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Help with sizing new mod con boiler

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Hi everyone I'm a homeowner in the market to replace my old cast iron boiler. I'm looking at going to a 95+ afue modcon boiler to take advantage of the rebates offered by massave. I live in Seekonk Massachusetts. My home is 1092 sqft ranch built in 1963. I've had blown insulation added to exterior walls, new ext. doors and replacment windows and vinyl siding with foam insulation. Attic is insulated with 8" of insulation. Basement is finished and heated with a minisplit heat pump. I currently use a 41 gallon indirect for dhw. I have 84 ft of baseboard on 1 zone and indirect on a second. I will be adding a 24x24 bedroom above a new attached garage at some point which will add another 576 sqft so 1668sqft total. I'm concerned about what size boiler to buy. We have 5 people total living in house which includes 3 teenage girls who love to take showers! House has 1.5 bath but will be adding 1 more full bath with addition. I dont want to oversize the boiler to avoid short cycling and not condensing enough. Based on some calculations I've done I would need a boiler that would mod down to about 17k btu. Does this sound correct? I will be increasing the indirect to a 50 gallon with the new install but was hoping for some help from the pros with sizing the boiler for my application. Looking into a Lochnivar knight boiler or Navien. Thanks in advance.

Comments

  • NY_Rob
    NY_Rob Member Posts: 1,370
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    It's good you have one zone- 84' of baseboard- it will prevent short cycling on a high turndown boiler like the HTP UFT-80W which can modulate down to 8K BTU's on low-fire.

    Currently 8K BTU's is the lowest turndown you'll find available.

    IIRC the small Loch boilers only have a 5:1 turndown vs. the 10:1 turdown on the HTP boilers, so even though the max output of the Loch is lower, the HTP offers the lowest min fire BTU output. 80K BTU's will heat your 50gal Indirect faster too.

    Being in Mass, you should have no problem finding a qualified HTP installer.

    http://www.htproducts.com/UFT-Boiler.html

    http://www.htproducts.com/literature/UFTWallFloor-Brochure.pdf
    Rich_49
  • Ktfinch2009
    Ktfinch2009 Member Posts: 22
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    Ok so 80k btu. thanks So how does the htp compare to the navien or lochnivar?
  • Gordy
    Gordy Member Posts: 9,546
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    The lochinvar 55 is like 7.5:1 tdr. Low end of 8.3k input.

    Remember that's input. IF you are getting full efficiency the output is going to be lower by 5% depending.

    They are both nice boilers you need to find the a good installer is the key to any installation.
  • bob eck
    bob eck Member Posts: 930
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    Check out the updated Lochinvar Knight condensing gas boiler.
    10-1 turndown
    Lochinvar
    WHB055N 55 max 8.3 min
    KHB055N 55 max 8.3 min
    SS Firetube heat exchanger
    15 year heat exchanger warranty
    How are you going to heat the new addition? If using copper baseboard look at using Heating Edge HE2 or HE3 copper baseboard that will help the boiler to condense most of the heating season.
    Ktfinch2009
  • NY_Rob
    NY_Rob Member Posts: 1,370
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    Ok so 80k btu. thanks

    Well, your home will be well below the 80K BTU output of the HTP and even the Loch. But since they can modulate down to 8K BTU's that's the important rating at this point. It's disappointing that there's no current mod-con that can fire lower than 8K BTU's but when you put in perspective- one stovetop burner on your kitchen range puts out 8K BTU's.

    So how does the htp compare to the navien or lochnivar?

    The Loch has been called the "Cadillac of boilers" and for good reason. It's feature rich, good quality and backed by a long established company witn nation wide support. Unfortunately, it's a more complicated install than the HTP and it probably twice the price of the HTP. In a simple setup like yours- you may not ever need the added features the Loch offers.
    The HTP UFT doesn't have some of the added features of the HTP, but it's robust, a bit less complex but probably half the price of the Loch. It's also represented by an established company (based in Mass) with a warranty comparable to the Loch. It's also pretty simple to service and if you look at it's install manual- you'll notice there's not much annual maintenance to be performed.

    The only added feature I know of on the Loch which maybe helpful to you for your new upstairs bedroom is the ability to heat different zones with different temp water. The downstairs may need 110F water and the upstairs may need 130F water. The ability to "dial in" each zone with it's own custom supply water temps is really nice. Is it necessary? Not at all, but if it's worth the extra $$$ to you it's certainly a nice option to have.

    Download the sales brochures and install/owners manuals for both. Give them a good going over and come back with questions regarding features, etc.




    Ktfinch2009
  • NY_Rob
    NY_Rob Member Posts: 1,370
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    I will be adding a 24x24 bedroom above a new attached garage at some point which will add another 576 sqft so 1668sqft total.

    Since the new addition will be exposed on three or four sides (I assuming) it will have different heatloss characteristics compared to the rest of the house.
    Insulate as well as you can with the best windows you can find.

    We have one area of our house that was added on after the fact, and it's exposed on three sides. It cools off (and heats up) faster than the rest of the house due to exposure to the elements.
    It originally had standard fin-tube copper baseboard. Being that copper fin-tube has no mass (other than the water in the 3/4" pipe), the radiators got cold within a few min of the zone closing. That portion of the house caused the boiler to cycle a lot from heating up and cooling down quickly.
    I finally bit the bullet and replaced the 18ft of fin-tube with 19' of cast iron baseboard radiator (Burnham Baseray) summer of 2017. Last winter was the first winter I went through with the cast iron baseboard.. and the upgrade in comfort was outstanding due to way less cycling of the t-stat. The boiler would run for longer and stay off much longer (both positive benefits) too due to the high mass of the cast iron baseboard filled with hot water retaining heat for 30+ min or longer after the t-stat was satisfied. So rather than the room temp swinging up and down all day long, it remained much more stable and comfortable with the cast iron baseboard.
    One other data point- cast iron heats more by radiation vs. fin-tube convection heat- so you "feel" the heat on your body vs. from the warm air in the room. If you are going to have open wall space give consideration to the cast iron baseboard, if you'r going to have furniture, etc... in front of all the walls then go with Heating Edge low temp copper baseboard (I have some of that installed in my house too and have nothing bad to say about it).



    Ktfinch2009
  • Ktfinch2009
    Ktfinch2009 Member Posts: 22
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    Thanks everyone. My contractor is pricing out the Loc and htp in the 80k btu size. Crazy how these are half the size of original boiler quoted. I definitely want the boiler to be as efficient as possible and not short cycle so I feel better now!
    DZoro
  • Ktfinch2009
    Ktfinch2009 Member Posts: 22
    edited November 2018
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    One more thing. The indirect I was quoted is an h2o 50 gallon stainless. Brochure is saying boiler output needs to be 133k btu. Will an 80k btu work with this tank?
  • Paul Pollets
    Paul Pollets Member Posts: 3,658
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    a 50 gallon indirect will be to small for teenage daughters' usage. try an 80g.
  • Ktfinch2009
    Ktfinch2009 Member Posts: 22
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    But will an 80k btu boiler be enough to run that size tank? According to specs of indirect I would need 155k btu boiler output.
  • Ironman
    Ironman Member Posts: 7,397
    edited November 2018
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    You only need 155k btus to get the full rated output of the indirect. At 155k, you'd have the capacity stored in the tank plus be able to make about 4 gpm continuously. With 80k input, you'd have what's stored plus be able to produce about 2gpm continuously. A water saver shower head uses about 2 gpm of hot.

    I've got six daughters plus a wife that fills the tub and soaks. We all got by on a 50 gal gas water heater. Obviously, they had to take turns and sometimes wait about 20 minutes if the ones before used up all of the hot water.

    As long as you don't have an unrestricted tub diverter, you should be fine with an 80k boiler + an indirect. I've done 80k + a 32 gal indirect countless times and never had a complaint.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
    NY_Rob
  • Ktfinch2009
    Ktfinch2009 Member Posts: 22
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    I figured a 50 or 60 gallon would be fine considering a 41 gallon boilermate has worked fine. Going up to a 50 or 60 should cover 1 more shower and a dishwasher no?
  • Ironman
    Ironman Member Posts: 7,397
    edited November 2018
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    By the way, the Utica h2o is a good tank. So is their SSC boiler.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
    Ktfinch2009
  • Ktfinch2009
    Ktfinch2009 Member Posts: 22
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    So a 50 gallon h2o and 80k btu boiler will work fine together? Right now my 94k btu cast iron boiler works great with 41 gallon boilermate. Basically looking for same results just a bit more storage and reliability of stainless with the h2o.
  • NY_Rob
    NY_Rob Member Posts: 1,370
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    I have a 30gal HTP SS Ultra indirect... two daughters and a wife. We have the 1.5gpm Niagara showerhead in the main bathroom. Have never ran out of hot water even with them taking 30min showers. As a matter of fact... the UFT-80W even cycles a few times with that low volume showerhread because the indirect hits setpoint while the shower is running because it's making hot water faster than the 1.5gpm showerhead is using it.

    That's the great thing about a mod-con using an indirect for DHW... you can use the full output of the boiler for DHW and only 1/10th output for spaceheating.
    Ironman
  • Ironman
    Ironman Member Posts: 7,397
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    I figured a 50 or 60 gallon would be fine considering a 41 gallon boilermate has worked fine. Going up to a 50 or 60 should cover 1 more shower and a dishwasher no?

    How often are you gonna have 2 showers and the dishwasher going simultaneously?

    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Ktfinch2009
    Ktfinch2009 Member Posts: 22
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    Showers I would say most mornings and here and there in the evenings. Dishwasher and showers not that often probably by coincidence.
  • hot_rod
    hot_rod Member Posts: 22,468
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    From a pure thermodynamic standpoint you size the boiler to the biggest load. Sometimes that is the DHW.

    The tricky part for homeowners id defining exactly what they want or need for DHW.

    If your family uses most of the DHW bin shorts periods, then a larger tank gives you "dump load" if it were an all day load like a working kitchen you would want to size to maintain a constant gpm of DHW, a tankless is better for that.

    Most families want dump load capability, morning and evening, so that larger tank size, at higher temperature, with a mix valve set to 115- 120 would be my suggestion. The boiler will recover slower if it is a small but, sized to the heating load, generally not even noticed.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    Ktfinch2009IronmanSolid_Fuel_Man
  • Alan (California Radiant) Forbes
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    We have a 50 gal. h2o indirect and a 50k boiler. There's only one bathroom, but can take back-to-back showers, no problem; Speakman shower head with flow restrictor removed; lots of water.

    There are two kinds of customers: Most of them don't care if they may have to schedule showers to get along with their indirect. Fewer say they never want to run out of hot water and you have to give them a larger water heater or an on-demand. You have to ask the question and you have to ask at the beginning of the job.

    I had one customer in San Francisco with a 6-bathroom house. The master bath shower had 6 body sprays, one overhead rain shower and 2 regular shower heads. Two hundred and twenty gallons of stored hot water gave them a 12 minute shower with everything on.
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
    Ktfinch2009
  • Ktfinch2009
    Ktfinch2009 Member Posts: 22
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    We do like to take very hot showers...lol it seems the general consensus is larger tank for hot water demand with the 80k mod con. We've had zero issues with boilermate 41 keeping up with demand. That model spec says 100k btu boiler needed to meet 170 gallon per hour. My boiler is 94k and we have zero issues with running out of hot water. Dropping down to 80k btu mod con and going to a 50 gal h2o tank worries me a bit but I'm no expert.
  • Rich_49
    Rich_49 Member Posts: 2,766
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    But will an 80k btu boiler be enough to run that size tank? According to specs of indirect I would need 155k btu boiler output.

    You should be storing in any tank at 140* or a bit higher and mixing down to your usable / comfortable temp . Those BTUh numbers listed are for the varying temp rises listed . Smaller boiler = larger tank .
    You didn't get what you didn't pay for and it will never be what you thought it would .
    Langans Plumbing & Heating LLC
    732-751-1560
    Serving most of New Jersey, Eastern Pa .
    Consultation, Design & Installation anywhere
    Rich McGrath 732-581-3833
    Ktfinch2009IronmanCanucker
  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,646
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    You can fill a bucket with your shower head, measure the water temp and calculate the gpm. Time how long it takes to fill the known volume of the bucket.

    Give us those numbers, and realistic shower times and how long in between showers. Simultaneous showers would just double the volumes.
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
    Ktfinch2009
  • Ktfinch2009
    Ktfinch2009 Member Posts: 22
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    Ironman said:

    If you're doing fine with a 94k btu boiler and a 41 gal indirect, why do yo think an 80k + a 50 gal won't be enough? It's your $$, and a larger tank won't hurt the system at all, but I can assure you the 80k + a 50 gal is more than sufficient.

    Code requires an ASME 1070 tempering vale to be installed. Do like Rich said and set the indirect stat to 140*+ and the tempering valve to 120* and you'll get about 15% more capacity and kill Legionella at the same time.

    I was hoping a 50 gal would be sufficient. I was more concerned with the 80k btu boiler heating the water quick enough and not running out of hot water.
  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,646
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    More than likely you will be fine. But it you really want to know, youd have to give us the numbers that I posted.
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
  • Ktfinch2009
    Ktfinch2009 Member Posts: 22
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    More than likely you will be fine. But it you really want to know, youd have to give us the numbers that I posted.

    When I measure temp do I run on highest setting for hot water at ea fixture? I did check temps at shower and faucets the other day but not gpm. Shower was at 114 degrees at highest setting. Faucets were at 136 degrees hot water only. Boilermate is set just below red area on dial if that makes any sense.
  • Gordy
    Gordy Member Posts: 9,546
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    Ironman said:

    If you're doing fine with a 94k btu boiler and a 41 gal indirect, why do yo think an 80k + a 50 gal won't be enough? It's your $$, and a larger tank won't hurt the system at all, but I can assure you the 80k + a 50 gal is more than sufficient.

    Code requires an ASME 1070 tempering valve to be installed. Do like Rich said and set the indirect stat to 140*+ and the tempering valve to 120* and you'll get about 15% more capacity and kill Legionella at the same time.

    I keep thinking about when the extra future full bath gets added. I don’t know the shower use habits now. Back to back times 5, or staggered intervals with some time between. Morning, or night etc. Which may be just because there is only one full bath. Add the extra bath then simultaneous back to back showers could be possible.

    I had two daughters at home with a standard 40 gal conventional gas WH. Never had issues with the 4 of us. Higher tank temps mixed down does add more capacity. That WH was 38k Input.

    Low flow shower heads do wonders also. Not only in saving hot water, but saving water. There are plenty out there where you can hardly tell a difference.

  • Ktfinch2009
    Ktfinch2009 Member Posts: 22
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    Right now with 1 bath we take showers back to back. We try to limit kids to a 5 min shower but they usually go over until we start screaming lol. there is always enough with 41 gallon boilermate. I know once additional bath is in we will def be taking simultaneous showers as kids will be getting ready on first fl bath and my wife and I in 2nd fl bath. We also had thoughts of adding a large soaking tub in our master bath but not sure now. Would a 60 or 80 gallon indirect be a better choice with 80k boiler? I just want to make sure smaller boiler will work well with larger indirect say 50 60 or 80 gallon.
  • Gordy
    Gordy Member Posts: 9,546
    edited November 2018
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    Here is food for thought. An HTP ssu 45 has a 1st hour rating of 212 gallons with 180 boiler water, and 141k boiler with 140 tank temp. That’s 42.4 gallons per person for 5 people of 140 degree hot water. Of course this is mixed down, and extends the gallons. That’s near a 17 minute shower each person with a 2.5 gpm shower head.........not counting the extended gallons from mixing down.

    The straight 45gallon capacity is more than a 4 minute shower each.
    Ktfinch2009Rich_49
  • Paul Pollets
    Paul Pollets Member Posts: 3,658
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    There's no teenage girl that I've known that takes a 4 minute shower. The larger tanks are not excessively higher in price. Why take the chance of running out? Yes, a larger boiler will recover the tank faster than a smaller one.
    Ironman
  • Ironman
    Ironman Member Posts: 7,397
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    Paul, my middle daughter would take a 7 minute shower even before she entered the Air Force. She's the rare exception. After years of concerted effort, we've finally got the oldest down from an hour to 35 minutes. :)
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
    Solid_Fuel_Man
  • hot_rod
    hot_rod Member Posts: 22,468
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    Run a 50 gallon up to 145, mix it down to 115, plenty hot for showers and washing dishes or clothes.

    Handy simulators at the Lochinvar site.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    Ktfinch2009Rich_49
  • Ktfinch2009
    Ktfinch2009 Member Posts: 22
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    Thanks guys!
  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,646
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    Dont let the 80k boiler scare you about size of the indirect. You could have a 500 gallon indirect fired with an 80k boiler, it would just take time to bring 500 gallons up to temp.

    Basically a larger volume tank makes up for less BTU input for large draws like simultaneous showers. As has been said, a conventional gas water heater is generally 40k or less BTU and 40 gallons and most people are fine with that.

    I'd go for the 80 gallon of you are considering a tub, the price difference from a 50g to an 80g is not much.

    I wouldn't run an indirect above 140 as it lowers boiler efficiency, caused more minerals to precipitate out of the water causing scale and reduced output, and increases standby losses. 140 is good to kill legionella.
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
    Ktfinch2009
  • hot_rod
    hot_rod Member Posts: 22,468
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    We do like to take very hot showers...lol it seems the general consensus is larger tank for hot water demand with the 80k mod con. We've had zero issues with boilermate 41 keeping up with demand. That model spec says 100k btu boiler needed to meet 170 gallon per hour. My boiler is 94k and we have zero issues with running out of hot water. Dropping down to 80k btu mod con and going to a 50 gal h2o tank worries me a bit but I'm no expert.

    Hot showers are not the issue, generally 103- 105 is plenty of temperature. The question becomes volume, how many gallons available.

    Comes a time you sit the clan down and have that awkward conversation... the 5 minute shower policy :)

    Families with high DHW usage always need 10 gallons more than what you give them.


    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    Ktfinch2009Gordy
  • bob eck
    bob eck Member Posts: 930
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    Make sure they pipe the new indirect with 1” supply and return.
    If they pipe it with 3/4” supply and return you might not get the full heat exchange and then the 50 or 80 gallon IDWH might not put out as many gpm as it should.
    Store water at 140 - 150 F and mix down to 120 F going out to your faucets.
    Install 1.5 GPM shower heads.
    Vaughn indirect water heater. With 80,000 BTU boiler and 100 degree temp rise. Incoming water temp 40 F going to 140F in your tank.
    First hour gallons are as follows.
    35 gallon model 129 gallons
    50 gallon model 140 gallons
    70 gallon model 161 gallons
    Mix down to 120F going out to showers can add 10% to 15% on numbers above.
    50 gallon IDWH with 1.5 GPM shower heads and you will be fine.
    Make sure it is pipes correctly.
    Makes sure the IDWH gets piriority over heating zones.
    Did your old boiler and indirect work to give the IDWH priority?
    I see no need for 70 or 80 gallon IDWH but it’s your $$$$

    Ktfinch2009
  • Ktfinch2009
    Ktfinch2009 Member Posts: 22
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    Will do and yes current system is set up for dhw priority.