Primary Loop on a condensing-style direct vent boiler

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by philtrap, Jun 19, 2013.

  1. philtrap

    philtrap New Member

    Messages:
    66
    Location:
    Long Island, NY
    Thanks for your time Dana - I have a graduation this weekend and will have to get back to you on the heat loss numbers. My neighbor sized it up and has them and he is using the same unit and has a very similar home.
  2. philtrap

    philtrap New Member

    Messages:
    66
    Location:
    Long Island, NY
    Thanks Dana - Don't worry, I won't tell you to Die. You have been very helpful, unlike Tom who has been nothing but condescending from my first post.

    I spoke to my neighbor very shortly last night a our daughters graduation and he said the 110 was not oversized for our homes. He has the same unit and that's why I bought it. As I noted before, his home is very similar to mine, with the exception that I added a one room extension, so my home is a little bigger. Both homes were built by the same builder in the same year. They are much more larger than what you quoted as average for Long Island.

    Anyway, he did the heat loss calculations when he installed his unit (I think it was 2 years ago) and he recommended that I get the same unit since it works great for him and that's why I bought it (actually thru his heating supply Co.). He has the loss cacl's somewhere, but could find them during our 5 minute talk last night and I don't think I really need them since he already did them and determined the unit to be the correct size. I think I'll be fine with the 110

    I do plan on installing a water heater to it, but not right away since the one I have works and is fairly new.

    Thanks again Dana
  3. Tom Sawyer

    Tom Sawyer In the Trades

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    3,227
    Location:
    Maine
    I may be condescending but I'm also right which is what really pisses you off. Your house would either have to be a mansion or have next to no insulation in it to need a boiler that big. Even getting your hot water off it using an indirect is not going to correct your and your neighbors mistakes. Anyway, I'll be helpful for a minute. Walk around your house and add up the total amount of fin tube. Multiply that by 590 and with an expected water temperature of 180 degrees, that will tell you how much heat your radiation is capable of delivering. Now figure out if you can get return water temperatures near or below 135(delta t) I'm betting you can't possibly accomplish anywhere near that without dropping the output temperature which of course delivers less heat and less output to the baseboard. Most folks ( and I guess some heat guys) don't understand that copper fin tube baseboard needs relatively high water temperatures because its not radiant, it's convective but, your neighbor the heat guy knows this right? Understand that if you can't get return temps low the unit will not condense and it will short cycle. All of which basically means that you have thrown a wad of cash out the window mistakenly believing that you have bought high efficiency equipment. If as I suspect, your radiation is fin tube, you would have been better off with a cast iron low mass boiler like maybe a Biase with a gas power burner. It would have run at the same or better efficiencies and it wouldn't short cycle.

    More help. You should have done the heat loss before you bought the equipment but even at that it still comes back around to the radiation. At this point a heat loss is only going to hammer the nail home.
    Last edited: Jun 22, 2013
  4. philtrap

    philtrap New Member

    Messages:
    66
    Location:
    Long Island, NY
    Thanks Tom - I'll take my own measurements... It will take some time since I'm having and going to many graduation parties this weekend.

    But, let's say you and Dana are right and my neighbor is wrong and the 110 is oversized (even though his doesn't short cycle). I can try to return it, but I'm not sure I will be able to since it's been unpacked and hung on the wall. The only connections made are the two 3" PVC vent. The box it came in is crushed. I won't know until Monday if they can swap it. The price difference between the two is only like $300.

    If I can't return it an am stuck with it, I'm asking, can I hook it up as designed, let it short cycle and maybe die in a few years and then replace it with the PT60 since all the fittings are the same? I know that's a waste, but if you guys are right it may be the only option at this point. Or is there something I can do to help it not short cycle, like add heating zone in my basement or screened in porch and make it 3 seasoned? I Know your going to say you need the heat loss info, but I'm just thinking out loud while trying to get thru this. You also said something about hooking it up like a regular boiler without a Primary Loop. Maybe that's an option.

    It's a tough situation because I'm getting different advise. My neighbor is saying it's OK and will be fine and you guys are saying it's way oversizes...

    Thanks,
  5. Tom Sawyer

    Tom Sawyer In the Trades

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    3,227
    Location:
    Maine
    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: Jun 22, 2013
  6. philtrap

    philtrap New Member

    Messages:
    66
    Location:
    Long Island, NY
    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.
  7. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,139
    Location:
    New England
    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.
  8. Dana

    Dana In the trades

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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.
  9. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,139
    Location:
    New England
    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.
  10. Tom Sawyer

    Tom Sawyer In the Trades

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    Maine
    It could but ain't that just throwing heat out the door?
  11. Dana

    Dana In the trades

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    That's more like throwing the heat under an asphalt rug, isn't it? :)

    For the number, length depth & duration of the snow events on L.I. the rationale for a snowmelting system in temperate Long Island is nowhere near what it is in northern New England. That sort of thing might be (very weak, on it's best day) rationale for a boiler that size, but doesn't fix any issues related to it's being so oversized for the space heating loads (which under any circumstances will be the preponderance of the fuel used, unless it's a super-insulated house with miniscule heat load, and it's primarily domestic hot water.)
  12. Tom Sawyer

    Tom Sawyer In the Trades

    Messages:
    3,227
    Location:
    Maine
    I guess that there really isn't a cheap and easy solution to the problem and unfortunately I see this situation time after time. Far too many people don't understand the concept or operational parameters on mod-con boilers and as a result an awful lot of them get a bad reputation when 99% of the time it's installation error, not the boiler. It would be the best option if he can return it and go with something substantially better matched to the radiation but again, assuming the baseboard is copper fin tube, most mod cons are not going to match well. I hate advising less efficient equipment but n this case a gas boiler in the high 80's would most likely perform better and cost about the same to operate. Biase with a Carlin EZ power gas burner would be my choice I think.
  13. Dana

    Dana In the trades

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    I'm not sure how to prescribe a solution without knowing the system or the actual heat load.

    The neighbor's AWOL heat load calculation but it doesn't take a huge amount of information to put an upper bound on the heat load, or the min-temp you'd likely be able to run without short-cycling at min-fire. The notion that the smaller boiler wouldn't cut it is dubious at best- it would even be adequate for most houses in Minnesota, where the design temps are into negative double-digits, even for the same 1950s vintage housing stock so ubiquitous on L.I.. Anything built since 1980 or so usually has even lower heat loads.

    Faith-based sizing leaves too much to be desired though:

    "He has the loss cacl's somewhere, but could find them during our 5 minute talk last night and I don't think I really need them since he already did them and determined the unit to be the correct size. I think I'll be fine with the 110."

    Really? No numbers, no descriptions of construction or size, or the size of the radiation, just an abiding faith in a neighbor? He must be a nice guy and all, but something is really wrong here- most averaged sized houses in ALASKA that would never need the full output of the -110 and could manage just fine with the -60. The true heat loads of most mid-sized houses with OK windows on L.I. pencil out to something a bit shy of 15 BTU/hr per square foot of conditioned space, using reasonable design temps. Even at non-condensing temps the -60 puts out about 54,000 BTU/hr, which would cover a typical sorta-code-min house in the 3500-4000' range, or a tighter-than-average house with better windows in the 5000'+ range. Can't fault the newbie for being easily led down this path by the licensed HVAC guy who should have known better, but oversizing still seems to be the rule rather than the exception in the hydronic heating trades- practically a TRADITION.

    I'm not sure why it's so difficult to convince people that their heat loads are as low as they really are. It may have something to do with the tradition of oversizing cast iron boilers by a minimum of 1.5x. Most I see are around 3x oversized for the actual loads, and 4x isn't rare. While oversizing cast-iron by 1.5x has no real down side, there's an efficiency penalty at 3x. The downsides of oversizing mod-cons by even 1.5x are more dramatic, throwing away the potential efficiency & comfort you might otherwise get out of it, and for no good reason. This forum and others are rife with examples of misbehavin' systems where the boiler is too big to heat the place efficiently, where the contractor or homeowner upsized it a bit "just to be sure". In some cases it's easier to just crank up the temp to make the problems go away, turning a 95% AFUE boiler into an 85% as-used AFUE boiler, but it's a shame when it comes to that, a situation I'm hoping philtrap can avoid with a bit of corrective steering.
  14. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,139
    Location:
    New England
    FWIW, snow melt under asphalt often doesn't work well...the asphalt (tar) really isn't a solid and it can be problematic...best is concrete. The snow-melting was somewhat of a jest, but for some people, knowing their driveway and potentially sidewalks will be safe to traverse is a major benefit, and priceless. What's the cost of throwing your back out once with the doctors' visits and maybe loss of work time, not counting the discomfort. Or, the heart attack it helps prevent. It is solely self-serving, but it certainly is nice. Depending on where you live, it could cost $100 a pop to clear snow, and a big storm might take a couple of pops to maintain passage. Would not pay for the materials and installation over its life for the average user, but again, it is nice! Depending on your labor costs, it might be a wash once installed, though. It won't go on strike, either!
  15. philtrap

    philtrap New Member

    Messages:
    66
    Location:
    Long Island, NY
    Thanks folks for all of your input. I'm doing a full heat loss study and will post it soon. I'm paying a guy to do it. I'm still not sure if I'm stuck with this 110...

    FYI - Another guy in my neighborhood with the same house had this heat loss calc done and had an Alpine 080 installed and it works great.
  16. Tom Sawyer

    Tom Sawyer In the Trades

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    3,227
    Location:
    Maine
    You have to remember that before indirect s most boilers had tankless coils in them so everyone oversized for the domestic hot water load.

    I think I would rather go with an AFUE 87 than short cycle a mod con or run it constantly at very high temperatures. The 87% stuff is built to take the constant temps.
  17. philtrap

    philtrap New Member

    Messages:
    66
    Location:
    Long Island, NY
    All right folks... So here is the deal....

    I'm still waiting for my heat loss data which I'll get to you as soon as I get it, but I'm pretty sure (maybe 99.999%) that you guys are correct and my neighbor is totally wrong and the -110 is oversized. I probably should have bought the -60 from the start. Now here is my dilemma...

    The supply house won't take it back since I don't have all the packing. I'm OK with this. I have 2 options:

    1) buy the -60 and sell the -110 for a loss of +/- $1,000 (which I'm OK with)

    2) use the -110 and let it short cycle and not be efficient a die in a few years and then replace it then. Both units are the same size and have the same connections so it would be an easy swap.

    Bottom line is I got screwed by trusting someone and now I'm in a jam. I'm still going to save money in the long run by doing it myself even if I throw away the -110.

    Thanks again and please make your answers and thoughts quick and to the point (i.e don't lecture me... It's bad enough you already have and I could have left this forum, walked away and no one else could learn from this mistake).
  18. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,139
    Location:
    New England
    You'll be more comfortable and save money with the smaller unit. You may not lose as much as you think on the one you have, but that's a crap shoot, especially if you need to move it quickly. While maybe illogical, a lot of people don't think about replacing a boiler until fall when they need to turn it back on, and it doesn't work. Then, you have the conscious thing regarding selling it to someone where it will probably be oversized. Call the manufacturer and ask if the one you have can be converted and what it costs...it may work out that it can be derated to provide the same functionality of the 60 by just changing a few parts, and the cost won't be as bad as losing the money on the one you have and replacing it. For optimum operation, you should have someone with a combustion analyzer come in to check and tweak (if necessary) the burner, especially if you can derate it, which changes parts.
  19. philtrap

    philtrap New Member

    Messages:
    66
    Location:
    Long Island, NY
    They said they don't recommend it. The Tech said I may be better off by getting a 50 Gallon buffer tank. I know it's still too early until I get the heat loss numbers, but he was confident that I would be able to make it work. I did call the supplier again and asked if they would take it back as an "opened box" item at half the price I paid. I think that's fair and may solve the problems...

    I wonder if there is a buffer tank the can be used for hot water...
  20. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,139
    Location:
    New England
    YEs, there are buffer tanks that also produce potable hot water.
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