How to make sense of the choices? Thread split off.

[54regcab]

What brand/age of unit are you talking about that the "high limit" kills the power to the stat? all the high limit does on any newer "board type" unit is break the circuit back to the board, the lugs on the board receive direct power from the thermostat.

Most of the ones we see this problem on are the newer Carrier/Bryant/Payne 80% units. Not sure of model numbers, but they are under 10 years old. It's the style that uses a thumbscrew to hold the large main door on, and the blower/board are behind a sub panel. Diagnostic code of "33" when it shuts off the burners on high limit and kills the 24V to the thermostat. Blower continues to run while on high limit. Once the furnace cools back down power is restored to the thermostat. This is not the only furnace that does this, just the most common one we see for high limit shutoff. When testing, the discharge air from the registers gets to about 130 degrees before the furnace kicks off on high limit.
http://www.sbeelectricandhvac.com/do...-310aav-error/

I've seen a few old pilot light furnaces cut the power to the primary side of the transformer when out on high limit. Not very common, I think most do like MrBill says and just kill the power to the gas valve. Not sure how many just kill power to the gas valve because we get very few calls on them. We see more of he induced draft style furnaces when it comes to high limit shutoffs, I'll start paying more attention to which brands/models I'm seeing.

[Joehvac25]

I bet the OP is wondering when the hell someone will give him some advise lol

[54regcab]

Here's the wiring diagram for the Carrier/Bryant/Payne furnaces I've been seeing:

Notice that the Limits switches are in series with the R terminal on the board. It would be interesting to know how many are done this way vs. just killing power to the gas valve.

[54regcab]

I bet the OP is wondering when the hell someone will give him some advise lol

Agreed, this thread has run away and has become more "tech to tech" than AOP. Maybe a mod could split them up for us?

[RoBoTeq]

I'm glad to see the hydronic coils are becoming more popular, and you are promoting them. Have you done any systems that use multiple air handlers? On your setup will the water heater be doing your domestic hot water also?

I thought I made it clear that hydronic coils have been used for quite a long time now. There is nothing new about hydronic coils or hydronic air handlers, they were available over 40 years ago when I started in the HVAC industry.

My tankless water heater will be taking care of my domestic water heating as well as auxilliary heating for my heat pump and spot floor heating for my bathrooms and kitchen area. In most cases when I have installed systems for new houses being built where natural gas was not available, I would install an oil or LP water heater for domestic hot water and to supply hot water coils for auxilliary heating for heat pumps. Did that for decades.

[RoBoTeq]

I would have like to have one of those furnaces for my own 1600sqft house. 50k isn't just for small houses, it can serve over 2,000sqft IF it's insulated right. Ask any of the building science guys about this one. Gas around here is cheap (less than 50 cents per therm), so anything over 80% doesn't have a reasonable payback time. The variable speed feature is mostly for comfort reasons, even though the payback time on that is long.

In a daily basis I work with the Smarthours project. We install the Smarthours thermostats and run service calls when the customers call in if it doesn't work correctly. We don't repair the HVAC units or install new ones as part of this project, we just determine if the problem is with the thermostat or HVAC unit. As part of the project we see the good, bad, and the ugly as far as installs go. Some installs are excellent, others are so bad we don't even do the thermostat install for liability reasons. Most are in-between.

Since our thermostats are system powered they go blank when a furnace shuts off on high limit, and then we get a service call. People are quick to blame our thermostat because the old battery power stat it replaced just let the furnace cycle on high limit and the customer didn't know the burners were switching off because warm air continued to come out of the vents and the thermostat "stayed on". Since the furnace is often oversized to begin with the house still stayed warm even though the burners were shut off on high limit 1/2 the time. After seeing a bunch of these through the winter it does give me prejudice to the combination over-sizing of furnaces and poor ductwork design.

So, basically, your entire perspective of the HVAC industry comes from the standpoint of how HVAC systems affect some setback thermostat that the company you work for sells. Not exactly a very biased view of the HVAC industry.

The only reason I keep asking you questions is that so many of your posts have very erroneous information about the HVAC industry, as well as about furnace sizing in them. If you are only seeing the HVAC industry from the perspective of a singular control that any HO can buy from any big box store or off of the internet, you really don't have a very accurate view of the HVAC industry at all.

[RoBoTeq]

I bet the OP is wondering when the hell someone will give him some advise lol

Plenty of advice has been given to the OP and the OP has already stated their appreciation for the advice that was given. Just exactly what have you contributed to this conversation?

[RoBoTeq]

Here's the wiring diagram for the Carrier/Bryant/Payne furnaces I've been seeing:

Notice that the Limits switches are in series with the R terminal on the board. It would be interesting to know how many are done this way vs. just killing power to the gas valve.

Once again, you show your ignorance of how furnaces work. Aside from the other safety features that come into play when the high limit switch is activated, just what else do you propose should be done?

[Joehvac25]

Plenty of advice has been given to the OP and the OP has already stated their appreciation for the advice that was given. Just exactly what have you contributed to this conversation?

I haven't contributed anything, I just sat back and read 3 pages of bickering back and forth with intermittent information being passed around. I think the OP was wondering about which furnace he should buy or how to pick a contractor, not about how a limit string works and whatever else.
In my opinion I would tell the OP to research company's rather than brands, find someone who will do the job right.
I'm not saying your input was faulted I was enjoying the debate myself.

[54regcab]

I thought I made it clear that hydronic coils have been used for quite a long time now. There is nothing new about hydronic coils or hydronic air handlers, they were available over 40 years ago when I started in the HVAC industry.

My tankless water heater will be taking care of my domestic water heating as well as auxilliary heating for my heat pump and spot floor heating for my bathrooms and kitchen area. In most cases when I have installed systems for new houses being built where natural gas was not available, I would install an oil or LP water heater for domestic hot water and to supply hot water coils for auxilliary heating for heat pumps. Did that for decades.

Never said that is was new, just becoming more popular. Only have seen a handful of them around here.

[54regcab]

Once again, you show your ignorance of how furnaces work. Aside from the other safety features that come into play when the high limit switch is activated, just what else do you propose should be done?

I was just trying to explain what I've been running into in the field. The fix to the high limit cutoff issue lies to resolving why the furnace is overheating to begin with. The limit itself is just a safety device.

[RoBoTeq]

I was just trying to explain what I've been running into in the field. The fix to the high limit cutoff issue lies to resolving why the furnace is overheating to begin with. The limit itself is just a safety device.

You continue to change the intent of your accusational posts and continue to avoid answering specific questions.

What exactly is it you are complaining about with the way the HVAC industry deals with furnaces when they reach their temperature limits?

What is it about furnaces that has you running around the internet, especially on DIY sites, complaining about the HVAC industry? There are ways to install less heat in any system, so why are you complaining that specific furnaces are not manufactured in lower capacities?

[RoBoTeq]

I was just trying to explain what I've been running into in the field. The fix to the high limit cutoff issue lies to resolving why the furnace is overheating to begin with. The limit itself is just a safety device.

You are giving advice on and making comments on things you evidentely do not fully understand. That sort of thing may be ok on the DIY sites you are on, but that sort of thing is not acceptable on a site where there are so many professionals in the HVAC industry who have to correct your misinformation.

When a furnace overheats, several things happen, depending on the model of the furnace. At the very least, more than just shutting off the gas valve occurs.

It doesn't matter what type of thermostat is installed on any furnace dependent on the thermostat to initiate furnace operation. All any such thermostat is is an on/off switch that is thermally controlled.