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Domestic hot water coil, tankless

LerchLerch Member Posts: 20
I have a Burnham V7 series oil fired boiler model PV74WBT with the tankless coil. During the summer months I typically turn down the temperature of the water in the boiler to conserve energy to what extent it might help. I typically lower the temperature from 160/180 to 120/140. This time around I've noticed that the hot water temperature is not consistent, and often too cold. I investigated the mixing valve which we've had problems with in the past (taco model 5000), but it is in good working order. I then spent some time measuring the temperature of the water coming out of the tankless coil using an infrared thermometer measuring the surface temperature of the copper pipe. The temperature is about 90-95 degrees. The surface of the tankless coil measures about 125 degrees. The temperature gauge on the boiler measures about 155 degrees, and as I mentioned before the temperature is set to 120/140 on the tiny dials on the Honeywell control. My question is this: What is a typical temperature drop through a tankless coil and/or how can I evaluate whether my tankless coil is doing it's job properly? I'd like to see hot water temperature of about 120 coming out of the tankless coil, but it looks like I'm losing more than that along the way. Any help would be appreciated. Thank you.


  • Jean-David BeyerJean-David Beyer Member Posts: 2,663
    I have part of the answer that you need.

    I do not know the temperature rise you can expect through a hot water coil. But as far as the mixing valve. To work, most of these require that the supply temperature be at least 15F hotter than the delivery temperature. So if you want 120F out, you must supply at least 135F in. The 15F number varies a little between manufacturers.

    I never found out what the cold supply needs to be. I suppose it could be 15F lower than the delivery temperature, but that is just a guess.

    I have a similar valve for my darkroom with a big (about 3" diameter) knob on it so I can easily adjust the delivery temperature. I usually set it to 75F and in the summer I cannot always get the delivered temperature that low. I am not sure what the cold water temperature is in the summer, but when it is 90F outside, I guess the cold water could be pretty warm.

    My valve is rated at a flow of 1/2 gallon per minute up to 2 1/2 gallon per minute. Below 1/2 gpm, it can on longer control the temperature between +|- 1/2F.
  • meplumbermeplumber Member Posts: 678

    It is probably time to put a rebuild kit in your mixing valve.

    My initial thought would be that the coil needs to be flushed.  Over time the coil becomes caked with sediment and this acts as insulation, not allowing the water to pickup the heat as it travels through the coil.

    Have a pro come out and take a look at it.  If he concurs, he will do an acid flush of the coil and you will be back up and running.

    Good Luck.
  • LerchLerch Member Posts: 20
    15 degrees sounds about right

    That's a good point. I'm seeing the temp after mixing about 80 degrees, when incoming temp is 95. The temp gauge on boiler runs from 138 to 153. I might just turn up the temp of the boiler water for now. I don't need 120 out. 100 would do.
  • LerchLerch Member Posts: 20
    Mixing valve is brand new

    I had some corrosion on the sliding surface of the mixing valve and so I replaced it with a new one, which is a slightly different design, perhaps less likely to corrode (piston is plastic).

    I too had thought that the coil may be plugged up. That's why I asked the question. I'd like to know how to test the coil before calling a pro, unless of course the testing procedure is out of reach for the DYI'er.
  • Jean-David BeyerJean-David Beyer Member Posts: 2,663
    I rebuilt my mixing valve once.

    Most of that valve is bronze. The piston is stainless steel. The control knob is plastic, but that is outside the valve. The O-ring seals look like neoprene. What I would call the main valve seat is Teflon. That seat seems to wear a little in about 10 years or so. Deposits seem to accumulate in spots. Some can be rubbed off. Some come off in a strong acetic acid solution. Others require mechanical force, but I fear that force could damage the surfaces, so I just replace those parts. Once every 10 years is not much of a burden. If I had to pay my contractors rates for labor, I would just replace the valve, I think.
  • icesailoricesailor Member Posts: 7,265
    Tankless Coils:

    If pennies were peanuts, you are saving peanuts by turning the water down on the boiler. You have turned the boiler operating temperature down so low that there isn't sufficient heat energy in the water to transfer heat to the coil. If the coil were really dirty, you could turn the operating control to 180 degrees and still not get a hot shower. It might get hot at first but it would go cold.

    Unless properly prepared by the original installer with places to hook hoses and isolate the coil, I could easily blow a morning, cleaning your coil and still not have it work as well as you say it did. Though I have my doubts that it did.

    You have the luxury of living with your mistakes. The only one who you may answer to is your significant other. If I did what you are doing, turning down the water and not having enough hot water, my life would be miserable. If I was paid to do this by a customer and I had the results you are having, I would be going back on a free service call. IMO, you have already spent money on things that may not have been broken.

    I only have storage tanks in my houses. Like the one I have today.

    My ears are burning when I contemplate my wife in a cool shower with shampoo in her hair and she can't get it out. My former teenagers were the same way.

    I have the limit stop on my Symmons S-96-1/2 shower valves with the limit stop sets so you just turn the handle to the stop, back it off slightly, and get in.

    But to each his own.
  • LerchLerch Member Posts: 20
    Taco Model 5000 has changed

    The old Taco 5000 mixing valve that I had included a metal piston, I think.  It's been a while since I took it apart, so may not be remembering correctly.  I'll take it apart tonight to have another look.  However, the new one definitely has a plastic piston, and is definitely different than the old one, including a convenient means of user adjustment - the green plastic cap just pulls off and has a triangular sort of wrench built into the cap.  The corrosion in the cylinder wall of the old one was quite bad, to the point where I think it interfered with the motion of the piston.
  • LerchLerch Member Posts: 20
    Sure, peanuts.

    Ice,  I hear what you're saying.  I'm an engineer by training, and tend to experimentation.  My wife likes to save energy.  She lived in Guatemala for a time, and they didn't have hot water there, and in many places no electricity either.  There are other things that I might do that would make my life miserable, but experimenting with saving energy at the boiler is generally not one of them, especially when its 95 degrees outside.  You're right, I have the luxury of living with my mistakes.

    I'm a little confused as to what you are referring to in the following sentence: "I could easily blow a morning, cleaning your coil and still not have it work as well as you say it did"  Did I say the coil was working well?  I don't know whether it is working well, and that is the reason for my post.

    At the moment, I have turned up the temperature at the Honeywell control, and the temperature out of the mixing valve has improved.  Maybe that's enough.  But, I would still like to know if there is a way to assess the condition of the tankless coil.
  • Steve_152Steve_152 Member Posts: 26
    edited August 2011
    tankless / check / fix

    Lerch,    i  also    have engineering  background  /  2nd. class    stationary  lic.  &  me.  master oil   tech. lic.  /     i think  ice  was   trying to nicely  tell you that  acid washing  inside of your coil  was not  a cost effective  solution /   it usually does not work well /  you need to do  prep. work that takes up time=$$$  /    i have  done   many &   not   often  worked   out well../    put in  new    flow restrictor   in     incoming  cold water supply  before   coil   rated  1/2   gal.  less than    coil   gpm    rating   &   new    honeywell  /  sparco     mixing  valve    {   this  valve is also  rated  as  anti-scald  }     //    turn up  aquastat...  /    if still n/g  ...   replace  coil  or  pipe  in   in-direct    tank...   ???    are you  on city water   or well ??  if well   you may have  more  minerals  in water  causing    build-up  both  inside & outside of coil  that would  tell me replace coil,   or take it to radiator shop  & pay them to  put in  cleaner tank  for  approx.  4 hours.  & hope for best        my  $.02    opinion...     Steve.      btw.....unless you    paint    copper pipe   black  you will not  get  accurate   temp.  with    remote    temp.    "heat-gun"
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