pumps and foam injection, but comparatively little about boilers and bypasses.

I'll be back on later, time to enjoy the weather and practice some pitching with my boy.

Thanks again, and as always, this site is amazing in its amount of knowledge and the generosity of its members.

Andy

hot_rod · June 2016

A 3 way thermostatic valve is about the least expensive way to assure return temperature protection. Bypass piping, or pumping will not unless it has a temperature sensitive control.

We build these mainly for wood and biomass boiler but also sell them for high mass distribution, boiler protection.

This explains the operation and tech details.

http://www.caleffi.com/sites/default/files/file/01223_na.pdf

Gordy · June 2016

So long as the boiler is able to get above 135* as read on the tridictator in a reasonably short time on fire up, and is above 135* at shut down the condensate will burn off.

I have a WM cgm 7 that is bypass piped with no valves other than the mixer for the radiant, and is servicing full radiant. That boiler has seen nothing over 95* return water for the last 23 years, and is fine.

Tell tail sign of condensing is flakes in the burner tray, and on the burners.

Yes the valve HR is talking about is guaranteed success verses bypass piping.

FranklinD · June 2016

Thank you all for your input. I was thinking along some of those lines myself...one thing the contractor pushed about this boiler was the 'option card' for outdoor reset...but the more I think about it, the more it seems to be geared towards a high-temp baseboard application.

Hatt & Gordy: that's basically what I've done with the bypass...throttled it so I have lower flow through the boiler, so that the output is about 30-40* above the return temp of, say, 90-100 during cold weather. That way the boiler temp at the end of the cycle is around 130-140. If that's good enough, I'm happy the way it is. I was just curious about that wording in Dan's book about the boiler getting up to high limit and shutting off to save fuel...because mine never hits high limit unless I set it to 140, and then it will just short cycle as the temp will drop quickly. But as was said, high boiler temp doesn't necessarily equal efficiency, either.

Looking at the burner tray, there's definitely crud in there, but it looks mainly like a few pine needles, dead bugs, pollen, chaff, and dust. I don't *think* any of it looks like iron scale.

Looking up in the chamber, through the pins, I can see one that appears to be almost white compared to all the others...but that could be anything.

Hotrod: I'm going to look into the thermostatic valves again...for some reason I always overlook them, even though I actually checked them out a few years back. Ha.

Thanks again!

Andy

SWEI · June 2016

FranklinD said:
one thing the contractor pushed about this boiler was the 'option card' for outdoor reset...but the more I think about it, the more it seems to be geared towards a high-temp baseboard application.

The option card provides partial reset functionality. Given that your house is being well heated on a design day with 150°F water, it would serve no real purpose in your system. A four way iSeries-R valve would be a much better fit IMO.

FranklinD · June 2016

I would love an iSeries valve with odr...I just can't justify the price at this point unfortunately. I'm still waiting for our 13 year old power vent water heater to die so I can install an indirect...but we have very good water so it could be awhile.

Part of this is my desire to play with and tweak things, the other is the hope for a little better comfort and a bit more fuel savings (not that they were all that bad last winter).

Andy

FranklinD · June 2016

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Gordy: just so I understand what you're saying (I don't know if the quote function worked, but this is about your post above)...

So if I understand this correctly, having relatively cool (95*) return water is okay, as long as 1) it's not returning a large glut of it to an already-hot boiler, and 2) the tridicator/boiler temp on the supply out of the boiler reaches a temp of 135* or above relatively quickly while running and at the end of a cycle.

That's basically how I had it set up last winter, though I want to add another thermometer and will tweak the valves a little bit.

Glad to hear that boilers can last awhile set up this way.

Thanks again,

Andy

Gordy · June 2016

It is all about how much "bypasses" the boiler.

With my radiant I have a mixing valve, and the bypass piping is piped into the bypass port of the mixer, and boiler supply into the boiler supply port of the mixer. With out valving the two branches. There is more of the return water sent to the mixer, and less sent to the boiler. So I'm pumping 15gpm, and 9 bypasses, and 6 hits the boiler. The mixing valve does the throttling for me.

My boiler is set at 160 HL. The mixed temp is any where up to 115* depending how long a heat call runs to satisfy the tstat. So the colder it gets outside the longer, or more frequent the call might be thus the higher the mixed supply temp goes. With return temp following at a 15 delta.

The boiler never cycles off HL. So I get a nice steady burn, and at the end of a call I'm sitting above 135 usually higher. Which burns off the condensate.

In your case your emitters require a higher supply temp than radiant, and you have no need for a mixing valve. So throttling valves would be utilized to deliver the temp the emitters require. In the end you will, or should have less bypassing the boiler than in my scenario. However the end result needed is the same get the boiler above 135' and try to keep it there after the call is satisfied.

What is hard on a boiler from a condensation point of view is coming out of deep setbacks with a closely matched boiler so it struggles for a bit longer to get above 135. Or another scenario is a deep set back zone that cuts lose while the boiler is up to temp, and pulls it below 135, and then it shuts down when the call is satisfied below 135. This is where boiler shock can come into play stressing the HX.

Many variables, that's why the mixer HR described is the 100% guarantee to successfully protect a boiler from an installers stand point. Then one can rest assured any of the above scenarios will not affect the boilers life.

Gordy · June 2016

I will add that I have a WM cgm 5 in my rental that feeds a radiant zone, a ci baseboard zone, and ci base board with a standing ci radiator zone. Three zones. One zone
is the garage, and storage its set at 60* when it calls it will suck the boiler down off HL to near 80 degrees. That boiler is 18 years old. However it had a cracked section due to boiler shock at 3 years old.....someone left a garage door open for an extended period, and the HX did not take kindly to that.

Boiler bypass question

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