Boiler bypass plumbing questions


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Old 02-27-10, 09:16 PM
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Boiler bypass plumbing questions

Hey All,
I've just discovered this great forum and was hoping to get some help with some basic boiler questions. I'm going to install a Burnham MPO 147 boiler w/Beckett AFG combo (147MBH) in my house w/2 zones(baseboard) and an IWH. I've decided to go with circulator pumps instead of zone valves because I like the redundency. My question for all of you is about Boiler by passing, the installation manuals aways show it but I never see it on any of my neighborhood boilers. The only reason I can see doing it with baseboard radiators is if I wanted to do some sort of Outdoor reset control. If I'm going to do bypassing shouldn't I do it right and set up a primary/secondary loop? Am I over thinking this, tyring to do the best when good is good enough? Any thoughts would be greatly appreciated. PS my old Boiler (Pensotti R1-30 (119MBH)) was heavily bypassed, a 1" copper pipe from supply to return with no valving at all????? If anyone is interested I can post a pic..
 
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Old 02-28-10, 03:08 PM
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The first thing we always urge is to DO THE HEAT LOSS CALCULATION! Unless you have a very large home, (i.e. 4000 sq ft) it's highly unlikely that you need that big a boiler. Do NOT size the new boiler off the old one...

The MPO boilers are more tolerant of cooler return water... there is a unique 'jet' arrangement on the return fitting that allows the boiler to accept somewhat cooler water without condensation issues... but a boiler bypass is really so little extra piping, why not just put it in anyway? ... and close the valve if you don't need it...

ODR: In spite of the fact that the MPO can tolerate somewhat cooler return water, you still don't want to do FULL reset. Since this is still a non-condensing boiler, you would be limited to PARTIAL RESET... meaning that the temperature can't go all the way down to room temp... you would need to set the bottom limit somewhere around 120-130 or so... rbeck may weigh in on this too... he knows a lot about the MPO boilers.

To do FULL 'SYSTEM' RESET, you would need a more elaborate primary/secondary arrangement.

The reason you never see it on neighborhood boilers is because the installers never read the manuals... they only use them as kneeling pads to keep the knees of their Levi's clean.

Fact is though, that if it's a fin-tube baseboard system, most of them don't actually NEED a bypass... fin-tube heats up pretty fast, and there's not a large volume of water...

Again... please don't install a boiler that's any bigger than you need!
 
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Old 02-28-10, 04:20 PM
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Thanks for the reply NJ,
Unfortunately my existing Pensotti R1-30 (119 MBH) boiler has a crack in the water jacket leaking into the fire box (white smoke, lots of blow-back) My heating company tried 3 times to get the burner running right and couldn't, they finally determined that the boiler was toast. They added stop leak and the boiler seems to be holding together but they said it could go at any time. I do trust their diagnosis so they recommended a Burnham MPO 147, they did not do any heat loss calculations, but I know that the old boiler works way too hard to keep up, if 2 zones call for heat it can barely maintain 140F. After getting 3 quotes for three different brands with about the same ratings, I realized I couldn't afford any of them. I kinda freaked and decided to do it myself, I found a real good deal on the MPO and bought it.. The boiler was sized with a planned 1000sqft addition in mind, so long story long, I'm married to the 147 because I already have it sitting next to my old boiler! I wish I had found this site sooner but...... I've got to go with what I've got. The good news is that I've learned alot in the last couple of weeks about hydronic heating and every day the old boiler last brings me closer to spring! Also having the new boiler right there allows me to fit up the manifolds so everything will be that much easier when the day comes to slide it in. I will install the by-pass piping and go from there. From your reply it doesn't sound like ODR is really beneficial unless you have a modulating burner to down fire the system to maintain a low temperature, is that right?
Thanks again for the help and sorry for the long reply
 
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Old 02-28-10, 04:39 PM
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Long reply? hardly! have you seen some of mine?

From your reply it doesn't sound like ODR is really beneficial unless you have a modulating burner to down fire the system to maintain a low temperature, is that right?
No, I didn't mean to imply that at all...

ODR will almost always be a benefit. The lower the water temp, the less fuel you burn to heat it up... it's just that because of the condensation issues with conventional boilers, you can't run the temp all the way down ... you have to keep it off the 'floor'. So your heating curve with an MPO might start at say 130, and on up to 180, depending of course on the outdoor temp, whereas with a condensing boiler, you could run all the way down to room temperature water if that's what was called for.

But again, there are piping arrangements that will allow you to run the space heating at lower temps, while running the boiler at a higher temp, out of condensing range.

There are technical essays at Tekmar that might help to explain a bit... they can all be found at this site:

tekmar Essays

And this one has some good explanations of the options:

http://www.tekmarcontrols.com/litera...robat/e021.pdf
 
 

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