radiant heating...

 Has anyone attempted to use a domestic tankless water heater as a source
for radiant heating?  I'm searching for ideas to back up my solar for
days of limited or no sun shine....

Thanks
John

  On Fri, 05 Sep 2008 20:45:33 -0500, john


My Paloma's manual warns in very strong language not to attempt that.  Knowing
how it works, I fairly sure the concern is with hot return water overheating
the gas regulator that varies the flame according to water flow.

Other brands that use electronic controls may be more suitable but then you
have the electronics to power and hope not to fail.

Why not just use a small hydronic boiler?  The cost shouldn't be that much
different.  You'd have a product designed for the application.  My Paloma, the
largest one they make, cost around $1200.  It's been a long time since I
bought one but I don't recall a hydronic boiler of similar capacity costing
much more.

You might also take a look at the current crop of wall-hanging
ceramic-surface-burner-equipped ventless propane heaters.  Most of the heat
output is in the form of infrared radiation which means that a fairly small
heater can keep you warm even in a fairly large room.

My primary heat source is wood but I have one of those heaters as backup.  I'm
having a senior moment, but I believe that it's rated at 18kBTU.  I can't get
far enough away from it to be comfortable when it's operating on high so it
usually operates on the middle setting.  It'll run for a couple of weeks on a
100 lb tank of propane.  This type of heater is cheap enough that you could
buy one for each room in a big house for the cost of a tankless heater or a
boiler.

John

--
John De Armond
See my website for my current email address
http://www.neon-john.com
http://www.johndearmond.com  <-- best little blog on the net!
Tellico Plains, Occupied TN
In theory, there is no difference between theory and practice.  In practice,
there is.

 Even though the plants would probably like the extra CO2, I want to keep
the combustion out of the greenhouse and in a more controlled area.  I
am not comfortable with any surfaces or heat source in the greenhouse
hot enough to start combustion; that's part of the appeal of the
hydronic system.

Valid concern with the demand water heater now that you mentioned it.  I
have seen commercial boilers that require a minimum return temp, but you
say that these have a max allowable entering temperature.  Considering
what they are designed for that makes sense.   I will investigate the
boiler idea further.

Thank you for your input, that may have $aved me a co$tly mi$take.

John




Neon John wrote:

there is.

 On Sat, 06 Sep 2008 13:27:57 -0500, john


The reason I chose the Paloma brand is that they were (are?) the only brand
that has true proportional flame control (constant water temperature
regardless of flow) without using electricity.  There is a fancy valve in the
unit that contains a diaphragm.  incoming water flows through an orifice,
creating a little differential pressure.  This acts on the diaphragm.
Connected to the diaphragm is a rod that operates the gas valve.  It's a plug
valve with the plug contoured to provide proportional gas flow.  The hot
outgoing water also passes through a separate part of the valve where a
bi-metal strip resides.  This strip acts to trim the gas flow to keep the
water temperature at the setpoint regardless of flow.

The concern is with hot water coming in contact with the rubber diaphragm.

There are two other types of tankless heaters.  On-off and electronic
proportional.  The on-off type has some sort of flow detector that turns the
gas full on.  The water temperature varies with flow.  The third type performs
like the Paloma except that it has electronics and requires wall power.

The second two types MIGHT work.  The on-off one most likey would.  You'd need
some external control, though, to control the water temperature.  No solid
evidence either way with the electronic ones.

The biggest problem I see with the tankless heaters is the total lack of
thermal mass.  That leaves you no room for error.  My Paloma has a 250,000 BTU
burner that impinges on a finned heat exchanger that looks like an automotive
heater core.  Just a few seconds of firing without water would melt the thing.
Just a few more seconds of firing without flow would boil the exchanger dry
and then melt it.  It's copper with soft solder connections.

You're most welcome.

John
--
John De Armond
See my website for my current email address
http://www.neon-john.com
http://www.johndearmond.com  <-- best little blog on the net!
Tellico Plains, Occupied TN
No one can be right all of the time, but it helps to be right most of the time.
-Robert Half

 
| The biggest problem I see with the tankless heaters is the total lack of
| thermal mass.  That leaves you no room for error.  My Paloma has a 250,000 BTU
| burner that impinges on a finned heat exchanger that looks like an automotive
| heater core.  Just a few seconds of firing without water would melt the thing.
| Just a few more seconds of firing without flow would boil the exchanger dry
| and then melt it.  It's copper with soft solder connections.

The issue I have with tankless heaters is that they depend on electricity
or gas (depending on your source) being available the instant you need it.
The electric ones are worse because of the possibility of hour-to-hour rate
changes.  I also want to minimize gas lines in my house (maybe the kitchen
will get one).  So I prefer going with small tanks at each bathroom and a
large central tank or two feeding the bathrooms.  I do want to work out ways
of having them fed by solar as the primary heat and electric as backup.  That
is something I will need to dive into someday.

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