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#20783 - 01/22/09 11:33 AM Thoughts on thermostat
Terry Haroldson Offline
newbie


Registered: 02/20/03
Posts: 3
Loc: North Vancouver
It has been awhile, since I posted here, but glad to see familiar faces still here.

I need to replace my current thermostat, so I am reconsidering whether to begin some automation. Here is my situation:

I have live in Vancouver, and have a heat pump as well as gas furnace (forced air). As you can imagine, most of the year the heat pump works fine, but when it drops below 5C (40F) or so (which this year has done for almost a month now!!), the heat pump is probably not too efficient, so I prefer to switch to the gas for primary heat. My current thermostat would always run the heat pump for 10 minutes or so before it figured out it can't do the job. Or hitting the increase temp by 3 degrees button on the thermostat will force the it into stage 2 (gas on, HP off). But I am tired of this.

So I am considering a standard thermostat (Totaline p374-1900, or Honeywell TH8320U1008). Both of these will allow adding an external temp sensor to lockout the heat pump below a preset temp, giving me the desired operation. But I am also considering the RSC TR40, but have 2 questions:

1. Does anyone know if the RCS TR40 provides the above functionality out of the box (ie: locking out the Stage 1 Heat Pump when external temp is below a set value). I don't see anything in the docs that suggest it might do that.

2. Assuming I can't do 1, then I have an Ocelot, SECU-16 and RLY8XA that I could use. Can someone outline briefly what it would take to do what I want. I am assuming I could set up a temp sensor off of the SECU-16, which could then send the appropriate command to the thermostat to go into Stage 2. But I am not familiar with the details... is it possible?

Thanks for any thoughts.

Terry

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#20784 - 01/22/09 12:07 PM Re: Thoughts on thermostat [Re: Terry Haroldson]
Guy Lavoie Administrator Offline
Beyond All Hope
*****

Registered: 12/21/02
Posts: 6548
Loc: Montreal, QC, Canada
The TR40 doesn't have any lockout feature for this. What you have is a "dual fuel" system and most thermostats don't have this functionality built-in. In my case, the hvac company had it set up with an external temperature sensor and a relay, to send the themostat's call for heat to either the heat pump or the oil furnace. The oil furnace was also set up as the "2nd stage" heating source that the thermostat (including a TR40) could call upon if the heatpump couldn't keep up.

Depending on the refrigerant used in your heat pump, it might still be effecient down to pretty low temperatures. An old R-22 heat pump can go down to about 0 deg C. and still be somewhat effecient, but a R-410a heat pump can go much lower. My R-410a based Carrier heat pump works all the way down to -12 deg C, and starts to need supplemental heat at around -5 or so.
_________________________
"If you don't know what you're doing, do it neatly..."

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#20785 - 01/22/09 01:37 PM Re: Thoughts on thermostat [Re: Guy Lavoie]
Terry Haroldson Offline
newbie


Registered: 02/20/03
Posts: 3
Loc: North Vancouver
Hi Guy,

Good to hear from you again.

I checked with my brother-in-law (the HVAC guy) and my heat pump uses R-22. I would be interested in how they configured that. One concern is to ensure that when stage 1 (HP) can't keep up, and calls stage 2 (Gas), that it automatically turns off Stage 1 right at that point. In my case the evap coil (for HP) is in the supply side of the ducting, so if Stage 1 and 2 are on at the same time, it could overload the heat pump. I think generally, the thermostat might keep both on thinking more in terms of an Electric furnace for backup.

Terry

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#20786 - 01/22/09 05:14 PM Re: Thoughts on thermostat [Re: Terry Haroldson]
Guy Lavoie Administrator Offline
Beyond All Hope
*****

Registered: 12/21/02
Posts: 6548
Loc: Montreal, QC, Canada
My setup is a modifed version of what is called a "bi-energy" system. Our power company (Hydro Quebec) has a special rate plan with a power meter that has two sets of dials and a heat sensor. The idea is that you normally heat with electricity (heat pump or heating elements) and you pay a reduced rate for all your electricity. When the probe senses that the outside temperature goes below -12 deg C, it switches your heating system to a non-electrical type (oil or gas) and the cost of your electricity also goes up significantly. A status LED tells you clearly that you're at the high rate.

Normally this is set up so that if the heat pump cannot keep up, it turns it off and brings in the oil furnace instead. But until you reach the outside temperature of -12, the electricity is still at the reduced rate, so I modified this so that if the heat pump cannot keep up, it adds a 9000W heating element to provide additionnal heat to what the heat pump is producing. These elements are located after the evaoporator coil, so the heat pump will not be damaged. It is only when the outside temp reaches -12 deg C that both of these turn off and I go to oil heat. I've had a 900 liter oil tank last two winters!

Now for specifics: I have a relay that is turned on when the power meter switches to the high rate. This disconnects the thermostat's compressor and 2nd stage (electric heat) signals from their normal destinations and sends them to the oil furnace instead. Both outputs are actually connected together when the relay is switched, so that if I happen to set the thermostat to emergency heat (ie: 2nd stage only) for some reason and I were to switch to the high rate, the oil furnace will still run.
_________________________
"If you don't know what you're doing, do it neatly..."

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#20790 - 01/23/09 07:00 AM Re: Thoughts on thermostat [Re: Guy Lavoie]
dahlheim Offline
junior


Registered: 12/31/08
Posts: 43
there is a sample program in the Leopard II manual which would serve to accomplish such a two-stage heating system using only a controller and relays. i've actually been experimenting with this over the past few days and it's working fine. i modified the program a bit based on my hardware. i use inexpensive gang-mount 5V temperature sensors wired to the secu-16 analog inputs, and elk-912b external relays switch the furnace(etc), the 912b's being controlled by the secu-16 relays.

of course, i plan on a safe backup to account for system crashes. i thought perhaps just a simple thermostat set to a lower setpoint wired in parallel with the relay circuit or something.

i work an almost random schedule so the typical scheduling thermostat doesn't work for me. instead, perhaps a computer can download my work schedule in .ics format and parse it, then adjust the thermostat for me. now that'd be some home automation...

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#20791 - 01/23/09 01:28 PM Re: Thoughts on thermostat [Re: dahlheim]
Terry Haroldson Offline
newbie


Registered: 02/20/03
Posts: 3
Loc: North Vancouver
Thank you for the responses. That gives somewhere to start. Terry
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#20802 - 01/25/09 10:07 AM Re: Thoughts on thermostat [Re: Terry Haroldson]
DonJ Offline
newbie


Registered: 10/31/08
Posts: 14
Which 5 v temperature sensors are you referring to?
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#20811 - 01/26/09 08:57 AM Re: Thoughts on thermostat [Re: DonJ]
dahlheim Offline
junior


Registered: 12/31/08
Posts: 43
well, i'm using a couple of these:

http://www.smarthomeusa.com/ShopByManufacturer/AES/Item/ATP-1000-WSF/

i like their design and they function well but i would imagine if you're so inclined you could build something (or find something? although i did do a moderate search) a little better. you see, these operate between +32F and +200F at a range of 0-5V, but as these things seem to go, it's not a linear relationship. it's interesting that the unit seems very accurate at 50-100F, but at those temperatures, the output is literally T/100, so 66F is 0.66V. plugging this into the analog input of a SECU doesn't of course provide the same kind of resolution. i use the formula ((SECU_PARAMETER)*1000/512)+4 to get my temperature, and it works within a couple of degrees, which is good enough for me.

by the way, if this interests anyone, it may be useful to note for future searchers:

i mentioned that i used the code in the Leo2 manual as a basis for my code. i did modify it in several ways. one of them involves a characteristic which is present in these sensors and may not be in, for example, the temp bobcat, when hooked up to a SECU16. when transitioning between two temperatures, the secu16 input for a short time reports rapid fluctuation between the two temperatures. i think that immediately acting on temperature changes as the code in the manual does could potentially damage a furnace if it's being asked to rapidly switch on/off like that. so, i coded in a timer to wait until the temp has been stable for a couple of minutes before turning the heat on/off in response.


Edited by dahlheim (01/26/09 01:51 PM)
Edit Reason: Additional info

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