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Thread: Mod-con or not?

  1. #1
    DIY Junior Member aspkiller's Avatar
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    Default Mod-con or not?

    I sure could use some condensing/non-condensing help before I replace my 18 year old gas combi boiler. Hopefully, some of you can offer some insight. Here are some of the facts:

    The house is located in the Lake Tahoe basin at 6,300 feet. It is 2,700 sq. ft. on three levels, reasonably well built and insulated and is 18 years old.

    Current system is a Trianco Heatmaker HW-M2-130. (130k In / 110k out = 84% AFUE). It has a built in 20 gal. tank with a double wall heat exchanger (required by code). There is a total of
    100 linear feet of 3/4Ē Copper /Aluminum baseboard radiators. There are three zones, one for each floor. The top floor has 40 l/f for kitchen, dining, living (high ceilings and most windows), the mid floor has 28 l/f for the MBR and bath, while the bottom floor has 34 l/f for the secondary bedrooms and laundry (which has the combi boiler, etc.).

    The system supplies DHW at the constant rate of 130 gph of 100 degree rise and the intermittent rate of 6.8 gpm of 60 degree rise. It has been quite sufficient to date. However, the house is
    rarely used in the dead of winter and then only by one or two people with minimum DHW requirements.

    Dec to Feb temperatures average 41 hi and 15 low and the mean temp for the six winter months is 32 degrees. Additionally, we keep the house at 55 degrees when we arenít there in the
    winter. A freeze alarm calls us when there is a problem. But, I would like to go back to the days when we didnít get those calls.

    Over the past couple of years, little nagging problems have begun to surface, suggesting that it might be time for a replacement. Were it not for the double wall DHW exchanger requirement, I
    would probably have opted for the Laars Endurance with DHW. But conforming to that requirement probably means going with boiler and an indirect double wall exchanger storage tank.

    Which leaves me with several questions:

    If I go with the indirect storage tank, will a mod-con boiler really be of any great advantage in our situation. Cost is secondary to reliability, as we really donít like having service people in the
    house when we arenít there.

    Should I split the system and go with a separate water heater that can be shut down or put into a winter mode? If so, then what type of boiler and HW heater would be best?

    Thanks for any and all advise offered

  2. #2
    Master Hot Water Mpls,MN BadgerBoilerMN's Avatar
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    Default heatmaker replacement

    You have one of the first condensing boilers ever built. The first generation condensing boilers HeatMaker and Glowcore- were not known for reliability, though there were obvious exceptions. Needless to say, the boiler is long overdue.

    A proper heat load should be performed first, as your current boiler has twice the output of the radiation. Boilers should not be upsized for domestic hot water.

    Modern condensing boilers are quite reliable if sized, installed and maintained by an experienced professional (someone you would trust in your house, even when you are not home). A ModCon with an indirect is one of the most efficient heat sources available and most comfortable as all condensing boilers now have built-in outdoor reset automatically adjusting boiler water temperature to the outdoor temperature improving operating efficiency and comfort at once.

    Double-wall heat exchangers are going away for residential applications as they are totally unnecessary and diminish heat transfer efficiency. You may find an enlightened plumber who can help you with this. Proof of this statement is the fact that Buderus (the largest boiler manufacturer in the world) sells indirect water heaters in 150 countries - as I recall - and does not make a double-wall indirect water heater.

    Which high efficiency condensing boiler is best depends on several factors including; local support, application, fuel available and radiation. For instance several European condensing boilers have operating water temperature limits, which may not satisfy a high temperature (radiation short) application.

  3. #3
    DIY Junior Member aspkiller's Avatar
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    Default

    Thanks. I also determined that the boiler is grossly over-sized for the baseboard load and was selected by the original contractor to meet DHW requirements.

    Me getting a code change to allow single wall HX wouldn't be worth the effort. But, I will check to see if there is anything already in the works.

    Also, I'm trying to determine if there are any substantial benefits in having the HX on the outside of the storage tank, which might make it easier to drain and bypass when we aren'there for several months at a time during the winter (and replace in the event of leaks). Or, does keeping a large, extremely well insulated heat sinc actually improve fuel and boiler
    efficiency? Our boiler / storage tank location is the laundry room, which is quite small, partially underground and very well insulated. Heat loss should be minimal.

    Lastly, fire tube or water tube?
    Last edited by aspkiller; 08-07-2011 at 10:08 AM.

  4. #4
    Retired Defense Industry Engineer jadnashua's Avatar
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    Default

    I have a Buderus, wall-hung, condensing boiler with an indirect. My indirect is larger than 'normal', since I wanted to be sure I could fill an oversized 6' tub and not impact other users. But, for most people, you can use a smaller indirect than you would if it was a self-contained WH, and they come in various shapes (rather than just round cylinders), so your packaging is much more flexible. I had a Trianco-Heatmaker and it was very unreliable. Plus, it was jetted to provide the minimum hot water, and that was barely adequate. Yours is setup with 60K more BTU than mine, so it is normal to get more hot water out of the thing.

    A large quantity of stagnent water in a tank that is cooled off probably isn't a great idea. But, some of the better indirects are so well insulated that they drop barely 1/4-degree per hour, and with no use, they may not call for the boiler more than once a day or so. There isn't that much reason to shut them off, as standby losses are small, and often, what they do lose goes into heating the house, so it isn't a total loss in the first place. My gas usage decreased about 35-40% when I went to the Buderus and indirect from the Trianco unit. Your Trianco stayed hot all the time winter/summer. A mod-con only fires when it needs to. Mine is often sitting at ambient in the summer since it has both low mass, and the indirect doesn't call for heat very often.

    One specific burner type isn't necessarily better than another...it's more in how well integrated and executed it is.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  5. #5
    DIY Junior Member aspkiller's Avatar
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    Default

    Thanks Jim-

    Still taking all the data in before calling a few locals to weigh in on the subject. Just want to be a little more educated so I can understand what will probably be several different approaches to the
    re-do.

    So, more input please, from any others with expertise in this area.

  6. #6
    Master Hot Water Mpls,MN BadgerBoilerMN's Avatar
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    Default

    Nearly all indirect water heater are super-insulated and even those that aren't lose little since the common flue is missing. Think thermos bottle.

    If you turn the temperature down on your vacation home water heater be sure to turn it up again long before you shower, as legionella can be a concern. I always include a potable water mixing valve on all my modcon boiler, indirect designs and keep the tank at 140įF.

  7. #7
    In the trades Dana's Avatar
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    Default

    Quote Originally Posted by aspkiller View Post
    Thanks. I also determined that the boiler is grossly over-sized for the baseboard load and was selected by the original contractor to meet DHW requirements.
    Just to be clear- the baseboards are the emitters, not the load, and the design-heating load cannot be determined by the amount of baseboard designed-in. The length of the baseboard relative to the actual heating load is what determines the water temp requirements, and with a mon-con you'd ideally be able to run the baseboard at 120F or lower, where it's putting out on the order of ~200BTU/foot )when the air near the floor is 65 (it'll put out considerably more per foot with 50-55F air), in order to be able to get some condensing efficiency out of it.

    But for a 2700' house to have a design heat load of only 100x 200= 20,000BTU/hr in your location would be unusual. At 160F average water temp you're looking at ~450BTU/foot (for a total of 45K, which is possible), and at 180F it would be more like 600BTU/foot (60K, which is probably more than the actual heat load if it's a tight well insulated house with low to moderate glazing fraction.) As BadgerBoilerMN says, to design it right you need to start with a careful heat loss calculation to determine your true design heat load, and work it back from there.

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