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Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 8136
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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I have a Carrier 59MN&A060 gas furnace with a Carrier

Customer Question

I have a Carrier 59MN&A060 gas furnace with a Carrier 25VNA036 HP/AC, with a Thermolec TU-20 Modulating plenum htr and a Infinity wifi communicating thermostat. I have access to a dual fuel program with my electric coop, whereby a get an excellent price for electricity 98+% of the time, with the abilty for them to signal the shut down of the heat pump and plenum heater. The thermolec is tied to the HP through an isolation relay so that it comes on when the heat pump actuates, but only provides heat if the plenum air temp falls below a preset temperature. My goal is to have the heat pump work down to -20 degrees and be augmented by the plenum heater. The gas furnace would only be called upon if the electric utility closes the contact to indicate the system should go to gas. As such, I cannot force the furnace only to start below a certain temp as the utility coop sends its request based upon usage, not based on outside temp. Today we got below 20 degrees (about the balance point for the HP) and the system automatically chooses gas...I have reviewed the installation and owners manual and it is not clear what I can change to get the system to operate as hoped for. Can you shed light on what I can do to force the system to use the heat pump (which should provide adequate heating when coupled with the plenum heater down to -40 degrees?
Submitted: 1 year ago.
Category: HVAC
Expert:  airheatman replied 1 year ago.

What is the model on the infinity? And do I understand you simply want to set the temp at which heatpump locks out and forces the backup heat on?

Customer: replied 1 year ago.
gas furnace 59mn7a060, HP 25vna036, wifi sysTXCCIT all Carrier. And to answer your second question, I already have the heat pump set to lock out at -20 degrees, I want to know how to force the system to stay in heat pump mode (rather than gas) unless the electric company closes the contact to indicate that they want to not use electricity for the heat pump or plenum heater.
Customer: replied 1 year ago.
20 November 2015 12:27
gas furnace 59mn7a060, HP 25vna036, wifi sysTXCCIT all Carrier. And to answer your second question, I already have the heat pump set to lock out at -20 degrees, I want to know how to force the system to stay in heat pump mode (rather than gas) unless the electric company closes the contact to indicate that they want to not use electricity for the heat pump or plenum heater.
Expert:  airheatman replied 1 year ago.

I will open this to other experts who may be able to assist.It is my understanding that this is what determines when heat pump locks out or is allowed to continue to be the main source of heat.

Expert:  Phil replied 1 year ago.


Running a heat pump below its optimal design parameters causes the heat pumps compressor to exceed its compression ratio design condition, and over heats the discharge gas and traces of oil, causing the oil to break down to an acidic black sludge .

I need to clear up some of these temperature figures... by -40 degrees, to you mean 40 below zero F.? if you mean +40F, then there is no problem running the heat pump in that range, it is well within its design conditions.

We can discuss other ways of reducing your utility bill if you like, in that case tell me a lot about the house construction, age and nearest city so that I can assess the climate conditions for you.

Customer: replied 1 year ago.
Hi Phil, Yes I did mean 40 below zero F. The Carrier 25VNA series are designed for cold weather. I have copied the relevent spec sheet data from there advanced product data sheet at hvac partenrs. The system software allows me to set the lockout down to -20 degrees F. So, how do I force the HP to remain the primary heat source (in conjunction with the plenum heater) unless the dual fuel utility curtailment contact is closed.SYSTEM DESIGN SUMMARY
1. Intended for outdoor installation with free air inlet and outlet. Outdoor fan external static pressure available is less than 0.01--in. wc.
2. This product is qualified for low ambient cooling operation (below 55_F / 12.8_C) with an Infinity User Interface ONLY.
3. The maximum outdoor operating ambient in cooling mode is 125_F (51.67_C).
4. Minimum outdoor operating air temperature for heating mode is –30_F (–34.4_C).
5. Maximum outdoor operating air temperature for heating mode is 66_F (18.9_C).
6. For reliable operation, unit should be level in all horizontal planes.
7. For interconnecting refrigerant tube lengths greater than 80 ft (23.4 m) and/or elevation differences between indoor and outdoor units
greater than 20 ft (6.1 m), consult Residential Piping and Longline Guideline and Service Manual available from equipment distributor.
8. If any refrigerant tubing is buried, provide a 6 in. (152.4 mm) vertical rise to the valve connections at the unit. Refrigerant tubing
lengths up to 36 in. (914.4 mm) may be buried without further consideration. Do not bury refrigerant lines longer than 36 in. (914.4
9. Use only copper wire for electric connection at unit. Aluminum and clad aluminum are not acceptable for the type of connector
10. Do not apply capillary tube indoor coils to these units.
11. Factory--supplied filter drier must be installed.
Expert:  Phil replied 1 year ago.

Hello again,

I can work with you this without a time limit, however I am unable to fathom that the system can be run to minus 40F in heat pump mode.

I only see minus 2F as the minimum operating condition on this chart. Assessing your best options will take me a little time.

Customer: replied 1 year ago.
I appreciate your help. We appear to have been on the ragged edge of capabilities when we added the plenum heater to the mix. The heat pump does not provide much capacity below 5 degrees F and is almost irrelevent, except that that the plenum heater should still provide the heat needed unless energy curtailment is requested. Please let me know if you come up with anything.
Expert:  Phil replied 1 year ago.

Hello again,

Tell me a lot about the house, age of the house, type of construction, nearest city.. etc.

As a person approaches extremes, minor changes can have exponentially benificial effects.

A combination of changes such as mattress warmers and night set back to 55F can also save massively on your utility bill.

Doing the math on your carrier unit might show that we can run it down 10F outside with some benefit, the life cycle costs of that approach will be determined by the local climate... if it only rarely gets that cold then it would be a benefit.

Older systems for instance were designed to have no more than 10% air leakage from the ductwork, if that has not been rectified, and insulation is marginal, and window frames and doors leaking, those alone can double you required heating capacity. You can have a infra red camera study done on your house that will expose those heat leaks (often free by an insulation company.)

Let me know, we can go from there.

If those can be corrected then what you have in place might well be more than sufficient. Especially if have a large kitchen exhaust fan that runs a lot, and do not have an HRV (heat recovery ventilator).

Customer: replied 1 year ago.
House is unoccupied during the months of Dec to March except for a couple of weekends, so temp will be set to 55 degrees F.
Biggest concern is keeping electric as primary as opposed to propane and having the gas come on if utility company gives us a contact closure. Propane tank is 250 gallon and will likely run out before spring, refill is questionable during winter, neighbors had to have 2 1000 gal tanks installed as winter resupply had been unreliable.
House is less than 10 yrs olld, R38 ceiling, R19 walls, double pane argon filled windows (which get covered over with R10 foam before i close up, I have used several calculators that all give me roughly 60,000BTU needs for coldest day (-40 F). Length of time that the temp is down to -40 varies quite a bit, some yers it only goes to -30 for a night of two, other times (last winter, it stayed between -10 (hi) and -40(low) for about two weeks with the -30 to -40 coming for several nights in that time period.
Electric coop can shut electric off for heat pump or electric heat for up to 4 hours at a time.
My first choice would be to have a software or hardware solution to allow the system to work with the following heating priorities, 1. heat pump, 2. heat pump with auxiliary electric heat, 3. electric plenum heat, 4. gas when cooperative called for it at any temperature, and gas below -20 F.
Customer: replied 1 year ago.
I think the real issue here is that the Carrier system is set up around outside temperatures. The only temperature that I need to set is when to stop using the heat pump (say -20F). At temperatures above that I need to use the heat pump as my heat source unless the electric coop says otherwise
Expert:  Phil replied 1 year ago.


Is the house on a slab or a raised foundation, if it is on a raised foundation can you tell me what insulation you have under the floor.

R-19 in the walls, and R-38 above the ceiling is less than marginal for a climate that cold, you can cut the heat loss significantly by adding more insulation .... and a radiant barrier

The other issue is the almost complete lack of high end HVAC service in some arctic type climates... if you have good service nearby they need to have a key to your house and authorization to do what it takes to keep it above 55F... a wi fi or land line alarm should be fit.

R-60 insulation would be a good idea as well, it would afford less heat loss if the electric utility shuts you down for 4 hours.

An infra red camera study would still be in order, there can voids in existing insulation detectable by those means.

Customer: replied 1 year ago.
Thanks, ***** ***** need to focus on the here and now, as I am closing up for the winter and hope to leave this weekend, so insulation changes or heat loss studies cant happen this winter. Can you help me with any software configuration change or rewiring that will address my heating priorities as listed above, or can you redirect me to someone who might be able to help with that?
Expert:  Phil replied 1 year ago.

Hello again, Leaving this weekend does not give you enough time to replace the zone controller.

I have done my best so far, if moving the thermostat and damper to zone 3 fails.. that leaves you (and I with no other options).

I don't think anyone else here will be able do any better than that, however I will opt out so that others here can see your question.

If moving to zone 3 solves the problem please rate my work positively. Thanks

Thanks for letting me work with you. When we are finished, if my answer has been polite, professional,and thorough, please click one of the ratings . If you still need help let me know!. Expert is not paid for assisting you until you rate answer. Feel free to follow up for30 days at no additional cost.

Customer: replied 1 year ago.
I do not have a zoned system, so moving to zone 3 is not a possibility.