Two stage furnaces and thermostats

[dcaster]

My LPG furnace developed a heat exchanger leak and so it has been replaced with a high efficiency two stage furnace. The old furnace was also a high efficiency furnace but just a one stage 100k BTU/ hour. The replacement is a 90k btu/hr / 63k btu/hr. Did not do a proper analysis of the size furnace needed, but based on how the new furnace seems to be working, the low stage is good to below 20 degrees F. right now. And I am going to put in more insulation so things will change a little. The new furnace has jumper settings for using a one stage thermostat with 5 or 12 minutes wait before kicking in to high, or for using a two stage thermostat.

Okay here is the question...........it seems to be that the furnace ought to run at the low stage unless the outside temperature is below about 20 degrees F. Then switch to high stage as long as the outside temperature is below 20 degrees. There is no mention of doing this in the furnace manual. So is this a bad idea for some reason.

It seems like a simple thermal switch located outside would be better than spending a bundle on a two stage thermostat.

Dan

[Steve]

Keep it simple.... use a 2 stage thermostat, and don't bother with the jumper settings. that way it will only give you what heat that the thermostat calls for.

[ilbebauck]

Dan, Generally speaking...it is better to operate from space demand which is directly linked to heat loss from the structure ...so, if you get full sunny days during the winter you can take advantage of the solar heat coming thru windows as well as heat produced within the house from food cooking, showers , etc... which *may keep you on first stage furnace operation longer thereby maximizing economy . This is the advantage of using a 2 stage space thermostat , whereas if you go the outdoor thermostat route you are basing operation on the outdoor temperature without considering possible heat generation within the house and from the sun . So, my vote is for you to go with true 2-stage heat operation via a 2 stage electronic programmable thermostat and hook it up to the furnace control board in like manner .

[MLD]

Bit confused on just how your furnace works---didn't know that any of them new anything about the outside temperature. The way that my 2 stage works is pretty simple---When there is a call for heat it runs on the 1st stage (I think it's about 70% of the furnace's capacity) and low fan speed for the first 10 minutes. If the thermostat still calls for heat after the 10 min. then the 2nd stage kicks in and the fan speed ramps up to 100%. On very cold days the house maintains a fairly even temperature because the furnace kicks on very frequently and most of the time cycles only on the 1st stage. MLD

[ilbebauck]

In residential, its only Heat Pump resistance heaters that are sometimes enabled/disabled off of outdoor temp. --- high efficiency gas fired furnaces are not...BUT, it is possible to modify such a system to accomplish it. The OP is asking if it would be beneficial for him to do it off of outdoor temp. since he accurately knows that first stage is good down to about 20 f. for his application ; he could very easily do it this way...but its not going to be the most economical . On your furnace....you are using a single stage Thermostat in conjuction with the 10 min. Time Delay feature of the furnace to bring in 2nd stage . In my opinion, 10 min. isnt long enough but you cant do much about that --- youd see better economy if you had a true 2 stage Thermostat operating the furnace off of true space demand and eliminating any time function . Of...if you were proficient , you could at least increase the 10 minutes to 20 minutes by wiring in an independent 20 minute Time Delay Relay for 2nd stage function, and keep your Single stage thermostat.

[danny burstein]

our high efficiency natural gas unit ("Lochinvar") has some sort of internal logic programming that modulates the flame performance. There's "zero", of course, and then 15 or so percent and up.

- I don't quite know the algorithm but it measures the outdoor temperature, and also the delta between the outgoing hot water and the incoming cooler pipes.

- when first installed the outdoor sensor was wired wrong, so the unit only fired up to the lowest setting... So it was getting a bit chilly by the by. He came by and disconnected it until he could get the factory rep on the phone...

[ilbebauck]

On a Boiler, it is called 'Hot Water Reset' (from outdoor temp) and it shifts the Hot Water output temp to a higher value as the outside temp decreases . For clarification, I was referring to FORCED AIR Heating Systems when i said in residential applications that its just Heat Pump electric resistance heaters that are sometimes enabled at a certain outside temp. value . A hot water systems such as yours is THE best for comfort as the outside walls of the structure are getting blanketed with virtually constant heat of varying degree ... but high efficiency gas Forced Air Heating does a more efficient job plus a/c, humidification, and air cleaning apparatus can be easily added to it making it a complete environmental system . Regards.

[danny burstein]

I love our hot water system, but with two complaints that are intrinsic to the design.

a: as you mentioned, it does great for heating but... doesn't do diddly squat for cooling.

b: It also only has the ability to raise the indoor temperature a couple of degrees/hr. So if we've been gone for a week and the house is at 50 or so degrees, it'll take quite some time to get comfortable.

(For now we've got a neighbor who can usually come by and kick it on early in the day we're coming back.)

[Steve]

[Quote] In residential, its only Heat Pump resistance heaters that are sometimes enabled/disabled off of outdoor temp. --- high efficiency gas fired furnaces are not...BUT, it is possible to modify such a system to accomplish it. [Quote]

OD temp *is* required for balance point changeover on dual fuel systems, but not for gas furnaces ... high efficiency or not.

Heat pump with resistance heat back-up will run the strips when needed during defrost cycle so it won't blow snowballs, or if the heat pump isn't keeping up with the heat loss.... no additional OD temp sensor is needed or required. FWIW, Rheem heat pumps have sensors built in that will shut down the heat pump and energize strips only when the OD temp drops below 6F.

[Daniel who wants to know]

In residential, its only Heat Pump resistance heaters that are sometimes enabled/disabled off of outdoor temp. --- high efficiency gas fired furnaces are not...BUT, it is possible to modify such a system to accomplish it. The OP is asking if it would be beneficial for him to do it off of outdoor temp. since he accurately knows that first stage is good down to about 20 f. for his application ; he could very easily do it this way...but its not going to be the most economical . On your furnace....you are using a single stage Thermostat in conjuction with the 10 min. Time Delay feature of the furnace to bring in 2nd stage . In my opinion, 10 min. isnt long enough but you cant do much about that --- youd see better economy if you had a true 2 stage Thermostat operating the furnace off of true space demand and eliminating any time function . Of...if you were proficient , you could at least increase the 10 minutes to 20 minutes by wiring in an independent 20 minute Time Delay Relay for 2nd

Or if you just want to be geeky, use 2 single stage thermostats, the first between R and W1 as per normal the second between W1 and W2. Set the first a couple degrees higher than the second, so that as long as the first stage is enough to keep Tstat 1 satisfied stage 2 never runs, if stage 1 can't keep up the temp drops and the Tstat for stage 2 kicks in.

[ilbebauck]

When Carrier air to air Heat Pumps were in their infancy, many of the installations operated with OD thermostats enabling the resistance heaters to operate in conjunction with the Outdoor Unit during normal operation . I still have the training material from that time depicting this.

[dcaster]

I am no expert, but do know that some hot water furnaces do have a sensor for outside temperature and adjusts the flame based on the outside temperature. In my case i was thinking of just a switch controlled by the outside temperature . And use that to switch the furnace to high heat when it is really cold outside. That is not mentioned in the furnace manual, but seems like it would work nicely.

=20 Dan

[dcaster]

You did bring up a good point. If it is sunny the house will stay warm without running the furnace when the temperature is forty degrees. If I do go the outside temperature switch I will put it in a box that has some solar gain built into it. My big problem with two stage thermostats is they seem to be very expensive. I may not be looking in the right places, but most of the two stage thermostats at Grainger are 200 + dollars. ( they do have some that are a little over

50 dollars. )

Dan

[Steve]

You did bring up a good point. If it is sunny the house will stay warm without running the furnace when the temperature is forty degrees. If I do go the outside temperature switch I will put it in a box that has some solar gain built into it. My big problem with two stage thermostats is they seem to be very expensive. I may not be looking in the right places, but most of the two stage thermostats at Grainger are 200 + dollars. ( they do have some that are a little over

50 dollars. )

Dan

Honeywell FocusPro 5000 TH5220D1003

[Tony Hwang]

Hmmm, That does not make sense. Buildings have different air tightness and insulation. Monitoring outside temp. won't do much for any thing. For humidity, Yes but temp., No. Furnace will do it's own thing depending on demand from 'stat.

[ilbebauck]

=A0 =A0 =A0 =A0 =A0 =A0Dan

Youre going to spend close to that regardless of what 2 stage programmable stat you go with. . If you want to go the cheap route, then your outdoor stat idea would be better than nothing.

[Steve]

Youre going to spend close to that regardless of what 2 stage programmable stat you go with. . If you want to go the cheap route, then your outdoor stat idea would be better than nothing.

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The Honeywell 2 stage stat I posted( Honeywell FocusPro 5000 TH5220D1003) is around $50 and will do the job. It will be less expensive and less of a PITA than running new wires and buying another stat and OD temp sensor that will support balance point operation.

[ilbebauck]

Well, if he can get a 2 stage stat for $50 , then absolutely that is the way to go. Ive never seen one that cheap.

[dcaster]

Ah, the Honeywell FocusPro is not programmable. I am amazed at how much two stage thermostats cost. I would think that there would be lots of models using a relatively cheap micro-controler.

Dan

[Steve]

Well, if he can get a 2 stage stat for $50 , then absolutely that is the way to go. Ive never seen one that cheap.

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There is a reason I posted that particular model.... Its a good quality stat, and my customers that have them have been very pleased. You can do a simple search on yahoo and find a bunch of places that have them for $50 or less. It also has an auto-changeover feature.... will change from heat to cool or vise versa on its own... nice to have in the fall and spring when your running the a/c during the day and heat at night.