Do certain boilers work better with indirect than others?

Discussion in 'Boiler Forum' started by dwassner, Jan 31, 2012.

  1. dwassner

    dwassner New Member

    Jan 31, 2012
    After a few searches on google with no results I thought I should just post my question here. We are about ready to close on a home that has a baseboard heating system. The boiler is from '66 and the lender is requiring that it be replaced, and the hot water tank is from '01 and will need replacement soon enough also. I want to go with an indirect hot water system. Are there any boilers that are geared towards this setup? Are there any features that certain manufacturers offer that would make them better with indirect than others? Any to stay away from? thanks
  2. Dana

    Dana In the trades

    Jan 14, 2009
    All hydronic boilers (including steam boilers) can be set up to run fine with an indirect, with controls to run the indirect as a separate heating zone. Most high-efficiency modulating-condensing gas/propane fired boilers have a built-in presumption that there is going to be an indirect, which may require higher output temps than the other zone-calls when operating under "outdoor reset" control (where it raises & lowers the output temp of the boiler in response to the outside air temperature.) But even simple minded cast-iron boilers work fine with an indirect. If yours is a smaller boiler (and almost all should be relatively small, in a house that is insulated and not gargantuan in size) it's better to control the indirect as a "priority zone" , which inhibits the space heating zones from drawing heat when the indirect is calling for heat, which results in much faster recovery times on the hot water tank.

    Any time you're buying a new boiler is the golden opportunity to "right size" the boiler for the space-heating load. Almost every boiler installed in 1966 was at least 2x more than was needed, and if the house has been tightened up and insulated, windows upgraded, etc. it's almost certainly going to be 3x oversized. There is a cost in operating efficiency when oversizing a boiler (even a modulating boiler), not just the up-front cost. Before you drop serious coin into a boiler swap, insist on a "Manual-J" type heat loss calculation, based on the 97.5th or 99th percentile outside design temperature, and no lower. If the HVAC guy doesn't know what you're talking about, find somebody who does. An example of what is NOT a heat loss calculation (but still common amongst installers) goes something like "Lessee, ya gots a 2400' house, times 30BTU/ft comes to 70,000BTU/hour." This type of calculation reliably oversizes the boiler by 2x or more, costing you more up-front, and every heating season thereafter in reduced efficiency.

    If yours is oil fired boiler there is a lower limit to the BTU output that oil-burners can run, and that may still be 2x oversized for the heat load. If that's the case there are good/better/best options there, but I won't elaborate without more info.

    If yours is a steam boiler, the boiler has to be sized for the size of the baseboard to function properly.

    If it's pumped hot water (likely) the amount of radiator or baseboard is not a factor in the boiler sizing, but it may factor into how the near-boiler plumbing is done. Don't let somebody talk you into something along the lines of "Lessee, ya got 125 feet of baseboard, times 600 BTU/ft comes to 75,000BTU/hr, that's the boiler you need." which is easily as-bad as the BTU/ft methods. If in fact you have 125feet of baseboard and the Manual-J type heat load calc says your heat load is 35,000, it means you can run the system at 140F instead of 180F and keep up even on the coldest day of the year, and get better efficiency out of even a tiny 2-plate cast iron beastie, but it also means that most of the time you could run it cooler than that, and get SUPERB efficiency out of a modulating condensing boiler (as long as it isn't oversized by more than 1.5x)

    Get multiple proposals from multiple contractors, and multiple heat loss calculations. Or, hire a hydronic heating designer to do the heat loss calc and spec the boiler, then put it out to bid to several installers. There are many good boilers out there- most important to YOU would be how much available support & local expertise there is for the product, both at the installer and distributor level. You can have the greatest boiler in the world, but if something craps out in January and the local installer has to wait to call tech support in California who has to wait until the main office in Germany opens for business to order the part, who in turn then has to order the part from distribution in Japan, you've got a problem if you happen to live in Wisconsin, even if it's still under warranty and everybody is real nice about it.

    To find competent installers it can be useful to find the local distributor for particular manufacturer and see if you can't get a short-list of installers from them. THEY know better than anyone who is installing them by the dozen, as well as who is calling tech support with stupid questions that are already answered in the manual, or should be part of general knowledge. (They won't be sending you to the latter. ;-) ) Mind you, the ability to install them competently is a different aspect than sizing them correctly. Those who insist on doing a heat loss calc using software that takes into account the wall & roof areas and R values, windows & doors etc. move to the head of the line. Those who respond positively when prompted to do a Manual-J (even if the charge you a few hundred to do the work) are at least on the consideration list. (There are a rare few who may offer to do a blower-door test to be sure to get the infiltration rates correct.) If all they want to know is how many square feet and want to throw out a number, move on- or it will end up costing you money in the end no matter what they charge you.

    To assess any Manual-J calc, first look at the outside design temperature they used for the calc. Too often well intentioned people will use a design temp 10-15F lower than the ASHRAE 99% number for your location. (With a zip code I can probably look it up or estimate it from weather data, or you can google the exact phrase "outside design temperature", along with the name of a decent sized city near you.) Most Manual-J methods end up oversizing by at least 20-30%, so even UNDERSIZING the boiler output by 10-15% has very low risk of leaving you cold, and is preferable to oversizing by 10% from both a comfort & efficiency point of view. Long burn times (and never short-cycling) are the hallmark of a right-sized and appropriately set up system, which will also have lower maintenance & longevity issues.
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  4. dwassner

    dwassner New Member

    Jan 31, 2012
    Thanks for the quick response. I will not be able to do any energy audits so as far as sizing the unit, I'm on my own. This is because of the lender, not because of me.

    Anyways, zip code is 14469, natural gas, the house is a 1240 sq ft. ranch, pumped hot water system, copper plumbing with no insulation. Most of the windows are old, not in the greatest shape. Insulating the attic is on my priority list. I will have to get into the house to crunch some numbers. Do you know of an online calc. that would be best to use?

    If you arrive at the size boiler you need, it seems like chances are you will end up either under or over-sized because of the difference each model puts out. Since the manufacturers do not offer a 45k, 50k, 55k, etc model, how do you decide what to go with if your in the middle of two outputs?
    Last edited: Jan 31, 2012
  5. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Sep 2, 2004
    Retired Systems engineer for defense industry.
    New England
    You want one that has a peak output at least what your load is on your 'maximum' design day (i.e., the coldest day you want it to be able to heat the house). Most people may not actually pick the absolute coldest day, but some percentage like 90% or so. Since it's almost never that cold for the whole day, if the house drops a degree or two, it isn't a huge deal. But, as you noted, most boilers only come in fixed sizes. The saving grace to some of this is the newer (well, not really all that new) mod-con boilser that can modulate their output to match the needs of the day. As a result, they tend to produce condensation, thus the nick-name modcon. Some of them can modulate down to 20% or so of their maximum firing rate. Ideally, for max comfort and efficiency, a boiler runs constantly at just the right output to keep you warm. A modcon with an outdoor reset (where it adjusts its output based on the return water temps, time of operation, and the outside temperature) ends up being the maximum in efficiency.

    Most indirect WH are set up as a separate zone, ususually a priority zone, so when they call for heat, they disable the space heating and apply all of its output to the WH. Once that's satisfied, it reverts back to 'normal' operation.
  6. mage182

    mage182 Member

    Feb 17, 2011
    New York
    I have a ModCon. I live in a cape that is almost the same size as your house. I live on Long Island so our heating needs and design day aren't really that much different.

    My experiences are discussed in this thread. I'd consider the whole thing good reading if you want to know more about ModCons.

    As far as heat loss calculators, I've used this one. The results have come out very close to what I also calced manually using a more mathematical approach.

    The plumbed I first hired to put in my boiler chose a unit that was comparable in output to what was there before. This turned out to be a little under double what I needed and has caused a lot of headaches since. In my opinion, doing the prelim work in choosing the correct size unit based on heat loss calc and finding a plumber to install it correctly are very important.
  7. Dana

    Dana In the trades

    Jan 14, 2009
    Using Rochester's 97.5th percentile design temp you'd be looking at +5F for design temp, or +1F for the 99th percentile. The design temps for Syracuse are similar, so the design temp used for the boiler sizing need not be any lower than 0F. Don't be surprised if somebody uses -5F or even -10F, but interpolate what the heat load would be at 0F or 5F from the BTU/hr number they come up with. (eg: Say they used -10F, and an interior temp of 70F, and came up with a heat load of 60,000BTU/hr. The interior to exterior delta is 70F - (-10F)= 80F. But at 0F outside temp the delta is 70F, so the heat load at 0F is 60,000 x (70/80)= 52,500 BTU/hr. )

    To come up with a crude heat load calc we'd need to know how many windows, and do they have (or WILL they have) exterior storms?

    Number of doors?

    Full basement?

    Type of wall construction/siding? (Brick cavity wall, 2x4 timber framed, 2x6 timber framed? etc.)

    Type and amount of insulation in walls, attic, floor, basement walls, etc (the "after" picture, not the "as-is right now" values.)

    Number of stories?

    Online calculators are really crude, and will oversize the sucker completely. If you have the time, Taco has a professional tool available as a freebie download here, but it may take a bit of experience to know what to enter for data, and when the defaults are way off. The ventilation rates use should be no more than 0.5 air exchanges per hour, and if you plan to do some blower-door testing & remedial air sealing before or during your insulation upgrades you can probably lower that to 0.25 ACH.

    Crown boiler has a freebie spreadsheet based heat loss calculator downloadable here.

    Don't be surprised if these tools deliver a number in the ~45KBTU/hr range, or higher, but if this is a 2x4 framed single-story mostly rectangular house with a modest amount of window area (not lots of huge single-pane picture-windows and multiple glass sliding doors etc.) it's fair to say that with modest air sealing and insulation efforts + storm windows as-needed, its unlikely that the true heat load at 0F will exceed ~40KBTU/hr, and may be shrinkable to under 25KBTU/hr, which is getting into the smallest of the 2-plate cast iron boilers, and under the peak output of the smallest modulating condensing boilers. With a mod-con the lowest-fire output is the more important number, and as long as the min-mod of the boiler is half your calculated heat loss (or lower) it will be tweakable to deliver long efficient burns even during the shoulder seasons, even if it's 2x oversized at high-fire.

    If your design temp heat load is under 30K you may be a candidate for using a combi-hot water heater (condensing or otherwise) rather than a boiler-proper, provided there is enough baseboard to deliver design-condition heat with 130-140F water. You can figure on at least ~300BTU/linear foot out of the most commonly use baseboard with 140F, so if there's more than 100' it'll probably make it with a combi-heater. You can tweak the temp up if it falls short- with 150F water it'll be delivering better than 350BTU/ft. But let's not get too far ahead- you (or your heating contractors) have some heat load calculating to do first.

    If you opt for a simple dumb cast-iron boiler and your calculated heat load comes between sizes, it's generally safe to go with the smaller unit as long as it's output is at least 75-80% of the calculated number, and the smaller unit will deliver higher as-used efficiency. Let's say your heat load calc comes in at 33KBTU/hr, and say, you're looking at the 2-plate Burnhams:

    P202: 37K-in, 27K-out

    P202X: 50K-in, 37K-out

    The bigger unit's output is about 10% higher than your calculated heat load, and the smaller unit is 22% UNDER your heat loss calc. You can get away with undersizing by 22%? How is that?

    These calculation methods overestimate reality by ~15-30% as a rule. Worst-case the smaller unit is undersized to the true load by about (22%-15%=) 7%, which means if it stays at your design temp ( 0F) for several hours the house will cool off to (100%-7%=) 93% of the delta between your design temp and 70F interior temp at which it was calculated. In this example that would be 0.93 x (70F-0F)= 65F, which is a perfectly comfortable room temp for sleeping, and since the design temps or lower occur almost exclusively during the pre-dawn hours, that's probably going to be OK even without auxiliary heat. Absolute worst case- say you're going through the cold snap of the century and the high for the day is a frigid five-below, you'd be able to make up the difference with two or three 1500W oil-filled electric space heaters. And as often than not undersizing the boiler by 20% relative to a Manual-J calc comes closer to a true "right sizing", but few HVAC contractors ever want to risk the 5AM phone call from the client, and consciously or otherwise build-in their bias into the calculation inputs (is that really R38 or R30 in the attic? How leaky IS that house anyway.) It doesn't take many tiny weights on the input-scale for a Manual-J to end up 50% oversized relative to reality. Being 50% oversized isn't an efficiency disaster, but you don't want to then increase it another 25% "just to be sure". (In California under Title 24 rules, up-sizing by more than 10% above a Manual-J calculation is a code violation, but it's easy for the contractors to bias it up by that tweaking the inputs to the software, and some will.)

    If your calculated heat load in this example turned out to be 30% high compared to the true heat load (it's possible- it happens all the time), your heat load is then 23K, and a 27K-out burner has 17% of margin at design temp- you're good down to -12F with no shortfall! But if you went with the 37K-out unit you'd be at 1.6x oversizing- still not a disaster, but any bigger than that would be slipping down the part-load efficiency curve, and it would under-perform relative to it's AFUE numbers. At 60% oversizing it would keep up at -42F, which may have happened since the last ice age, but probably not in the last century. This is the basis of the "10% max oversizing" rule, and it's a valid one.
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