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10-10-2012, 02:06 PM #1New Guest
- Join Date
- Oct 2012
New Bryant Evolution Extreme Heat Pump + Modulating 97 Furnace
My wife and I just built a new home in Minnesota and had the Bryant Evolution Extreme Heat Pump installed, paired with a 97% efficient furnace with the modulating speed blower.
I'm new to the world of heat pumps, so I'm curious about a couple things.
1. When on "Hybrid Heat" the furnace seems to run more often than I think would be necessary, given that our temperatures have only been getting down to the low 30s. Does this happen to help "warm up" the system in some way when it's just turning on? If I select "Heat Pump Only" on the evolution control, the heat pump does a fine job keeping the house warm without the furnace.
This leads to what might be the bigger question...
2. How does the "Hybrid" system know when to switch between the furnace and the heat pump? Is it based on the # of degrees it is trying to warm the air inside, or the exterior temperature, or some other factor? For the lowest operating cost, would I be best served to leave it on the "Hybrid" mode or keep it on "Heat Pump Only" mode until the nighttime temps get closer to the single digits?
3. With this sort of system, will I see lower operating costs by having the program drop the temperature just a few degrees during the day, then bring it back up before we get home in the evening, or would it be more advantageous to set the temperature very low while we're gone, to minimize the amount the system needs to run while we're gone, then turn it way back up before we get home, in essence, have it not work much at all, then a big workload to warm it up, or a lower but more consistent workload?
10-10-2012, 03:17 PM #2New Guest
- Join Date
- Oct 2012
By the way, our heat pump is the 20.5 SEER/13.0 HSPF model. Natural Gas costs .58/therm currently, and electricity rate is $0.06975/kwh.
10-10-2012, 03:55 PM #3
Both your gas and eletric are very low. If the heat pump was sized correctly, the thermal balance point is probably around 30F. Especially since effciency takes a hit around this tmeprature since it needs ot go into defrost. IT might be runnign hte furnace during defrost and then just staying on the furnace.
You economic balance point is also probably around 30-35F, but I'll let on of the other pros crunch the numbers.
WHen it switches depends on how it was set-up. I think you can select a temperature, or just let it stage based on capaicty and demand. SO when the controller sense that the heat pump is falling behind... it will bring on the furnace for more capacity.
The use of setbakcs will alos make it run on the furnace more. A heat pump might be able ot keep up at 30F if sized correctly, but it definitely won;t change hte tmeprature very fast, expecially in a larger well insulated home.
Overall, setbacks should be used for comfort, but for the most part, don't save energy and in the summer can even use more energy. WIth a hybrid system, in the coldest part of winter, you could save a little turning it down at night, when it would run the furnace anyway. But in the afternoon when it gets over 30F< you want it to run the heat pump as much as possible. IN general, whe nthe heat pump is running above abotu 30F with your low electric rates, it costs less than the furnace. When you use a setback, you force the system to "recover" whcih will often cause it to run the furnace. Make sense?
However, that being said, if you defer your heat pump run time to late afternoon, the heat pump will be more effcient at that time. The opposite is true in summer. THe worst time to run the system is 2-8PM.
My opinion, just set the temperature for comfort first, then in the afternoon weekdays when yoru away, set it back maybe 1-2F in the winter. Do not set back for cooling except for comfort in the evening.
10-10-2012, 05:27 PM #4
If your infinity control is set to comfort, set it to economy.
Next, find out what the COP of your heat pump is at different temps. With your posted rates, your heat pump has to operating at a COP of 2.75 or higher to cost less to use then your gas furnace. And i calculated that only using 95% efficiency for your furnace.Contractor locator map
How many times must one fix something before it is fixed?
10-11-2012, 06:00 AM #5Professional Member*
- Join Date
- Nov 2004
- SW FL
Economic balance point is near 27'F.Designer Dan
It's Not Rocket Science, But It is SCIENCE with "Some Art".
Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities
10-11-2012, 08:01 AM #6
Many of these new systems, calculate the recovery time into the startup temp to provide the temperature requested by the setback or setup stat settings. A big benefit from summer setup is the large amount dehumidification from a longer cool down cycle priour to occupancy. Also high indoor temps mean less latent cooling to maintain <50%RH.
These rates utility rates are really low, about 50% off typical. Heat pump will not compete anytime defrost is required which is usually <36^F.
Regards TBBear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
Use +Merv 10 air filter. -Don't forget the "Golden Rule"
10-11-2012, 08:37 AM #7
Air cooled condenser have reduced effciency and capacity as the outdoor temeprature increases. Therefore, while you will reduce you totla heat gain over the period by using a afternoon setback and will redcue shorter cycles, you are deferring your operating period to a time of day when the unit will be the least efficient, and have the least capacity to recover. Further, if sized correctly, a AC unit won't be able to recover and on a larger home with more thermal mass, could take 8-10 hours to recover at or above design conditions. Even longer with vented attics and wall materials like brick & stone that will store up quite a bit of heat. TO make matters worse, durign this recovery period, the internal heat loads are usually at a peak. People come home, cook, watch TV, uses do laundry, dishes, shower, turn on lights...etc. You internal heat load probably is 2-3X the average load used in a Manual J. This is why properly sized equipment often seems undersized to many people... and why furnaces rarely if ever seem undersized. You have all these internal loads going on, and design conditions usually only last for a few hours early on a couple cold morning... then the sun comes out and warms it up and people start using appliances, etc. Also, thermal mass and structures like basements and slabs are tricky to determine the actual heat loss and heat system tend ot get oversized a little for worst case scenrios with these.
My experience with a properly sized system, was that in hot weather, setbacks just weren't possible. If I let the structure warm-up to lets say 76F and I wanted to 73F for sleeping that evening, it could be well past midnight before it recovered. IF I kept it 74F, it had little trouble recovering.
You are correct however that dehumidification benefits from a long run time during a recovery. I'm not saying that you nessesarily eliminate recoveries. I also personally like to sleep when it's cooler and drop the temp in the evneing before bedtime. In my home at least with it's large amount of mass and relatively small AC unit, these PM recovery periods give me 60+ minute run times. My total run time for 100 days starting in late June through September , September, was right around 1000 hours or about 40%. It was over 50% until September rolled around.