Need Help specing water heater (not which brand is best?)

Discussion in 'Water Heater Forum, Tanks' started by whatnoise, Oct 18, 2011.

  1. whatnoise

    whatnoise New Member

    Messages:
    1
    Location:
    Tennessee
    I am in the process of replacing my water heater and like anything I do I first research, research, research....

    A little back ground...

    I currently have a 50 gal. nat. gas Rheem water heater. While supplying the hot water to all of the plumbing fixtures in the house, it also does double duty, supplying hot water to my heater. The heater is a First Co. Aquatherm "Combo Heater".

    Specifically the water heater is...
    Rheemglas Fury 50gal
    Model # 21V50-60N
    Input BTU of 60,000
    FHR of 89 gallons

    The "Combo Heater" is...
    First Company
    Model # 36MBXB-HW
    Max heat output 51,100 BTU @ 140* water temp
    Max inlet water temp of 180*
    Air handler of approx. 1200 CFM (best guess according to literature from the First Co. website)
    Flow rate of hot water through unit approx. 4 GPM (best guess according to literature from the First Co. website)
    From what I could gather (also from the website), the water returning to the the water heater has a heat loss of 10 -20 degrees.

    The home...
    2 adults and 1 (for now) child (16 months old)
    3 sinks (1 kitchen, 2 bathroom)
    2 bathtubs
    1 shower
    1 dishwasher
    1 clothes washer
    1 heater**
    The home is located near Memphis, TN so it is a mixed cooling/heating climate.

    I am wanting to replace the water heater for several reasons...
    1) I have a leak somewhere on the return line to the w.h. from the heater caused by...
    1a) inadequate bracing of copper lines between w.h. and heater.
    1b) inadequate "flooring" beneath the w.h. (I can rock the w.h. back and forth with ease because the sheathing used is so thin.)
    2) Pipes have corrosion at other joints.
    3) The water heater is full of sediment (I think). After using hot water and the burner fires up you can hear a loud noise. I assume this to be sediment "boiling" in the bottom of the tank.
    4) A couple of years ago I was having trouble keeping the pilot lit. I put 2 new thermocouples in it with no luck. I ended up turning the temp up quite a bit and it hasn't gone out since.
    5) In the winter while heating the house taking 2 showers back to back usually the second shower starts to go cold. From living in the house for 4 years it seems the heater would work more efficiently with a better water heater.
    6) The water heater is nearly 9 years old

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    Obviously I need a new water heater that at the very least matches the specs of the current one. My biggest consideration being the fact it has to supply hot water for the heater in the winter.

    While looking a the First Co. website I came across the following info for sizing a water heater to use with the heater...

    ""Proper water heater sizing should consider both the gallon capacity AND the BTU input of the water heater.

    To determine water heater GALLON CAPACITY: A minimum 40 gallon high recovery and/or high efficiency gas or oil-fired water heater is recommended. The following volume sizing guide is satisfactory in most areas of the country:

    600-800 CFM air handlers - Minimum 40 gallon water heater
    1000-1200 CFM air handlers - Minimum 50 gallon water heater
    1400-1600 CFM air handlers - Either two 40 gallon water heaters piped together, one high input 50 gallon (63,000 to 75,000 BTU input), or one 72 to 75 gallon
    2000 CFM air handlers -Any combination of water heaters having at least 105,000 BTU OUTPUT.

    To determine water heater BTU INPUT: (Assumes a water heater recovery efficiency of 76%). For mild climates...BTU INPUT = structure's heat loss x 1.51. For colder climates...BTU INPUT = structure's heat loss x 1.58.""

    Calculating the heat loss of my house is way over my head, so looking at the old heater ( and knowing it is just barely enough to do the job), and the above info...
    I think I need a water heater with at least...
    50 gal
    60,000 BTU input
    FHR of 90

    Through much of my reading I am most interested in Bradford White, with Rheem/Rudd a close second.

    I have been looking and have come across several water heaters I am interested in...

    1) Bradford White M-2-XR504T6FBN 48 gal. Extra Recovery
    65,000 BTU input
    FHR of 101
    Recovery of 69 GPH
    Recovery Efficiency of 79%

    2) Bradford White M-2-XR65T6FBN 65 gal. Extra Recovery
    65,000 BTU input
    FHR of 126
    Recovery of 69 GPH
    Recovery Efficiency of 79%

    3) Bradford White GX-2-25S6BN 25 gal. High Performance
    78,000 BTU input
    FHR of 155
    Recovery of 84 GPH
    Recovery Efficiency of 82%
    ***I am guessing even with the great specs that the 25 gallon capacity is too small right?


    4) Bradford White GX-1-55S6BN 55 gal. High Performance
    80,000 BTU input
    FHR of 200
    Recovery of 86 GPH
    Recovery Efficiency of 82%


    Sorry for such a long post. Any and all advice is very welcome.

    Thanks, Stephen
  2. PumpHeat80

    PumpHeat80 New Member

    Messages:
    3
    Location:
    CA
  3. leejosepho

    leejosepho DIY scratch-pad engineer

    As you likely understand, those matters should be addressed even if you do not replace the heater at the same time.

    You might try connecting a hose to the drain valve and see what kind of flow you get, but some of the pros here might know more about what that kind of noise cound indicate.

    That would be a matter of recovery rate, and I have heard of people using electric water heaters and then wiring them in such a way that both elements are working whenever the WH is energized. However, I believe gas water heaters already heat from bottom to top.

    There are heaters made specifically for that purpose, and a search for "hydronic" here in these forums might provide you with some more information about them.
  4. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,534
    Location:
    Cave Creek, Arizona
    1. You are NOT going to run a domestic water heater at 180 degrees, so you can disregard that specification.
    2. If the heater is rated at 52,000 btus, and the heater only inputs 60,000 that does NOT leave much leeway for the domestic water heater demands.
    3. The last two heaters DO run at high temperatures to achieve their ratings, BUT they also incorporate a "mixing" valve to moderate the temperature and thus achieve the hourly capacity they show.

    4. The "return temperature" of the heater depends on the amount of heat extracted from the water, and the lower the input temperature, the greater the heat lost will be.
  5. Dana

    Dana In the trades

    Messages:
    2,812
    Location:
    01609
    The lower the temp, the less heat gets pulled out of by the air handler coil. If it's rated to pull 52K out at 140F, it'll be pulling about half that at 120F. One has to be careful when using a standard gas HW heater in combi configurations like that to keep the return water temp from the air handler well above 120F, otherwise you can severely shorten the life of the HW heater due to excessive exhaust condensation in the center-flue heat exchanger. (Measure it- with 140F out, and a 20F delta-T you may need to bump up the storage temp.)

    By setting the tank temp high and using an aquastat on the air handler to cut power the blower whenever the exit temp drops below 125F you can save the HW heater, and keep the air handler from robbing too much from the showering performance.

    If you need more showering performance, a drainwater heat recovery heat exchanger on the shower that feeds both the cold input to the tank and the cold side to the shower will more than double the apparent performance of the system. With a 60K, burner behind it, with drainwater heat recovery you could literally shower all day long under a 2gpm showerhead at typical TN incoming water temps.

    With higher storage temps comes higher standby losses, and it looks as if the system is already above the insulation, outside of conditioned space, which also raises the standby loss. It may be worth spending the bucks on a condensing tank heater with a big burner (A.O.Smith Vertex 100 or similar), and running a lower storage temp. That way you can go lower temp on the air handler return and get mid-90s efficiency out of it in space heating mode without damaging the water heater, and with the bigger burner you could just let it "share" with the shower rather than forcing a cut-out. As long as you have 105-110F water in the tank you can still take a hot shower, and at those temps the air handler would be taking less than 15KBTU out of the system, leaving ~80-85K of output for the shower. You'd have lower standby loss, higher combustion efficiency, and you'd never run out of showering water. It's a coupla grand just for the HW heater, but it's really more like the "right" solution, and it's harder to screw up. These units come with purpose-built side ports designed specifically for space heating applications.

    With any of them be sure to measure and do the math on how fat the fuel supply plumbing needs to be for the rated input BTUs- when in doubt, go fatter. There's no guarantee that the original installer got it right even for the 60K burner, but if it's on the edge at 60K it may be undersized for 80-100K burners. The standard Vertex has a 76K burner, which would not give you all-day showering performance without a drainwater heat exchanger, but probably WOULD with one.

    I have a 4" x 48" DWHR unit on my combi system running at 130F max temp, and a modulating burner than runs ~45K max in space heating mode (all zones calling for heat), but bumps up to ~55-60K under heavy showering use with the heating system running. It will run pretty much all day long in the shoulder seasons, but in the coldest part of the winter the 110F cut-out on the airhandler will sometimes trips after 15-25 minutes of shower (ab)use. It never runs out of shower capacity, and I suspect my winter incoming temps are more than 10F colder than yours.
  6. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,947
    Location:
    New England
    One thing to keep in mind also is that IF you properly setup your antiscald shower control, it is done with the water heater set and fully heated. Then, you are supposed to adjust the limit device (a screw, cam, or some other type of stop) to limit it to a safe temperature. This means that you will likely never be able to drain the tank, as the valve is preventing you from using only hot...i.e., it will always be mixing some cold in, based on the full temp of the tank, not the diluted temp after running for awhile. This is one reason I like a thermostatically controlled valve...once properly adjusted, it can adjust itself to all hot, if need be, to try to maintain the set temperature (some can't adjust that far, though). And, as the tank cools off, you may never know, since the valve is continually compensating as the tank's water cools; you'd only know once the valve got to it's max hot, minimum cold input control limits.
  7. Dana

    Dana In the trades

    Messages:
    2,812
    Location:
    01609
    Yep, thermostatic mixing valves can go all the way to hot when the hot temp is at or below the adjusted setting, but will deliver well-controlled output temps over a wide range of flow when the tank temp is above the adjustment temp.
  8. ITSec

    ITSec New Member

    Messages:
    17
    Location:
    Virginia
    Thanksgiving day my Apollo water heater became a leaker, which meant I lost hot water and heat at the worst possible time. I was planning on replacing it summer 2012, guess it couldn't wait. I was going to purchase a Bradford White from PexSupply since they sell BW to the public, but my wife couldn't wait for the water heater to be delivered. So I started looking at the box stores and picked up a Whirlpool from Lowes; it's actually made by American Water Heater Company with a Whirlpool badge. It has 2" insulation, 12yr warranty, .67 EF and I replaced the alum anode rod with mag. The other water heater I was looking at was the Kenmore Power Miser 12 from Sears, it has 2" insulation and 2 anode rods, but Sears had none in stock. (Lowes 10% military discount & Sears had 10% discount using promo code "water") Whole 4-day weekend spent researching, buying, and installing water heater :(

    This thread caught my eye because in OP's pictures, it looks like he has a normal water heater with T fittings at the top to send hot water to the air handler and back. Is that configuration possible on a normal water heater and not a special hydro-heat Apollo water heater? The Apollo water heater has 2 additional ports on the side to send hot water to air handler and cool water back. Apollo went out-of-business and their site redirects you to State.

    The water that came out of the coils in the air handler was nasty, had rusty and grayish-black water drain out. Hopefully I wasn't showering in that water.

    BTW Bradford White has one combo anode rod in the hot port, so that might be a PITA to replace.

    From a 2009 post a user said whirlpool and sears water heaters are junk, http://www.weilhammerplumbing.com/products/ damn it! Oh well I needed one ASAP.
    Last edited: Nov 28, 2011
  9. Dana

    Dana In the trades

    Messages:
    2,812
    Location:
    01609
    The T-fitting approach may work well enough so long as you don't run the HW heater in condensing mode with too low a return water temp. Purpose-built hot water combi-heaters have features designed into them to help avoid self-destruction in space-heating mode.
  10. ITSec

    ITSec New Member

    Messages:
    17
    Location:
    Virginia
    Guess I purchased the wrong water heater, I assumed apollo was the only company making dual purpose water heaters. That Vertex is a sweet looking water heater but doesn't fit the budget. The water heater I have now only has 40,000 BTUs and the old one was 65,000 BTUs, so I don't even think it is wise to connect my current water heater to the air handler.
  11. ITSec

    ITSec New Member

    Messages:
    17
    Location:
    Virginia
    Hey whatnoise I found a good website for calculating heat loss, http://www.moorepage.net/heatloss.html the website explains it very well and there is an excel spreadsheet where you can play around with the numbers.
  12. Dana

    Dana In the trades

    Messages:
    2,812
    Location:
    01609
    That's a pretty lousy heat loss calculator with many inherent built in errors, e.g. A 2x6 16" o.c. studwall with R19 batts isn't an R19 wall, but will in fact average ~ R14 due to the thermal bridging of the framing. There are similar issue with the treatment of other assemblies, and the agree to just punt on the air infiltration factor, despite it being one of THE major heating/cooling load factors.

    Taco has a much better freebie downloadable from their site, but it's targeted at HVAC pros, and there may be a bit of a learning curve.

    If you have a heating season's worth (or mid winter month's) of fuel use history and some hard meter reading begin/end dates, you can do a far better job of determining the heat load at design temp. By looking up the daily heating degree-day weather history for the billing period(s) and calculating heating fuel used per heating degree-day. It's fairly simple arithmetic to convert that to fuel use per heating degree-hour, and calculating what the fuel use needs to be at design temp. For a non-condensing hot water heater we can assume that the heat going into the system is no more than 80% of the fuel content.

    To figure out your design temp, you can either google the exact phrase "outside design temperature" and the name of the nearest mid-sized city, or go to Weatherspark.com ferret out the minimum winter temps were over the past 5 years and average them, that'll be pretty close to the 99th percentile number.

    eg: Let's say you live in Richmond VA, and your billing was from 12 January through 14 February, and the base-65F degree-days at the nearest Richmond weather station came in a 962HDD, and you had used 220 therms. That's 220/962= 0.229 therms per degree day. At 100,000 BTU/therm that's 22900BTU/HDD. In a 24 hour day that would be 954 BTU per degree hour.

    Multiple google sources place the 99th percentile design temp for Richmond at +14F. The difference between 14F and 65 F is 51 degrees, so from a fuel use point of view that's 954 x 51= 48.7KBTU/hour. But with an 80% efficiency burner that's 0.8 x 48.7K= 38.8KBTU/hr needed by the house.

    This is just an example your calculated heat load using real numbers could be either higher or lower than that (hopefully lower, if you have a relatively tight, reasonably insulated house), but that's pretty much an upper bound, since at least 10% of the fuel probably went into heating domestic hot water, and your as-used efficiency was probably more like 75%, or even lower, depending on the duty cycle of the heating system during the billing period.
  13. ITSec

    ITSec New Member

    Messages:
    17
    Location:
    Virginia
    Wow Dana, very informative; it's going to take a few times reading your post to understand but i'll get it.
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