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Indirect heat loss

TonyS Member Posts: 849
trying to sway an older couple away from a indirect water heater into a new Navien Tankless. They each take a shower every day in the evening, very little other hot water used. I am trying to estimate the standby loss. A 40 gal indirect will hold 333 lbs of water and at a loss of say 1 degree an hour would be an 8000 btu loss in a 24 hr period but what about the 5 section cast iron boiler? It may hold 5 gallons of water but what about the 300 lbs of cast iron? How many btus is there? Anyone have a way to figure this?


  • Tank HL

    I know that UniR was looking into that very thing not long ago. If you do a search back you may find something interesting,, he`s a very smart guy!
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    It depends, as usual.

    With my W-M Ultra-3 80KBTU/hr boiler, the heat exchanger holds about 3 quarts of water. It runs cold-start, so it can be assumed that any residual heat in there is lost unless it is going to do heating of the building at the same time. But that is not a lot of lost heat. The indirect water heater I have loses 1/2 degree F per hour according to the manufacturer. So say 42 gallons lost there. Say also that there are a 3 gallons in the 1" pipe between the two units ( it is probably less than that). The pipe, though insulated, loses all its heat by the time the water asks for heat again (for me, about once a day). So we need to heat 4 gallons from garage temperature up to 180F and 42 gallons from 108F to 120F. This can be computed, but I do not have the results handy.

    With a C.I. boiler that you may or may not cold start, YMMV, as they say.
  • CMadatMe
    CMadatMe Member Posts: 3,086
    Keep it Simple

    I take it this unit qualifies for the federal tax credit. So start there. Also how about the fact that the boiler is OFF in the summer months. No need to heat up that boiler. If your unit is like the Noritz we have had a huge success with, the burner modulates. It only makes the hot water you need when you need it. No standing pilot like the boiler (if that is the boiler installed).

    You have to sell the convience while showing savings in terms the customer can understand. Calculating btus' and trying to get someone that doesn't know what a btu means is like when DNA evidence first started. People get overwhelmed and tune you off. They can see that their boiler is off in the summer time and can grasp that. They can understand the instantaneous only comes on when you need water because they can see that and you can show them the pilot doesn't stay on.  Just my two cents. 

    There was an error rendering this rich post.

  • TonyS
    TonyS Member Posts: 849
    No argument here

    I agree the Navien unit is 98% and like you said it only makes the hot water you want at exactly the temperature you want  and exactly the amount you want at 98% efficiency. Pretty much the epitome of fossil fuel water heating. I was just thinking about it today, figuring pounds per gallon x temp. But the boiler loss is more than the 5 gallons it holds, its probably closer to its weight in water which would be close to 400 pounds wet, which is a loss of another 48 gallons of water combined with the 40 gallons in the indirect and im sure the boiler is loosing more than 1 degree an hour. This could easily exceed 20,000 btus a day in heat loss. That is a gallon of oil a week. The argument that the heat is lost to the structure in winter is offset by the fact heat is also added the the structure in the summer when it has to be removed so that point is moot. It is very easily possible in a case with an older couple with no kids the heatloss is more than the actual hot water used.  
  • TonyS
    TonyS Member Posts: 849
    Interesting articles

    But they are comparing the tankless to a standard gas water heater not an indirect. So there is definitely are big difference in cost and installation especially if its a replacement. The price for a indirect is much closer. I would imagine if there are 4 to 6 people in the house, standby loss isn't as big an issue, but the less people that live in the house using hot water, as in my example of 2 elderly people , each one shower every 24 hours starts to consume allot of standby energy. Another problem I see with their test is they said the calcium can block the tank-less. This is true but at least the tank-less has provisions to clean the unit and keep it very efficient to its death as the tank type heater doesn't suffer flow restriction ,it still suffers from decreased efficiency because the calcium builds up on the bottom of the tank, inhibiting heat transfer and raising stack temperature with almost no way to clean it out. The cold water sandwich doesn't bother me and I give my customers a choice, Navien makes a unit that has a circulator and a 1 quart storage tank that remains at temperature( yes there is standby loss but very little) the circulator can be used for recirculating through the piping or turned to recirculate internally, either way on this model there is no minimum flow. I find consumer report articles are not that good. I saw a complaint on their treadmill test where they tested 2 units that were made by the same company and sold under different names. These were identical machines! They gave them 2 completely diff rent ratings.
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    Low use heat loss with indirect DWH.

    I live alone in my house. I am 71, so you decide if I am elderly or not. I have a 38 gallon indirect fired DWH that claims to lose 1/2 degree per hour. (Temperature not specified, though it should be; imagine I set it to run at the ambient temperature: there would be no heat loss.) Mine is set to give out 120F water at the nearest tap inside the house.

    The boiler holds 3 quarts of water, is run cold-start, and the piping is insulated between the boiler and the DHW heater. I take about one shower a day with a low-flow shower head for, say, 10 minutes (I never really timed it). I normally run the washing machine twice a week, but set to use cold water. The diswasher goes about once a week. As far as I can tell, my boiler runs for a little over 5 minutes once a day. I wish my computer could connect to the boiler so I could get these measurements. In any case, my gas bills for the recent months, when I do not heat the house, are like this:

    May 15 to May 28: 3.15 therms Gas $3.47; $1.63 delivery; $3.30 customer charge  (first installed May 15)

    May 28 to June 29: 6.28 therms Gas $6.91; $3.25 delivery; $8.25 customer charge

    June 29 to July 20 3.14 therms Gas $3.46; $1.63 delivery; $5.78 customer charge.

    They are fooling around with the meter reading dates at the moment because the meters are being equipped with remote sensing so the meter reader can just drive down the street and read the meters by radio.

    In this interval, no home heating was done, so the entire bill is my hot water heating cost. So I figure it costs me about $0.50/day to heat my hot water; about 1/2 what it cost to do it with electricity. To figure out actual energy cost, you might not wish to include the customer charge, that seems to be about $0.26/day. And there is a lot of noise in these data because of varying period for each bill, etc. If I have the patience, I may continue to keep these bills. More interesting to me will come this winter when the heating season begins. I wlll need to get the slope right on my outdoor reset for one thing.

    One interesting thing, to me at least, is that the water heater company and the boiler company (W-M for both) recommend setting the hot water in the boiler to 190F for fastest recovery. It recovers so fast I reduced this to 180F. Though right now it scarcely matters because it recovers so fast the boiler never gets up to 180 at all. It normally gets to around 170F and is done. Since it spends so much time at lower temperatures, the condensing in the boiler actually causes the condensate pump to run sometimes.
  • TonyS
    TonyS Member Posts: 849
    Im sure that is true

    with a small modcon but what about a 5 section oil boiler? Have you been to Weils website? They have a energy calculator that gives you the payback period for different boilers. Its fun to play with but a big eye opener. Take care Tony
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    I tried W-M's payback calculator

    I tried W-M's payback calculator, but it was useless to me. For one thing, it assumed my existing boiler used the same fuel as the one with which I wanted to replace it. But I wanted to switch fuels as well.

    Second, it assumed my existing boiler would last throughout the life of the new boiler, where I was concerned that it would not, being about 55 years old at the time.

    It was not interested if I was going to use outdoor setback. It was not interested if I wanted to use an indirect fired hot water heater,  etc.

    Furthermore I could not enter that I was willing to pay a little extra to protect the environment.
  • TonyS
    TonyS Member Posts: 849
    It is based on all

    things being equal fuel, indirect ,ect. and your correct, if you are replacing a boiler that has to be replaced anyway you have to subtract the difference in price. As for you wanting to pay a little more for the enviroment that you can figure out after it calculates the numbers, that being said, I have to congradulate you on being one of the few over seventy crew that I know that is concerned about the enviroment. Have you always felt this way or just having feelings of guilt after a lifetime of v-8s LOL.
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    Have you always felt this way

    I have always felt this way; at least since high school (1952-1956).

    I have never owned a V-8. All my cars have been 4-cylinder models between 1300cc and 1600cc in size, except my two recent ones (Honda Accords; 1980 and 1996) whose engine size I do not know. I walk short distances that many others drive, and I take the train in preference to driving, even for transcontinental trips from near NYC to Seattle, for example.

    I use compact fluorescent lights in most of the fixtures of my house except for four 7 1/2 watt bulbs in the safelights of my darkroom, and seven 75watt floodlights in my kitchen that are on dimmers.
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666

    I have some dear friends who are 9 years old, 12 years old, 13 years old, and 18 years old. I do not what them suffering and dying from contamination I could have refrained from producing. I know it takes "all of us", not just me to fix this, but I have to start somewhere, and changing my behavior is a lot easier than changing the behavior of a bunch of politicians.
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    If you mean me,

    Then yes, it does qualify for the federal tax credit. And the New Jersey tax credit too ($300) that in spite of government bureaucracy has already been paid into my bank.

    Also, the state will help me pay for getting my oil tank out of the ground. But not until the DEP  approves of the remediation from the leak.
  • hot_rod
    hot_rod Member Posts: 18,908
    I question that 1/2 per hr.

    on the heatloss of indirect tanks. Mine is in a very small, and warm room, over 80F all summer and I hear it kick on at least once every night. No draw, no recirc and a check valve in the mix valve on top.

    1/2 degree per hour should be 4 degrees over night. My setpoint control that operates the tank to boiler is set at a 10 delta t. So....?

    Maybe a couple wraps of bubble foil insulation.

    I've noticed a trend towards 3-4" of insulation in Euro tanks, and some different materials being tried, like a nylon fleece insulation instead of foam or fiberglass.

    Another thing common on Euro tanks is the way the hot and cold nipples go into the tank. Generally on the side and with a drop leg to prevent convection currents. That seems like a simple and workable feature. They also use "convection brake" nipples like shown in this picture.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    I do not really know.

    I was just quoting Weil-McLain. Besides that, the unit was manufactured in Belgium, so it may be one of the Euro tanks to which you refer. The domestic water pipes, both supply and delivery rise about a foot from the top of the water heater, and then drop about 5 feet to pick up the cold water and deliver the hot. I have put 1/2 inch black rubber foam insulation around these pipes except where the ball valves and the ground strap are. I run it at about 120F which lowers the temperature loss some compared with those who run it at higher temperatures. W-M claim 2" of CFC-free insulation.
  • KevinCorr
    KevinCorr Member Posts: 106

    It seems that it would be more efficient to insulate the indirect than to have another fire with a separate heater, which means another fuel supply,, vent, maintenance. I don't plan to try to prove it.
This discussion has been closed.