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Re: Radiant Floor Heat Water Heater ? - Page 5

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Posted by daestrom on December 12, 2003, 12:38 am

The question came up when 'News' was describing a 'condensing gas boiler'.
He was trying to explain that it is really just a water heater used for
heating the water in a hydronic heating loop.  The term 'boiler' is just a
hold over from old days of steam-heat.

Anyway, these units are more efficient than older style units, in part,
because they use a slow circulating system for the hydronic heating tubing.
With a slower flow rate, the return water from the loop is cooler.  This
cooler water (in the range of around 54C in 'News's example) will extract
more energy out of the flue gases than if the returning water was still
quite hot (say, 80-90C).

The reason it can extract a lot more energy is because the cooler the water
is, the more water-vapour can be condensed out of the flue gasses.  Now,
someone asked why the water-vapour didn't condense out of the flue gasses at
100C, after all, water boils at 100C (near sea-level), so it should condense
at 100C as well.  Right???

If you have a bottle/tank/container filled with steam at atmospheric
pressure, it *will* condense at 100C, just like we all learned in school.
BUT, if it is a mixture of steam and other gasses, things get more
complicated.  If the container is filled with 1/2 steam and 1/2 air (by
volume), then the partial pressure of air is 7.35 psi, and the partial
pressure of steam is only 7.35.  When steam is at a partial pressure of 7.35
psi, it won't condense at 100C.  You must cool the mixture until the
temperature is such that the saturation pressure is equal to (or below) the
*actual* partial pressure of the steam.

Like moisture and humidity in the air.  On a typical summer day, the air is
warm and has some moisture in it.  When night comes and the air cools, the
saturation pressure drops and some of the moisture condenses out on things
as dew.  The 'dew point temperature' is the temperature you have to cool the
air down to, in order to get any moisture to condense out.  The following
morning, as the air warms up, it is able to hold more moisture, so things
dry off again.  Never get near 100C, but moisture condenses/evaporates with
the mild temperature change from night to day.

The flue gasses of a NG fired furnace or water heater ('boiler') are a
mixture of air from the intake, CO2, and steam.  *IF* the furnace/'boiler'
can cool them down far enough to get some of the steam to condense, then the
latent-heat-of-vaporization the steam has, will be given over to the
furnace.  This is a significant fraction of the total energy liberated in
burning the NG.  High-efficiency force-air gas furnaces get this because the
return air of the home is cool enough to cool the flue gasses down to the
point where a fair amount of the steam/moisture condenses.  But with a
water-heater, the return water temperature *could* be too high to condense
very much of the flue gas steam.

So, 'News' was trying to point out that the return water has to be
relatively cool in order to get the 'efficiency boost' of condensing flue
gas steam.

Now, does *anybody* have any more questions?????  ;-)


Posted by offgridman@cs.com on December 12, 2003, 6:03 am
Thanks for the detailed explaination.


daestrom wrote:

Posted by News on December 13, 2003, 12:38 pm

I wasn't trying, I did a simple overview quite well, having just re-read it.


The flow rates have little to do with it.  It is the return temperature that
matters.  Some boilers have electronic control systems that modulate the
internal pump to achieve the lowest return temperature possible.  In fact at
certain times having the pump speed very high will reduce the return

The maximum speed of a domestic system is determined by:

1)  The noise emitted by the system
2)  Errosion of the innner wals of the pipes due to higher water speeds

Problems can arise due to excessive water speeds, which in extreme
conditions can cause premature tube failure by one of several mechanisms
including erosion/corrosion and/or cavitation. The recommended maximum water
velocity with good design practice and installation is 2ms-1 irrespective of
tube outside diameter.

So, this may act against a control system attempting to gain maximum

The cooler the return water more efficient.  Condensing starts at around


That is why electronics are used to get the return water as low as possible,
and/or engineering the heating system to give the lowest return temp,mp as
possible - usually but having larger emitters.  BTW, I have a condensing
boiler and oversized the radiators to assist in lowering the return temp.  I
also installed an outside weather compensator which lowers or raises the
return temperature of the return water to outside conditions.  Most of the
time it is below 54C.

I think I did that easy enough, without going into minute detail.

Who killed JFK? :)


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Posted by daestrom on December 13, 2003, 3:15 pm


I'm sure those that you left with a few questions already have.  And they
still had questions.

You explain that the electronic control systems modulate the pump speed (and
thus the flow rate) in order to control the return temperature.  Yet you
start out with "The flow rates have little to do with it."  Don't you think
that might be a bit confusing to people?


Well, if you *had* explained it well enough, then people wouldn't be asking
why the water vapor doesn't condense at 100C instead of 54C, would they?.
Apparently not everyone understood why it is necessary to lower the return
temperature so low to get some of the water vapor to condense.

I think Nick was expressing the question of why doesn't it condense at 100C.
(I suspect Nick already knows the reasons, but was trying to get you to
explain it better for the benefit of others, he *did* add a smiley)  He put
it a bit sarcasticly by asking, "Water boils at 54 C in the UK? ;-)"
Similarly, 'offgridman'  wondered why we were talking at 54 instead of 100.
If you look at the responses to your explanation, you would see that you
left some people confused with some questions (which you didn't bother to
answer).  Or did you just assume everyone understood you, and they were just
poking fun??

Although *you* think you did a good job, you didn't look at your audience's
reaction to get any feedback.  If you had, you might have found that you're
explanation wasn't as well understood as you thought.


Posted by News on December 13, 2003, 4:24 pm

Probably.  The point is that flow rates do not have to be slow.  It is the
temperature that matters, irrespective of flow.


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