Hot Topics: Nest Thermostat Troubleshooting

open nest thermostat

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Original Post: Heat not working after swapping out old 7 wire t'stat for Nest

lijoe408 - Member

Hi, I recently swapped out a very old thermostat for a new nest unit. AC was working fine, but i found that with the heater, the until turns on but doesn't blow hot air. I've been looking at the old setup and everything seems to have been wired the same way to the best of ability. Is there something wrong that I'm missing?


thermostat wiring board

open nest thermostat

PJmax - Group Moderator

Welcome to the forums.

The wiring looks correct. Tells us about the furnace.

Typically the thermostat calls for heat and the furnace turns the blower on.

In AC mode, the thermostat controls the blower.

Make and model would be good. The ID tag is inside on the side wall near the burner.

lijoe408 - Thread Starter

Thanks. I looked around on the furnace but there was no model number. There is a sticker that looks like it wore off a long time ago as the unit is pretty old. It is a Lennox pulse furnace.

furnace panel

roughneck77 - Member

Not to stray from your original question, but the pulse is getting very old at this point in time.

I’ve worked on many.

My first question when I see one is when was the last time the heat exchanger was tested? There is a very specific test that needs to be done to verify it isn’t cracked.

lijoe408 - Thread Starter

Not sure about the heat exchanger. This is a home that I'm renting. Any possible reasons that you can think of as to why it isn't blowing heat?

I should mention that I never previously used the heat either with the old thermostat. The only thing I've ever used is the AC and it's been fine with the Nest.

PJmax - Group Moderator

The Pulse furnaces have undergone recalls and safety breakdowns. Most are no longer in use.

Those that are get checked every year.

You as a renter should not be changing a thermostat in a rental unit.

roughneck77 - Member

As said above you should not be performing work or modifications to the equipment as a renter.

That thermostat isn’t very good anyway, they have a long history of problems.

Ask your landlord when was the last time the furnace had a checkup. It’s required with those furnaces, and your safety.

lijoe408 - Thread Starter

Good to know on the pulse furnace. I already previously had the thermostat and google home setup. I asked the landlord if i could swap it out prior to moving it and got written permission to do so. The previous thermostat was extremely worn and old.

PJmax - Group Moderator

These Pulse furnaces were ahead of their times. Many had electronically controlled blower motors. Typically the blower is controlled from the furnace board via a timed function in heat mode. There are other methods listed as options depending on exact model. The manual below is for one of the common pulse series.

Lennox G21 pulse.

roughneck77 - Member

I’d worry about making sure the furnace was sound first before trying to get the nest to work.

The other thing to consider is the lack of parts available for the Pulse these days (a lot of them are obsolete) coupled with the fact that the Nest has been known to occasionally cause damage to electronics in equipment.

I myself have had a Nest short and cause a control board failure, more then once.

I know those devices are very popular with the DIY crowd but I really dislike them for the problems they’ve caused my customers in the past.