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dcaster@krl.org posted on December 25, 2011, 4:29 pm
My LPG furnace developed a heat exchanger leak and so it has been
replaced with a high efficiency two stage furnace. The old furnace
was also a high efficiency furnace but just a one stage 100k BTU/
hour. The replacement is a 90k btu/hr / 63k btu/hr. Did not do a
proper analysis of the size furnace needed, but based on how the new
furnace seems to be working, the low stage is good to below 20 degrees
F. right now. And I am going to put in more insulation so things
will change a little. The new furnace has jumper settings for using
a one stage thermostat with 5 or 12 minutes wait before kicking in to
high, or for using a two stage thermostat.
Okay here is the question...........it seems to be that the furnace
ought to run at the low stage unless the outside temperature is below
about 20 degrees F. Then switch to high stage as long as the outside
temperature is below 20 degrees. There is no mention of doing this in
the furnace manual. So is this a bad idea for some reason.
It seems like a simple thermal switch located outside would be better
than spending a bundle on a two stage thermostat.
Dan
replaced with a high efficiency two stage furnace. The old furnace
was also a high efficiency furnace but just a one stage 100k BTU/
hour. The replacement is a 90k btu/hr / 63k btu/hr. Did not do a
proper analysis of the size furnace needed, but based on how the new
furnace seems to be working, the low stage is good to below 20 degrees
F. right now. And I am going to put in more insulation so things
will change a little. The new furnace has jumper settings for using
a one stage thermostat with 5 or 12 minutes wait before kicking in to
high, or for using a two stage thermostat.
Okay here is the question...........it seems to be that the furnace
ought to run at the low stage unless the outside temperature is below
about 20 degrees F. Then switch to high stage as long as the outside
temperature is below 20 degrees. There is no mention of doing this in
the furnace manual. So is this a bad idea for some reason.
It seems like a simple thermal switch located outside would be better
than spending a bundle on a two stage thermostat.
Dan
Steve posted on
Re: Two stage furnaces and thermostats
Keep it simple.... use a 2 stage thermostat, and don't bother with the
jumper settings. that way it will only give you what heat that the
thermostat calls for.
IlBeBauck@gmail.com posted on
Re: Two stage furnaces and thermostats
Dan, Generally speaking...it is better to operate from space demand
which is directly linked to heat loss from the structure ...so, if
you get full sunny days during the winter you can take advantage of
the solar heat coming thru windows as well as heat produced within
the house from food cooking, showers , etc... which *may keep you on
first stage furnace operation longer thereby maximizing economy .
This is the advantage of using a 2 stage space thermostat , whereas
if you go the outdoor thermostat route you are basing operation on
the outdoor temperature without considering possible heat generation
within the house and from the sun . So, my vote is for you to go
with true 2-stage heat operation via a 2 stage electronic
programmable thermostat and hook it up to the furnace control board
in like manner .
Re: Two stage furnaces and thermostats
You did bring up a good point. If it is sunny the house will stay
warm without running the furnace when the temperature is forty
degrees. If I do go the outside temperature switch I will put it in a
box that has some solar gain built into it. My big problem with two
stage thermostats is they seem to be very expensive. I may not be
looking in the right places, but most of the two stage thermostats at
Grainger are 200 + dollars. ( they do have some that are a little over
50 dollars. )
Dan
Re: Two stage furnaces and thermostats
You did bring up a good point. If it is sunny the house will stay
warm without running the furnace when the temperature is forty
degrees. If I do go the outside temperature switch I will put it in a
box that has some solar gain built into it. My big problem with two
stage thermostats is they seem to be very expensive. I may not be
looking in the right places, but most of the two stage thermostats at
Grainger are 200 + dollars. ( they do have some that are a little over
50 dollars. )
Dan
Honeywell FocusPro 5000 TH5220D1003
Re: Two stage furnaces and thermostats
Youre going to spend close to that regardless of what 2 stage
programmable stat you go with. . If you want to go the cheap route,
then your outdoor stat idea would be better than nothing.
Re: Two stage furnaces and thermostats
Youre going to spend close to that regardless of what 2 stage
programmable stat you go with. . If you want to go the cheap route,
then your outdoor stat idea would be better than nothing.
------------------------------------------------
The Honeywell 2 stage stat I posted( Honeywell FocusPro 5000 TH5220D1003) is
around $50 and will do the job. It will be less expensive and less of a PITA
than running new wires and buying another stat and OD temp sensor that will
support balance point operation.
Re: Two stage furnaces and thermostats
Well, if he can get a 2 stage stat for $50 , then absolutely that is
the way to go. Ive never seen one that cheap.
Re: Two stage furnaces and thermostats
Ah, the Honeywell FocusPro is not programmable. I am amazed at how
much two stage thermostats cost. I would think that there would be
lots of models using a relatively cheap micro-controler.
Dan
Re: Two stage furnaces and thermostats
Ah, the Honeywell FocusPro is not programmable. I am amazed at how
much two stage thermostats cost. I would think that there would be
lots of models using a relatively cheap micro-controler.
Dan
------------------------------------
Do you want a cheap stat?? or a good one?? In the last couple of months I
have replaced several Lux and Hunter stats that failed because they are
junk. Out of all of the Honeywell stats I have installed, I have only had to
warranty a half dozen. The H'well stats have a 5 year warranty on them too.
Re: Two stage furnaces and thermostats
Well, if he can get a 2 stage stat for $50 , then absolutely that is
the way to go. Ive never seen one that cheap.
--------------------------------------------------
There is a reason I posted that particular model.... Its a good quality
stat, and my customers that have them have been very pleased. You can do a
simple search on yahoo and find a bunch of places that have them for $50 or
less.
It also has an auto-changeover feature.... will change from heat to cool or
vise versa on its own... nice to have in the fall and spring when your
running the a/c during the day and heat at night.
Re: Two stage furnaces and thermostats
I personally think an auto changeover feature on a thermostat for
residential is not a good idea ; it could turn on the a/c in cold
weather and possibily destroy the compressor if there exists
interally generated heat from cooking, many people in the house at one
time, or from a lamp too near the thermostat. Id only go with auto
changeover if there was a low ambient cutout switch on the a/c unit.
Re: Two stage furnaces and thermostats
I personally think an auto changeover feature on a thermostat for
residential is not a good idea ; it could turn on the a/c in cold
weather and possibily destroy the compressor if there exists
interally generated heat from cooking, many people in the house at one
time, or from a lamp too near the thermostat. Id only go with auto
changeover if there was a low ambient cutout switch on the a/c unit.
---------------------------------------------------------------\
Think 5 degree dead band....
I have been putting in both the FocusPro and the VisionPro for many years...
I have never had a problem with compressors. But then the refrigerant charge
is properly balanced by subcooling and superheat, correctly sized ductwork,
etc.
The only way I can think of that you would have a problem with the a/c
trying to come on in the winter is if the stat is in a closed kitchen when
your doing holiday cooking.... not a good idea
Re: Two stage furnaces and thermostats
A 5 f. dead band is not adequate protection ; if the heating
setpoint is low like at 65 f. the sun hitting the thermostat
directly or a wood burning fireplace , prolonged oven cooking,
etc...could easily push the temperature to 71 f and higher which would
trigger the mechanical cooling at an outdoor temperature cold enough
to cause equipment harm . Shutting off the a/c breaker is always a
good idea in the winter , regardless .
Re: Two stage furnaces and thermostats
A 5 f. dead band is not adequate protection ; if the heating
setpoint is low like at 65 f. the sun hitting the thermostat
directly or a wood burning fireplace , prolonged oven cooking,
etc...could easily push the temperature to 71 f and higher which would
trigger the mechanical cooling at an outdoor temperature cold enough
to cause equipment harm . Shutting off the a/c breaker is always a
good idea in the winter , regardless .
---------------------------------------------------
The 5 degree deadband is a *MINIMUM*... it will not get any closer than 5
degrees. Normal settings are 76F for cooling and 70 for heat. If you drop
the cooling below 75F, the 70F set point will drop accordingly. likewise, if
the heat set point is raised above 71F, the cooling setpoint will rise
according also. When I program a control, the customer will *NOT* be able to
get the set point temps any closer than 5 degrees. I also set the
temperature stops in the control so it will not go below 70F in cooling, nor
above 75F in heating modes.
Its only common sense......and actually reading the directions.
MLD posted on
Re: Two stage furnaces and thermostats
Bit confused on just how your furnace works---didn't know that any of them
new anything about the outside temperature.
The way that my 2 stage works is pretty simple---When there is a call for
heat it runs on the 1st stage (I think it's about 70% of the furnace's
capacity) and low fan speed for the first 10 minutes. If the thermostat
still calls for heat after the 10 min. then the 2nd stage kicks in and the
fan speed ramps up to 100%. On very cold days the house maintains a fairly
even temperature because the furnace kicks on very frequently and most of
the time cycles only on the 1st stage.
MLD
Re: Two stage furnaces and thermostats
In residential, its only Heat Pump resistance heaters that are
sometimes enabled/disabled off of outdoor temp. --- high efficiency
gas fired furnaces are not...BUT, it is possible to modify such a
system to accomplish it. The OP is asking if it would be beneficial
for him to do it off of outdoor temp. since he accurately knows that
first stage is good down to about 20 f. for his application ; he
could very easily do it this way...but its not going to be the most
economical . On your furnace....you are using a single stage
Thermostat in conjuction with the 10 min. Time Delay feature of the
furnace to bring in 2nd stage . In my opinion, 10 min. isnt long
enough but you cant do much about that --- youd see better economy
if you had a true 2 stage Thermostat operating the furnace off of
true space demand and eliminating any time function . Of...if you
were proficient , you could at least increase the 10 minutes to 20
minutes by wiring in an independent 20 minute Time Delay Relay for 2nd
stage function, and keep your Single stage thermostat.
danny burstein posted on
Re: Two stage furnaces and thermostats
our high efficiency natural gas unit ("Lochinvar") has some
sort of internal logic programming that modulates the flame
performance. There's "zero", of course, and then 15 or so percent
and up.
- I don't quite know the algorithm but it measures the outdoor
temperature, and also the delta between the outgoing hot water
and the incoming cooler pipes.
- when first installed the outdoor sensor was wired wrong, so
the unit only fired up to the lowest setting... So it was
getting a bit chilly by the by. He came by and disconnected
it until he could get the factory rep on the phone...
--
_____________________________________________________
Knowledge may be power, but communications is the key
dannyb@panix.com
[to foil spammers, my address has been double rot-13 encoded]
Re: Two stage furnaces and thermostats
On a Boiler, it is called 'Hot Water Reset' (from outdoor temp) and
it shifts the Hot Water output temp to a higher value as the outside
temp decreases . For clarification, I was referring to FORCED AIR
Heating Systems when i said in residential applications that its just
Heat Pump electric resistance heaters that are sometimes enabled at a
certain outside temp. value . A hot water systems such as yours is
THE best for comfort as the outside walls of the structure are
getting blanketed with virtually constant heat of varying degree ...
but high efficiency gas Forced Air Heating does a more efficient job
plus a/c, humidification, and air cleaning apparatus can be easily
added to it making it a complete environmental system . Regards.
Re: Two stage furnaces and thermostats
I love our hot water system, but with two complaints that
are intrinsic to the design.
a: as you mentioned, it does great for heating but... doesn't
do diddly squat for cooling.
b: It also only has the ability to raise the indoor temperature
a couple of degrees/hr. So if we've been gone for a week
and the house is at 50 or so degrees, it'll take quite some
time to get comfortable.
(For now we've got a neighbor who can usually come by
and kick it on early in the day we're coming back.)
--
_____________________________________________________
Knowledge may be power, but communications is the key
dannyb@panix.com
[to foil spammers, my address has been double rot-13 encoded]
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