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  1. #1
    Join Date
    Dec 2005
    Posts
    7

    Unhappy

    Looking for a better Thermostat to replace a Honeywell Chronotherm III (early 90s) which thinks it is controlling a simple heat pump, not a dual fuel system. The system is a heat pump (Carrier 38BY030300), 80% eff. oil furnace (Armstrong Ultra 80 L6B84/95C12-5A) but no heat strips. There is an external bulb thermostat which sets the crossover temp. between HP & furnace.

    Here's how I want the system to fucntion:
    Set back 10 deg colder at night
    Temperature to be at setpoint within 10 minutes of scheduled time
    Furnace to handle large temp. cahnges - if too cold for HP to accomplish in reasonable time
    HP to maintain temps - even when very cold

    Don't want:
    Furnace to run only a few minutes when HP goes into defrost
    Cold air blowing when HP defrosts
    HP to come on so early (> 2 hours) in the morning in order to get to set point. Pros/Cons?

    Considering using RobertShaw 9720i or 9870i, and White-Rodgers 1F95-371 or 1F95-377. Wondering if de-humidification option on 9870i is worth the extra $.

  2. #2
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,913
    Most people that have duel fuel, got it to save on heating cost.

    But you seem to want to use the more expensive fuel, gas to recover.


    Even if it takes the HP 2 1/2 hours to recover, chances are it would still cost less then using the gas furnace.

    Have you looked at the honetwell vision pro 8321.
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    How many times must one fix something before it is fixed?

  3. #3
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,086
    Sorry, forget the Vision Pro. He wants backup heat to come on. VP won't do that.

    I've found more reliability out of W-R 1F90 than the Invensys models. The 1F95-371 is an impressive stat. Summer humidity control requires VS blower or a jury-rigged fan slow down method.

  4. #4
    Join Date
    Dec 2005
    Posts
    7
    Wouldn't mind using the HP more for better economy, i.e. to maintain temps, but really don't want it going into defrost and running furnace briefly or alternatively, blowing cold air - the current setup, as fuel smell usually accompanied short runs of the furnace - perhaps due to 20' chimney flue.

    Wondered how these stats typically handle the HP defrost cycle? Would it be wise to add heating strips for just the defrost? If so, is a stat with 3 stage heating required?

  5. #5
    Join Date
    Dec 2005
    Posts
    7
    Beenthere,
    Looked at the VisionPro, but prefer buttons to touchscreens.

    BaldLoonie,
    Can a standard 3 speed blower be used for hunidity control by
    utilizing the slowest speed? Liked the intertmittent fan feature on the Invensys stats - to recirculate air while temps allowed to rise when in summer cooling mode. Do the W-R stats have the smae feature, or they just fan ON programmable?

  6. #6
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,086
    Defrost heat is a function of your wiring, not the stat.

    As for humidity control, if you are currently using a high speed for cool, and that isn't always the right speed, you can rig up a relay to slow the blower if the stat calls for dehumidify.

    I don't think the W-R has an intermittent fan feature.

  7. #7
    Join Date
    Mar 2004
    Posts
    1,042
    My vote is for the Carrier dual-fuel thermostat. When properly configured, you can set it up with a range of outdoor temperatures where it can choose between the two heat sources based on demand. But below that temperature range, it's only fossil fuel, and above that range it's only heat pump. To prevent the furnace from running very short cycles to provide defrost tempering, they finish the entire call for heat with the furnace after a defrost. Once the setpoint is satisfied and it cycles off, it can go back to the usual logic. (If you really want, you can also set it with a single switchover temperature, or let it decide based on demand without any cutoff temperatures, but those options don't work quite as well.)

    They accomplish some nice stuff with this setup.
    -No short furnace cycles but without sacrificing defrost tempering. How? Once a defrost cycle is initiated, it always finishes. They detect this by noticing that during heat pump heating, there is a suddenly a call for backup heat that the thermostat knows it didn't initiate. If the call for heat is satisfied during the defrost, the system keeps doing what it's doing until the defrost completes. (On systems with multistage backup, it can step down to a lower backup heating stage during that condition to avoid overshooting while it's waiting for the defrost to terminate.) A lot of old-school dual fuel setups will shut down the heat pump right in the middle of the defrost cycle if that happens.
    -Direct control of two-stage fossil fuel backup. You get to pick which stage to use for defrost tempering. (The Thermidistat model can handle all four stages of two stage heat pump with two stage fossil fuel backup setup).

    If you want humidity control, Carrier's Thermidistat can do all the above, and also have additional control logic for humidity control. When properly configured with compatible equipment (yours may not support every goodie), it can not only provide some overcooling when needed, it can also select lower blower speeds during cooling mode, and keep the blower from running for that extra 30-60 seconds after a call for cooling. I'm not sure about the others, but the VisionPro line so far only has one dehumidification strategy, and that's the overcooling.

    All of these options will require some low-voltage wiring. Both Carrier units require the use of an outdoor temperature sensor to do dual fuel control.

    I don't recommend using continuous fan operation during cooling season except in arid climates. It just re-evaporates moisture off of the cooling coil, pushing indoor humidity up just when you were trying to keep it down. These models don't have an intermittent fan feature either.

    I have read about one creative setup where they installed a duct heater downstream of a dual fuel system, and wired the duct heater solely for defrost tempering. Unless the system was defrosting, the elements stayed off and it acted like a normal dual fuel system. To me, that seems like a lot of trouble for very limited benefit, though.

    Finally, you can't get the full benefit out of a heat pump using big setbacks like that; depending on the control system, it either won't catch up on time, or you'll switch to fossil fuel in a rush to catch up. Unless you can't sleep without it being cold in the house, try much smaller setbacks (or none). Even with the "finish heating with fossil fuel after a defrost cycle" logic, you still want minimal setbacks. Say you are several degrees from your setpoint when the time comes to recover; the heat pump starts up, runs for 10 minutes, and then needs to defrost. The furnace starts and runs until it completely catches up! If you were only one degree behind, the furnace still runs until it catches up, but that amount of time is much less, getting you to an off-cycle and back onto the heat pump much sooner. Result: less gas/oil consumption, more heat pump runtime (which we can assume is a more economical heat source for you or else you wouldn't have gotten dual fuel!).

  8. #8
    Join Date
    Nov 2005
    Location
    USA
    Posts
    529
    I second the Carrier tstat recommendation. Have it, love it.
    The posts and comments made by me are in no way affiliated with any company or organization. They are simply my personal opinions.

  9. #9
    Join Date
    Sep 2003
    Location
    Northern Ohio
    Posts
    104
    The Carrier T-stat is the most versitle one out there and I've tried quite a few and keep going back to the Carrier/Bryant p/n TSTATCCPDF01-B and you will also need the outdoor sensor accessory

  10. #10
    Join Date
    Dec 2005
    Posts
    7
    "Finally, you can't get the full benefit out of a heat pump using big setbacks like that; depending on the control system, it either won't catch up on time, or you'll switch to fossil fuel in a rush to catch up. "

    wyounger,

    So what is the benefit of using a programmable stat with a heat pump?

  11. #11
    Join Date
    Mar 2004
    Posts
    1,042
    You use the programmability part in cooling season, where you get the same benefits as always.

    To me the *ideal* system would watch outdoor temperature and decide how far it should set back based on learned history of how long it takes to recover. Some systems will tell you they do that, but will only start recovering up to a max of 90-120 minutes in advance. In weather at or below the balance point, though, you don't want your heat pump to cycle off for a setback period, only to have to use more expensive heat sources later to catch up on work it could have done. So the ideal system in my book would be programmable, and would set back on days that were warm enough that the heat pump would be able to recover without any aux heat. On days that weren't warm enough for that, it would count the entire setback period as "In Recovery", as Honeywell might say, and not set back at all. Then the heat pump runs nonstop, maximizing what you get out of your cheapest heat source, and minimizing how much work you're going to need your strip heat or furnace to do later. That's especially true for overnight setbacks. How much recovery can a heat pump do at 5-6 am, the coldest hour of the day? Probably none. So if you let it lose ground overnight, like 10-11 pm when the setback period might begin and you might still be above the balance point, it's just going to take more aux to catch back up in the morning.

    I haven't found anything with such smarts. To some extent thermostat manufacturers are constrained in their creativity by the consumer's ability to understand what the thermostat is doing. If the consumer catches the stat doing something that they find counterintuitive, they're going to think it's screwy, even if in fact it's doing something that's perfectly logical and will save the consumer money. If the consumer doesn't understand it, it still counts as a problem.

    For that matter a smart thermostat could learn that "we're probably going to go below the balance point tonight, and I'm probably going to have to use some aux heat in the morning." With a long-term perspective like that, it might make sense to actually heat *beyond* the desired no-setback setpoint with the heat pump during the day (when COPs are highest), again knowing you will lose some ground by morning without using some aux heat. Just like when you are driving a gutless vehicle and a really big hill is coming, you probably try to be going faster than usual when you get to the base of the hill, knowing that you're going to be losing speed the whole way up. At the same time, can you imagine how many complaints they would get if they started selling people thermostats that ran the heat pump *extra* during the day when the people aren't home, contrary to popular logic? Most people would freak out over that. The utilities and government have been telling them that more setback is always better, but that is really only true with regular AC, regular fossil fuel heat, and ok, electric-furnace heat, even. Heat pumps need to run around the clock under a lot of winter conditions, though, and all a setback is going to do is shut down your cheapest heat source so you can pay more to catch up later with another heat source.

    Depending on the weather, you can get away with *some* setbacks during heating season with a heat pump. During spring and fall, I set back my own heat pump two degrees while I'm at work and two degrees while I'm asleep. With the 90 minute recovery time that my system allows (and an auxiliary heat lockout in place just to be sure) my system will recover OK without any aux heat during those conditions. During the winter, though, that's not so much the case, and so I use no setbacks at all. The heat pump gets more run time during the day and overnight that way, and it doesn't have so much catching up to do when it doesn't really have any excess capacity with which to catch up.

    [Edited by wyounger on 01-26-2006 at 01:16 PM]

  12. #12
    Join Date
    Jan 2003
    Location
    Olathe Kansas
    Posts
    246
    This is only an opinion. Say we have two systems-- everything indentically the same including heat loss. Now that we have two systems the same lets put two very different thermostats on to control the systems. The first will have a fancy set back thermostat with smart recovery and the second will have the a thermostat with no smart recovery. It's 20 deg. outside and we have a set back of 5 deg. The second thermostat is going to trip 2nd stage immediately until it comes upto within 2 deg. of set point. The first thermostat wil start early and is going to use first stage 30 to 60 min with no aux. and then go into second stage because it could'nt make set point. I'm going venture to say that the system with the set back thermostat probably is going to use about the same energy as second system. I don't think that the fancy thermostats are some kind huge energy saving device that manufacturers want the public to believe. Hope you all can figure out what it is I'm tying to explain.

  13. #13
    Join Date
    Mar 2004
    Posts
    1,042
    I tend to agree, at least during colder weather. The concept works a lot better with fossil fuel heat. Then the idea is just to have the thermostat figure out how early to start so that the house will hit the desired temperature at the same time, even under varying weather conditions. Without it you either come home to a house that is just starting to recover, or you try to guesstimate how much earlier the system should start to try to recover, probably starting way too early sometimes and way too late other times. That's no big sweat when you have a heating system that can recover from a typical setback in an hour or less. That's just not the story with heat pumps, though, in many cases.

    I think it does make a big difference though when the weather is milder. If the heat pump can recover your 5 degrees in 90 minutes without aux heat, the smart recovery thermostat will let it. The old-school thermostat will still use aux until it's two degrees from the setpoint, unless you have a lockout on your strips.

    This is why I keep trying to come up with more and more elaborate control logic to get that last little bit out of the heat pump. In the end, though, just using minimal setbacks, or no setback at all, has darn near the same effect, at least until the point that you're starting to use aux to maintain your setpoint even during the day when you're away at work. To me it's a shame to have in the same 24 hours both the use of aux heat and also the heat pump cycling off. If the stat understood when you wanted strict temperature control and when you would allow it to be a little more creative, you could better maximize your HP output and minimize your use of backup.

    I think programmable stats are good stuff for cooling, though, and great in the case of oversized cooling or fossil fuel heating equipment. Since efficiency (and latent control) is only achieved during longer runtimes, and oversized equipment doesn't get any long runtimes while maintaining one temperature, the benefit of setbacks is more pronounced in those cases. The more you can get oversized equipment to make one long go of recovery after a setback, and the less short-cycling while maintaining temperature, the more chance it has to work efficiently and provide some latent control.

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