[chrisv2]

I have a Honeywell TH4201D thermostat, with a Rheem heat pump that has two auxiliary electric heat strips.

I'm a little confused about wiring the T-stat.

Pictures of t-stat guide and heat pump guide, are below. Air handler says "jumper W1 and W2 for maximum heat rise".

Questions:

a. Is it correct that W1 and W2 have nothing to do with the outdoor unit - that all the outdoor unit cares about is wire Y (compressor contactor), and either activating or deactivating the reversing valve (in my case, wire B for heat)?

b. Should I jumper W1 to W2 at the air handler, and E to Aux at the thermostat? Based on reading, it seems a lot of techs do this - what exactly does it accomplish?

c. What do I do with wire L from the T-stat? It says "L is powered continually when set to emergency heat". I see no where to wire it, either at the air handler or the outdoor unit.

Thanks in advance for any help you can give me to understand this wiring.

 

[user_12345a]

It sounds like your heat strips are staged. Ideally, you should use a thermostat that can stage aux heat.

When jumped some may use timed staging, others may bring both stages on at once. they use sequencers so there's a few minute delay probably on most.

thermostat staging is better for comfort.

Aux heat is used in defrost, and having them jumpered, you'll bring the entire kit on during defrost cycles and waste energy heating with electricity when you only need to temper the air.

You can jumper them at the air handler though and use that stat. not recommended.

E and aux can be jumpered at the stat if you don't have separate E wire. This is just for emergency heat mode.

Don't need L if it wasn't wired.

When the B terminal label is used you have to set the stat to energize change over valve in heat mode - default is cool.

 

[chrisv2]

Thanks - that purple wire D from the outdoor unit is used to turn on electric heat during defrost; I do have that connected.

So the fact that my t-stat has an AUX terminal doesn't mean that it can stage Aux heat?

Or are you saying I should look for a new thermostat that has both a W1 and a W2 terminal? I'm willing to change thermostats but I have no idea what to look for. It has to be simple/non-programmable (for my elderly mother).

 

[user_12345a]

You need a stat that can stage backup and run heatpump at the same time.

Check the manuals and look at heatpump wiring.

Most consumer stats will not be able to do this.

When there's one aux terminal, the stat doesn't have control over aux heat output. With separate terminal it will only bring on the second stage when actually needed to maintain setpoint.

 

[chrisv2]

OK, I think I understand. I still don't know which alternate t-stat I could use, I spent all morning looking for one online lain:

Is there any chance that my Honeywell t-stat will "know enough" that if more than a couple degrees of heat rise is needed, it will call for AUX -- but if less than a couple degrees of heat needed -- then it will just run the compressor?

I thought I read somewhere that it did measure the differential and would stage AUX heat based on how far the desired room temp is from the current temp.

(edit) yeah, I just talked to Honeywell. They told me that the t-stat has an algorithm that will first call for Y to be energized (compressor only), then if after a certain period of time, or too large a temperature gap - then it will call for AUX. So I think I'm ok here.

Here's my own version of the wiring diagram. If you see anything wrong let me know - thanks!

 

[supers05]

If you actually have 2 stages of heat inside the air handler, it's better to use a stat that can control them without the jumper. That'll work, but you'll get more comfort and could save some electricity with the better stat.

You're looking for a 3 heat stat in the chart below. You only need 1 cool, but I don't think they sell a 3H/1C, so a 3H/2C will do just fine. There's lots of options to choose from. Some of them require a C (common) wire. I didn't see in your post if you have that available, so this is just a reminder to check.

https://forwardthinking.honeywell.com/related_links/50-1376.pdf


Cheers!

 

[user_12345a]

"Is there any chance that my Honeywell t-stat will "know enough" that if more than a couple degrees of heat rise is needed, it will call for AUX -- but if less than a couple degrees of heat needed -- then it will just run the compressor?"

It uses a complex algorithm, tracking rate of change and bringing the aux on when it senses that it's not keeping up. or if there's a big difference.

with w1 and w2 done separately, does this a step further, only bringing on w2 when first stage plus heatpump can't keep up.

not all 3 heat stats will do it, a lot may be for 2-stages of heatpump heat plus one of aux heat.

 

[chrisv2]

Thanks guys. I think I'm going to stick with the existing thermostat since even though it is programmable, it is very simple and mom knows how to use it (in manual mode).

The 3H/2C stats I looked at are all a lot more complex in terms of user interface, which will not work out well for her. Also, not sure if this matters or not but the 3H stats do say "two stages of heat pump plus one stage of AUX", as user_12345a pointed out.

The downside, as stated, is that instead of staging two 3.5K heat strips, it is going to run as a single 7K heat strip every time AUX is called for. Hopefully that won't be a huge waste of energy.

As I posted earlier unless someone sees a glaring error, this is my wiring diagram as of now:

 

[user_12345a]

What stat is being used now?

Why is it being replaced?

Presumably the original has W1 and W2 for aux and the new one you've decided to use just has one.

 

[chrisv2]

See wiring diagram above; stat now (and before) is the Honeywell TH4210D.

What has been changed is the air handler and the condenser units. This is where both W1 and W2 were introduced...

 

[user_12345a]

should be okay only 7.5kw it shouldn't make that much of a difference, it's not a lot of backup to begin with.

Unless the unit is 1.5 ton, even with the fully 7.5kw it will just temper the air in defrost mode. The output is just over what a 2 ton heatpump will put out at 47f.

If you're doing the work yourself, I trust that it's being charged/commissioned in accordance with manufacturer's instructions.

 

[chrisv2]

It's a 2-ton. 950 sq/ft condo on first floor, in NY.

Yes I'm working with a licensed contractor, although kind of a newbie. I'm handling the wiring, duct transition work, etc. He's doing all the copper work and refrigerant.

 

[user_12345a]

A lot for that size but the extra capacity will be nice in heating mode.

May have issues dehumidifying in cooling mode though.

 

[chrisv2]

The old system was also a 2-ton, albeit R-22 (new one is R-410a of course). I did a detailed heat loss/gain calc and we seem to be right on the money. In fact, with only 7KW of aux heat it might be a little tight in winter if/when we see any below zero days.

The old system had 10KW of aux heat but was wired incorrectly with too light of gauge wiring. Should have never passed inspection. There was no easy way to rewire that circuit so I decided to play it safe and lower the load. If we really need it I can always add a run of electric baseboard heat easier than it would have been to rewire the air handler.

 

[user_12345a]

For a/c, if you need 2 tons for around 900 sq ft, something is messed up with the load calc or the condo.

Heatpump sized for a/c because the capacity drops below rating below 47f anyway and you run into dehumidification problems if grossly oversized for cooling.

But the 2 ton will need less backup heat than 1.5. So it's not all bad.


The backup i'm sure you got right.

 

[sktn77a]

Yes, I think you've got it right as the priority in NY will be heating for most of the year with just a month or so of cooling. user_12345 is correct though, you probably won't get optimal dehumidification with 1 ton per 450 sq ft.