Been looking at the specs for a Carrier 58MCB ‘Condensing’ furnace. They have what is called a “Condensate Trap”.
Why can’t any condensate just be drained out to the sewer line? What is the purpose of the ‘trap’??
Also, they say that the trap must be protected from freezing. So, what happens if your gone for a long time and the electricity goes off?
They also say that the furnace must operate in the 60° – 80° range. Since I’m needing a furnace for a vacation home, I would like to operate the furnace in the 40° range when we’re not there.
http://xpedio.carrier.com/idc/groups/public/documents/techlit/58mcb-1si.pdf?SMSESSION=NO
PlaneWood by Mike_in_Katy (maker of fine sawdust!)
PlaneWood
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We've had literally worlds of trouble with both of ours downstairs - I could go on for hours.
Not happy with Carrier, but that's what we put in 14 years ago, and we've kept trying to make the #$@%& things work - not looking forward to this heating season.
Forrest
What problems and what model? Black control board?
The trap keeps combustion fumes from blowing through the condensate drain. It's critical to the safe operation of the unit. For freeze protection it can be protected by heat tape as described in the instructions, but of course this won't protect it in the event of a prolonged power failure. (But note that water pipes in the house won't be protected either.)
For your operating conditions a conventional furnace may be a better choice, or possibly even a gravity wall furnace (since it requires no power).
Dan -
Water pipes in the walls will be sloped and will be drained with an underground valve into the septic tank before leaving for long periods. The electric hot water tank will have extra insulation and will be left at a low temp setting. The electricity does go off occasionally in this rural area but rarely ever for more than 24 hrs. The last time it was off for that long was 10 years ago.
PlaneWood by Mike_in_Katy (maker of fine sawdust!)PlaneWood
You should be able to insulate well enough (if not the house then just the utility area) to assure above-freezing temps inside for 48-72 hours, even in below-zero weather. And the condensate contains enough contaminants that it's probably not going to freeze above 25F. If you really wanted to get paranoid you could install a battery-powered heater setup for the trap. Or just buy an extra trap to have on hand, just in case it freezes.However, I'd set the temp a bit higher than 40, both to give you more safety margin and because temps that low are hard on the furnishings.And place an electric space heater in the utility area, in case the furnace fails.
If Tyranny and Oppression come to this land, it will be in the guise of fighting a foreign enemy. --James Madison
Another thought: If you get the slightly more expensive unit with the variable speed DC fan motor, it draws (based on my quickie measurements) less than an amp on low speed. (Haven't measured it on high speed yet.) It wouldn't take much of a UPS to keep it going through a power failure.
If Tyranny and Oppression come to this land, it will be in the guise of fighting a foreign enemy. --James Madison
A condensing furnace has drain taps at the bottom of the outlet of the secondary heat exchanger, which during operation is under a sight negative pressure, and at the discharge of the inducer blower, which during operation is at a positive pressure. As Dan mentioned, the trap seals these drains to prevent air leaking in and products of combustion from leaking out. If the trap is not present, the unit will most likely not fire, because in the startup logic, flow is proven via a pressure switch in the combustion system. No trap, no pressure, the furnace assumes no combustion air flow, therefore no fire and no heat.
Most basic thermostats do not control down to 40 degrees. If this is for a vacation home (near Katy TX?) why bother with high efficiency? Get an 80% eff furnace for 2/3 cost or less.
Useful info, thx>>because in the startup logic, flow is proven Flow of????Re the trap on the draft inducer - since the inducer is connected to the flue, (Think of it as a great big "drain" without a trap and sloped the wrong way) would a pressure switch work there not prove if the trap is dry? (I'm thinking maybe yes, given the weight of the air in the flue, but I'd prefer to know rather than suppose!)
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"Flow of????" Air through the sealed combustion system.
Typically there is a low pressure switch tapped into the bottom of the outlet of the secondary heat exchanger and one on the outlet of the inducer blower. On a call for heat, depending on the fan control board and the gas valve/ignition control circuitry, the logic for my funace with a Honeywell 2-stage SmaertValve system goes MOL as follows:
t-stat send a 24 vac signal to the W (or W1 for two stage) term. The high pressure switch is "polled", if it is open, the sequence continues. The inducer blower is started, flow through the HX is verified by closure of the low pressure switch tapped into the secondary HX drain pan, and high pressure switch closure indicating flow through the inducer sufficient to exhaust the products of combustion. The pilot lights. Feedback though a thermocouple verifies this and the gas valve opens and fires the unit. A bonnet temperature sensor indicates the HX is hot and the unit blower is started.
There are more safeties in a typical system, high temp limit , roll out switch, gas pressure switch, etc.
The drain is usually tapped into a rubber connector between the fan discharge and the PVC pipe, the pressure tap is on the side of the volute.
Edited 10/11/2006 12:13 pm by Tim
"If the trap is not present, the unit will most likely not fire, because in the startup logic, flow is proven via a pressure switch in the combustion system. No trap, no pressure, the furnace assumes no combustion air flow, therefore no fire and no heat."Does that mean that the trap needs to be primed before the furnace can be used the first time or if it has been sitting for a long time.
"flow of" is the airflow through the heat exchangers caused by the combustion fan. In theory, the trap should be primed before startup. Cold return air can cause problems with the heat exchanger. If there is a danger of freezing, don't use a condensing appliance. All of the above is standard for every manufacturer.
Bill -
Yes, according to the Carrier Specs, the trap has to be primed before startup.
All the electronics they put in these to ensure proper operation, I'm afraid, is just more source for potential failures.
I'm also looking at a Geothermal AC/heat pump by Waterfurnace. Conceptually they're pretty simple as long as you have a constant temp water source. I have a spring nearby that I'd like to take advantage of. Providing 5-7 GPM of 65° water from it would be feasible.
PlaneWood by Mike_in_Katy (maker of fine sawdust!)PlaneWood
"Does that mean that the trap needs to be primed before the furnace can be used the first time or if it has been sitting for a long time."
In some equipment, it means exactly that, but in most cases, no.
Tim -
No, the house will be in Oklahoma. Most of the Honeywell digital thermostats will control down to 40°.
The thing I like about the Carrier condensing furnaces is that the combustion air can be vented out through a PVC pipe out the soffit. I'm hoping to not have any pipes going through the metal roof.
Studor vents can be used that terminate in the walls or attic. That will prevent the need for water drainage vent lines going through the roof.
I'm also looking into a Geothermal AC/heat-pump by Waterfurnace. I have a spring nearby with a constant 65° water source. That means the heat pump will work at max efficiency regardless of how low the temp gets. And, no outside coils needed that might frost up and freeze over.
Cost of electricity in this rural area is dirt cheap, while propane is very high. 7 cents/KWH for electricity and $2 / gallon for propane (this winter).
PlaneWood by Mike_in_Katy (maker of fine sawdust!)PlaneWood
Oklahoma, Texas, same difference. (the Sooners and Longhorns should be smoked out by that one ;?p) Neither is close to the (not-so-)Great White North.
The below 60 operation of any gas burning appliance with a heat exchanger will tend to cause short term condensation on the heat exchanger during every startup at those temperatures and will shorten its life.
Water Furnace is one of many products in the geoexhange HP market. Other major players are Trane, McQuay and ClimateMaster. I like the closed-loop, vertical bore configuration.
I pay $0.085/kwh and $1.46 gal in rural northern IL.
You save some money by running at lower temps, but think of it as lack of insurance.
If your house loses a degree an hour, the difference between 40 and 65 is 25 hours.
Most insurance policies require inspection every 24 hours or no coverage.