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Thread: Have some issues with hot water boiler system

  1. #31
    In the trades Dana's Avatar
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    Quote Originally Posted by jasesun23 View Post
    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?

    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.

  2. #32
    In the Trades Tom Sawyer's Avatar
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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
    [B]No, plumbing ain't rocket science. Unlike rocket science, plumbing requires a license[B]

  3. #33

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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 by jasesun23; 12-26-2012 at 11:36 AM.

  4. #34
    In the trades Dana's Avatar
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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 by Dana; 12-26-2012 at 02:26 PM.

  5. #35

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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

  6. #36
    Retired Defense Industry Engineer jadnashua's Avatar
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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 by jadnashua; 12-26-2012 at 03:20 PM.
    Jim DeBruycker
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  7. #37

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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.

  8. #38
    Retired Defense Industry Engineer jadnashua's Avatar
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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).
    Jim DeBruycker
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    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  9. #39
    In the trades Dana's Avatar
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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.)

  10. #40
    In the Trades Tom Sawyer's Avatar
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    Rule #1 Water always flows to the path of least resistance
    Rule #2 See rule #1
    [B]No, plumbing ain't rocket science. Unlike rocket science, plumbing requires a license[B]

  11. #41

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    Dana- they are branches. I have 2 Zones but one is a separate back room/enclose sunroom that was added later with copper pipe and slant fin baseboard. The main zone with the 2 branches do have shut off valves right below where the return pipe thermometers are. I could close one and restrict its flow a little to get the temperature to match up better. Pics below

    I left the boiler off all day today. PSI was around 9 at 78 degrees (taken from return pipe well). I've been messing around all week checking psi at the drain valve so a little water gets removed each time. I've been removing little amounts of air each day on upstairs radiators and I took the expansion tank off today to confirm its psi (~12 depend on which gauge I use) which removed a little water. So with that amount of water and air it could of dropped it to 9psi, or there could be a small leak. Will keep my eye on it over the next week or two to figure out if its a leak.

    Thank you everyone for your time and help and if I don't post anything in the next couple days have a great New Years.

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  12. #42
    In the trades Dana's Avatar
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    It doesn't much matter if the return water branches match. The valves are there for getting the room temps reasonably matched & comfortable on both branches. Gate valves like that see excessive wear when used for adjusting flow (ball valves do much better), and may not seal completely after years of use in a half-way position. If you're happy with the room-to-room temperatures, just leave them alone. Tweaking them will have no effect on the temperature the boiler actually sees.

    If you're seeing 9psi in the basement it may start to suck in air at the top of the system, depending on just what the elevation difference is between the top of the system and the pressure gauge.

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