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Thread: Primary Loop on a condensing-style direct vent boiler

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    DIY Member philtrap's Avatar
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    Question Primary Loop on a condensing-style direct vent boiler

    Hi folks - I'm a first time poster and thanks in advance for your input...

    I'm installing a Triangular Tube Prestige Solo (PT110) boiler and have 2 questions...

    1) Is there minimum length of pipe from the boiler where the primary loop ends? I mean, can I place the closely spaced tees sic inches from the boiler, or is it better to make a larger loop so there is more flow and water running thru the primary loop?

    2) The boiler has 1" fittings and my piping is 3/4". Where is the best place to step down to 3/4"? Would it be OK to go directly from the 1" stubs on the boiler to 3/4" with a step 1" to 3/4" fitting or should I run 1" pipe for a while or maybe to the closely spaced tees?

    Thanks

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    In the Trades Tom Sawyer's Avatar
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    You should read the installation instructions. You absolutely cannot reduce the primary inlet/outlet or the primary piping to 3/4" It appears that you do not have an understanding of hydronic theory. It would be a shame to spend all that money and time and then have to tear it all out and do it all over again. You should find a competent technician and have the job done right.
    [B]No, plumbing ain't rocket science. Unlike rocket science, plumbing requires a license[B]

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    DIY Member philtrap's Avatar
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    Thanks Tom. I do understand that the primary loop must be 1". I asked the question before finishing the directions and from looking at the way the oil boiler was installed (with 1-1/4" boiler in/outs to eventually 3/4") I was just thinking of an easier way of doing it with all the 3/4" supplies I have. I do understand hydraulics and thermodynamics and have taken classes in these subjects too. My main question is in reference to question #1 which the directions does not address. Do you know if there is an optimum length for the primary loop with the closely spaced tees. I can place the tees 12 inches from the boiler, or would, say, 4 feet be better. Would the four feet create too much heat loss and the opposite hold true for the 12" length.

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    In the Trades Tom Sawyer's Avatar
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    Quote Originally Posted by philtrap View Post
    Thanks Tom. I do understand that the primary loop must be 1". I asked the question before finishing the directions and from looking at the way the oil boiler was installed (with 1-1/4" boiler in/outs to eventually 3/4") I was just thinking of an easier way of doing it with all the 3/4" supplies I have. I do understand hydraulics and thermodynamics and have taken classes in these subjects too. My main question is in reference to question #1 which the directions does not address. Do you know if there is an optimum length for the primary loop with the closely spaced tees. I can place the tees 12 inches from the boiler, or would, say, 4 feet be better. Would the four feet create too much heat loss and the opposite hold true for the 12" length.
    Why would 4' create too much "heat loss"?
    [B]No, plumbing ain't rocket science. Unlike rocket science, plumbing requires a license[B]

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    DIY Member philtrap's Avatar
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    I'm just asking if there is an optimal length. I know if it was 1,000 feet away it wouldn't be good and if the tees were right at the in/out of the boiler it probably wouldn't be good either.

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    In the Trades Tom Sawyer's Avatar
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    Quote; I do understand hydraulics and thermodynamics and have taken classes in these subjects too.

    So you should know the answer then.

    www.heatinghelp.com Buy Dan Holohans's book
    [B]No, plumbing ain't rocket science. Unlike rocket science, plumbing requires a license[B]

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    In the Trades Tom Sawyer's Avatar
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    There are several options but they are only band aids. I'm boarding a plane for Kansas City so I'm not going to be able to post for a few hrs. Maybe Dana will chime in
    Last edited by Tom Sawyer; 06-22-2013 at 09:36 AM.

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    DIY Member philtrap's Avatar
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    Thanks Tom - I'm fine with band aids. As long as it works, I'm OK. I'm even OK with buying a replacement in 4-5 years, so whatever works to make it run properly (if they won't take it back) I'm fine with that. Even if I have to place a zone that runs all the time facing the exterior of the house to heat a shed or even the yard, I'll do it I just want it to work and condense as designed.

    I'm now in a rock and a hard place with my neighbor for this, but I'll take the lumps and pay for whatever extras I need.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    How about running snow-melting for the sidewalk and/or driveway! You'd probably want to use a heat exchanger so you wouldn't need antifreeze in the whole system. In reality, on a cold day, your house probably only requires maybe 30-40KBTU. On an absolute coldest day, a bit more. The only way to tell for sure would be to run a decent load analysis. Unless you're running a spa where your hot water usage is constant, you do not take the IWH in as part of the system load, since first, it stores some energy, and you typically have some time to reheat the tank after usage. To get condensing, you need cool return, and that's limited by how much heat you can radiate, and by how much you are putting into it at the beginning. As stated, the type of heating devices can dictate your input temps to the system - some can handle lower temperatures, but some just don't work well unless it is higher - higher means less likely to return cool enough to condense. On an average day, I doubt you'll get much condensation. For decent efficiency, you want relatively long burn cycles, and when the boiler is way bigger than needed, that just can't happen unless you bandaide the thing, maybe a buffer tank. As a result, you won't be reaching expected efficiency levels. Each start cycle takes a hit on efficiency, and you also lose some when it shuts down. Ideally, it could just run for a long period, just meeting your needs, and adjust as those needs varied. Can't happen when it's too big.
    Jim DeBruycker
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  10. #10
    In the trades Dana's Avatar
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    Until you divulge the information necessary to make the heat load calculations and the zone radiation information we're kinda stuck here. So far we don't even know how big your house is, the number of zones, and the amount of radiation per zone (or whole-house.) Got a zip code, with a winter gas bill that includes the meter reading dates & fuel use?

    I'm sure you can make the think work without short-cycling at a non-condensing temperature, but that's not why you would ever buy a condensing boiler. With a condensing boiler you'd like to be able to set it up so that it can run nearly-continuous burns at a temperature low enough to get 95%+ efficiency whenever it's kinda-cold outside, say 35F, and only run at a higher non-condensing temp (if ever) when it's at or below your 99% outside design temp.

    Adding zones won't stop it from short cycling, nor will adding a snow melting zone. It's condensing boiler's 101:

    The min-fire in on the thing is about 30KBTU/hr- the size of the radiation of the SMALLEST zone relative to that 30KBTU/hr that will determine the lowest temperature you can run without it short-cycling. The min-fire input of the Solo 60 is about 16KBTU/hr. The lower you can run the system temperature, the more condensing you can get out of it, but in general you won't get ANY condensing performance until your average water temps are below 130-135F, a temperature at which fin-tube baseboard can only deliver ~300 BTU/hr per foot. At 30,000BTU/hr and 95% efficiency the boiler's dumping 28,500 BTU/hr into the system, so at the beginning of condensing temps the smallest zone needs to be able to deliver that heat to the room without heating up or it'll quickly go over the reset curve temp and turn off, making a short cycle. So at 300BTU per hour per foot that means your shortest baseboard zone needs to be at least a large fraction of (28,500/300=) 95 feet long.

    With the -60 at 95% efficiency the output is about 0.95 x 16,000= 15,200 BTU/hr, and the smallest zone only needs to be a large fraction of (15,200/300=) 51 feet long.

    If your smallest zone is actually 15' long, you'll have to bandaid something even with the -60, but you'd be totally screwed with the -110. Most bandaids involve adding thermal mass to the system in an appropriate way to stretch out the minimum burn times, or adding radiation, reducing the number of zones by tying them together, etc. But you can't cheat the basic physics- it needs to run at low temp to get condensing efficiency, and it needs to at least approximately balance the BTU-in with the BTU-out, even at those lower operating temperatures.

    But so far you've given us nothing to work with to figure any of this out- you haven't even shared your neighbor's heat load numbers (and at what outside design temp he used), nothing about hour house design/construction, square feet of window, etc.

    Using a standard indirect for hot water won't fix any of these problems, but using a "reverse indirect" can make a real difference in burn-times, since it's not a separate zone, but rather a massive hydraulic separator for the heating system.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    Humm, snow-melting a 300' driveway could make a difference, but as said, it depends on your smallest zone and the lowest output available...it's unlikely the 110 can go low enough and the only time it would likely ever go to max would be on maybe reheating the indirect, should you have one.
    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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