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03-24-2008, 05:20 PM #1New Guest
- Join Date
- Mar 2008
Auxiliary Heat Control During Subzero Weather
I am a homeowner looking for technical details so I can understand my HVAC system and properly diagnose faulty operation, etc. I am fairly knowledgeable but agree that HVAC DIY is not wise. I am having an 18 SEER Amana R410A split heat pump system installed by an HVAC contractor. This system is comprised of a 4-ton ASZ18 compressor, a White-Rodgers 1F95-1277 T-Stat, and a variable speed AEPF426016 air handler equipped with 20 kW of resistance heat strips (HKR-20CB kit). It’s replacing an 25-year-old Trane heat pump system that provided good service over the years but the Amana 18 SEER unit should provide a significant savings over the approx. 8 SEER unit it’s replacing. I live in the Chicago area where below zero operation occurs several times a year. The Trane compressor ran during all non-emergency heat demand. I was told that this is to prevent “dry” startups on very cold days. It was equipped with a hard start kit and a crankcase heater. I think my electric-energy-electric-utility costs have been relatively competitive with equivalent gas-energy-electric-utility homes in my area.
The new system has 2 stages for both heating and cooling (Y1 and Y2) and up to 4 stages of resistance heat (W1 and maybe W2). The Duel Fuel T-Stat option controls the second stage of auxiliary heat (W2) using an adjustable Outdoor Remote temperature setting but this option cuts off the compressor. The ASZ18 is efficient well below zero versus resistance heat so the compressor should continue to run. From the data I’ve seen for the ASZ18048, pure resistance heat will be more expensive to operate in subzero weather. I will not use this option unless below zero operation is an issue with respect to compressor life or the lifetime warranty on the Copeland compressor. Does this make sense? If not, please explain.
My contractor has explained that the auxiliary resistance heat consists of 4 stages of 5 kW heat strips controlled by time sequencing. As he explained, a T-Stat call for auxiliary heat (W1) will start a progressive process where additional stages of heat strips are energized after every 10 minutes that the T-Stat demand is not satisfied. I have not been able to find a written explanation of this process anywhere and the Amana/Goodman technical support people are not willing to discuss any of this with a lowly homeowner. From the schematics, I see that the heater strips have 2 circuits that can possibly be independently and externally controlled, which corresponds to the T-Stat and air handler terminals (W1 and W2). It’s not clear to me how the time sequencing operates. Is the progression 5, 10, 15, 20 kW? If the heat strips are controlled by W1 only, is the progression 10 then 20 kW (heat strip pairs only)? Would it make sense to use an independent outdoor thermostat to control W1 and/or W2 for subzero operation? Assuming my HVAC system is properly sized, what is the best auxiliary heat setup for my location and equipment? Is there a way to keep the compressor running if T-Stat W2 is used? Thanks for your help!!
03-24-2008, 05:50 PM #2
I believe on the air handlers control board, there is a jumper that must be cut to use 2 stage electric heat.
If that stat can handle 2 stage Hp, and 2 stage electric heat.
Ask your installer about up grading to the Honeywell IAQ stat, it can handle 4 stages of heat. It can also enhance humidity control in the summer, by slowing the VS blower.
03-24-2008, 07:38 PM #3
I think maybe you and your installer are not talking about the same thing. From what you've written, your main concern is economy of operation. Your statement is that the HP will be more efficient to operate down to some temperature below zero and you'd rather have that type of operation. You're contractor, if I'm reading things properly, is simply telling you that at some set outdoor temperature, the HP is locked out and you're on straight resistance heaters, 2-stage or not. Those are two different topics of discussion.
I'm assuming your installing company did a Manaul 'J' load calculation. He then selected a HP that will deliver the needed Btu's per hour down to some sub-zero number. If you don't know what the number is, you need to ask. Below that number (design heat load) you'll need auxilliary heat that will deliver either sufficient supplemental heat, that when added to the output of the HP at that temperature, exceeds the BTU needs of the home. If on the other hand, the HP is to be locked out (turned off) below that design temperature, then you need auxilliary heaters equivalent to the TOTAL Btu need of the home down to the minimum design temperature. So if your old Trane system used the HP output PLUS the resistance heat output to give you the needed Btu's below zero, then the new system needs to either do the same (HP + Electric) or the electric resistance heaters need to be sufficiently sized to handle the entire load without any help from the HP.
As to economy of operation, your installer should be able to give you the exact temperature at which the HP becomes more expensive to operate than the electric heaters based on the cost of electricity in your area.If YOU want change, YOU have to first change.
If you are waiting for the 'other guy' to change first, just remember, you're the 'other guy's' other guy. To continue to expect real change when you keep acting the same way as always, is folly. Won't happen. Real change will only happen when a majority of the people change the way they vote!
03-25-2008, 05:44 AM #4
As to economy of operation, your installer should be able to give you the exact temperature at which the HP becomes more expensive to operate than the electric heaters based on the cost of electricity in your area.
The cost of electricity has no bearing on which is cheaper.
They both use electricty.
When the HP's COP is less then 1, is when the strip heaters will be cheaper.
I'll guess you were thinking of duel fuel.
03-25-2008, 10:46 AM #5Regular Guest
- Join Date
- Jun 2007
- The Twilight Zone
To the OP:
- All Amana/Goodman systems are ARI-rated with the optional TXV (thermostatic expansion valve) installed in the air handler. The AEPF comes with a check flowrater expansion valve (F in the model# means flowrater). Get the optional TXV for peak efficiency in cooling mode.
- Additionally, the 1" filter rack in that air handler is poorly designed. Get a separate 4" media filter cabinet to avoid the monthly aggravation that I go through ripping out the old filter and surgically re-installing a new filter.
Best to you.
03-25-2008, 04:31 PM #6New Guest
- Join Date
- Mar 2008
Auxiliary Heat Control During Subzero Weather - Reply
Thanks to all who responded. I appreciate your expertise.
The White-Rodgers 1F95-1277 can handle 4 stages of heat (Y1, Y2, W1, & W2) but the forth stage (Dual Fuel, W2) locks out the compressor. The Amana compressor data for the ASZ18, which can be found on the Amana website, has of COP of 1.61 for High Stage heat at -10°F. I don’t see any mechanical reason to lock out the compressor; do you see any mechanical reason? If not, I will turn off the Dual Fuel option and W2 will never be energized. I verified this information with White-Rodgers. Locking out the compressor in very cold weather is my first issue. I am ok with W2 not being usable. There are other ways to deal with this, which is my second issue.
By the way, my AEPF will get the optional TXV. I believe it’s the only way you can get 18 SEER out of this split system. Yes, the 1” filter rack is very, very poorly designed. And yes, heating and cooling calculations have been performed for my home. Second stage compressor heat is good to about 20°F outdoor temp, maybe lower. At that point, the addition of 10 kW of resistance heat should be good down to about -10°F. The final stage of 10 kW resistance heat should be more than enough to handle the all time record low of -26°F for this area. This setup should also be able to handle Emergency resistance demand with the compressor off. I don’t want any more tonnage because it will be way oversized for cooling and I will have difficulty controlling humidity. Thanks but these are not my issues for this post.
Yes, I am very concerned about efficient/economic operation. With a COP of 1.61 at -10°F, I definitely want to limit resistance heat to those times when the compressor can’t handle the heat loss. Assuming it’s not malfunctioning, I don’t think there will ever be a time when the compressor heat will be more expensive than resistance heat. However, I think I should understand how resistance heat is controlled so I can properly choose the best setup options for my situation and have my contractor install those options and the proper controls for them. This same knowledge is also needed in order to recognize faulty operation. Heat pumps are a rare breed in the Chicago area. I’m afraid I will get the standard setup if I don’t ask for something special that may be better. I have a feeling that factory settings and default options are typically set up for the South. For example, Amana and Goodman do not have snow stands for their compressors. I struggled with that one but that is also not an issue for this post.
As explained in my original post, there are four 5 kW heating elements but only 2 control circuits of 10 kW each (W1 and W2). These circuits are supposedly sequenced but I’m not sure how sequencing works. If the W1/W2 jumper is not cut and W2 (2nd stage resistance) is time sequenced, then an outdoor T-Stat may not be needed. Otherwise, W2 must be controlled by an outdoor thermostat. I believe these are my only options if I don’t activate the T-Stat Dual Fuel feature, which I do not plan to do. I definitely don’t want 20 kW of heat with the first stage of auxiliary or emergency resistance heat.
I’m having difficulties verifying how the HKR-20CB heat strips are sequenced. Are they progressively sequenced in 5 kW increments, 10 kW increments, or not sequenced at all? If they are sequenced, assuming they are time sequenced, what are the time increments? This is the information I need to make an intelligent decision on whether to use an outdoor thermostat. I think I may be happy with time sequencing because 20 kW of resistance heat would be rarely used in my situation but I don’t want it to activate too soon. Any information you have on resistance heat control and sequencing would be appreciated. This is my second issue. Thanks for your help.
03-25-2008, 07:21 PM #7
Since you don't have duel fuel, don't activate it on the thermostat.
Most 20KW strip heaters are energized in 10KW increments.
Although there is nothing wrong with that WR thermostat.
The IAQ stat will give you the control your looking for. And save you from having to get an OD stat to wire in also.
03-26-2008, 01:00 AM #8Regular Guest
- Join Date
- Jun 2007
- Metro St. Louis
I would think that you could set up your thermostat to do two stages of aux heat - with the compressor running on both.
However, I'm not sure that you can stage the 4 elements in the way it was described. I've got the MBE blower instead of the AEPF, and it can only do two stages of strips.
I also looked at the ASZ18 - almost put it in instead, but it's specs aren't quite as good in heat mode.
Here's a link that might help: