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Indirect HW inadequate

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Steve Garson_2
Steve Garson_2 Member Posts: 712
I have an Amtrol Boilermate 41 gallon indirect connected to a WM GV-4 with priority to the water heater. I have the heater on high temp with a mixing valve, but when filling our bathtub, the water goes cold before its full and we need to wait another 10+ minutes to add more hot water. The circulator on the GV-4 is the only circulator, with 1" pipe to HW Heater, but all the other zones are off while the water heater heats up.

I measured the delta-t on the loop heating the water, which is around 50 degrees - 180* in and 130* out. The specs on the water heater make it sound like I should have more hot water.

What might be my problem? Might it just be that the heat exchanger needs cleaning? Do I need a circulator just for the HW heater?

Thanks for your insights!

Steve
Steve from Denver, CO

Comments

  • Alan (California Radiant) Forbes
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    Has the indirect been able to fill the bathtub in the past? Was it a sudden change or one that happened over time?

    I don't think it's the pump that's the problem or the HX if the boiler is able to satisfy and heat the indirect to its setpoint temperature. Do these indirects have dip tubes? If yes, maybe it's broken.

    I could also be that the aquastat on the indirect has lost its calibration and is no longer heating the indirect to the desired temperature.
    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
  • mattmia2
    mattmia2 Member Posts: 9,850
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    Did it ever work?

    Is the boiler continuously firing during the draw or does it shut off on high limit? What is the boiler temp itself. The boiler temp and if it is firing will tell you some about if a cleaner or bigger hx in the indirect or more flow would help of if all the heat the boiler is making is going in to the indirect.
  • mikeapolis
    mikeapolis Member Posts: 46
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    Is this a new install? Maybe your Tee is too closely spaced and is satisfying the boiler aquatstat temp and leaving the sidearm cold. Pictures if you have them would be great
  • Steve Garson_2
    Steve Garson_2 Member Posts: 712
    edited March 2022
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    Thanks for your comments. We recently moved into this house. Based on the boiler serial number, I would guess that the boiler is 30 years old. The HW heater was likely installed at the same time, just a guess, of course! The boiler does shut off after the indirect hits its high limit. Boiler water temp is 190*. The HW aquastat heats the water to 150* which is then tempered with the thermostatic mixing valve. Photos to follow.
    Steve from Denver, CO
  • Henry
    Henry Member Posts: 998
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    The DT at 50F is way to high. The flow rate is too low. The DT should be 20 to 25 F. The indirect should have its own circulator.
    MikeAmanndelta T
  • MikeL_2
    MikeL_2 Member Posts: 503
    edited March 2022
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         What's the capacity of the bathtub? Public or private water supply? What are the physical characteristics of the water supply? Do you run out of hot water during a shower?  Cold start boiler or does it maintain a low limit. Does the hot water pipe leaving the water heater feel cool to the touch when you run out of hot water at the tub valve? Single handle tub valve?
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 8,157
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    To test Gallon per Minute (GPM) at the tub, I place a 5 gallon bucket that I have marker at each 1/2 gallon with a permanent sharpie marker. Take out the cellphone and use the stopwatch app. Simultaneously turn on the hot only faucet and start the timer. At one minute, if the bucket overflows, you are greater than 5 GPM. If 1 minute passes and the bucket does not overflow, turn off the water at 1 minute on the timer, then I look at the water lever in the bucket and see what hash mark is close to the water level.

    Sometimes you have to really put your mind to the big problems in the world.

    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

    Solid_Fuel_ManZman
  • Steamhead
    Steamhead Member Posts: 16,950
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    Henry said:

    The DT at 50F is way to high. The flow rate is too low. The DT should be 20 to 25 F. The indirect should have its own circulator.

    I agree.
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
    Consulting
    Zman
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 8,157
    edited March 2022
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    There is a number applied to water heater ratings. It is called the First Hour Rating. When a tankless water heater is compared to a tank type water heater the tankless uses the Gallon per Minute at a given temperature rise and multiply it by 60 (because there are 60 minutes in one hour) Thank you Mr.Obvious

    When you use a tank type water heater you use the recovery rate of the heater and add 80% of the storage capacity of the heater. So a 40 gallon water heater that has a 20 gallon per hour recovery rate (like maybe an electric) would have a first hour rating of 52 gallons.
    The reason I bring this up is to reinforce the fact that you have about 30 gallons of hot water ready to go as it sits there in the tank waiting for use. At 5 GPM flow rate to fill the tub you only have 6 minutes of Hot water available until you start to run cold. The recovery rate of that water heater is relatively fast considering there is a 105,000 BTU burner connected to it. The thing is that there is a sequence of operation that takes time to get to the rated recovery rate.

    1. The water tank temperature needs to drop to activate the water heater thermostat. 30 seconds
    2. The boiler gets a call for DHW ----------------------------------------------0 seconds
    3. the boiler goes thru the pre-purge and start up cycle-----------------------------60 seconds
    4. The burner lights and starts to heat the cold water in the boiler ------------------- 10 seconds
    5. The circulator is circulating the cold boiler water for those 10 seconds and an additional 110 seconds
    6. The water in the boiler is now up to temperature and and heat is being transferred to the indirect

    A total of almost 4 minutes (3.666 munutes) has passed and you are finally at the point where the tank recovery will take over and heat the water at its specified recovery rate. Now since you may be using the water at a rate faster than the recovery rate of the tank/boiler combination, you will start to feel luke warm or even cool water within 8 minutes.

    There is another possibility with that Amtrol Tank. The heat exchanger may be fouled. I read the manual and found this in troubleshooting

    The trouble is, I read the rest of the IO manual and there is nothing that actually describes the procedure for cleaning a fouled heat exchanger. It can be fouled on the boiler side or the DHW water side or both. If you have an old one, it may be more trouble than it's worth to maintain, after all these years without maintenance.

    I would test the tub GPM first to see if there is a greater draw than the rated capacity

    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

    hot_rodSolid_Fuel_Man
  • mattmia2
    mattmia2 Member Posts: 9,850
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    To test Gallon per Minute (GPM) at the tub, I place a 5 gallon bucket that I have marker at each 1/2 gallon with a permanent sharpie marker. Take out the cellphone and use the stopwatch app. Simultaneously turn on the hot only faucet and start the timer. At one minute, if the bucket overflows, you are greater than 5 GPM. If 1 minute passes and the bucket does not overflow, turn off the water at 1 minute on the timer, then I look at the water lever in the bucket and see what hash mark is close to the water level.

    Sometimes you have to really put your mind to the big problems in the world.

    Sometimes i will weigh the bucket with the water in it because I know water is very nearly 8 lb/gallon
  • hot_rod
    hot_rod Member Posts: 22,396
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    30years old? I'll bet the finned copper coil is fouled. The coil is removable, but at that age...
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    Zmanmikeapolis
  • Steve Garson_2
    Steve Garson_2 Member Posts: 712
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    All good information. I just measured the flow to the tub: 4GPM.

    If I give the HW heater it's own circulator, will I be able to fill the tub without stopping? Is doing this pointless? If a circulator is added, would the water be flowing through the WM circulator to the HW circulator? The tub faucet is a one-handle faucet that does not offer flow reduction.

    Should I simply live with it? My wife is the one who takes baths.

    Ed - good explanation with the timing. For showers, we never have a problem with hot water.

    Steve
    Steve from Denver, CO
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 8,157
    edited March 2022
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    Now I'm not sure what WM GV boiler you have but a 4 section (GV-4) probably has a 100,000 to 110,000 BTU burner in it (depending on the series, but the old ones were 105 input with maybe 80,000 to 85,000 NET output. (Net rating allows for loss thru the piping) Since water weigh is around 8.3# (depending on the temperature) and you have lets say 82,000 BTU available every hour to use for recovery, and you are probably increasing the temperature to 120° (after mixing). If you have incoming water at 50 degrees (we were 58° year round in souther NJ) the math looks like this.

    Gallon = 8.3 pounds
    Temperature rise = 70°
    Available BTU per hour = 82,000
    Available BTU per minute = 1366.6. (82,000/hr. ÷ 60 minutes)
    (BTU of 1366.6 ÷ 8.3 pounds) ÷ 70° temp rise = 2.35 GPM.

    If you get near 3 GPM to fill the tub, and you still have insufficient hot water then you will need to look into the maintenance of the heat exchanger in the Amtrol Tank.

    Final thought, if there is air in the boiler side of the exchanger. the heat transfer will be compromised. Make sure the DHW zone has no air in the system. Air will be obvious if there is a gurgling noise when the pump is running.




    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

  • Steve Garson_2
    Steve Garson_2 Member Posts: 712
    edited March 2022
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    Which explains why the showers are now problem. Great explanation!

    But it sounds like I'll get faster recovery with a circulator specifically for the water heater. Thanks.
    Steve from Denver, CO
    EdTheHeaterManMikeAmann
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 8,157
    edited March 2022
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    All good information. I just measured the flow to the tub: 4GPM.

    If I give the HW heater it's own circulator, will I be able to fill the tub without stopping? Is doing this pointless? If a circulator is added, would the water be flowing through the WM circulator to the HW circulator? The tub faucet is a one-handle faucet that does not offer flow reduction.

    Should I simply live with it? My wife is the one who takes baths.

    Ed - good explanation with the timing. For showers, we never have a problem with hot water.

    Steve

    Is there a service valve on the faucet? Sometimes they are built in to the valve body so you can service the cartridge. If there is a valve... then try closing the valve down until you get about 3 GPM flow. This will give you more time for the recovery of 2.3 GPM to get started earlier. If you time everything just right, you will be able to live with ti just the way it is.

    I assumed you had a dedicated circulator already. If you have a zone valve, what model valve do you have? If it is the old Taco Zone valve like this one, add another 60 to 90 seconds to the above sequence of operation for the zone valve to open before the boiler fires. That will really put you behind the eight-ball.

    You don't need a dedicated circulator, Change that valve to a Honeywell or some other fast action valve. You will gain over a minute in recovery time lag

    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

  • MikeAmann
    MikeAmann Member Posts: 998
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    His indirect coil should require 180* water at least 8 gpm.
    The CV of the zone valve will inhibit the necessary flow.
    Go with the dedicated circulator.
    Your 50* delta T says so.
    delta T
  • Alan (California Radiant) Forbes
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    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
  • mattmia2
    mattmia2 Member Posts: 9,850
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    MikeAmann said:

    His indirect coil should require 180* water at least 8 gpm.
    The CV of the zone valve will inhibit the necessary flow.
    Go with the dedicated circulator.
    Your 50* delta T says so.

    Ok. so lets work this out. Let's say the net output of the boiler is 80,000 btu/hr

    btu/hr = gpm * 500 * delta t
    80,000 = gpm * 500 * 50
    80,000 = gpm * 25000
    3.2 = gpm

    So as long as you have 3.2 gpm at your 50 degree delta t you are moving 80,000 btu/hr in to the water
    This could have implications in preventing condensing in the boiler, but it won't make any more hot water.

    Without knowing the flow and the output of the boiler you can't determine if more flow will increase output.

    The coil in the tank is arranged so the return to the boiler is where the 50 degree or so cold water enters the tank. There is plenty of approach to transfer the heat from the boiler water to the incoming water at a relatively low flow. 8 gpm will only help you transfer wise if you have a very large boiler.
  • JohnNY
    JohnNY Member Posts: 3,244
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    Isn't there a simple math rule-of-thumb for calculating hot water rise involving 50,000 and 1 GPM? Is that a 70° rise? Good morning, everyone. Happy Sunday.
    -John
    Contact John "JohnNY" Cataneo, NYC Master Plumber, Lic 1784
    Consulting & Troubleshooting
    Heating in NYC or NJ.
    Classes
  • Zman
    Zman Member Posts: 7,589
    edited March 2022
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    A few thoughts:
    • Amtrol indirects have fairly small heat exchangers and are known to clog. Normally I would look there first.
    • Your delta T numbers on the boiler side are indicating low flow, not a HX issue.
    • Does the boiler run constantly at high fire during the DHW call? If so, you wont likely improve your situation by making circ or indirect tank changes.
    • I know that it may be difficult to get your spouse on board, but... if you fill the tub more slowly, you won't run out of hot water.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • hot_rod
    hot_rod Member Posts: 22,396
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    Consuelo said:

    mattmia2 said:

    MikeAmann said:

    His indirect coil should require 180* water at least 8 gpm.
    The CV of the zone valve will inhibit the necessary flow.
    Go with the dedicated circulator.
    Your 50* delta T says so.

    Ok. so lets work this out. Let's say the net output of the boiler is 80,000 btu/hr

    btu/hr = gpm * 500 * delta t
    80,000 = gpm * 500 * 50
    80,000 = gpm * 25000
    3.2 = gpm

    So as long as you have 3.2 gpm at your 50 degree delta t you are moving 80,000 btu/hr in to the water
    This could have implications in preventing condensing in the boiler, but it won't make any more hot water.

    Without knowing the flow and the output of the boiler you can't determine if more flow will increase output.

    The coil in the tank is arranged so the return to the boiler is where the 50 degree or so cold water enters the tank. There is plenty of approach to transfer the heat from the boiler water to the incoming water at a relatively low flow. 8 gpm will only help you transfer wise if you have a very large boiler.
    The average water temperature in the indirect coil is 155F. Yes, you are correct that the boiler will produce no more hot water provided the coil can transfer the full 80K. However, as the tank climbs above approx. 125F, the capability of transferring 80K is reduced. Once the tank climbs to approx. 135F, the capability might be down to 40K. The result is the boiler will cycle to recover the tank and, it might cycle for hours if a tank temperature of 150F is desired. This, of course, is after the demand has ended.

    If there is a desire to keep the tank at 150F (and there certainly is), higher flow to the tank is recommended.

    I have watched an 80K mod-con modulate downward to approx. 50% output near the end of the recovery (150 tank temperature) and this is with a dedicated circulator, 1" lines, and a 10F DT between supply and return. Most of them cannot transfer 80K if the DT between the coil and the tank drops below approx. 35F.
    Good point on how heat transfer drops as the tank temperature increases. Often we assume that full boiler output is being transfered until the tank reaches setpoint.

    You see that in solar thermal as the tank nears setpoint. The array doesn't provide quantity or quality energy, like a boiler can.

    But also, as cold enters the tank for any DHW draw you get that wide delta again, for the amount of cold that enters around the coil.

    Be interesting to see how the tank performance is actually tested by manufacturers, the hydronic formula doesn't tell the whole story.

    I've heard standby loss of less than 1 degree/hr was done in a room with ambient of 80F. Not a typical basement temperature :)
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • mattmia2
    mattmia2 Member Posts: 9,850
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    Consuelo said:

    mattmia2 said:

    MikeAmann said:

    His indirect coil should require 180* water at least 8 gpm.
    The CV of the zone valve will inhibit the necessary flow.
    Go with the dedicated circulator.
    Your 50* delta T says so.

    Ok. so lets work this out. Let's say the net output of the boiler is 80,000 btu/hr

    btu/hr = gpm * 500 * delta t
    80,000 = gpm * 500 * 50
    80,000 = gpm * 25000
    3.2 = gpm

    So as long as you have 3.2 gpm at your 50 degree delta t you are moving 80,000 btu/hr in to the water
    This could have implications in preventing condensing in the boiler, but it won't make any more hot water.

    Without knowing the flow and the output of the boiler you can't determine if more flow will increase output.

    The coil in the tank is arranged so the return to the boiler is where the 50 degree or so cold water enters the tank. There is plenty of approach to transfer the heat from the boiler water to the incoming water at a relatively low flow. 8 gpm will only help you transfer wise if you have a very large boiler.
    The average water temperature in the indirect coil is 155F. Yes, you are correct that the boiler will produce no more hot water provided the coil can transfer the full 80K. However, as the tank climbs above approx. 125F, the capability of transferring 80K is reduced. Once the tank climbs to approx. 135F, the capability might be down to 40K. The result is the boiler will cycle to recover the tank and, it might cycle for hours if a tank temperature of 150F is desired. This, of course, is after the demand has ended.

    If there is a desire to keep the tank at 150F (and there certainly is), higher flow to the tank is recommended.

    I have watched an 80K mod-con modulate downward to approx. 50% output near the end of the recovery (150 tank temperature) and this is with a dedicated circulator, 1" lines, and a 10F DT between supply and return. Most of them cannot transfer 80K if the DT between the coil and the tank drops below approx. 35F.
    But this is only a problem if you are trying to run back to back large draws, right? With a ci boiler longer cycles are desirable but not an actual problem unless you have back to back large draws. On the other hand the lower return water temp is very desirable with a mod con.
  • Alan (California Radiant) Forbes
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    My 50 gallon indirect can't fill my tub either, but I have a different set of circumstances:
    - 50,000 BTU input boiler
    - dedicated DHW circulator
    - old-fashioned, American Standard deep tub
    - no tempering valve, 125° setpoint
    In my opinion, I don't think changing your zone valve to a pump is going to get you anywhere. As @Solid_Fuel_Man said, if you want to remedy the bathtub issue, get a larger indirect.
    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
    mattmia2Larry WeingartenSolid_Fuel_Man
  • MikeAmann
    MikeAmann Member Posts: 998
    edited March 2022
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    A simple workaround (don't let the wife know) is to install a 3 gpm flow restrictor in the HW line going to the tub. Or you can make one yourself. It's just a small hole of the proper size.
    https://geohydrosupply.com/merrill-flow-regulator-3-0-gpm-3-4-fpt/




    ZmanEdTheHeaterManbucksnort