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A-1 Tech
A-1 Tech, Service Manager
Category: HVAC
Satisfied Customers: 526
Experience:  I have been in the HVAC business for over 40 years.
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hello. i am trying to install a venstar T5800. from a honeywell.

This answer was rated:

hello. i am trying to install a venstar T5800. from a honeywell. what do i do with the old Rc wire?

A-1 Tech :

Hello, my name is XXXXX XXXXX I will assist you today

Customer:

hi ron

A-1 Tech :

Your old thermostar has an Rc and Rh or Rc and R ?

Customer:

Rcband R

Customer:

Oops Rc and R

Customer:

What was hooked up was W,Y,G,Rc, R and C

A-1 Tech :

Okay, you should have a wire that fed the R or the Rc with a jumper across. Is that correct?

Customer:

uh no jumper. two separate wires. The Red from AC went to Rc, The red from heat went to R

A-1 Tech :

That indicates you have a 2 transformer system which is rare. Most a/c units don't have a transformer

A-1 Tech :

You may not be able to have a 2 transformer system with this t stat, but you can convert it to a 1 transformer system.

A-1 Tech :

Let me look at your new stat wiring diagram. just a minute please.

Customer:

hmm. i had a c wire run because i was using a battery operated tstat prior.

Customer:

it worked with the old honeywell. there is no power to the venstar now

Customer:

are you saying Rc is a 24Volt input?

A-1 Tech :

yes

A-1 Tech :

Rc is 24 volt input for cooling

Customer:

so should i hook that up to the c on the venstar?

A-1 Tech :

The C is the 24 volt common or neutral and will be needed for the stat to funtion

A-1 Tech :

I would put the wire going to Rc on the R terminal and tape off the wire that was going to the R terminal on the old stat.

A-1 Tech :

If you accidently shorted the Rc wire or the R wire, you may have blown a fuse at the furnace circuit board

A-1 Tech :

can you jumper the Rc to the G and see if the blower comes on?

Customer:

i dont think so. let me quickly hook up the old one. ill get back to you in 5

A-1 Tech :

okay

Customer:

ok. back. both ac and heat come on now

Customer:

so the old one works

A-1 Tech :

great

A-1 Tech :

you have the R and Rc connected?

Customer:

yes

A-1 Tech :

Does your heat work also?

Customer:

seems to be

A-1 Tech :

I see you said it did, sorry.

A-1 Tech :

Can you access the heat ? And what type heat do you have?

Customer:

thats ok. i had a nest tstat. wouldnt work until i hooked up a c. when i did it worked fine for a while until the baseplate blew out.

Customer:

never get a nest.

Customer:

i have gas hot water heat

A-1 Tech :

We can try to convert this to a 1 transformer system if you would like.

Customer:

i dont think anything is working now. ill get back to you in a few

A-1 Tech :

The only problem I have with this is we don't know the size of your transformers.

A-1 Tech :

okay

Customer:

the heat works. the blower motor for the ac does not turn on now. prolly blew the fuse in it

A-1 Tech :

does the a/c and the heat use the same blower?

Customer:

no.

Customer:

gas hot water heat, rheem electric ac

Customer:

two separate systems

A-1 Tech :

I understand. So the thermostat controls the fan on the a/c only?

Customer:

the tstat call for cooling but nothing happens

Customer:

the tstat controls the blower which then passes on the call for cooling to the compressor

Customer:

but the fan doesnt come on at all now. the blower unit is dead

A-1 Tech :

Does the a/c unit run?

Customer:

my understanding is the fan should come on if i switch it on at the tstat even if i dont want cooling, correct?

A-1 Tech :

that is correct

Customer:

the compressor never turns on.

A-1 Tech :

the fan and compressor are on the same transformer

Customer:

i think the call to the compressor is relayed through the blower unit. yes i beleive they are

Customer:

so im basically stuck until i get the blower unit up again

Customer:

correct>?

A-1 Tech :

Typ, the Rc is the hot and on a call for cooling, it energizes the Y and the G terminal with the Y being cooling and G being the fan

Customer:

i shouldve just watched football :-)

A-1 Tech :

Sorry for your troubles.

A-1 Tech :

Where do you live?

Customer:

ny

A-1 Tech :

is it warm there now?

Customer:

its not a big deal. i will have my HVAC guy fix the fuse. my setup with my old honeywell tstat was easy. venstar should give you better instructions,

Customer:

its about 72 out

A-1 Tech :

Here is what I would do

Customer:

ok

Customer:

jumper the rc to the c?

A-1 Tech :

You have a 2 transformer system and trying to make it work on a 1 transformer t-stat.

A-1 Tech :

Rc to C would be a direct short

A-1 Tech :

R and Rc are both hot wires.

A-1 Tech :

C is a common/neutral

A-1 Tech :

I would have the a/c guy change you over to a 1 transformer system

Customer:

hm ok where does my c go to now?

A-1 Tech :

We could do that now, but I don't know if the transformers are large enough

A-1 Tech :

the C still goes to the C terminal

Customer:

its not going to help now since i blew that fuse, correct?

Customer:

and why was it ok with my old tstat?

A-1 Tech :

well, if the transformer for the heat were large enough, it would. But we don't know

A-1 Tech :

Because the old stat had a place for 2 hot wires. One to rhe R and the other to the Rc

Customer:

i dont want to mess with my heating unot. its a new traingle tube with microprocessors etc. i was told not to mess with the transformer on that one,.

Customer:

understood. so when i switched from heat to cool it just used the RC wire rather thab the r, correct?

A-1 Tech :

It can still be changed over to single transformer.

A-1 Tech :

yes, that is correct

A-1 Tech :

R is hot wire for heat and Rc is hot wire for cool

Customer:

gotcha so how would i do that?

A-1 Tech :

keep the transformer for the heat

Customer:

the R wire?

A-1 Tech :

BUT, instead of the hot going through the thermostat, it would go through a set of relay contacts.

A-1 Tech :

correct, the R

A-1 Tech :

then, use the wire going to the Rc terminal and put it on the R terminal on the stat.

A-1 Tech :

would be normal operation for cooling. But when it calls for heat, it would energize the relay which in turn closes the contacts bringing the boiler on

A-1 Tech :

right now, it turns the boiler on without the relay

A-1 Tech :

we need the relay to seperate the 2 transformers.

A-1 Tech :

give me a little bit and I will draw this out for you if you would like

Customer:

sounds complicated. ok. great

A-1 Tech :

your HVAC tech should know how to do it

A-1 Tech :

I will show your boiler as energizing a gas valve as I don't know what all controlls you have on it.

Customer:

yes he should. just pisses me off that venstar doesnt clarify this. as i said its a fairly standard installation around here,

A-1 Tech :

I agree,

A-1 Tech :

They could have put a Rh for heat and Rc for cool

Customer:

exactly. thats what the Nest Tstat had. but thats a crap product,

A-1 Tech :

give me a bit and I will draw up your existing and the way it will need to be done.

Customer:

awesome. thnKA

Customer:

THANKS

A-1 Tech :

will be a rough sketch

A-1 Tech :

I will attach it here as a picture

Customer:

k

A-1 Tech :

okay, going to scan these, then I will post.

Customer:

ok

A-1 Tech :

Full Size Image

A-1 Tech :

Full Size Image

A-1 Tech :

This may not be exact, your cooling transformer may be in the cooling unit and not the blower. Just the location of wiring would change.

Customer:

thanks. looks simple enough. ill rate you so you get paid. thanks again. if you would like to know how it turns out send me your email adress. [email protected]

A-1 Tech :

we can't give out our email, but you can come back to this question and let me know. I would appreciate that.

Customer:

ok. how would i do that?

A-1 Tech :

your email shows as just x to me

A-1 Tech :

I will put you down for a follow up in a couple days. you will get an email

Customer:

how about this? tmatwellatgmaildotcom

Customer:

ok great thanks again for your help

A-1 Tech :

Thank you and good luck. This will work for you

A-1 Tech and 3 other HVAC Specialists are ready to help you
Thank you for the positive rating. If I can assist you further please let me know.

Ron