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Community and Q&A

Replacement Boiler Calculations

punkdr | Posted in Energy Efficiency and Durability on

Hello all,

I have read Dana s instructions on how to calculate the size for replacement boiler.  I did my own calculations but am a little shaky on that and was looking to see if I’m in the ballpark.

here is my information

7500 sqft house with 9 zones smallest zone is over garage room  About 400 sq ft…radiant part is pex in concrete over plywood, with hardwood and tile as finish surface.

99% outdoor design temperature =12 F

39.9 deka therms associated with 1040 degree days.(65 degrees)

current boiler is burnham that is rated 164k btu/hr input…has 2 outputs on name plate one is DOE 136 kbtu and the other is h20 118 kbtu/hr(I assume this is  the one you should use for this calculation).

radiant heat is somewhat exotic in my locale, and have had poor luck with the local folks, so I’m trying to do all the homework myself.

I am thinking the HTP RGH 100kbtu (2:1 turn down ratio) could possibly be a good option. It is relatively inexpensive and from my limited understanding of these systems might better handle the micro zones?

Is there a better option for a heat source considering cost/benefit.

also the current boiler has 1 1/4 inch inlet hooked to the same sized manifold…the htp has only 3/4” inlet, would this be likely to cause an issue?

Any suggestions/directions would be greatly appreciated!

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  1. punkdr | | #1

    Dana, Jon, anybody?

    Or recommendations for someone I can pay for a consultation?

  2. Expert Member
    Akos | | #2

    That unit is meant for domestic hot water. You can use it in a combi application on an open system, but then you need a heat exchanger to isolate. Technically it can work as a boiler, but it is against code to use it as one.

    Your best bet is to stick to their modcon and install a small buffer tank if you are worried about microzoning. The buffer tank adds very little to the cost and even a 20gal unit makes a big difference.

    The size of the pipe connections doesn't matter provided the unit has the BTU output needed for your setup. My boiler at home connect with 1" pipe to the existing 4" trunk. It does look a bit wrong, but works without issues.

  3. Jon_R | | #3

    > HTP RGH

    It lists space heating as an application, so should be fine if used only for that. But keep in mind that fuel use calculations don't produce a Manual J result and need upwards adjustment. Completing the calculation, 100K btu/hr input leaves little margin for things like windy day infiltration being much higher than average and days below design temp. In your case, I'd approximately match the 118 kbtu/hr old boiler output (which I assume always met the load). +1 on a boiler (with outdoor reset) and a buffer tank.

  4. punkdr | | #4

    Thanks guys,

    We truly appreciate using your time and expertise answers these questions.

    I was thinking of going conservatively on the btu sizing because the old boiler would only run 7-8 minutes max before the aqua stat would cut the gas, even though the call for heat would not be satisfied...the pump would then keep running until the aquastat in the boiler would drop below the cut off temperature. So, the system couldn’t radiate the heat out as fast as it could make it. It was always able to keep up however.

    I do have an ac/forced air gas furnace that covers about 1/3 of the total square footage, so if it gets too cold/windy, this can be used to supplement, although I would anticipate this would be needed only rarely.

    Thank you again both for your insights....I will check out the boiler options and a buffer tank.

    1. Jon_R | | #5

      > have an ac/forced air gas furnace
      This would provide the margin you need to use a 100K btu/hr boiler.

  5. Expert Member
    Akos | | #6

    Unless you are talking about something different, this unit:
    It is only rated for combi heating in an open system.

    For a closed system, you need a boiler, or if you want to use the above one, you need to have it on potable water loop with something like a Taco X block to isolate it from your space heat.

    Since the cost of a modcon and these is comparable, I don't see why you would go for this. Something like a UFT-100 or UFT-120 has very good turndown, at most you might have to combine some of the small zones or just add a small buffer tank.

    The efficiency of the condensing water heaters also drops off if you do need to run at at full capacity, the nameplate efficiency is for domestic hot water use, not steady state operation at rated BTU. You can get reasonable efficiency out of them for space heating at lower fire, but it won't match that of a modcon with outdoor reset.

  6. punkdr | | #7

    Hi Akos and Jon,

    Thought you might be interested in this additional information.

    From the htp brochure: “The Crossover can be used in several applications, such as space heating, recirculation, solar water heating and geothermal applications. ”

    The reason I was considering the htp water heater is that It potentially seemed like a less complicated install...less efficiency likely, but less things to go wrong? Internet reviews seem positive and like it is reliable....Push water in get hotter water out.

    As I mentioned have had terrible luck with the radiant heat folks here. One flooded my house, one fried all my valves, and one was so clueless that they just made guesses about how to repair the system...

    I will be having my regular Hvac guy install the boiler (radiant heat is not his specialty). However he has proven track record as one of the smarter and better installers in Hvac.

    So anyway, I called HTP and they said that none of the RGH products were ok to use for space heating in any format and attaching them to any device for space heating would void the warranty...

    Whether this is true or not, it doesn’t give me much faith in HTP technical/customer service. Either the brochure or the tech support has to be wrong!

    So at this point I am leaning towards a condensing burnham boiler with an outdoor reset, and keeping my fingers crossed!

    Thanks again for your input.

    1. Expert Member
      Akos | | #8

      HTP was listing those for combi heat a while back, my guess is that there were some warranty issues. Standard power vented hot water tanks, when used for combi heat, wear out the blower.

      The Burham modcon has a 5:1 turn down. It takes a pretty big floor heat zone to use up 20k btu of heat especially with outdoor reset. The standard 2" concrete cover for above deck pipes doesn't have enough mass to add sufficient buffer capacity. Plan on a buffer tank, run through the sizing here:

      If you go with a 10:1 turn down unit, you can skip the buffer tank.

  7. Expert Member
    Dana Dorsett | | #9

    >39.9 deka therms associated with 1040 degree days.(65 degrees)

    That's about 40,000 BTU/HDD input. At (136KBTU/164KBTU=) 83% efficiency on the existing burner make that 33,200 BTU/HDD of output (= load) , or (/24=) 1333 BTU per degree-hour.

    A presumptive balance point of 65F and a 99% design temp of +12F is (65F -12F= ) 53F heating degrees, for a design heat load of about 1383BTU/F-hr x 53F = ~73,300 BTU/hr, which is stills credible for a reasonably tight code-min 7500 ' house. (The BTU/hr per square foot ratios are usually lower on larger houses than smaller houses.)

    That's pretty close to the DOE output of HTP's UFT-080W, but it's still reasonable to step it up to the UFT-100W, which can still modulate down to 10,000 BTU/hr-in, 9500 BTU/hr-out. There is enough thermal mass in the heat exchanger volume in that series of boilers to avoid short cycling on zone calls in a micro-zoned house, as long as the radiation/load ratio is similar zone by zone, and the outdoor reset gets dialed in, ensuring that zone calls usually overlap even during the shoulder seasons. There are other 10:1 turn down mod-cons that would work too.

  8. punkdr | | #10

    Thank you Dana, I always appreciate your insights.

    Hopefully, I’m not wearing out my welcome, but a couple of more questions, for you folks, if I may.

    I have attached a schematic of my current system. Changing over from a cast iron to a mod con, do I need the four-way mixing valve, or should I remove it, or just turn it all the way down?

    Also, do I need an additional pump on the boiler side of the four-way valve to affect a primary/secondary sort of circulation? If so I would probably go for a unit with that pump built in.

    Instead of the buffer tank I thought I would use the indirect (It’s already there)as a “tempering tank”, that would run in conjunction with the micro zone.

    Again thanks for your help and insights

  9. Expert Member
    Dana Dorsett | | #11

    If the radiation to load ratio is about the same for all zones you don't really need a buffer tank or a mixing valve if using a 10:1 turn-down condensing boiler. Simply dialing in the outdoor reset curve to near-perfection would guarantee overlap on zone calls and nearly continuous burns for most of the heating season.

    Most fire-tube mod-cons don't need to be plumbed primary/secondary, and will work just fine over a wide range of flow. Just be sure that the design satisfies the minimum flow requirements of the boiler when any ONE zone is calling for heat. That may require over-pumping it a bit given how micro-zoned the system is. Water tube type boilers would definitely need to be set up primary/secondary on this system.

    The indirect should be set up as a priority zone with it's own pump, and would normally override the outdoor reset temperature when called. So even when 100F water is keeping up with the heating load, the indirect could still be getting 150F or 180F water on the heat exchanger to bring the temp up quickly at the full-fire (or nearly so) output of the boiler. All mod-cons have the hooks in the controls to be able to do this.

    As drawn I don't see how the indirect can ever receive the full-temp output of the boiler, or even any flow. Isn't there another pump?

  10. punkdr | | #12

    Hi Dana, thanks for your quick reply.

    It’s probably not clear from the drawing, but the indirect has been abandoned, (One morning my wife was super upset when there was no heat and no hot water!) so there is a direct fired gas hot water heater plumbed downstream in the domestic hot water supply from this indirect boiler.

    I left the indirect tank in as I did want to use it as a tempering tank because the boiler room was always 10 to 15° warmer than the house. I never got around to ripping off the insulation though.

    The 5:1 turndown boilers are definitely cheaper than the 10:1.

    I was thinking that when the zone For the room over the garage called for heat( which is does more frequently than any other zone in spite of my attempts at improved insulation and air sealing) i would synchronize that with opening the valve to the indirect and let it (passively?) dump some heat into that indirect

    I have one other microzone, but it is located near in the center of the house, and receives a lot of heating from the adjacent zones. It hardly ever calls for heat. Therefore, I was just gonna leave that one alone.

    The rest of the zones I think are fairly evenly balanced.

    There was a pump for the indirect, but it got removed when it was decommissioned.

    If I left the mixing valve in and just turned it down would it hurt my performance much? (Just trying to minimize the amount of re piping that needs to be done).

    Also, how do figure out the minimum flow for one zone? Turn on the pump And open only one zone and see how long It takes to fill a 5 gallon bucket?

    Thank you again for your help!

  11. punkdr | | #13

    Dana, Jon, Akos,

    I thought I would let you know how things went and leave some feedback for any future readers of the thread.

    I ended up getting an awesome deal on a HTP 140- that price I figured whatever efficiency I could get from a slightly smaller unit, would never offset the price savings.

    Since that unit has a 10:1 turndown ratio, I didn’t install a tempering tank. I did put a thermostat on the smallest zone that only allows for max 3 calls per hour. From my time spent sitting in the boiler room, I noted that the smallest zone almost always overlapped another zone call.

    Jon’s point about the wind is spot on. We had an extremely windy winter this year, and during those windy spells I used the boiler only, so I could assess how good I was on sizing. Lesson is that wind, if you have an average house (mine is built to typical 1995 standards as far as airtightness) increases the heating load to a surprising degree!

    The boiler alone was able to keep up, but just barely.
    It would run for 8plus hours at a go.

    I hooked it up for an outdoor reset, but haven’t used it. This season I ran at a set 140 for the boiler temperature, and it worked pretty darn good. As far as I could tell is was never not condensing.

    Since it stayed in condensing mode essentially always, and seemed to have fairly long burns, I am wondering do I need an outdoor reset?

    If so, what do I set the lower boiler temperature limit to? Most of the info I could find suggested a 1 or 1.5 degree there a better way to figure that out? I guess my question is what is the process to dial in the reset curve to perfection?

    Anyway, thanks again for all your help!

    You folks are an invaluable resource for helping us homeowners get it right.

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