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Thread: New Boiler and Indirect water heater question...

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  1. #1
    DIY Junior Member Maxadine1's Avatar
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    Default New Boiler and Indirect water heater question...

    Goodmorning,

    I have a question about replacing my old boiler and hot water tank. Our boiler is original to our house (mid 80's) the hot water tank is under 10 years old. We have gotten 2 quotes for an high efficient boiler and indirect water tank. Both are at opposite ends of the spectrum. The first is for a Burham Alpine 105 with 105,000 BTUH input and a SSU45 Superstor and the second is for a Burham Alpine 150 with 150,000 BTUH input and a SSU80 Superstor. My question is do we need an 80gal tank? is 45 to little? We have gas heat. 4 zones. Our house is about 2500sqft with 3 1/2 baths. We have not done a heat loss yet. Do you have any thoughts? Our old system is an old Burnham boiler with a 40 gal tank. We have 2 adults and an infant presently living in the house.

    thanks
    Kyle

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    The larger system is almost certainly oversized, and the smaller one quoted is likely as well. You don't need to upsize the boiler based on the size of the indirect. Generally, it is set up as a priority zone, and all of the available heat will go to reheat the water when that is required. If you don't have a big soaking tub, you can likely get by with the 45g tank. But, if all three showers might be running, and those users like to take long, hot ones, you may prefer something larger. Sounds like that is unlikely, at least in the near term. You could compromise and go with say a 60g if you wanted to hedge your bets. The good thing is that, other than size, indirects tend to be quite well insulated, so you don't have a lot of standby losses like you could (relatively speaking) with a gas fired stand alone WH.

    Without a good heat load analysis, sizing the boiler is all kind of useless.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3
    In the trades Dana's Avatar
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    What Jim said. I live in a not-super-insulated 1923 house about that size in Worcester, and the -105 would be nearly 3x oversized for my design condition heat load, and I have a colder outdoor design temp than you. Get a room-by-room heat load.

    Also since it has 4 zones, what it the total amount of radiation (baseboard, radiant floor, whatever) on the SMALLEST zones?

    If the minimum-fire output of the boiler isn't small enough you can short-cycle it into low efficiency running it at condensing temps if it can't dump that heat into the room at those temps, cutting into the efficiency of the boiler. With any system you'll have fewer problems and can get higher average efficiency by going with the SMALLEST modulating-condensing boiler that actually meets the design condition heat load.

    At 120F fin-tube baseboard puts out about 200BTU/foot, @ 140F it's ~300BTU/ft, @ 180F it's ~600BTU/ft.

    The min-input of the Alpine-105 is 21KBTU/hr, so at ~ 120F running in condensing mode it's min output is ~20,000BTU/hr.

    If your smallest zone has 40' of baseboard, the balance point would be at 20,000/40= 500BTU/ft, which takes water temps of 165-170F. But at 120F the baseboard is only emitting 40 x 200= 8000 BTU/hr....

    ...so you have 12000BTU/hour more heat going into the system than the baseboard can deliver into the room. What's gonna happen?

    The temperature of the loop rises until the boiler turns off because it's overshot the outdoor reset curve temp, then refires when the temp has dropped sufficiently repeat a gazillion time until the thermostat stops calling for heat. How bad is it?

    In most boilers the temperature hysteresis is a fixed number of a handful of degrees- lets call it 5F. In 40 foot baseboard with 3/4" pipe you have about 7lbs of water, double that for the boiler mass and distribution plumbing, for about 15lbs of water as the total thermal mass of the zone. At 12000BTU/hr (3.3 BTU/second) of excess heat, to raise 15lbs of water that 5F degrees only takes (15lbs x 5F)/3.3= 25 seconds, after which the boiler stops, waits, refires repeat. Running it that way throws away a fixed amount of heat with every flue purge & ignition cycle, and wears out the boiler.

    The min-mod output of the smaller Alpine 80 is ~15000BTU/hour, so a 40' zone that balances at (15000/40=) 375 BTU/hr. The baseboard at 120F dumping 8000BTU into the zone, so your excess now is only (15000-8000=) 7000 BTU/hr (or 1.9BTU/second). The minimum burn time then is (15x 5)/1.9= 39 seconds- still not great, but substantially better than 25 seconds. But if you bumped the temp to 140F the baseboard would be delivering (300BTU/ft x 40=) 12000BTU/hr, so the excess is only 3000BTU/hr, (0.83BTU/second) and your min-burns are 72 seconds, which is much less of a disaster, whereas the bigger boiler with the 20,000BTU min-output would still be short-cycling like crazy on zone calls.

    Unless you can run the boiler at 140F or less with burns of well over a minute (5 or fewer burns per hour would be GREAT), you won't be able to get the condensing efficiency without abusing the boiler, and that's why smaller=better, especially with low-mass heat emitters on a multi-zoned system.

    But do the math on your actual zones to figure it out. If you have big old-fashioned radiators you have a lot more water-mass buffering it, and the min-burn times soar, not a problem. If you have radiant slab floors, even better! But if you have even one or two stubby zones shorter than 40' of baseboard you have to think about how you want to go about fixing "the problem". And it IS a problem even for the Alpine-80- a BIG problem for the Alpine 150! Adding mass is one way, adding radiation is another, combining zones is yet another, but ideally you'd be able to set the temperature floor of the curve at ~120F for baseboard, if you want to hit 95% efficiency.

    With 3 people and even a 50KBTU/hr boiler behind it you won't need to upsize the tank, if the indirect is controlled as a "priority zone". A 50K boiler is nearly 2x the burner of a typical 40 gallon gas-fired standalone. You'd only need to upsize the tank if you were installing a big soaking-tub or spa or something.

  4. #4
    In the trades Dana's Avatar
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    BTW: With the exact meter-reading dates and the therms used it's possible to put a fairly firm upper bound on what the whole-house heat load is, using the boiler as the measuring instrument. The base-65F heating degree days can be added up for a weather station near you on degreedays.net, and you can convert therms/degree-day into BTU/degree-hour, and multilply by the number of degrees between your heating design temp (+11F) and 65F (so you're looking a 54 heating-degrees. A mid-80s atmospheric-drafted boiler ran at about 80% efficiency steady-state, so 25 years later call it 75%.

    eg: Say, between 5 Feb and 6 March you used 132 therms, and you looked up and found out that was 628 heating degree days. That's 132/628=0.21therms per degree-day.

    A therm is 100,000BTU, so that's 21,000BTU/degree-day

    There a 24 hours/day, so that's (21000/24=) 875BTU per heating degree-hour...

    ...as source fuel BTUs, but in an old boiler running at ~75% efficiency only 3/4 of that was heat applied to the house, so the house needs:

    0.75 x 875= 656 BTU/degree hour as boiler output

    Your 99% design temp in Weymouth is +11F, so at (65-11=) 54 heating degrees your whole house heat load is not more than:

    54 x 656= 35,424 BTU/hour

    That's a realistic number for a reasonably tight '80s house with 2x6 framing and double-pane windows. If the place leaks air like crazy, the basement has no insulation and you have a bunch of single panes it might be over 50KBTU/hr @ +11F, but it would be unusual for a house that size to be over the 73KBTU/hr DOE output of the Alpine 80 if it's insulated, and the windows actually have GLASS in them. :-) But run the numbers on a wintertime gas bill- this method of estimation only under shoots if you've been running auxilliary heating like a wood stove or something fairly often. Most of the time its an overshoot (due to ignoring the hot-water usage.)

    If your numbers come up even 2x that in the example (possible, I suppose), the Alpine-80 is still your boiler, assuming you stick with Burhnam.

    If 3x the example (not very likely) go ahead with the Alpine 105, (or better yet- fix the heat leaks on that house- it's an efficiency-disaster, truly!)

    It would have to be almost 4x that amount to rationalize going with the Alpine 150.

  5. #5

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    I have a Burnham Alpine with a 50 gal Burnham indirect tank in my cape. It's oversized by a lot. The tank really isn't a problem because as stated above the standby loss is very little. I go away skiing for 4 day weekends and return to find that the tank has only cycled once during that period on non-use due to heat loss.

    The boiler is a different issue. After doing a LOT of reading here, on other sites, and the Alpine manual I've gotten my unit to stop short cycling for the most part. This all could have been avoided if I had spent more time in the beginning of this project finding an adequately qualified installer that would do a correct heat loss calc and correctly size the unit needed. With the heating season coming to an end, that is my suggestion. You won't need heat again soon until October. Get 10 estimates if you have to. Get 20. Ask them for detailed heat loss calcs and their justification for choosing the model they did. The numbers won't lie.

    Dana is right to ask about zone sizes too. I switched to hydronic in my house and put in two zones on the first floor. After this heating season I'll be combining them to increase the load. I'm also pulling out all the fin tube I put in and replacing it with cast iron baseboard that I've been collecting off of Craiglist. That should totally cure my short cycling issues. If you don't have enough heating mass you'll never heat rooms with 125 degree water which is what my boiler made most of this mild winter. In that case the unit will heat the water to ~125, then turn off. The pump will stay on as long as the zone is calling for heat. Then when the water reaches ~110 the boiler will cycle on again. That isn't a situation you want to be in.

    That being said, I've been very happy with the Alpine unit. I would recommend it as an alternative to the much more expensive Buderus models.

    If you really want to test the contractors you're getting quotes from, you can talk to them about smart pumps. I've read about some installs using them and the ability for the pumps to sense temperature and vary the GPM output of the pump for each zone has yielded even more money saving results.

  6. #6
    Master Hot Water Mpls,MN BadgerBoilerMN's Avatar
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    Every properly installed hot water boiler - be it for a new house or retrofit - must be properly sized to the heat load. Every savvy contractor will produce the load - or a sample thereof - before ordering your new boiler. An indirect water heater (the best way to heat water when a boiler is the main heat source) is sized to the DHW plumbing load. An indirect-fired water heater must have the heating capacity to match the output of the boiler or short-cycling will occur.

    Over-sizing boilers is unfortunately standard practice in the industry. Over-sizing indirect water heaters, is of little consequence, other than the initial up-front cost of the indirect.

    To clear up some confusion; The Burnham Alpine is a wall-hung, low-mass, sealed-combustion boiler, which uses the Gianonni stainless steel, water-tube heat exchanger. As such, great care must be taken in the design of the near-piping to avoid low-flow and subsequent flashing (pings, hisses, rumbles, etc.). The water-tube boiler is also susceptible to plugging from old system debris e.g. rust from old radiators and other ferrous metals. Flushing the old system and installing a "y" strainer may be in order.

    The use of ECM "smart" pumps, is an advanced design skill and should be avoided on the secondary side of any water tube condensing boiler without considerable experience. Though the percentage of energy savings can be above 50% (we use them all the time) the real savings in a typical residence is in the dozen dollar per year range.

    As mentioned, when converting from a high to low-mass boiler, load size matters. In any conversion the radiation should be measured and zone loads taken into account. We use low-load strategies (no, not the over-sold buffer tank) to control boiler cycling and proper load management. Though cast iron baseboard will add mass (contributing to load) it does not guarantee proper load (heat load calculations do this best).

    Mixing up mass and load is a common mistake. We heat many homes with very low "mass" radiant ceilings, which will never operate above body temperature here in Minneapolis.

    As for finding a contractor it would be unwise to wait until November, in fact March is the perfect month to buy a new boiler. 20 new boiler installation quotes is not only irresponsible it is an unnecessary waste of your time and theirs. Contractors have to spend money to give you a "free" replacement boiler bid. For every boiler the contractor doesn't sell, he must add the cost of the estimate to future boiler jobs, driving up the cost for everyone.

    We suggest a better way.

    http://www.badgerboilerservice.com/contractor.html


    http://www.badgerboilerservice.com/boilerchoice.html

  7. #7
    In the trades Dana's Avatar
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    No argument from this corner with anything BadgerBoilerMN offered up!

    But doing the napkin math on both the zone-radiation and the fuel-use against heating degree days & design temp is a dead-easy backstop against ridiculous or not-well considered proposals, which are the most common kind, when offered "free". The notion that ANY house that size in Weymouth MA needs a 137KBU/hr-output boiler like the Alpine 150 is preposterous, whereas the 96KBTU/hr-out Alpine 105 is merely ludicrous.

    Maybe in an extreme cold snap and it actually hit -72F or -113F or something it might need the output of the -105, but that's not exactly a rational outside design temperature to use for Weymouth.

    Get a room by room, zone by zone heat loss calc done (even if you have to hire BadgerBoiler to do it), have a hydronic heating designer (more than a licensed plumber) make the necessary adjustments to the system design, and put it out to bid.

    Just dropping in either of those boilers will likely cost you more in the unnecessary additional up-front cost of a larger boiler, and in reduced efficiency than the cost of a heat loss calc & system design adjustments to make it work right from the get-go. (Not everybody is up for playing "junior hydronic designer" like some of us. ;-) )

  8. #8
    DIY Junior Member Maxadine1's Avatar
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    Thanks so much for your input!
    So after a bunch of quotes we finally got a heat loss done. It came to 50000. Now we were told by other companies that it would take a few hours and cost about 3-400$ to have it done. The company we went with it took them about 15 minutes? Does that sound about right? Not sure about the cost. Just a refresher we have a mid 80's garrison just south of Boston. It is 2500 sq feet approx. Windows are new and it is very well insulated. We have 4 zones of heat. The company suggested a Bosch 100 boiler and indirect water heater. Any thoughts?

    thanks
    Kyle

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