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System circulated position with a mod/con

CanecreekCanecreek Member Posts: 21
First of all high everyone.

I have visited your post many times and have a question I don't seem to be able to find an answer for. I am retrofitting an old oil boiler system with a mod/con and my current zone pumps are on the return lines. All the pipe plans I have been looking at show the system or zone circulators on the supply upstream of the boiler. I need to know if I will need to change this config or if it will be ok as is. The current system's supply and return pipe directly to and from the boiler. I will be adding a Low loss header with the new boiler and am concerned about how the position of the pumps may affect the header flow. I will be greatful for any help. I have read a lot of posts on this site and you guys seam to know what you are talking about


  • ZmanZman Member Posts: 3,476
    edited October 2013
    expansion tank?

    Circs on the return side are not a problem by themselves. It is important to put the expansion tank in the right place. Which boiler and how were you thinking of piping it?

    What type of zone?

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • CanecreekCanecreek Member Posts: 21
    edited October 2013
    System circulator position with a mod/con

    Mabe a Burderus gb142-45 or Knight wm 97-155. Current system appears plumbed correctly with the expansion tank on the supply with an air separator on top. I searched some more since my last post and understand the reason for locating the expansion tank and air separator on the supply in front of the system circulator is to maximize air removal. My current supply leads about 20 ft from the boiler then splits into the 2 zones. There are no current zone valves and the zone circulators are located just before the circuit returns to a single pipe which is about 5 ft from the boiler return. There are 2 radiators supplied prior to the zone split so plumbing new circulators or zone valves at the split will be involved. If I replace the current circulators (located on the zone returns) with zone valves and add a variable speed pump to the supply would that work? I am tring to avoid excessive plumbing but I do want my system to function correctly. I plan to use a low loss header so that would put the current circulators with in 5 ft or so of the llh. From what I have learned so far that may reduce my air removal due to the system pressure being close to its highest not far before the air separator and expansion tank. I am looking for solutions that will not require opening that 20 ft run to add zone valves or zone circulators. I can do it but it will be a pain. That part of the system piping is in the crawl space.

    Here is a little background on this project since I know these questions will be asked. I have done heat loss calculations based both on a sq ft method and with the slant fin calculator. My house is about 5000 ft with about 2200 up stairs. It was built in 1959 but at the time was the largest best built house in my little town. It was not insulated but has 1 1/4" thick plaster, 2x6 studs and 1" thick siding all of which is very tight. All the windows and single pane but are casements and very tight. The doors are all weather stripped with bronze and nice and tight. I am able to heat most of the house quite comfortably with a 28,000 bth unvented gas heater during moderate weather (1st and last 5-6 weeks of the season. My heat loss calculations come about 175,000btuh max with slant fin and about the same using a basic square footage method. The original boiler was a giant oil fired unit of unknown output and there have been 2 installed since with the current boiler rated at 229,000 btus. This boiler is the one I have lived with since it was here when I bought the house. The 2 nd boiler was still in the basement when I got here and I separated the heat exchanger to get it out (750 lbs). In doing so I noticed the heat exchanger was completely rotted out. At the time I knew very little about boilers but after some education I understand it likely corroded due to condensate fron short cycling. I timed the current boiler last season and it was running for about 3-4 minutes and cycling at 6 or 7 minutes. My fuel bills have been astronomical. I learned how to calculate my radiator footage and learned I have about 1800 sq ft of radiators which is very nice for my mod/con plan. I will be sealing my crawl space since it is all below grade and I have some nice French drains. I know that the slant fin calculator usually comes up about 25% over so I am looking for a boiler that will doe about 140,000 and turndown 5:1. I am trying to get my boiler at or below my max output requirement so as to have it condensate as much as possible. I want to be able to get good long burn cycles even when the weather is mild so I am actually more concerned about how low my boiler will turn down than it's ability to keep up on those 3 or 4 nights a year it gets really cold. If it can't keep up durring a really cold snap (fairly rare in NC but does happen) I will have a 19,000-28,000 btu unvented heater to make up the difference. My math says I can run at about 135-140 deg at max keeping the boiler condensating at all times. I hope to run a 35 - 40 delta and may have to put some thermostatic valves on some of my radiators in the front of my zones to get even heat. I have purchased a 40 gal indirect DHW heater and will make sure I have all nesacery components to make this system function at peak performance.

    I am a retired building contractor so learning building systems is part of what I do. I have done a lot of home work and will continue to learn until I am sure I know what I am doing which is why I am here. I have read enough post here to know collectively you guys know what you are talking about. After reading all this I look forward to hearing about where I am wrong and what to do about it. Thanks for the help

    It is amazing what you can Lean if you just read a little!

    I now know that I will have no problem with a 20 delta t on my radiators and a 40 on the boiler which should make my radiators a little more even. I feel a little lazy asking some of these questions that I could just search and read a little and find the answers. My plan is to have the longest firing times I can creating the lowest high temps possible
  • ZmanZman Member Posts: 3,476
    Right direction

    I think you are off to a good start.

    I would take a hard look at a firetube boiler like the Lochinvar WH155. I like stainless better than aluminum and the firetube is less likely to get clogged up with the stuff that floats around in older systems.

    I believe the new lochinvar controller can control a variable speed pump. This would help you get to you target delta t, always a challenge with boilers.

    A separator like this would also be a good call

    Looks like it is time to start drawing,

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • CanecreekCanecreek Member Posts: 21
    System circulater position with a mod/con

    Thanks Carl,

    Separator is exactly what I want. The Burderus has a llh included but it mounts horizontally requiring dirt and air removal another way. Was already leaning toward the Knight because of SS heat exchanger, more flexibility with the control and they make them in Tennasee which is right next door if I need parts. Still not sure about zone issue. Would this separator make pumps on the return work ok or still too much pressure to get a good air release. I will be moving to VFD circulator or circulators.

  • SWEISWEI Member Posts: 7,356
    edited October 2013
    Two kinds of Knights

    The KB and WB series are based on a Giannoni water-tube heat exchanger.

    The WH series are based on the newer fire-tube HX design.
  • CanecreekCanecreek Member Posts: 21
    Which Knight heat exchanger

    What is the difference?

    What about zone circ location?
  • SWEISWEI Member Posts: 7,356
    edited October 2013
    Heat exchangers

    the fire-tube design permits higher water flow rates, is much less susceptible to water quality issues, and requires less frequent maintenance.  In an existing system with cast iron radiation, they can usually be direct pumped just like the cast iron boiler they replace.

    I would seriously consider adding TRVs to your radiators.  This will eliminate zone valves or pumps, external controls, and thermostats from your budget and deliver superior comfort with individual temperature control at each radiator.  With proper sizing, you should be able to run both your radiators and your boiler using a 30ºF ∆T.

    Standard heat loss calculations do not properly account for thermal mass (e.g. your plaster walls.)  Given that you are comfortable with 28k during those shoulder season  days, I'd take another hard look at the boiler sizing.  If you have the specific dates and location, you could look up the degree-days and derive BTUs per degree-day per square foot, then use that to project a design day.  It's entirely possible you can get by with a 120k or even a 100k boiler.  The bigger the boiler, the higher its minimum firing rate.  A 175k mod/con with a 5:1 turndown will bottom out at 35k while a 100k bottoms out at 22k.  The lower that minimum number, the longer the boiler will run and the more efficiently and comfortably it will heat your house.
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