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Matching boiler and Indirect DHW tank

Yeshwant Member Posts: 20
Boiler would use the existing chimney (with PVC pipe, of course, in it).
What are the consequences of using a 50 KBTUH boiler with a 20 gall indirect tank?


  • Yeshwant
    Yeshwant Member Posts: 20
    Matching boiler and Indirect DHW tank

    The heat loss of my 1500 sq ft brick townhouse is 43,200 BTU/ hr.
    I was thinking of replacing the current boiler with a wall hung 50,000 BTU Knight mod/con and the smallest Indirect DHW tank (Superstor 20 gall.)
    However, the tank requires the boiler to be 80,000 BTU/hr.

    What are my options here? If I oversize the boiler (even though it says 5:1 turndown) is it going to be able to modulate as far down as a 50,000 BTU would in the Spring/ Fall seasons? If I get a 50,000 boiler, will the DHW supply suffer? (There are only two persons in the house and two showers max at any time, the DW will heat it's own water, the Washer will be high-efficiency as well).
  • Norm Harvey
    Norm Harvey Member Posts: 684

    You could go with a wall mounted tankless DHW unit.
    "We see the world as WE are, not as IT is, because it is the "I" behind the EYE that does the seeing"
  • reduced output.

    Call HTP 800 323 9651. They can give you the exact numbers. I see they list the SSU-20 with a 80,000 btu boiler 180 degree water at 168 gallons of 115 degree water for the first hour or 121 gallons of 140 degree water for the fist hour rating.

    Looking at a standard 50 gallon gas water heater you are only getting 91 gallons for the first hour.

    So even with the reduced rate you should have enough hot water. The SSU-20 recovery time is only about 13 minutes.
  • hot_rod
    hot_rod Member Posts: 21,880
    Boiler will be plenty for that tank

    it is still more output then a typical gas fired water heater of the 30- 50 gallon type.

    I have a Munchkin 50K wall hung driving an 80 gallon Heat-Flo tank. plenty of dump load and about a 30 minute recovery when cold. the 20 or 30 gallon would work well with that boiler.

    Another option... I used a Triangle Tube "Mini Smart" on a 90 Prestige last year. It is basically an external flash tank to heat DHW instantanously. Supplies about 2-2.5 GPM depending on incoming water temperature.

    The 'Mini" is the small tank below the boiler in this pic.

    hot rod
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Mark Biro
    Mark Biro Member Posts: 46
    Small boiler, large DHW tax credit

    I am simply a homeowner, so take this for what it's worth. But I DID install a small mod-con boiler in my house this fall, and have been learning off these Wall discussions. (Thanks, Wallies!)

    Norm Harvey may be on to something (i.e. an instantaneous heater) in his reply to you.

    And the gummint has blessed THAT method of generating DHW with tax credits to the end of THIS year. (Complaint: and NOT other, efficient methods.) To $300 off your taxes, if I remember right. You have a few shopping days left, if you care to pursue it. ;-) Google will turn up the details.
  • Go for it...

    I have something similar in my house. We can't both shower at the same time unless we're both in the shower at the same time, in which case only one of us can get wet at a time... Does that make sense? Didn't think so.

    In any case, you can take back to back to back to back showers and STILL have DHW. It just can't handle two simultaneous showers for any length of time due to demand exceeding capacity.

    I say GO FOR IT.


  • What does "The tank requires the boiler to be 80kBTU/hr" really mean?

    The tank doesn't require anything. What determines your requirements is your DHW load. That requires a certain number of BTUs, over a certain timeframe. You store a portion of those BTUs (in the tank) and meet the rest with the boiler output. The more your store, the less you have to produce. However, storage has some practical upper limits.

    two showers with regular flow heads, for ten minutes each is 10 min x 2.5 gal/min x 8.4 lb/gal x your temperature rise.. assuming 45 degree water to 110 at the fixture, 65 = 13,650 BTUs. To meet that with no storage, multiply by the number of those in an hour to get the boiler output in BTUs/hr that you need. in this case, those are ten minute showers, multiply by six, that would be almost 82kBTUs/hr... slightly over the 80k boiler.

    with 20 gallons of water at 120, that's 120-110 at the fixture, 10 degrees x 20 gallons x 8.4 lbs/gallon = 1680 stored BTUs. 3360 for 130 in the tank, 5040 for 140 stored temp. That's 1/5 to 1/3rd of your shower load stored. Double your tank size, double your storage. Some combination of storage and boiler size will meet your required demand.

    Use low flow showerheads, cut your usage a lot, that's an option too.

    That said, the next size knight should modulate down just about as far as the 50k size so it shouldn't hurt to upsize in this case. I just get driven nuts by the "tank requires the boiler" comments (especially when it comes from people who should know better)... tanks don't require anything. Outputs require BTUs, and it's a question of how many you can generate, how fast you can exchange them into the tank, and how many you are taking out.. period.
  • Waste heat recovery...

    Like the GFX heat exchanger would help scads too...

  • Yeshwant
    Yeshwant Member Posts: 20
    Thanks for the replies, jokes and calculations.

    It's great to get your input.
    Forgot to mention that we currently have a 30 gall conventional gas water heater which is quite sufficient for our needs, except when we want to fill the tub, when we raise the water heater thermostat a little bit in advance. Sort of old-fashioned, like getting water ready on the stove for a bath. We also have a dual flow 2.35 gpm showerhead. The flow can be decreased to 1 gpm when soaping up.

    NRT. Rob, I am still trying to understand your calculations. It appears from them that two 10 minute simultaneous showers (total approx 28,000 BTU used in 10 minutes) would require me to store 20,000 btu i.e. 20 gall of 130 degree water (since a 50,000 btu/hr boiler can put out 8,300 BTU in 10 minutes only.) If that is right, then that would agree with the consensus about this being a workable combination.
    The GFX also sounds like a cool idea, but may cost a little to retrofit.

  • Ah, I messed up, that 13k figure I mention was only for one shower, not for two. Your 28k total is correct.. sorry for my mistake.

    You only have about 3300 BTUs stored in a 20 gallon tank at 130 degrees (remember you want 110 at the fixture most likely: that's 20 gallons x 20 degree differential x 8.4 lb/gallon = 3360 BTUs before the tank is too cold).

    That leaves 25k to the boiler, in ten minutes, that's 6x25 + 150kBTUs/hr of output that the boiler must produce AND that the tank must accept in heat transfer.

    I would consider that very unlikely with a 20 gallon tank in both cases.
  • Trust me....

    You WILL have more hot water than you had before. You might still need to boost temperature prior to filling large volume tub, or do like my wife does. She starts filling the tub at 10;00 PM at a very low flow rate. (~ 1 GPM). By the time the weather is on (20 minutes), her tub is nice and hot and ready for her occupation.

    I keep our tanks at 140 degrees F to avoid any more legionella issues. Don't EVEN want to go through that again...

  • tim smith
    tim smith Member Posts: 2,743
    ?? go up to larger tank

    I would just increase my storage capacity. Don't worry about the min boiler size for tank as this is just to get rated output of tank. Does not mean it won't work. Maybe go to a ssu 45 tank or equiv. Tim

  • Mark, he wants two simultaneous showers. Not just "more than he had before".
  • Rob...

    He said he is currently working just fine with a 30 gallon tank too...

    That means that his first hour capability is 80% of 30 gallons, or 24 gallons, plus recovery, which is probably a net output of 24,000 btuh, or 28 recovery gallons per hour @ 100 degree rise, for a total first hour output of 53 GPH of 100 degree rise water.

    If he goes with a 20 gallon tank, netting 16 gallons output plus 95% output of 50K btuH, resulting in 47,500 btuH, or 57 GPH of 100 degree rise water, plus his 16 gallons of net storage would total 73 gallons of first hour capacity. He WILL have more fire power than he had before. No?

    Stretching the numbers a little further, assuming that he has code required shower heads (2.5 GPM), and assuming that 80% of that flow is how water, that equates to 2 GPM of hot water per head. The typical (non teen) shower lasts 7 to 10 minutes, so with 2 heads running simultaneously, that would be 40 gallons of hot water necessary to satisfy two people showering at the same time. FWIW, I'd probably go with the bigger 45 gallon tank just to be sure. It gets expensive doing it twice. With a little diversity, the 20 gallon tank would work fine, but its not my house :-)

    I guess what I am saying, is if he was adequately satisfied with his 30 gallon 30 K water heater, he will most certainly be satisfied with his new confirguration that results in 20 more GPH of hot water output.

    The only caution I would drop into the conversation is that if he intends to fill his clothes washer with 100% hot water, and he wants to shower two people at the same time, he may have an issue.

    But like most things in life, it just requires a little adjustment. Like, DON'T FLUSH THE TOILET WHILE ANYONE IS IN THE SHOWER, or DON"T START THE CLOTHES WASHER UNTIL I"M OUT OF THE SHOWER...

    Also, in our calculations, we all assume that people like 110 degree F water out of the head. A national study found 105 to be the typical showering temperature. Cutting fine hairs off a camels back for sure, but none the less, import to the calculations.

    In reality, there is typically a lot of diversity as it pertains to showering loads. The only exception would be in the ski/lodging industry. Everyone comes off the slope at the same time and wants to shower before apres ski party, and everyone wakes up at the same time to hit the slopes. Little to no load diversity, and rarely do you find a heating system with adequate capacity to handle the simultaneous loads.

    Shower with a friend and save water and energy!

  • Yeshwant
    Yeshwant Member Posts: 20
    Reviving my old post on the indirect sizing question

    Getting close to the actual installation and I'm still having difficulties.
    My current Kenmore Miser 30 gall, 40,000 BTU/hr input, gas-fired water heater with 40.9 gall/hour 1st hour rise over 90 F, can supply enough water for two simultaneous showers (through low flow showerheads). The water cools slightly at the end of the two showers. That is the most gpm required in this house. No jacuzzi, etc.
    I feel a 50K boiler and 20 or 30 gall indirect should be enough.
    But, a heating contractor I got a recent bid from, refused to install any other combination than the Knight 80 KBTU/H and the 20 gall Superstor claiming, inspite of my reasoning, that that was what the 20 gall indirect was rated for. The heat loss for the house never goes over 42K and will decrease even further as we blow-in insulation into the crawl space under the roof. I also plan to get a wood stove for one floor for very cold days, so that would take down the heat loss further. I am also wondering about solar preheat for the indirect.
    In a real life situation, is there a difference between the Knight WBN 50 that modulates down to 10,000 BTU/hr and the WBN 80 that only goes to 16 KBTU/H? Should I go with the 80K boiler and the 20 gall indirect?
  • Ted_9
    Ted_9 Member Posts: 1,718

    SuperStor info.
  • Tony_23
    Tony_23 Member Posts: 1,033
    My 2 cents

    I'd stick with the Knight 50 and SS 20. Install a mixing valve on the IDWH. Honeywell makes a nice one special for water heaters. Makes a 20 act like a 30.

    If the contractor isn't comfortable doing a 50, find someone else.
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