View Full Version : Heat pump with a gas furnace back up
04-27-2005, 01:46 PM
I'm a refrigeration guy who has never worked on a residential heat pump. However I am closing on a house that has an air to air heat pump with a gas furnace back up. Can anyone tell me if the sequence of events that is supposed to happen when the thermostats call for back up heat.
I know that with electric heat the two systems can operate together because the electric heat is downstream of the evaporator, but I read that with gas furnace back up heat the 2 should never operate at the same time.
I haven't closed on the hose yet so I cannot be there to check things, but I did notice a lot of frost on the outdoor coil when I was there for the building inspection.
It was a cool rainy day. I have been told that there will be no defrost cycle when the heat pump operates in conjunction with a gas furnace, because that would cause high head pressures as the evaporator absorbed heat from the furnace.
I also know that the AC coil is downstream of the heat exchanger and that is the correct way to do it. Otherwise cold air could cause condensation in the heat exchanger and ruin it.
Knowledge replies would be most appreciated.
04-27-2005, 02:04 PM
Operation depends upon the dual fuel control. Some run the pump as 1st stage and gas as 2nd stage. Others base it solely on outdoor temp running pump if warm and gas if cold.
Most do defrost, the short time the furnace runs usually doesn't get hot enough to hurt anything. Some brands include a plenum sensor to shut off the gas if it does get too warm during defrost.
04-27-2005, 02:14 PM
Originally posted by BaldLoonie
Others base it solely on outdoor temp running pump if warm and gas if cold.
I think this is the way it works. It's hard to say. The homeowner knew nothing. I was concerned because it didn't look like a heat pump thermostat on the wall.
What about the frost? How much frost is too much on a 45° rainy day?
04-27-2005, 03:26 PM
Frosting can occur in that weather, how much is hard to say. 100% humidity and a coil around freezing. Should be a HP stat though.
04-27-2005, 04:10 PM
Yeah, I'd expect frosting in that weather too, but I'd also expect it to defrost. I don't ever recall seeing one that didn't defrost, but anything is possible I guess. Every time I wire up one of these systems I always ask the customer what their expectations are, or what the salesman promised? That way I have a better idea of which way to control it because there are a few different ways it can be done. You didn't mention a brand name. That might help us because most everyone has some type of add-on heatpump kit for a situation such as yours. It is just so hard to be specific because there are so many different ways to do it. Oh by the way, if you haven't closed on that house yet, you may want to have a third party check it out for you. That is especially true since you already have questions about its operation.
04-28-2005, 08:50 AM
I don't know the model. I am mainly just interested in the normal sequence of events.
The frost doesn't concern me. It was raining and cold that day. The thing only looks at the defrost thermostat every 90 minutes. The amount of frost that I seen could have easily accumulated in 90 minutes. Especially with the rain. There was not frost on the suction line or the compressor.
I guess there could be a problem with the defrost relay, defrost thermostat, or the timer. I know that the reversing valve is good, because we ran it in the cooling mode, I heard it energize and reverse. The liq line was warm and the gas line got cold leaving the evaporator.
I leveraged the guy an extra $400 at closing by telling him it needed to be repaired or at least checked. He chose to pay the money instead of calling a contractor. Of course there were other things wrong with the house as well. I ended up getting $4500 at closing all together. I'm keeping that money.
Third party?? That's you!! And that's as much as I will spend on a third party. I been doing commercial refrigeration for 15 years. I think I can take care of this one with a little help from this forum and my books.
04-28-2005, 10:44 AM
If it does turn out to have a single stage stat, you're going to want to toss it when you find out how unresponsive that setup is going to be when it's 40 degrees outside. It might be ok if you set the thermostat and leave it at the same spot straight through heating season, but with no way for the stat to respond to larger demands in mild weather, it's going to annoy you.
Sequence of operation varies depending on your exact control system, but many fossil fuel kits aren't much more than an outdoor thermostat that switches fuel based on outside temp. Combine that with a single stage stat and see how responsive the system can be at 40 degrees. You better set that thermostat and not want to increase it until the weather changes, because you're not going to get enough output to get anywhere. There are better systems, but I don't see how you can do very well at that job without at least a two stage thermostat. Having that, though, means you need a way to prevent frequent switching between heating stages. A two-stage stat is set up based on the assumption that it's no big deal to kick on the second stage for a few minutes here and there, since it's probably going to be strip heat. That won't fly with dual fuel, though, because to switch stages you are stopping one piece of equipment and starting another. All the efficiency goes away if you can't stick with one or the other for long enough to get into its efficient operating range.
I think even the most basic fossil fuel kits can defrost and probably run the furnace for defrost tempering. That's a good thing, because cold blow is annnnnoying. You may find that the installer didn't actually wire things up to provide defrost tempering, though, assuming that a dual fuel system won't be defrosting much anyway. That depends a lot of how aggressive you are about setting the balance point down low, though. When I run near my 30 degree balance point, my system will start a call with the heat pump, run it for 90 minutes straight just treading water, then defrost (with tempering) and finish the call for heat with the furnace. If the balance point were higher, especially with a demand defrost setup (which I don't have), though, you wouldn't see so much defrosting. The nice thing about the better setup is that during that 90 minutes of treading water, if I get chilly and bump the thermostat setting, it will switch over to the furnace immediately.
While you can potentially overheat the indoor coil while running the furnace for defrost tempering, you won't really see that. The furnace doesn't get the call for heat until the defrost initiates. OK, wait 30 seconds for inducer prepurge, then wait a minute for the HX to heat up, and then the blower turns on and warm air starts hitting the indoor coil. Since the blower was off and the heat pump has been cooling for 90 seconds already, the mass of the indoor coil is now COLD. Then you've got to heat the mass of the indoor coil back up while still working against the cooling that the HP is still providing to that coil. By the time the indoor coil could really start getting hot, the defrost is probably done.
If you do end up replacing the stat, go for a real dual fuel thermostat and toss the old fossil fuel kit. Benefits of the dual fuel stats? They have a better understanding of the difference between the two heating stages, and can be smarter about changes in demand or returning from setbacks when you're close to the fuel changeover point. It's hard for a regular HP stat to do as good a job in recovery from setbacks, because it never knows how much output the HP will be able to provide. Dual fuel stats always come with outside temperature sensors, so they have a shot at doing decent anticipation of how long recovery may take. They also generally finish a call for heat with the furnace after a defrost, given the logic that once the HX is hot and the heat pump has lost its rythm anyway, you might as well finish the job with the furnace. You can set a fixed balance point if you want, or you can let the stat decide it based on demand within a certain window (for example, my heat pump is allowed to run at >30 degrees ODT, and the furnace is allowed to run at <45, so it chooses fuel based on demand but only between 30 and 45). You can adjust the balance point, if you decide you want to, without even setting foot outside.
I have my defrost timer set for 90 minutes, and that works fine around here. If the heat pump runs for 90 minutes straight on the rare day with freezing or near-freezing rain, the coil will be almost completely frosted over by the time it defrosts, but that doesn't seem to bother the discharge temperature, even at minute number 89. Then boy, oh boy, does it put up a steam cloud, and then the heat pump gets to rest and really dry out while the furnace finishes the call for heat.
It's a nifty system, I think, with almost any control system, even a fossil fuel kit. Some people got it and then bypassed them because of issues like cold blow, but that's not an inherent problem with the system, just some setups. Depending on utility rates and local climate, it can really save on heating costs. It lets you use the heat pump when it can be most efficient and the furnace when it can be most efficient. The furnace never has to run a short cycle, because lighter demand is always handled by the heat pump. You get more blower operation indoors, because the heat pump runs long cycles and the furnace doesn't have a chance to short cycle, so you get more even temps indoors and better IAQ without having to actually use a constant fan setting. The only place where it doesn't look so great operationally is in climates where you need very frequent defrosts (Pacific Northwest, right on a lake, etc.).
[Edited by wyounger on 04-28-2005 at 02:36 PM]
04-28-2005, 11:19 AM
Thanks a lot. Everything I read covers mainly how the system works with electric heat. That seems pretty simple.
REGARDING THE GAS FURNACE SYSTEM ONLY. My next question is about the indoor fan during defrost and changeover. When the heat pump goes into defrost and the furnace kicks in, which fan speed is supposed to be running.
Does the high speed fan stay running, because the fan relay is still energized, or is there another relay that locks out the indoor fan relay coil to allow the low speed fan to run with the furnace.
Or does the fan switch energize another relay to switch fan speed only when the heat exchanger is warm enough.
That's an answer that I couldn't find in my books.
04-28-2005, 02:28 PM
Hmm. Good question. First, though, keep in mind that heating airflow isn't necessarily low and cooling/fan isn't necessarily high. It depends on what equipment you're running.
I've always done dual fuel with dual fuel thermostats; first a Carrier programmable, then the Infinity system.
I don't think (gas) heating and cooling on my first dual fuel system used different fan speeds in the first place. I'm trying desperately to remember if that Carrier dual fuel thermostat setup could stop the indoor blower during the first stages of a defrost. I think it did stop the blower until the furnace heated up, but it's been long enough that my memory is hazy on that one.
On my Infinity system, it tempers for defrost using low fire. My low heat fan speed is so close to that of the heat pump fan speed, though (800 versus 700 cfm, or something like that) that I haven't noticed exactly which it uses for defrosting. I'd have to watch the diagnostics during a defrost, and I just don't remember. I have noticed that Infinity actually stops the blower from the initiation of a defrost until the furnace is heated up. I do vaguely remember reading somewhere, though, that with Infinity systems it uses whichever airflow rate is higher.
I also don't know if all Infinity dual fuel systems use low furnace heating for defrost tempering, or if in some configurations it uses high stage. It's not selectable, but I can see where if you had a three ton heat pump on a 40k input furnace (there are some valid matches like that), you would hope that it would know that you'd need every bit of furnace output to avoid cold blow.
Oh... I forgot another advantage of dual fuel thermostats. They know when a defrost is initiated, and even if they satisfy during the defrost (lag time, setpoint change, etc.), they will keep the heat pump and furnace running until the defrost actually completes. With a fossil fuel kit you could potentially have the call for heat end at just the wrong time, shutting everything down before the defrost actually finished.
As for the operation you might see with a fossil fuel kit, I guess that would depend on the exact kit. I've not played with those to have any answer. I would expect the simplest solution to be the answer, so the blower will probably run at HP/AC speed nonstop until the furnace is hot. At that point, hmm... whichever of the two speeds is higher. Just a wild guess. :)
There are certainly some folks on the board that have used lots of fossil fuel kits who can answer that part better.
Here's a schematic for one kit that I found in a quick and dirty web search: http://www.jacksonsystems.com/pdf/ffk100operation.pdf
[Edited by wyounger on 04-28-2005 at 02:35 PM]
04-28-2005, 02:32 PM
Thanks. We're getting there. Like I said, I can't even get in the house to check these things for myself. I'm just trying to get on overview of the possibilities so I'll know what to look for.
04-28-2005, 02:40 PM
Aha! I found the manual online for the Carrier dual fuel stat I had in my last house. Sequence of operation and everything.
The Thermidistat manual has a more complete explanation of how Carrier thinks dual fuel should be handled, but I don't think it operates dual fuel systems any differently than the more basic model does.
[Edited by wyounger on 04-28-2005 at 03:18 PM]
04-28-2005, 03:25 PM
Thanks a lot man. I really appreciate that.
I'll look those over. This web site is great.
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