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Thread: Boiler decision - help wanted

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  1. #1
    DIY Junior Member abernat's Avatar
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    Default Boiler decision - help wanted

    Some background: I own a 1600 square foot house in Madison, WI. When we bought it it was set up as a two-flat, upstairs and downstairs. Each floor is heated with a boiler and cast iron radiators. The current setup is a pair of Bryant (?) boilers, 112K BTU in and 93K BTU out. I have no idea how old they are, as they came with the house (so > 5 years) and we have no documentation.

    Recently, we added insulation to the walls (3.5" cellulose, dense-pack) and attic (10"). We've been steadily replacing windows but there are a large number of original windows. We plastic them in the winter. Before this there was no wall insulation and minimal (2-3") in the attic.

    Since everything I've seen says that our boiler setup is now massively overkill, and we definitely get nowhere near 100% utilization even in the middle of winter, I'd like to make this system more efficient. However, the budget is _really_ tight right now as my wife is out of work. So I'm looking to make do with what I have. My two options are as follows, and I'd like advice on which one to select.

    Option 1: rework the system to use a single boiler. If the system was oversized to begin with (pre-insulation) then a single boiler should be able to handle the load easily.

    Option 2: purchase a used boiler and switch to it. This just came up. The replacement is a <brand unknown for now> 140K in, 90% efficient, PVC vented unit, 7 years old. The PVC venting is definitely attractive, although it would not mean we could seal the chimney up as we still have a gas water heater. The 90% is more attractive. Unit cost for that is a cheap $600.

    I'm also looking into an intellisense unit. However, budget is a factor.

    Which option will be the more efficient? Any other things I should look at?

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    Combining the systems is probably a good idea unless you expect to split the house again. You might want to consider an indirect WH when you get a little money in hand, much more efficient than a typical standalone tank, and faster recovery, too. 93/112=~83%, so not all that much to gain on a new one unless it is also able to modulate. A modulating, condensing boiler is generally designed to cold start, and makes use of an indirect more efficient since during the summer, it only needs to turn on when you need to reheat the WH tank, not stay on, when you have little need to space heat. Mine generally only turns on once a day during the summer, after morning showers unless I'm doing laundry.
    Jim DeBruycker
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    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    DIY Junior Member abernat's Avatar
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    We are not planning to re-split the house, no. It started as a 2-flat and we're recombining it into a single family home.

    WRT efficiency, the opinion seems to be that efficiency drops over time. My wife thinks the current boiler is ~25 years old, so would it still be kicking out 83%? Is there any way to tell? We have been getting it serviced every year, but that's only for the past 3-4 years, and I have no idea what happened before then.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    The efficiency is a maximum theoretical number, and especially with oversized units, you'll never reach that - same is true with a new(er) one. Those numbers are based on the unit constantly firing, and on an oversized unit, you'll get a lot of inefficiency with it turning on for a short time, cooling off, then restarting. So, just combining the zones so the one has to run longer will help all by itself. Total efficiency doesn't need to drop over time. Depends a little bit on if you have leaks and need to add water frequently. Ideally, once originally filled, you never have to add water, and any minerals that were in it have deposited themselves. If you have to add a lot, you can get deposits thick enough to affect the heat transfer, and thus the efficiency. If the burner is checked and adjusted and cleaned, there shouldn't be much difference over time otherwise.

    There's no guarantee the used, newer boiler is working properly. If I was going to go to the trouble of replacing, I'd want to go with a new one. Through the end of this year, (it might get extended again) you can get up to a $1500 tax credit for efficiency upgrades. Now, to qualify, you need a quite efficient unit, and that extra efficency costs extra. You'd qualify with an indirect WH, too. That's a 30% credit on hardware, up to the max of $1500.
    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 abernat's Avatar
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    Unfortunately, I think a new one is out of the budget. Even with the 1500 credit we're still talking multiple thousands. Also, our taxes are likely to be low this year (one of the benefits, I suppose, of not making a lot of money) and so we wouldn't even see the total 1500.

    So it looks like option 1 is the way to go. Use the current equipment, and up the load so that we don't have two units sitting idle all of the time.

    Thanks!

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    DIY Junior Member david_griffin's Avatar
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    Quote Originally Posted by abernat View Post
    We are not planning to re-split the house, no. It started as a 2-flat and we're recombining it into a single family home.
    .
    If I were you I would install a single heating unit. There are plenty of very efficient German made boilers for example Buderus.

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    Homeowner Thatguy's Avatar
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    Quote Originally Posted by abernat View Post
    Option 1: rework the system to use a single boiler. If the system was oversized to begin with (pre-insulation) then a single boiler should be able to handle the load easily.
    What's the cost to rework?
    What's your yearly cost to heat?

    Neglecting the changed costs for maintenance/repairs for 1 vs. 2 boilers:

    If it's $1500/yr for a 60% efficient two boiler setup then a mythical 100% eff. setup would cost you $1500 x 0.6 = $900/yr.
    If the single boiler setup is 80% efficient then this setup would cost you $900/0.8 = $1125/yr.
    If the cost of rework is $2000 then the rework would be paid off in $2000/($1500-$1125) = 5.3 years.
    Last edited by Thatguy; 10-18-2010 at 11:16 AM.

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    DIY Junior Member abernat's Avatar
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    Reworking is free; I'll be doing the work myself. I'm simply replacing existing piping runs and adding a manifold to the one boiler. The other will be disconnected and abandoned in place. Well, okay, it'll cost piping, but that's in the small number of hundreds. And I know I had short-cycle problems with the downstairs boiler that this will hopefully fix.

    The plot thickens: the $600 replacement is a Munchkin 199M boiler in "mint" condition (from the seller). The web indicates that the 199M is a 92% modulating boiler; this seems to match with the PVC venting (doesn't that require a condensing unit for safety?). It also appears as though it supports an indirect tank. Given this new information, would it make sense to snap this up? A 7-year-old mod/con vs a (guessing) 25-year-old 80% efficient unit?

    A side question - I was looking at indirect storage tanks, and they are *very* expensive. Is it possible to use a normal water heater as an indirect storage? I can't see how the plumbing would work, but it would be significantly cheaper.

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    Homeowner Thatguy's Avatar
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    Replace the $2000 with $600 and use your own numbers to see how long it takes for you to make up the purchase price.
    A water heater may lose 50 w continuously through the insulation and it probably makes sense to wrap it in a thermal blanket.

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    DIY Junior Member abernat's Avatar
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    I definitely see where you are going. One complication - in real terms, in a place like Madison (frozen north, heh), how much does the "modulating" nature get us?

    As of last year we spent ~$100 in gas, per month, for the downstairs boiler. This was including a _ton_ of short-cycling as we kept a single bedroom heated due to a new child. The upstairs also reported ~$100 in gas. So $200/month for 5 months = $1000 heating costs per cold season. If that's at 80% efficiency then going to a 90% will save us $100. However, since I know neither boiler ran very often, I'm wondering if the effective efficiency was lower (is "effective efficiency" a term?). If we were actually getting 60%, then we'd be saving $350 a year going to a 90%. If the modulation helped, we might get better than that.

    I simply don't know enough about boiler behavior to tell. I've seen a lot of references here to oversized boilers and horrible efficiency - is that a situation I'm in? Am I looking at the 6-year range (at which point I'd rather stick with what I have and know works) or a 2-year range (at which point the upgrade is a no-brainer)?

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    Retired Defense Industry Engineer jadnashua's Avatar
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    A modulating boiler works best if it is burning just enough so that it essentially runs all the time to match the heat loss. You get better comfort, and higher efficiency. The cooler you can run things, and match your load, the higher the efficiency. So, if you need say 20K BTU, and the thing can modulate to that, that's where it will run. The lower it runs, the higher the efficiency.

    Now, why did they take that thing out? You have no idea if it still works properly, and probably have a taillight warranty - good until you can see their taillights.
    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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    Quote Originally Posted by abernat View Post
    I definitely see where you are going. One complication - in real terms, in a place like Madison (frozen north, heh), how much does the "modulating" nature get us?

    As of last year we spent ~$100 in gas, per month, for the downstairs boiler. This was including a _ton_ of short-cycling as we kept a single bedroom heated due to a new child. The upstairs also reported ~$100 in gas. So $200/month for 5 months = $1000 heating costs per cold season. If that's at 80% efficiency then going to a 90% will save us $100. However, since I know neither boiler ran very often, I'm wondering if the effective efficiency was lower (is "effective efficiency" a term?). If we were actually getting 60%, then we'd be saving $350 a year going to a 90%. If the modulation helped, we might get better than that.

    I simply don't know enough about boiler behavior to tell. I've seen a lot of references here to oversized boilers and horrible efficiency - is that a situation I'm in? Am I looking at the 6-year range (at which point I'd rather stick with what I have and know works) or a 2-year range (at which point the upgrade is a no-brainer)?
    The efficiency you get out of a mod con has more to do with the temperature at which you can run the radiators and still stay warm, not so much the outdoor temp or anything else, as long as you can establish a reasonable minimal burn time (or modulate for some very long burn times). If the two cast-iron beasts were keeping the place warm, a 199K Munchkin is probably at least 2x oversized for your peak load (and may be 3-4x) but your AVERAGE load during the heating season on a binned-hourly basis will be about 1/4 to 1/2 the peak. It's not always -7F (or whatever your outside design temp is), and even if your radiation requires 160F or 180F water to deliver the peak load, odds are it can deliver the average heat load at 140F or less, which WILL get you into the condensing range with a 20F delta-T on the radiation.

    The lowest-modulated output of a 199K Munchkin is about 2/3 that of your current boiler, and if set up with a buffer tank, wouldn't short-cycle, and would deliver better than 90% efficiency as compared to the likely sub-70% efficiency of your dual cast iron setup. And since you're talking about an indirect HW heater, if that buffer tank was a TurboMax or Ergomax "reverse indirect" set up as a buffering pre-heater for an electric tank, the boiler would be doing all of the heavy-lifting on the DHW front during the heating season, while still running low temp enough to get the condensing benefit:



    The smaller ErgoMaxes are under $1500, and have enough mass to keep a 199K Munchkin out of short cycling itself into an inefficient range. If it turns out in practice that you need more mass in the buffer, you can plumb a cheap electric tank heater (not hooked up electrically) in series with the boiler loop on the return path between the reverse-indirect and the boiler.

    You can't really use an electric heater as a indirect. You'd need an extra pump and an external heat exchanger, and some controls, and by the end you'd have paid as much for Rube-Goldberg contraption of an indirect that had less capacity. What you're buying with an indirect is a well-designed heat exchanger & control arrangement, not just an insulated tank.


    Even with a 4x oversized mod-con you'll likely cut your fuel use by at least 30%, as long as the min-burn is on the order of 10minutes or longer. If the min-burn is 5 minutes or less you'll probably reap only 25% savings. But a lot depends on the mass, size & type of radiation. Big water-filled radiators can still put out a lot of heat with 90-100F water, where you'll see combustion efficiencies north of 95% (even with a 92% AFUE boiler), whereas fin-tube baseboard really craps out once you drop below 120F. But as long as the RETURN water coming back to the boiler is 120F or less you'll be getting better than 90% raw combustion efficiency, and if your min-burn is 10min+ the AFUE will pretty much be your average combustion efficiency.

    Odds are pretty good that the combined output of your dual-cast-iron is at least 3x your true heat load, and that the smaller one could carry the entire load by itself. You might start by hacking the system to run it all on the smaller boiler, and use the fuel use tracked against heating degree-day weather data to measure your actual whole-house heat load over the next season while saving up for a mod-con. Measure it in therms/ccf/decatherms, whatever they bill it out as, not dollars, since boiler input/output is measured in BTUs or kilowatts, not dollars. With only one boiler on the system it'll be possible to apply the proper derating for oversizing, and it'll be very real efficiency boost as well, even with short-cycling during the shoulder season. The December-February billing will have the least error, but the heating degree-day data must be to the EXACT meter reading dates no the billing period dates or calendar months.

    For an example of how that type of measurement can be done see http://www.terrylove.com/forums/show...e-Size-too-big
    Last edited by Dana; 10-19-2010 at 08:01 AM.

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