Nest thermostat to Big Maxx

[Kallen425]

I replaced my old thermostat with a Nest E thermostat for my Big Maxx 50,000 btu. I am using four wires...R,W,G,and C. Nest says this is correct but the thermostat won’t power on. I checked the connections at and inside the the heater. They all seem correct. I tried searching the forums with no luck. Anyone get a nest to work with their Big Maxx?

 

[mike93lx]

Are you getting 24v to the thermostat? Do you have a multimeter?

 

[manwithtools]

That's 24 volts AC you are looking for by the way.

 

[brewchief]

I've seen some unit heaters with a C terminal that isn't actually hooked to anything.

See if you have 24v ac at R snd C at the heater.

Sent from my SM-G965U using Tapatalk

 

[dogdog]

This bix maxx heater ? I think there was some one asking similar question few days ago... and the Nest might have drawn too much power that the tiny transformer can supply power... Nest draws more power , next one is that Echobee... so either change out the transformer if you can or allowed to or use that isolation relay...

but yes confirm if you have 24V on R and C... then test to make sure it works if you jump R and W... manually if it works then you either hooked up the lines wrong (happens when I did the echobee) or not enough power.

https://www.garagejournal.com/forum/showthread.php?t=438122


https://www.mrheater.com/downloads/dl/file/id/255/mr_heater_big_maxx_unit_heater_manual_2018.pdf

MAXIMUM LOAD NOT TO EXCEED 20VA AT 24V,

^^^
page 13/68

 

[Kallen425]

Thanks guys, I’ll check tomorrow to see what kind of voltage I’m getting.

 

[Kallen425]

Well I'm only getting 20 volts at the thermostat. Looks like I'll need an isolation relay

 

[dogdog]

Well I'm only getting 20 volts at the thermostat. Looks like I'll need an isolation relay

do you get 20V with the Nest hooked up ? or 20V without...

with the nest hooked up, then most likely a chance of the nest drawing too much power than the tiny transformer can handle... Check the voltage without... means the wires unhooked.

without the nest hooked up then you might have wiring problem like too thin gauge or too long of a run...

do you get the same 20V voltage at the Unit ? or 24V.

keep in mind isolation relay needs a separate transformer in addition to the relay... I think there was one that is already build in, transformer/relay.. forgot the brand.

Homedepot sells those transformers (40VA one honeywell brand) but not the relay...transformer I think was $50-ish at HD.... Amazon or ebay market place have the cheapest. $18+tax for transformer and about $12 for the relay I think...

 

[Kallen425]

20v without the nest hooked up and 20v at the unit. The thermostat wire run is just over 20’

 

[dogdog]

20v without the nest hooked up and 20v at the unit. The thermostat wire run is just over 20’

20v at the unit is not normal, maybe give them a call... not sure if you are adventurous enough to go find that transformer and measure it's output... it says 24V on page 13 of their manual.

see if you jump R and W together manually and if it works... ?your heater should operates.... Normally that is all the thermostat do, a relay to jump the two together when call for heat.

normally they (hvac stuff) are 24Vac... so... either a short somewhere within that unit, bad transformer..... bad meter or bad user (jk, but could happen... I did it many times)... or maybe you have AC issues ?

you are measuring between R and C right not R and W.

 

[fitter30]

Measure incoming voltage also look at transformer might have different line voltage taps.

 

[LS6 Tommy]

Measure incoming voltage also look at transformer might have different line voltage taps.

X2. If you're set up for 240VAC and running on 120VAC the xformer will have low output. Even if you can get the output voltage up to 24 VAC, the xformer may not be capable of carrying the current load. 20Va is about 1/2 of what average HVAC systems have. IIRC, nest t-stats require 20-30VAC.

Tommy