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mod-con boiler as back up

gg2 Member Posts: 15
I am looking at using an OWB to heat a 20-30 gallon buffer tank through a heat exchanger. The buffer tank will feed the radiant floor pex system. When the OWB is off, I want to use a mod con on-demand propane boiler to take over in a re-circulating loop with the 30 gal buffer tank and the radiant system:. (6 loops of 1/2 inch pex each 250 feet long)

A Quietside 120 ODW mod con can accept input water as high as 120 degrees but requires at least 29 psi input at peak demand. A Taco-9 recirculation pump delivers 8 gpm at 34 feet of head and is recommended by Micheal Chandler in a similar system.

Question: Would this pump provide the 29 psi needed by the mod con?

Is there a formula that includes gpm, head, friction, pipe size....to calculate psi?

(From what I have read, mod-cons are generally not used in closed systems because of the pressure requirement)


  • SWEI
    SWEI Member Posts: 7,356
    Has a heat loss calculation been done?

    Most of the advice in wood boiler forums is junk, and some of it is downright hazardous.

    Read this http://www.caleffi.us/en_US/caleffi/Details/Magazines/pdf/idronics_10_us.pdf and come back with questions.
  • Zman
    Zman Member Posts: 7,467

    I think you got some really solid advise about this in your previous post http://www.heatinghelp.com/forum-thread/141990/Domestic-HWH-for-OWB-backup

    You are still trying to heat your home with a grossly over sized wood boiler and a domestic water heater?

    Mr. Chandler designs and builds very nice homes. It is unfortunate that he knows absolutely nothing about heating.

    Check out this wall of shame boiler (hot water heater) room http://www.chandlerdesignbuild.com/indexFull.php?id=affordableGreenResidence&t=Affordable%20Green%20Prototype%20House%201

    Or this fine open heating diagram http://www.chandlerdesignbuild.com/indexFull.php?id=vidraMoody&t=The%20Vidra-Moody%20Residence

    As has been said before:

    Start with a heat loss

    Size your wood boiler and storage tank based on the heat loss

    Size a back up boiler (not water heater) based on heat loss

    Tie your domestic water into the system via Indirect

    Design an efficient ,comfortable heating system using outdoor reset mixing.

    Water Heaters are for domestic water, boilers are for heating spaces


    The other option is to follow the advise of those who agree with you.

    A Taco 009 will push less than 0.5 GPM at 35 feet or 8GPM at 12 feet.

    A Quiteside 120 ODW is a water heater not a mod/con. It is not designed or rated to heat your house.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • gg2
    gg2 Member Posts: 15
    follow up

    I appreciate those who responded with constructive advice. Carl's summary pretty much nails it. Bottom line is: This is my retirement home I have built from scratch myself from the hole in the ground - up. I have designed everything as practical and simple as I can so I can understand it and repair it after I am walking with a cane. I want the heating system to be the same way. Seems like there are many methodologies. A lot of folks have tried to sell me systems that do everything except cook my breakfast. Don't need that. So, following Carl's advice...re-iterated by others:

    1. Heat loss using three separate formulas comes up between 50 - 60k btus.. (Experts on this site have told me that is high for a 1200 sq ft house with full basement)

    Add to that btus needed for DHW. If I go indirect DHW off boiler. Should I be looking at about an 80K btu unit?

    2. The Calleffi diagrams showing boiler interface now make sense. Bob says the mod-con BOILER will work for back up. Two questions:

    What are disadvantages over conventional boiler?

    What is a good simple basic product line of mod con?

    3. What exactly is a hydrolink? (Seigenthaler mentions it and shows it in diagrams but never says what it does. Must be common knowledge to those in the business)
  • SWEI
    SWEI Member Posts: 7,356
    Sounds like you're doing some reading

    which is a really good thing.  That Idronics issue is worth its weight in gold.

    > Heat loss using three separate formulas comes up between 50 - 60k btus..

    (Experts on this site have told me that is high for a 1200 sq ft

    house with full basement)

    It does sound a bit high, but the good news is you're already at the smallest mod/con size offered by most manufacturers.  If you do get closer to your real heat loss, take a look at the minimum firing rate of the boilers you're considering.  If you can get to 1/3rd of your design day heat loss or lower, things will work out better.

    > Add to that btus needed for DHW.

    NO.  DO NOT OVERSIZE YOUR HEATING BOILER.  Buy an indirect large enough to meet your DHW demand using the BTUs available from the correctly-sized heating boiler.  Remember, a standard gas tank water heater only puts out 24,000-30,000 BTU/hr and most electrics only put out 15,400 BTU/hr.  A similarly sized indirect will put perform and out-live either.
  • Ironman
    Ironman Member Posts: 7,180
    Heat Loss

    Your heat loss numbers are extremely high at 50 btu's per square foot. I don't think I've ever come across a house that leaky in my 40 + years in this trade. And I've worked on some very old ones that you could literally feel the wind blow through.

    Most new homes in the mid-Atlantic to lower New England area require 22-25 btu's pr. sq. ft.

    Where are you located? Unless you're in ME or Alaska, I can't see you needing more than 30-35k btu's. And, as SWEI has pointed out, you don't add the domestic load to the heat loss. You give it priority over space heating in the control setup and size the storage larger if you have an unusually high load. The exception to this being a combi or an on-demand where the sizing is matched to domestic because there is no storage.

    We seldom use anything larger than a 30gal. indirect, even on large homes, and have never had a complaint. Much to the contrary, we here how pleased they are that they always have hot water.

    40-50k btu's is the smallest C.I. Or mod/con that's available. So, you're probably gonna be over-sized even with that. As to which to choose, the extra cost of venting a C.I. through the roof can cause its total cost to start approaching the cost of some of the cheaper mod/cons. Some will argue that the mod/con is more likely to break down due to its complexity, but that has not been my observation or experience - yet. We replace far more electric vent dampers and ignition controls on C.I. boilers than we do comparable components on mod/cons.

    The Hydrolink is a manifold/ low loss header that provides hydraulic separation between the boiler and secondary loops. This may, or may not, be necessary depending on the boiler you choose and the piping arrangement used.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • gg2
    gg2 Member Posts: 15
    mod-cons and mass

    Great information. Thank you. A few points:

    1 Heat loss for the main floor comes to about 20k btu/hour, perhaps due to log walls and lots of windows. The basement calculation is what shoots it through the roof, and I don't plan on heating the entire basement most of the time...just the utility room and the guest room when the kids visit. (It has four zones of slab pex) (Cabin is located upstate NY)

    2. I found an on-demand mod con by Peerless that seems flexible and simpler, smaller ones are PF-50 and PF-80. From Bob's comments, sounds like the PF-50 would do it. I really like the option of PVC vent out through my box beam as opposed to building a channel through my Den and venting out through the roof.

    3. Combination units are nice but I like the dependability of plumbing as a load, the indirect HWH.....not dependent on adequate flow rates or other finicky gadgetry. Rinnai, Baxi, Peerless and TT all have the option of prioritizing the Domestic HW ( As Bob pointed out) Peerless also sells indirects to match their mod-con.

    4. Lastly, idronics 2012 poses options for interfacing your indoor pressurized auxiliary boiler with your atmospheric OWB. ( require heat exchanger ) (Figure 5-8 Page 22)

    Question: Would there be an advantage to adding Mass at the heat exchanger on the indoor pressurized side? Perhaps an 80 plus gallon tank? It would give a large mass of 160-165 degree hot water for the mod con boiler to draw from as opposed to just drawing through the plumbed heat exchanger. This water would be mixed down anyway before it hits the radiant system and the mod con would never fire as long as the OWB was cooking. The tank temperature would reduce to radiant floor temperature when the OWB was off.

    Would including this 80 gal mass in the system help prevent short cycling by the mod con (OWB off line) ? Would it also increase efficiency as the return water would cool some in the storage tank before it is called back into service?

    (Note: idronics page 29 figure 6-13 discusses this concept with a fully pressurized system.

    Also: Steve Goldie addresses working Mass to your advantage in the Jan-Feb 2012 Edition of Modern Hydronics, ....artical called "Mass Matters"
  • RobG
    RobG Member Posts: 1,850
    Crazy Question

    What is your budget?

  • SWEI
    SWEI Member Posts: 7,356
    Heat loss for the main floor comes to about 20k btu/hour

    The PF-50 has a minimum firing rate of 16k BTU/hr.  If this were my house, I'd be looking for something with a lower minimum firing rate.

    Unless your basement is uninsulated and your ground temps are scary low, I'm still having trouble seeing how you got to 50k, much less 60k.
  • gg2
    gg2 Member Posts: 15

    Building the house with money I have saved for 40 years. Budget plan for the heating system was to keep it simple so I can understand it and maintain it, use good quality materials, and balance cost with results.
  • gg2
    gg2 Member Posts: 15
    firing rate

    Swei: I havent found anything that fires less than 16 . I will keep looking. Do you know of anything? Would increasing the mass on the supply side reduce the cycling and mitigate the issue? I am sure you are right on the basement...I don't know why they come out so high.
  • SWEI
    SWEI Member Posts: 7,356
    Firing rates

    Here's something I recently posted in another thread:

    Most currently available mod/con boilers have a minimum firing rate of

    16-17k for their smallest model.  Here are some of the lowest minimum

    modulation rates [in square brackets) of which I am currently aware

    (followed by their maximum output capacity in parenthesis):

    Viessmann 200-W B2HA 19   [11,580]    (64,655)

    Lochinvar WHN055                [10,450]    (53,250)

    Lochinvar Cadet CDN040        [8,545]     (37,600)

    Daikin Altherma                        [5,000]    (28,760)

    As far as domestic hot water, keep in mind that a typical gas tank

    heater puts out 24,000 - 28,000 BTU/hr and the typical electric only

    15,400 BTU/hr.  An indirect water heater fired by any of the above heat

    sources will offer faster recovery.
  • RobG
    RobG Member Posts: 1,850

    You are looking at 20 to 30k or more in materials alone. At that price paying to have a designer come in and do a layout for you (including a heat loss :) would be well worth the time and money. Trying to return items (especially from Internet vendors is a pain in the butt and supply houses aren't much better (especially when dealing with a homeowner (if they will sell to you at all). Better to have an itemized list and not have to worry about it.

    P.S. Do not use one of the Internet  designer / suppliers. You can work with someone over the phone and through e-mail until you get the right design for you. If you need a name, come here and we can recommend several people.


  • gg2
    gg2 Member Posts: 15

    Thanks Rob. I can get a good discount at two of the local supply houses through a plumber friend. All my heat pex lines are run. I made my domestic distribution manifolds myself and pex is run to all the fixtures. All that was well over 2g. Those copper fittings add up and thin out your wallet but I didn't like the overpriced black pvc pre-made manifolds. Everything ends in a good size utility room in the basement. The big expense lies ahead with the heating units and accessories. I haven't put a pencil to the cost yet until I had the system design. Wow! Didn't realize it could run that high. Guess I should shop around hard for prices. As much as I like the challenge of figuring this stuff out myself, I will run the design by a pro before I move forward. I can see that professional design could save money in the long run.
  • gg2
    gg2 Member Posts: 15
    Follow up questions

    Experts on this site have recommend the house system be a closed pressurized loop. Diagrams in idronics and other credible pubs show pumps that circulate the water through the mod con or conventional boiler. Most mod cons need a little pressure...15-18 psi with a max of 30 or so. What generates and maintains this pressure in the closed system?
  • SWEI
    SWEI Member Posts: 7,356
    Pressure in a closed system

    Is created when the system is filled, and maintained by the combination of the sealed nature of the system and the expansion tank (which keeps that pressure constant as the fluid temperature changes.)
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