Have some issues with hot water boiler system

Discussion in 'Boiler Forum' started by jasesun23, Dec 18, 2012.

  1. Dana

    Dana In the trades

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    The numbers you got out of Watts selector is only if you want to let it run all the way up to the 30psi limit, which you don't really want to do.

    You have more than 50 gallons of water total in the system, even if you were draining it from the very bottom of the boiler. The boiler has something like 5 gallons or more in it all by itself.

    Using the Watts tool, set volume to 55 gal, both the pre-charge & autofill to 12psi, and pressure relief number to 20psi (the very high end of the pressure you'd ACTUALLY want to see if you're starting at 12psi when the system is stone-cold). Using 100F/200F for temperature range and the tool gives you a minimum expansion tank volume volume of 8.8/1.6 for volume/acceptance. If you dial the temperature range back 100F/180F it gives you 6.6/1.2, which gets you within 7.2/2.9 limits of the Flex2Pro HTX60.

    The HTX30 is only good for 4.8/1.6 which is marginal at best. If the system was 12psi at a cold start of 65F and the high-limit set to 170F, the minimum sizes calculated by the Watts tool is 3.88/1.4 for a 30psi relief valve. Bumping just the temp to just 180F brings it to the hard limit on acceptance volume for the -HTX30. If the system actually has more water than 55gallons you're toast (as the symptoms seem to suggest!).

    The HTX60 should keep it from spitting water in your operating range unless the true system volume exceeds 55 gallons by more than just a little bit.

    Purging air from a fully drained multi-story system can be a bit of a pain. Note that you'll be starting with COLD 50-55F water- you may have to bleed more water out of some as the thing comes up to the operating temperature range to keep it down to ~12psi @ 100F boiler temp. Hopefully you'll find a bleeder valve (or valves) at the top floor of the system- open it up during the initial fill and fill until it stop gurgling & spitting air. You will likely need to run several 10s of gallons out of the bleeder before enough air is purged to get it to that point, and you're guaranteed to still have air elsewhere in the system, so this may be an iterative process. It may flow fine for awhile, then develop a bubble at the top of the system interfering with flow, which you can then bleed off with the bleeder valve until it's running clear with no fizz, readjust the pressure, repeat as-needed. (Be really careful if bleeding it while the system is hot.) Bleeder valves are often (but not always) located at a radiator, and many/most systems will have an automatic air trap/vent to aid in purging the last bits of air as the bubbles work their way to the top of the system while it's running once you have it purged well enough to get reliable flow.

    The temperature gauge on the return water line is important for monitoring the operation of the system. It's OK if it's running cooler on start up, but if it spends most or all of it's burn time with return water below 130F it can damage the boiler with condensation on the heat exchanger plates or damage a masonry/terra-cotta flue with flue condensation. Natural gas exhaust condensation is mildly acidic, causing corrosion on iron boilers an breaking down the mortar holding the masonry together if it's chronically condensing. Most systems are set up with a 20-25F delta-T, so if the boiler's output is normally 160-170F, the return water temps will be reliably above 130F, except during start ups (which is fine.) If you swap in a condensing boiler you would then want to lower the temperatures as much as possible to reap the highest efficiency, which is why they use acid-resistant plastic exhaust venting rather than venting into masonry chimneys.
  2. jasesun23

    jasesun23 Member

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    Dana great info- thanks.

    I played around with the Watts calculator. Temperature doesn't change it that much, even if I use room temp as the lowest setting not aquastat temp. But moving that pressure release valve number from 30 (which is what I entered) down to 20 really makes it bigger. Also the drain is all the way on the bottom of the boiler (included pic). Not sure how much more water was left in the actual system. Good news is there is now a Amtrol Extrol Model 60 on there along with an Auto Air vent on the branch of piping going to the expansion tank.
    The figures for the Extrol Model 60 are 7.6/2.5. Also from this site ( http://www.pexuniverse.com/docs/pdf/amtrol-extrol-expantion-tank-specs.pdf ) they tell you what model Extrol should be used depending on BTUs and based on Max temp. I've including a screen shot of those charts below. My system matches up well with the model I have now.

    I'm going to reread the second part of your reply to fully comprehend it since you are spot on with a lot of it

    Drain.jpg
    Fullscreen capture 12212012 71124 PM.jpg
  3. Tom Sawyer

    Tom Sawyer In the Trades

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    Awful lot of conversation and figgerin here when anyone that works with this stuff could have told you that your expansion tank was too small. What you have there looks like an old steam to forced hot water conversion or a monoflow system, both of which have large piping and radiators and lots of water.
  4. BadgerBoilerMN

    BadgerBoilerMN Master Hot Water Mpls,MN

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    You have a leak. Adding fresh water to a closed loop hydronic heating system is never a good thing.

    12 psi is common and sufficient and typically will not change regardless of pump or operating status (that is what the expansion tank if for). The system or boiler pressure relief valve should never go of, weep or leak. The expansion tank is correct. A new condensing boiler is the right and only choice. Be sure that you have a Manual 'J' heat load analysis performed before buying any boiler from any body.

    Merry Christmas to all.
  5. jasesun23

    jasesun23 Member

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    Tom Sawyer - It is a venturi monoflow system. I'm pretty sure it was always hot water.

    BadgerBoiler - New boiler or not I'd still have to fix the leak if there was one in the system right?

    With the new expansion tank there has not been a discharge of water for 2 days . However the pressure has consistently been at 22-23psi while running. Is there any chance I need a bigger expansion tank to get the psi down while running?
  6. BadgerBoilerMN

    BadgerBoilerMN Master Hot Water Mpls,MN

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    Many older boilers develop slow leaks between sections and over the burners. Weeping between cycles water lost is evaporated and never seen. Just a thought. Oh, and many pressure gauges lie.

    Lower the pressure until the highest vent sucks air and add 3-5 psi...call me in the morning.
  7. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    You need water pressure in the boiler for two reasons: to keep the water from flashing to steam when passing by the heat exchanger and to keep things from boiling and enough to keep things from boiling when passing through the pump and creating cavitation there. Cavitation is both noisy and can damage parts. Excessive pressure puts a lot of stress on things - they work best when the pressure is stable. The expansion tank normally keeps the pressure within a pound or two unless there's a problem elsewhere.
  8. jasesun23

    jasesun23 Member

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    Jim - I'm getting some advice on some other forums that a 10psi increase is perfectly acceptable in a system with 50 gallons in it. And that systems that only change a couple psi are the new 10-12 gallon systems. I'm a little confused at this point.

    BagerBoiler- I understand that you say the boiler itself might be leaking. Not really following what you mean when you say "Lower the pressure until the highest vent sucks air and add 3-5 psi" . With the fill valve shut off, if the pressure never goes below 12psi I should be okay right? Right now I am still getting some air in the top radiators from draining the system. Once all air from the system has been removed if the pressure doesn't drop below 12psi when cold I am good right?
  9. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    If the expansion tank is sized properly, the pressure shouldn't rise much between hot and cold. Older boilers don't like big pressure swings (well, most of them don't, new or old) - it can literally stretch the bolts holding the heat exchanger when hot, and when it cools, those bolts may not spring back, thus leaving less tension on the sections of the heat exchanger. This can allow it to leak. But, since it is a part that gets hot, that liquid usually just evaporates or is boiled off the outside, and doesn't show up as a leak unless you look very carefully.
  10. BadgerBoilerMN

    BadgerBoilerMN Master Hot Water Mpls,MN

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    I believe they are referring to a #60 (60#) diaphragm expansion tank. I doubt very much that it is required but would start by lowering the max operating temperature to confirm. The existing expansion tank is bad or you have a valve over-filling. These are the only two choices. If the 30# pressure relief valve goes off the resulting pressure should remain the same through a 24 hours cycle.

    No scoop is necessary in the typical cast iron radiated system.
  11. Dana

    Dana In the trades

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    Pressure-cycling an old cast iron boiler >10psi just plain isn't good for it. Pressure swings that high may not stress a water-tube boiler or mod-con very much, but even with new 50psi-rated cast iron boiler keeping it's operating pressure range withing a 5psi window and running the system at the lowest pressure that doesn't suck air is the right way to go every time. It's not as if there's significant up-charge for going larger. Going with the absolute minimum size that works is poor economy over the lifetime of the system.

    Badger's "Lower the pressure until the highest vent sucks air and add 3-5 psi" recommendation works. It defines the minimum operating pressure that still works for the system. With the fill valve shut, if you slowly drain the until the vents a the top just begin to pull in air, the pressure at the boiler will still be something significant. The static pressure at the boiler's gauge will be about 0.43psi per vertical foot between the gauge an the highest vent in the system. If that happens to be 25', it'll be about 11psi. If its 30' it'll be about 13psi. Whatever that turns out to be, that's the minimum pressure it can be with the system cool & idling, with the water at fully cooled idle, say 60-65F (and not 45F water from a super-cold fill.) Adding a few psi from there guarantees that no point will any point in the system be at negative pressure (whether it's hot or cold, pumping or not), and pulling air into the system to add to corrosion, or vapor-locking the system from flowing properly.

    With the system buttoned up and filled, with an expansion tank properly sized and charged, if it continues to lose pressure over the course of a week it's losing water somewhere. It would not be surprising on a boiler that age to have weeping at the plate seams when the system is cold (especially if it's had 57 years of 15psi+ pressure cycling.) As long as it's not puddling a properly operating auto-fill valve would mask the symptom and get you through the season. But whether it's leaking or not, swapping in a newer more efficient boiler in the intermediate-term is going to be economic.

    The high mass of the system means that it would never short cycle even if the new boiler was oversized, but it's still a good idea to right-size the replacement boiler, and it'll almost certainly have lower output than this one. In 1955 systems were typically at least 2x oversized for the heat load, even at the less insulated less tight and single-paned windows of the day. If there have been any improvements on insulation, air tightness and windows since the system was installed it could easily be more than 3x oversized. A careful room-by-room Manual-J type heat load calculation based on the construction type and exterior surface areas, etc, &/or using the billing to measure fuel use against heating degree-day data between the meter-reading dates could put some hard upper bounds on the boiler size. Oversized boilers cost more up front, and deliver less comfort, even when the mass of the system is keeping it from falling off an efficiency cliff by short-cycling.
  12. Tom Sawyer

    Tom Sawyer In the Trades

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    Look at the size of your mains and radiators. I'd bet someone took a sizeable galvy expansion tank out and replaced it with a #30xtrol when it should have been a 60. If the feeder is set right the system should be operation between 12 and 18 lbs
  13. jasesun23

    jasesun23 Member

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    Hi Guys. Hope everyone enjoyed their holidays.

    Just an update. Boiler has been running for a couple days since they replaced the 30 and put in the 60 expansion tank. No water has been discharged. Max psi or should I say running psi has been steady at around 22-23psi. Grabbed a couple gauges to make sure PSI on the boiler was accurate (ex. boiler gauge 20psi, 2nd gauge 19psi, 3rd gauge 18psi). So that were all fairly close. PSI seems always steady with the temperature for ex. 170-22ps. 130-18psi. 100-14psi.

    Tom Sawyer- The old expansion tank which is still up in the ceiling and good working condition looks to be at least 4 feet long and around 1.5 feet wide. So it is pretty big.

    The fill valve has been closed for over a week so i'm taking the feed out of the picture (though the feeder seems to be working correctly). It has been very cold lately so I haven't gotten a chance to see what the cold PSI is. I now follow getting the minimum PSI lowing the PSI until air is sucked in by the top radiator, then adding a couple PSI on top of that. I will do that when there is a window where the boiler is off long enough and I'm not freezing.

    As far as the boiler leaking I will keep an eye on it over the next week or two and make sure it doesn't drop below whatever I finally set the cold psi setting to. Be it 11,12,or 13.

    I'm just making up a scenario here since we haven't pinpointed if there are other issue - Fill valve is off so feed is out of the picture. Relief valve is working perfectly at 30psi. There are no leaks in the system. Assuming those statements are true what would be causing that 10-12psi jump from cold to hot boiler. Is there a chance the 60 expansion tank is too small? I am going to double check the pressure on it in a few days (but lets consider it correct). The piping is all there for the old expansion tank, id just need to connect it with a union, would anyone recommend that?

    Dana- if the boiler turns out to be leaking I will certainly consider a new system but right now I'd like to trouble shoot this one first.

    Badger- people on the other forum were saying 50 gallons in the system. That a 3-5psi increase from cold to hot is not a hard fast rule. And that 10psi is perfectly normal for a system with this many gallons in it. Not saying they are right just repeating info I'm being told.
    Last edited: Dec 26, 2012
  14. Dana

    Dana In the trades

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    If I'm reading your notation correctly it's running

    22psi @ 170F

    which drops to

    14psi @ 100F

    That yields a (crudest possible) linear approximation of ~0.11psi per degree F. So if it drops another 30 degrees it may lose another (0.11 x 30=) 3.3psi which makes it ~11psi when cold. It may actually be a slight bit higher than that due to the non-linear aspects of the tank, but you won't be looking at 10psi, and it won't be13psi. With the auto-fill set to 12psi it'll run about where it is now when it fires up in the fall- give or take a pound or two, not more, and the overall swings in pressure will be about the same.

    As long as it's not slowly losing pressure over the next few weeks it's not leaking, at which point it's time to declare victory and move on. But start planning for the replacement BEFORE it reaches a crisis.

    Modulating condensing boilers and water-tube boilers don't have the plate-flexing/bolt-stretching issues, and as such are more tolerant of pressure swings. It it's running a sub 10psi swing in pressure while operating it's a lot better than the 15+ swing it had been seeing, if not quite as polite as a sub 5F swing. The max pressure on the boiler may well have been rated 50psi on day-1, but there's no point in pressurizing just to it to see if (or at what pressure) it leaks.

    Keep your wintertime fuel bills, and when you have a month or two of billing where the system hasn't been down for days for the repairs we can use that information to determine what your real heat load is, and the oversizing factor of the current boiler. That's an important data-point to have when looking at a boiler replacement, since the oversizing factor will tell you your as-used AFUE efficiency, and what you might expect to get out of a right-sized boiler (modulating-condensing or otherwise.)

    The SunRad radiators work great at low temp, and you have a running (27 + 40 + 86 + 67 + 50 + 50 + 27 + 27 + 54 + 36 =) 464" of radiator that has a specified 1 square foot of surface area per running inch. That's a lot of square feet! At an average water temperature (AWT) of 150F those radiators deliver 110 BTU/hr per inch or 51,010, which may be well above the actual heat load at the 99% outside design temp. The fact that the system is cooling off to 100F before it calls for heat when it's not exactly warm out tells me that your time-averaged system water temp over the course of these so-so ~35F days is probably under 130F, and it may be under 120F. Your heat load at NYC's +15F design temp is only ~1.6x what it is at ~35F.


    At 120F AWT they would still be delivering about 28,000 BTU/hr, and you'd be well into the 90s on combustion efficiency. (If 28K has been your average heat load of recent days, odds are your design heat load is under 45K, maybe under 40K.) I'm guessing that your average (rather than peak) heat load even in winter is under 28,000BTU/hr and you could even break 95% AFUE using the outdoor reset control function of a mod-con boiler with a bit of tweaking on the response curve. (This is nearly impossible to hit with baseboards or under-sized radiators.) With a fuel-use /heating-degree-day based heat load estimate we'd be able to get pretty close to the real numbers. It's highly likely you would see more than a 35% reduction in fuel use by going to a mod-con, (and steadier room temperatures, higher comfort to boot!)
    Last edited: Dec 26, 2012
  15. jasesun23

    jasesun23 Member

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    Again Dana I'm probably giving you misinformation. The temps i'm giving you are from the square temperature gauge on the boiler. It never drops, or at least i've never seen it drop below 95. But at 95 degrees on the boiler thermometer if its been off long enough it may well be 60-70 degrees on the return pipe thermometers. I should probably pay more attention to them to give you accurate readings. I assume the thermometer on the front of the boiler is almost always going to be hotter than the pipe thermometers which are returning water after it has cooled going through the system and the boiler is where the flames are.
    The numbers I gave you were just to show that when the system is at its coolest its at ~12psi and at its hottest its running at 22psi. And the rise in psi seems directly follow the rise in temp, which makes sense. Now with the 60 tank I'm getting a 10psi swing not the 15psi plus swing that I was getting with the 30 tank. That why I made the assumption that if I went to a even bigger tank or connected the old one I may get the 5psi swing we are talking about.

    I follow what you are saying about a new boiler and it makes good sense. I will keep it in mind.

    thanks
    jay
  16. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    The pressure in the expansion tank should be your normal operating pressure. TO measure it, you must have a valve open so the water in the tank is not pressing on the bladder. WHen all closed up, the air pressure in the tank will be the same as the water pressure pushing on it. You want to determine the pressure with it not being compressed by the water. The pressure in the tank will rise relatively linearly until you get the volume of air down, then rise very quickly. The bigger the tank, the more likely you'll be in the linear, low slope area. If the air pressure is too low, it will fill up with water quicker, flex more, and wear out more. Most of them come with a nominal 14-15psi, but you can adjust that for your system.
    Last edited: Dec 26, 2012
  17. jasesun23

    jasesun23 Member

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    This is on a separate area then we've been discussing but since I'm becoming a pro on my boiler thanks to everyones help I'm curious about this. Zone 1 which is the whole house starts off as 1 main pipe and then breaks off into 2. One covers I believe the front radiators of the house and the other the back. Then the two pipes return right before the boiler where they are merged into one. Each pipe about 3 feet above where it comes back into the boiler has a thermometer. Old school mercury thermometers then stick into a well that goes into the pipe. I think there is graphite in the well to help transfer the heat to get an accurate reading. Anyway one pipe is always cooler than the other. For example right now the temperature at the boiler is 160-162. The temp of one pipe is 144 the other is 154. I remember Dana saying that I'd have to be careful that theres not air still in the system after draining it? Could this be an issue. Or is just one pipe have better circulation than the other so its get more of the hot water quicker?

    thanks jay.
  18. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    It could be several things that I can think of (and I'm sure others I can't): the total length of one branch and the radiators on it is longer and has more square feet of radiators on it, or, the flow isn't balanced, or one side of the house is colder than the other (say the windier side?). If it is a balance thing, you'd need to add valves in each line to help balance each. You might also want to swap the thermometers to verify you get the same results. It might just be that the heat transfer material isn't providing a good transfer, and the water is returning at the same temp. If the radiators have shutoff valves, make sure they are all open the same amount (normally, fully opened).
  19. Dana

    Dana In the trades

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    The different return branches would likely not be exactly equal temperatures. The lower-temp branch probably has more radiator length on the loop than the warmer one:

    more radiator = more heat extracted from the water = lower return temp

    Since they mix before entering the boiler, the water entering the boiler will be at some intermediate temp. Anything over ~130F is fine for the boiler, but for a terra-cotta lined flue you'd want to be a bit warmer than that. If the system always comes up to the 144-154F return water temps on every burn you have plenty of margin. When the system hasn't fired for awhile it'll take several minutes before it gets up to temp, but that's fine. It would be an issue if the thermostat(s) were usually satisfied before reaching that temp, and it was chronically running with return water temps in the 120s or lower.

    If there's air in the system there are potential flow issues, but if all radiators are getting warm you have enough flow. With vents at the top of the system any remaining air will eventually find it's way out, long as there is flow on both loops.

    Are they true zones, with separate thermostat controls, or is this just a branch? If the latter, there's a good chance that the system has valves to throttle back flow on one or both branches as a means of balancing room temperature differences. (If you constrict flow on one it forces more flow into the other.)
  20. Tom Sawyer

    Tom Sawyer In the Trades

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    Rule #1 Water always flows to the path of least resistance
    Rule #2 See rule #1
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