Conference 7.286::home_work

The questions here and in the next note, 4124, are different, though clearly
related.  The base note here is talking about the temperature setting on 
a tankless hot water system.  I can understand how the system might get up to
175, since the temperature in the heating pipes is likely to be even higher (our
cutoff is around 190 or 200).  But that's much too high for domestic hot water.
Perhaps someone with a tankless system can explain how temperture of the
domestic supply is controlled.  (An obvious guess is a mixing valve.)

As for the questions in 4124, traditional hot water settings are 120 and 140.
140 is recommended for automatic dishwashers that don't have their own 
temperature booster; many newer ones do.  120 is recommended for homes that
don't have automatic dishwashers, or for ones where the dishwasher has a
booster.  In either case, a temperature limiting valve in the shower can
probably be set to prevent burning.  I don't know about all the other water
faucets, but I wouldn't be surprised if it were possible to get limiting valves
for them.

Personally, I use 140, and would continue to use 140 even with a more modern
dishwasher having a booster.  There have been some claims that lower water
temperatures in hot water heaters are associated with Legionnaire's disease,
which is why I prefer the hotter temperature.

   Gary

    If you have a "tankless" system like mine, there *is* a mixing valve
    (as .1 suggested).  Look for the pipe that brings water to the boiler
    from your well or the street.  Just before this cold water pipe reaches
    the boiler, a T-fitting connects this cold water pipe to the
    hot-water-pipe-coming-from-the-boiler-to-your-hot-water-faucets.  And
    in this short connection there is a "mixing valve" which lets a
    user-determined amount of cold water to mix with the 190 degree (or
    whatever) water coming from the boiler.  Opening this valve more fully
    lets more cold water into your hot water.  Closing it keeps your hot
    water hotter.  Fiddle with this valve until your hot water is as hot as
    you wish.

    We have a tankless system also.  There are actually two temperature
    controls for the system, plus a couple of other places where you
    can control the hot water temperature.
    
    There's a boiler temperature control, usually out of sight someplace,
    that controls the temperature of water circulated through your
    hot water radiators.  It's usually set high, at around 190 or 200;
    I set ours back to 160, for improved heating efficiency.
    
    Then there's an aquastat that applies mainly in summer when the
    thermostat never calls for heat.  The aquastat determines what
    the temperature of the water in the "tankless tank" needs to
    be in order to prevent the furnace from coming on to heat the
    water.  Ours is set at 140.  This must be somewhat lower than
    the boiler temperature control or else the system does not work
    at all.
    
    Then there's the mixing valve to temper the temperature of
    the water coming out of the "tankless tank".  You want this
    set somewhat lower than that temperature of the aquastat.
    Otherwise hot water comes undiluted directly from the
    tank, and the rate of flow may exceed the heating capacity
    of the boiler. You'll also want it it a bit higher than
    the temperature you want delivered to your faucets to accommodate
    heat losses along the pipe.
    
    Finally, modern faucets have some kind of temperature control
    on them as well to control maximum temperature for safety
    reasons.
    
    Incidentally, it's possible to put a switch on the aquastat
    to control when the furnace is allowed to come on just to
    satisfy hot water demands, while leaving the normal heating
    function unimpaired.
    
    Rich

    re: .-1
   > Finally, modern faucets have some kind of temperature control
   > on them as well to control maximum temperature for safety
   > reasons.
    
    The only thing that I've seen like this are anti-scald valves
    on showers.  The way they work is if the cold water supply is cut off
    somehow, they also cut off the hot water.
    
    I haven't seen this kind of thing on regular ol' faucets (except
    special ones made for darkrooms).
    
    Rich
    
    

re: .3

I thought that it was more efficient to have your FHW temperature setting set
high, because oil burners functioned more efficiently when allowed to run
longer.  Anyone have more information on this tangential issue?

   Gary

    Regarding .5,
    there are two efficiencies to be concerned about.
    One is the efficiency of converting heat from your
    burner flame into heat in your boiler water.  The
    second is the efficiency of transferring heat from
    your boiler water (via radiators) into the air
    in your room.
    
    All other things being equal, efficiency of heat
    transfer improves as the temperature differences
    increase.  So the first efficiency (heating water)
    improves as you lower the FHW temperature, because
    the difference in temperature increases.  However,
    that decreases the other efficiency, because now the
    radiator temperature is closer to room temperature.
    
    So you have a choice: send more heat up the chimney,
    or have it take longer to warm up the house and
    run the circulator longer.
    
    If you elect to warm up the house faster (sacrifice
    burner efficiency), then the burner will run longer
    as it struggles to keep the water near the boiling
    point.  If you elect burner efficiency the burner
    will come on and off because the boiler water will not
    cool down so much as it circulates through the heating
    system.
    
    Personally, I elect burner efficiency.

    I also have a tankless hot water FHW system.  I didn't really notice
    any kind of "backwash" valve at that funny temp mixing valve where the
    cold is let into the hot to lower the hot tap water.
    How does the higher pressure very hot tap water not go into the cold tap
    water through that mixing valve and hence raise the cold tap temperature?
    (Unless of course there _is_ a "anti syphon type of valve that is built
    into the mixing valve.  Sure doesn't look that way though.)
    
    Steve...................................................................

    I need to replace my water heater tank.
    
    I currently have a 50 gallon, electric hot water tank.
    
    
    To replace with a 40 gallon, electric tank will
    cost me about $500.
    
    To convert to a 40 gallon, oil-burner heated tank
    will cost about $1200.  (I currently heat by oil, 
    forced-hot air).
    
    
    Does anybody have any charts, formulas, etc.  for comparing
    the 2 water heating methods?
    
    
    Thanks,
    Bob F.

    Lots of notes on the subject. Did you check?
    
    Marc H.