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Thread: Why would you use prority DHW , Can I do this ? 80K Gas Weil-Mclain ultra

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    DIY Junior Member charlie p's Avatar
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    Default Why would you use prority DHW , Can I do this ? 80K Gas Weil-Mclain ultra

    Im replacing 100k oil burner with an 80K Gas WeilMclain ultra.
    The 100k was over sized as we found out from a heat loss of 55k
    My oil burner has 3 zones ,that includes DHW from a 40gal boiler mate.
    the DHW is not zoned priority so if the other 2 zones and the DHW are all calling for heat ,they all get it.
    I haven't ever had a problem with running out of hot water so could I still run that setup with a 80k
    Is there a need to do priority
    Thanks for input
    Last edited by charlie p; 03-24-2013 at 12:55 PM.

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    The modern boilers with outside reset often do not run at max temp. Your old one might have. Being oversized, and at a higher temp meant it might keep up with the demand on the IWH. The Ultra, on the other hand, if set up reasonably, probably will never run at max temp unless it just can't keep up at those lower temps. This is likely to mean the IWH may not get as much heat as the old setup. Most of them are designed to provide priority zone for the DHW, which does two things: runs the boiler up to max, and shuts off the other zones until the DHW is restored. Some have a max limit, so it will switch back to space heating if the call for DHW is extensive, but not all. In general, that max override is not needed. While it may work without the priority zone, if the logic is calling for say 130-degree water for space heating, and the tank wanted to be 140, it would run forever and not achieve the setpoint for the IWH - the tank can't get hotter than the supply, and setting it up as a priority zone forces the boiler to high fire to get it there as fast as it can.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    DIY Junior Member charlie p's Avatar
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    Quote Originally Posted by jadnashua View Post
    The modern boilers with outside reset often do not run at max temp. Your old one might have. Being oversized, and at a higher temp meant it might keep up with the demand on the IWH. The Ultra, on the other hand, if set up reasonably, probably will never run at max temp unless it just can't keep up at those lower temps. This is likely to mean the IWH may not get as much heat as the old setup. Most of them are designed to provide priority zone for the DHW, which does two things: runs the boiler up to max, and shuts off the other zones until the DHW is restored. Some have a max limit, so it will switch back to space heating if the call for DHW is extensive, but not all. In general, that max override is not needed. While it may work without the priority zone, if the logic is calling for say 130-degree water for space heating, and the tank wanted to be 140, it would run forever and not achieve the setpoint for the IWH - the tank can't get hotter than the supply, and setting it up as a priority zone forces the boiler to high fire to get it there as fast as it can.
    I'm pretty sure (after reading it a dozen times ) I understand
    Is it normal for DHW to be set at 140
    From what Iv seen over the past 10 years with the boiler mate it has a smart control that reads 120,I can set it to what I want but that's what the guy who installed it had it set at
    Am I wrong thinking that's the temp the water in the tank gets to? Or is it the water temp that goes out to the showers or both?
    As of now I'm looking to be up and running Mon or Tues,,,cant wait. oil tank probably has 15 gallons,temps at night are cold enough to use the new boiler for another month or 2 befor summer sets in
    On top of the new boiler I insulated the pipes and had the guys redo the 1 zone that was a figure 8 and added a baseboard to the back room that didn't have heat.
    As I said,,,I cant wait

  4. #4
    Retired Defense Industry Engineer jadnashua's Avatar
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    120-degrees is a fairly normal water temp for non-boiler situations. It also aligns with the normal code requirements to keep it at or below that. BUT, it is not uncommon to set the IWH to 140 or even higher. This requires a tempering valve to be installed on the outlet, typically adjusted to 120 or so. Having hotter water isn't a bad thing, as in an indirect, with their good insulation, there isn't all that much standby loss and it functionally makes the tank appear bigger. If it's convenient, it's also nice to be able to run that full-temp water (before the tempering valve) directly to the dishwasher, and maybe the washing machine, since they do clean better with hotter water. If you set your new boiler to run at 180-190 or so (mine will jump to 192 or so, and normally runs 120-130 for heating purposes), you'll lose a lot of the efficiency of the thing. Most of the time, you'll only need much less than that. But, for max comfort, and efficiency, longer boiler runs are necessary. That can't happen unless the boiler temp is lower most of the time. This can mean the IWH won't ever recover to the setpoint, or it will take a very long time verses running at a higher temp. It would be folly to force the new boiler to run at a high, constant temp just to keep the IWH hot. When setup as a priority zone, the boiler will run at whatever temp the house needs, then switch all of its heat and raise the setpoint to keep and reheat the IWH so you can maintain your output.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    In the trades Dana's Avatar
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    In the late winter when your incoming water temps are 40-45F an 80K 95% burner can deliver 2gpm of 120F water pretty much forever, so long other zones aren't calling for heat and there are no other large hot water draws in progress. If the other zones are pulling 25KBTU/hr at the same time you're pulling an endless shower it eventually drops below body temp (a hint that it's time to get out of the shower.:-). With the potential of multiple simultaneous hot water draws it's really better to run it as a priority zone to limit the chances that it'll fall behind.

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    Master Hot Water Mpls,MN BadgerBoilerMN's Avatar
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    Most condensing boilers will provide for priority domestic hot water (DHW) changing set point (boiler water temperature) to 180F or so. This will satisfy nearly any indirect water heater (driven by the condensing boiler) in a matter of minutes. Since most hydronic systems may idle off for hours while space heating, without notice, the priority is almost always a good idea. Certain manufactures will time the DHW call and switch back and forth should the need arise e.g. IBC, Viessmann, but this is rarely necessary except in the coldest climates (not Minneapolis) or the highest DHW loads i.e. big tubs or lots of teenagers (OK, maybe Mpls).

    Outdoor reset does not effect DHW in any way, since all condensing boilers will have an option to change setpoint higher even if the installer decides to let space heating circulators run while the boiler is trying to satisfy the incidental DHW load.

    We almost never up-size a boiler up from the heat load or let DHW call run wild.

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    DIY Junior Member charlie p's Avatar
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    Any comments,Name:  IMG_3001.jpg
Views: 139
Size:  45.0 KBName:  IMG_2999.jpg
Views: 145
Size:  34.7 KB
    I really have no ideal what I'm looking at.
    The empty space to the right of the boiler is where the Boiler mate is going

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