Hybrid Car – More Fun with Less Gas

Solar + Tankless Gas for DHW: A good combination? - Page 2

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Posted by Steve Shantz on May 4, 2006, 2:01 am

I would agree with the advice you are getting, and I'll try to give
some good reasons why tankless is superior...

It is my humble opinion that using a gas water heater with a constant
gas pilot is a very bad idea, especially if you can get all or most of
your heat from solar.  At current gas prices, the pilot costs you about
35 cents per day... whether you need it or not.  (I actually went out
and spent 1/2 hour counting the revs on the gas meter!)  This cost is
especially bad in the spring, summer and fall where you will likely get
most or all of your heat from solar, and the pilot is mostly

The pilot keeps the tank warm, and actually heats up the water coming
from the solar pre-heater, if it is heated by the sun most of the way.
In this manner, the heat of the pilot is not wasted.  However, if you
have already heated the water all the way with the pre-heater, the gas
water heater pilot is a suprisingly large source of waste.  This is
something nobody seems to talk about.

The alternatives are to use an instant heater, but as mentioned in
previous posts, it must be a modulating heater.  Good ones are not
inexpensive.  A tempering valve is a must.

Several companies also make high efficiency gas water heating furnaces
with controls to heat hot water in a small tank.  I personally think
that this is the best idea.  On demand from a thermostat in the hot
water tank, the hot water from the furnace is sent through a heat
exchange coil inside the tank.  You get a finishing tank that is always
hot enough, and you only have one gas appliance, and a very efficient
one at that.  The main problem here is that if you are already set up
for forced air, this water heating furnace isn't going to work unless
you substantially re-do (or worse) your entire heating system.

I already have a hot water furnace, and when I get some $$$$ I'm going
to switch out my old inefficient furnace with a high efficiency furnace
/ water heater.

I would advise against a thermosiphon.  Lots of good research has shown
that they are very inefficient, by design, and they have a very small
surface area, which tends to make your glycol run much hotter that
otherwise would be required.


Posted by News on May 10, 2006, 11:28 pm

Is this a priority system?  A small hot water tank will not do a shower or
bath, but will act as a buffer and prevent burner cycling.  If there is DHW
demand does the furnace divert all the hot water into the tank as it being
depleted of hot water and switch off the space heating aspect?

Posted by Steve Shantz on May 11, 2006, 2:50 am
 You've got it right.  The system diverts the full energy output of the
furnace to heating the water.  The idea is that while water is being
heated, the thermal mass of the house will keep things comfortable
until the water demand is satisfied. More info at
http://www.nythermal.com/Products/boilers/boilers.htm    I'm looking at
the Trinity model.

The heat exchanger tanks I've seen start at 20 gallons, up to as large
a size as you want.  My thoughts are that if one has 80,000 BTU's /hr
of heating power, 20 gallons should stay hot regardless how many hot
water faucets are on.  3 GPM * 8 lb/Gallon * 60 min / hour  * 60 degree
F rise = 86400 BTU/hr to continuously heat cold water at 3 gallons/min.
 This is worst case, when there is no preheating from the solar
pre-heating tank.  Even in northern Indiana, I only had two weeks of
zero pre-heating last year (my first season).  Most of the winter, I
could pre-heat between 25 - 50% of my DHW requirements.

I think the idea has merits.  For larger houses, the furnace and tank
can be increased in size as required.


Posted by SJC on May 11, 2006, 4:12 am

  Seisco makes electric tankless that just mount on a wall. Connect 220V =
and they
can take a flow of water up 30F on pass through. Might work well with a =


Posted by News on May 11, 2006, 1:16 pm

It is a boiler, heating water rather than a furnace heating air.  I see they
have a combi version plate heat exchanger - on demand and tankless DHW.
These are the norm in Europe but with all inside the case.  This version
requires an external pump and blending valve. The European versions modulate
the burner to achieve a setpoint temperature output eliminating the blending

You could use one of these boilers to heat a forced air unit that has a
copper coil water to air heat exchanger and DHW using a small tank.  A
diverter valve will pump all the boilers energy into the tank. An anti-cycle
tank thermostat can be fitted to prevent cycling of the boiler when taps of
turned on momentarily as people do with the kitchen sink tap.

The combi version can be used have input from a solar heated DHW tank -
check with makers first.  In doing this I would always have a modulating
burner rather than a blending valve for the DHW.

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