Dual Fuel Hybrid System Question..

[SyNtAxx]

I recently had a dual fuel HVAC system installed. The house originally has oil hot water and baseboard heat.

I had a heat pump installed with the oil heat as the Aux. Heat\Emergency Heat. I have a 3 ton matched indoor\outdoor unit. With a Honeywell VisionPro TH8320 thermostat with an outdoor sensor mounted out on the outside unit near the electric hookups.

Recently I've noticed now that it is getting much cooler at night and early mornings that when the heat pump is running I can watch the outside temperature as displayed on the thermostat drop as much as 20* and thus locking out the compressor and switching over to oil heat prematurely. I currently have the lock out set at 25* and would rather not have to compensate for this inaccurate temp reading by lowering the lockout points.

My question is this, according to the install papers for the outdoor sensor the it should be mounted in a place not in direct sunlight or drafty areas, perhaps under the roof overhang or soffeting. My theory is that when the heat pump comes on and draws the now much colder air through the coils and over the sensor it is reading much colder then it really is. For comparative purposes I've placed another wireless temp sensor outside to get a second reading and thay are not anywhere near each other in temps when the heat pump is operating.

My Questions are:

Is the outdoor temp sensor in the ideal location or should it be moved?

Can the cool air being drawn over the sensor skew the readings? ( I think it can and the mounting location can be considered "drafty" but others argue not)

Is the sensor faulty?

Any opinions or advice is always appreciated. :)

[Dent]

Are you wondering if the heat pump is cooling the air around it so much that it is dropping the temperature below the cutoff?

If so, sounds like a ventilation problem. If the air directly around the heat pump is below the cutoff, the the pump should shut itself off, because it's not operating efficiently.

If the heat pump cannot pull air through it that is warmer than the cutoff, it should not be running.

[SyNtAxx]

No. What I am trying to say is that when the heat pump started this morning for example the initial outside temp read as 37*F and quickly dropped to about 18*F over the course of 8 minutes as indicated by the indoor unit. I know for a fact due to the second thermostat outside in the same location that is was not 18*F but ~36-38*F.

There is plently of space all around the unit and not air flow obstructions\vegetation issues.

I am just wondering that as the unit draws air in over the coils that it is drawing that air over the sensor and it is "seeing" 18*F when it really isnt that cold. Or is there an interference issue since the sensor is located near the 220V hookups? Or is the sensor faulty? When the unit shuts off after a call for heat the temperature sensor then shows the ambient temp within a degree or two of the other thermostat i have outside as a reference.

Make sense?

[Sonicview]

I would say try moving the sensor to a different location... then you could eliminate that as being the cause... there is a thread in this forum which discusses the best place to mount an outdoor sensor.. they are very sensitive and should be mounted in a vented enclosed box on the north side of your house, preferably out of any direct sunlight and located 3ft off the ground. that should give you the best results... mounting them in or near the outdoor unit is a very easy way to get it done faster becasue all of your wires are there, but it's not necessarily the best way.

[beenthere]

If the outdoor units air is being drawn or blown over the outdoor sensor. Then it is not in the right place.

Under the sofit can help.

[SyNtAxx]

Resurecting this old thread. The outdoor temp sensor was moved and that resolved the issue.

Now I have a few more questions.

Now that I have used the system for a few years I've noticed a something rather important missing. NO heat strips for defrost mode.

Here is the situation, heat pump is primary heat source in winter with the oil baseboard heat as the aux\emergency heat.

Problem: When the unit goes into defrost mode the AC is basically cooling the home I just heated for a few minutes. Frustrating.

Question: I am willing to install the heatstrips to cover the defrost mode cold air situation, BUT is it possible to be able to use the heat strips for say defrost mode and leave the oil in place for Aux\Emergency or does it have to be all or nothing here?

Thermostat: Honeywell VisionPro TH8320

I expect electric rate to climb here this year outside of Philadelphia PA.

Thank you in advance.

[beenthere]

Got a few systems with electric strip only used for defrost. The boiler provides all aux and or emergency heat.

[garya505]

Got a few systems with electric strip only used for defrost. The boiler provides all aux and or emergency heat.

Too bad this doesn't work with a gas furnace too. Seems like overkill to have to fire up a gas furnace just for the short time required for defrost.

[beenthere]

Too bad this doesn't work with a gas furnace too. Seems like overkill to have to fire up a gas furnace just for the short time required for defrost.


Often, the gas is still cheaper then what the electric strip heater would be.