Thermostat Wiring Assistance Request

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Old 03-19-09, 07:46 PM
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Thermostat Wiring Assistance Request

I have a Lennox residential HVAC system in our new home and want to change to a programmable thermostat. I cannot figure out the wiring equivalency from the base thermostat to the one I am thinking of buying. The particulars follow:

My current Merit 51M33 thermostat controls a 13HPD series heat pump (2H/1C w/external temp sensor on or inside the heat pump) and a G43UF series gas furnace. I'm thinking about upgrading to a Comfort Sense 5000 programmable thermostat, model X414701, because buying a Lennox brand is the only thing I can find that is clearly compatible with the heat pump external temp sensor.

Wiring for my 51M33 is O,R,G,Y1,C, W1 (jumped to E) and a pair of "T" wires for the heat pump outdoor temp sensor. I need help with figuring out the wiring to the X414701 and want to figure this out before I buy the new thermostat. I've looked at the instructions for the X414701 (referring to the multistage heat pump with auxiliary back up section instructions, presuming this is my configuration). Problem is there is no clear match to the 51M33 wiring configuration.

I tried asking about this on a forum for a Lennox parts dealer but got not response. I contacted the dealer's helpdesk -- their "sales" team said they were just salespersons and could not provide technical assistance but that I should be able to figure it out. They did, however, inform me that the external heat pump temp sensor I now have would have to be replaced with the one that is required for the x4147 programmable thermostat, for another $40.

I am not in love with the higher price associated w/buying the Lennox programmable thermstat (compared to a Honeywell 8000 series), but I cannot find another brand that is clearly compatible with using the outside temp sensor, and I prefer to not bypass that feature.

Can anyone help me walk through this or am I doomed to paying over $150 more for a local company to install in 20 minutes?

Thanks,
-Doug
 
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Old 03-19-09, 08:27 PM
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Welcome aboard Doug.

May I ask you why you want to change from a non programed to a programed t-stat?

I am afraid you won't not be able to use the outdoor sensor on anything else other than the t-stat you have now.
 
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Old 03-20-09, 05:05 PM
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Hi Jay,

We are not home during the day, would like to set back when not home and readjust before we arrive home is a primary reason. We used our automatic/programmed thermostat at our previous home and really liked it. I had originally purchased a LuxPro programmable (PSP721U) that is suppose to work with2H/1C heatpump/high efficiency furnace. But I learned after the fact that it doesn't use external temp sensor. Their tech support said that was fine, but I'm not comfortable with that idea b/c I don't want to run heat pump when it's below mid 30s outside - in KC it gets into single digits. And I don't want to have to manually set it to emergency heat - in my mind it partially defeats the heat pump and gas furnace combination strengths.

I can live with changing the outdoor sensor that is mated to the programmable Lennox t-stat I listed, and unless someone tells me otherwise, I don't anticipate physically changing it being that hard. But I'm really confused about getting the right wiring configuration for the new thermostat.

Thanks, Doug
 
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Old 03-20-09, 07:39 PM
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If you do a set back on heat pump, you are not really saving anything at all.

WHen you set it back, it may call for the gas to come on to help the heat pump to recover.

If you really must have to do the set back, I'd suggest the Honeywell IAQ t-stat.. With that one, you can lock out the gas heat, and let the heat pump do it's own recovery, but it going to have to work harder and longer.. again. no savings.

My cousin used to live in KC, and he had a heat pump, and when I told him about the set back, he stopped doing it, and he saw the savings than he thought he was doing by doing the set back in the day time hours.
 
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Old 03-21-09, 12:32 PM
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Jay - I apologize in advance if I am being a little slow on the uptake here, but I have some clarifying questions that I will appreciate your expertise with.

It's my interpretation from looking up info. about my heat pump model (seems to be a lower end efficiency unit) that is paired with a 90% high efficiency furnace that I listed in previous post, it may be less expensive at low temps to run the furnace than the heat pump for where I live. Hearsay is that heat pumps are not as savings productive in KC as they are further south - kind of on the edge thing. But again, I'm a lay person making interpretation from what I've read and what I heard from the builder's heating/cooling tech who I had out to check the blower noise level with the heat pump. That said, what follows is what I think I know and my questions:

-Winters can be pretty cold - down to 0 or below at night, and teens during the day for 2-4 weeks. Otherwise avg. in 20s-low 30s during the day. My limited understanding (don't know if I'm right or not) is that the heat pump is set to not run below about 36 or 37 degrees. (I thought the tech told me that b/c I have nothing in writing about it.) So, I presume the high efficiency furnace automatically handles everything below the threshold temp - could that be right? And at those times when the heat pump would be the primary heat source (say 37+ degrees outside), if the temp gradient between the thermostat setting and inside temp is > 2 degrees the furnace does the warm up and temp maintenance is via the heat pump. Hence the emergency heat mode from set back to warm up. So I guess another question is if the outside temp was in the teens, I am pre-supposing the heat pump is inactive and it's all furnace - is that possible? And if so, would the set back point you made be mute - I'd be saving during set back and spending with the initial warm up if everything is running entirely with the furnace anyway? For example, say it's dead of winter (below 30 degrees), we usually set the thermostat back to 65 while sleeping/away for 8-10 hours & bump to 69 or 70 while home.

- Summers are humid and can be hot for months. Talking 90 - 98 daytime and not much cooler at night. This will be round the clock ac with the heatpump unit I have for a 2-3 months. Usually set back is 82-84, and at home temp is set to 77-79. Wouldn't setback for 8-10 hours during the day (5-7 hrs at night) be cost effective?

Given my equipment, the set-up, my region, etc. is set back for winter & summer not cost effective? And if you think set back may have savings value, would I still be better off with the thermostat you recommended vs. paying $350. for the proprietary Lennox branded unit & its mated temp probe?

I know this is a number of questions but I sure have been a challenge finding an impartial resource that I can interpret in basic terms.

Thanks again, Jay, for your patience with helping me understand what I'm doing and determine cost-wise/efficiency what makes sense for my situation.

-Doug
 

Last edited by KCDoug; 03-21-09 at 12:58 PM.
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Old 03-21-09, 04:46 PM
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Give me your make and model of the your unit outside, also the furnace itself.

Also what is your rates for both gas and electric?

I am going to have to find the program back and plug those info in to see where the savings are at.
 
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Old 03-22-09, 10:03 AM
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Jay:

My entire HVAC system is Lennox. Home living area is approx. 2,650 sq.ft.

*Heat Pump: 13HPD (their basic unit, don't know tonage) 13HPD Heat Pump | Heat Pumps | Heat Pumps Residential Efficient | Lennox Residential

*Gas Furnace: G43U (base high efficiency unit. Don't know exact size), Furnace | Gas Furnaces | Home Furnace | Residential Furnace | G43 Gas Furnace | Lennox Residential

My utility rates vary monthly, but I averaged an Oct bill (first full month) and a Feb. bill to come up with these that include all the misc fees/tariffs but excluding tax:

* Electricity: $.086/KWH (it'll probably be higher for summer)

* Gas (dropped between Oct & Feb bills, bet it goes up again next winter): $1.6/CCF (hundred cubic feet).

Thanks, Jay.

-Doug
 
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Old 03-23-09, 12:22 PM
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Ok, plugged the numbers in,

The Heat pump is cheaper to run than gas.

For every 100,000 BTU of heat the system puts out, this what it cost you.

HP- 73

Gas- $1.76

Heat pump works good with temp down to into the teens. A good t-stat like Honeywell VisionPro can call for gas as needed when the heat pump falls behind.

Let the Heat pump keep the temp steady, and then you won't need to run gas to recover.
 
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Old 03-23-09, 10:23 PM
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Ok stick with me here and I will try not to get us lost..

First if it was my system I would find my balance point of my Heat pump and operating cost of my Heat pump vs My gas furnace. Purchase a Honeywell Vision Pro 8000 and an out door sensor.(I use this thermostat and outdoor sensor and it has the option to control your dual fuel system based on the set points that you program into it.)

But onto other things first I guess.....
Lets say what you have is 13HPD−042 that is a Lennox 13 SEER heat pump in 3.5tons.

That units heat/btu output at 65F is 44.0 kBtuh
at 45F it is 35 kBtuh
at 25F is is 25.1 kBtuh
at 5F it is 18.5 kBtuh

Now lets say that your heat loss is 35 kBtuh. this means that you need 35 kBtuh to maintain your home at 70 so as you can see by the time you hit 45F your at your balance point of what your heat pump can do.

Now you need back up heat. So yes this system is still making heat to as low as 5F but as you can see it is less than half of what you need to keep your house at 70F, so its 25F outside and your only running on your heatpump...making 25.1kBtuh... it is running and running and running.. and getting even further behind the heat loss of the house.
So now is it cheaper to fire that gas furnace up and heat the house in a few mins or keep the heat pump working.. spending your money on elect?

Note that when there is a wide temperature differential, e.g., when an air-source heat pump is used to heat a house on a very cold winter day, it takes more work to move the same amount of heat indoors than on a mild day. Ultimately, due to Carnot efficiency limits, the heat pump's performance will approach 1.0 as the outdoor-to-indoor temperature difference increases.

So the lower the temp outside.. the warmer the temp in the house.. the lower the COP (this unit is 2.9 at 65F)

at 65F your cost for 100,000 is Heat pump: $0.87 and your gas is $1.69

at
25F the Heat Pump is now at $1.02 per 100,000 and your furnace is still at $1.69

But as we see above your Heat Pump is only making 25.1kBtuh and the house is losing 35.

So you would be spending the $1.02 per 100,000 but the house would be losing it faster than the Heat Pump can make it. So your indoor temp is now falling.. and falling.. and falling.

So get the model number of your unit (need the size) and we can figure your balance point for your system.

All of these numbers will change based on ...
1- The model number of your outdoor unit
2- The model number of your indoor coil
3- The model number of your furnace.

Oh and I work for one of the Largest Lennox Service Compaines in the Country
 

Last edited by cyberdead; 03-23-09 at 10:40 PM.
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Old 03-24-09, 09:17 PM
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Jay - thanks very much for your help. If I were to go with the Honeywell thermostat, are you saying to still keep the temp. setting constant w/o doing set back at night or while away for 8-10 hours? And I'm still no clear about doing set back for summer when running the heat pump for ac?

Cyberdead - and thank you for joining in - I think I understood your detailed explanation and I would be interested in your further calculations.

1. The model # of my outdoor unit (heat pump) - all I found on the info. on the outside of the heat pump (and the installation manual) was model #13HPD, the only other visible number on the outside of the unit is 036-230-01. Have no idea the tonnage size and could not find anything obvious about size on the outside. (The service manual is for 1.5-5 tons).

2. How would I find # for indoor coil? I did see a sticker on the riser(?) with a model #C-33-43C-25-3. Could that be it?

3. My furnace model # is G43UF.

- I currently have a Merit 51M33 thermostat and was considering going to a set back - but Jay suggested that it may not be cost efficient to do a set back - and our conversation has progressed from there.

As I've mentioned, another key question that has evolved is whether or not to use a set back thermostat or is it more cost efficient to maintain a constant temp? And my question applies to both summer (where ac can run 24/7 for 3 months) and winter. Details are in previous post.

Thanks Jay and Cyberdead for your time and assistance.
 
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Old 03-25-09, 09:15 PM
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ok here is the chart you need.. now you need to find the heat loss of your home.. you can download a program for this or you may already have it.
This is for the closest coil and furnace that I could match to what you have.
Once you have the heat loss of your home find the btu on the chart below and its corresponding temp and then set your outdoor sensor no lower than this..

Due to the cost of gas in your area vs elect you can set your outdoor stat as low as the system can meet the btu's of your home.
Dont forget.. to use this you need the heat loss of your home
Do not confuse Economic Efficiency with Thermal Balance Point you can run your heat pump cheaper than you can run your furnace down to about 10F but it will not be making enough heat to heat your home.





And if you are spending more time in gas heat... I would surely run a night set back.. and do the same in the summer months.
 

Last edited by cyberdead; 03-25-09 at 09:37 PM.
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Old 03-27-09, 10:06 PM
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Thanks for the info. I'll Google for the heat loss programming and work with it over the next week, and then apply the chart you provided.

Still not clear on whether I will benefit from using a set back thermostat in the winter or leave my temp setting static - particularly if I find with the aforementioned calculations I learn that I could effectively run the electric heat pump greater than 50% of the time.

What about summer using the cooling cycle for this particular heat pump; is it more cost efficient to set back (raise the temp) for the 8-10 hours during the day when we are not home?

And all this boils down to whether or not it's worth buying a set back thermostat?

Thanks,
-Doug
 
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Old 03-28-09, 06:18 AM
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You can be the judge on the set back for cooling, but with my own view in my home now, and other homes in the past, set back didn't show any savings at all, and lost of comfort.

I even had the power company remove my "Saver switch" (Where the power co cycles the compressor on high demand time) since I thought it was harder on my system (on and off, on and off), and it fell behind on very hot days.
 
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Old 03-29-09, 12:57 PM
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I have a HP and use the set back settings on my Vision Pro.. works great..

Do you like Ford or Chevy????
 
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Old 12-07-09, 08:29 PM
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Doug - I have the same HVAC configuration as you do and am considering doing what you wanted to do. I didn't see closure on the thread so I am curious about what you ended up with. I am guessing that you live in the Avignon subdivision and we might be neighbors.

KCBrewer
 
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Old 03-14-10, 01:45 PM
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Choosing the right Programmable Thermostat

KCDoug, how did you choose your Programmable Thermostat? I have a Bryant system so do I need to call them to see what they recommend or can I just go to Lowe's to have them tell me what will work?
 
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Old 03-14-10, 06:39 PM
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Originally Posted by D. Wolfie View Post
I have a Bryant system so do I need to call them to see what they recommend or can I just go to Lowe's to have them tell me what will work?
Depends on what model of furnace, and A/C you have. A basic model from Honeywell will work.

I don't like Hunter that Lowes sells. Hunters are only good for their fans. Go to Home Depot and get Honeywell, or get a Pro line.
 
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