Abby Normal wrote
That is just plain wrong. The whole point of a set back is that less is pumped at the setback temp because the losses are lower. How much lower depends on how well insulated the house is.
Wrong again.
Not if you one of the systems sized so that doesnt happen.
Again, not if you have more than one system.
Abby Normal pontificates:
I disagree.
Nick
Newsgroups: alt.home.repair, sci.engr.heat-vent-ac, misc.consumers.frugal-living From: "Rod Speed" - Find messages by this author
Date: Sat, 17 Dec 2005 07:47:00 +1100 Subject: Re: prog. therm. and heat pump questions Reply | Reply to Author | Forward | Print | Individual Message | Show original | Report Abuse
The whole point of set back is to lower the temperature for a while.This would imply that the house cools down and the systems stops running during this cool down period. Then it would run less per hour due to the setback indoor temperature until such a time that it need to start warming up the house so that it was back up to temperature perhaps when the occupants awoke or perhaps returned home from work.
The problem is shortly after the temperature dropped to the setback temperature, the heat pump would end up running steady just to get back up to temperature.
Typically in the temperate states where air source heat pumps are used, they are sized with the cooling load in mind and use the heat strips. One sized for the full heat load will be grossly oversized for cooling resulting in summer time humidity control problems.
No not wrong just unsucessful in educating you.
So you are saying size one for the cooling load, one for the heating load and one for speedy recovery from set back then?
Three heat pumps now. Plus all the ductwork and backdraft dampers. Let's try to keep this practical and not go to hypothetical extremes to prove this is possible. Noah er I mean Nick is a bad influence on you.
Well like I said if you want to install three heat pumps, knock yourself out.
Newsgroups: alt.home.repair, sci.engr.heat-vent-ac, misc.consumers.frugal-living From: "Rod Speed" - Find messages by this author
Date: Sat, 17 Dec 2005 07:47:00 +1100 Subject: Re: prog. therm. and heat pump questions Reply | Reply to Author | Forward | Print | Individual Message | Show original | Report Abuse
The whole point of set back is to lower the temperature for a while.This would imply that the house cools down and the systems stops running during this cool down period. Then it would run less per hour due to the setback indoor temperature until such a time that it need to start warming up the house so that it was back up to temperature perhaps when the occupants awoke or perhaps returned home from work.
The problem is shortly after the temperature dropped to the setback temperature, the heat pump would end up running steady just to get back up to temperature.
Typically in the temperate states where air source heat pumps are used, they are sized with the cooling load in mind and use the heat strips. One sized for the full heat load will be grossly oversized for cooling resulting in summer time humidity control problems.
No not wrong just unsucessful in educating you.
So you are saying size one for the cooling load, one for the heating load and one for speedy recovery from set back then?
Three heat pumps now. Plus all the ductwork and backdraft dampers. Let's try to keep this practical and not go to hypothetical extremes to prove this is possible. Noah er I mean Nick is a bad influence on you.
Well like I said if you want to install three heat pumps, knock yourself out.
Figure out any Biot numbers yet Nick?
Abby Normal wrote
To reduce the amount of energy consumed to keep it at the higher temp.
Correct.
Correct.
Wrong with the situation being discussed, with more than one system so it doesnt take a long time to get it up to the normal temp again.
And what was being discussed was having more than one system and doing that so it doesnt take a long time to get back to the normal temp and doesnt use the heat strips to do that.
Yep, completely wrong.
You cant even keep track of what was actually being discussed.
Yep, I mentioned 3 for a reason.
You dont have to duplicate/triplicate all of those.
Wasnt doing anything like that.
Nothing to do with Nick at all.
Two and three was what was being discussed.
Your claim that setback has no value is just plain wrong with THAT situation.
That was honest of you.
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If you installed two complete heat pumps to save on and speed recovery, the added cost of the second system would be more than the savings provided by not using any strip heat. If you installed a slightly oversized 2 stage heat pump, it would run in low stage all summer providing about 65% total cooling capacity. Then it would run in high stage (High Capacity) during heating season when it was below 45 degrees outside. You could possibly set back in this case. Note that installing a two stage heat pump usually adds about $2,000.00 to an average installation. The average life of a quality, properly installed air to air heat pump is 15-20 years nationally. If you would save $2,000.00 over that time span, then install the two stage system. If you install two complete systems, add about $6000.00 to $10,000.00 to the cost of the first system, assuming you had room for the extra ducts and registers and equipment. Tough way to save money. Like going around your elbow to get to your asshole. (Or is it the other way around:-)? ).
Strretch
Yes, that's why I said it would only make sense if the extra was surplus etc.
You wouldnt need all the ducts and registered duplicated if you just used it for the faster return from the setback temp.
Yes, that's why I said it would only make sense if the extra was surplus etc.
That makes sense. Then again heat strips have their place. It isn't hard to think of a situation where they would save money (assuming net.thought.police still allow thinking), eg in a house with lots of heat loss and little thermal mass and a wimpy heat pump, eg 1000 Btu/h-F and 1000 Btu/F and a (70-30)1000 = 40K Btu/h heat pump on a 30 F night. If the heat pump can only keep up with a 40 F temp diff, no setback is possible, so a 10 hour night requires 400K Btu of heat, or 200K Btu of electrical energy with a COP of 2. With heat strips, the house can be 50 F most of the night, with 100K Btu of heat pump electrical energy and 1000Btu/F(70-50) = 20K Btu of 8 AM strip energy, so the strips save
80K Btu, ie 23 kWH of electrical energy, and the low-cap heat pump is cheaper.Why not a single-speed compressor with low and high airflows?
Nick
Nick,
If you check the data, or measure it yourself, Strips cost 2 to 3.5 times as much per BTU as the heat pump. So you set back by shutting off the cheap heat (Heat Pump Compressor), and recover by using the expensive strip heat. Setback increases the electric bill if the heat strips come on during recovery. In addition to warming the air during recovery, you also have to warm the entire thermal mass of the house and all it's contents. THAT is ia big part of the added power consumption.
Here in Myrtle Beach, SC, a lot of homeowners are relocated yankees (like me), used to setback. They setback in the winter and their electric bills go up. They call me to check their system, I explain proper operation, they stop using setback in the winter and their electric bill goes back down. Happens every winter, many times.
Many heat pump setback thermostats lock out strips during the beginning of setback recovery. On those, recovery takes a LONG time. After a number of hours, the strips are allowed back on, recovery occurs and electric bill goes up.
Note that recovery without strips only works in mild weather, so setback efficiency depends on your climate. It does not work well in my climate.
By the way, my wife, like many women, has to make several potty trips to pee every night. If I installed another setback thermostat and her tush gets cold on a regular basis every night, I will get the cold shoulder when it comes to sex. That alone is enough reason not to use a setback thermostat.
Stretch
"Stretch" wrote
I have a 12 year old thermostat (Chronotherm III) that has the adaptive recovery feature as you suggested. When the system comes on (when coming out of a setback), it will try to reach the set point without bringing on the auxilary heat strips. It will only bring them on if it "looks" like it's not going to meet the set temp at the time set. Seems to work pretty well, but if it's down in the 'teens, forget it. That's when the gas logs kick in anyway.....;-]
Stretch wrote
Nope, not with a very lossy house that loses more heat at the higher temp over the setback time than that 2/3.5 times.
Thats always been a furphy.
Depends entirely on the thermal mass of the house.
Doesnt mean that there arent situations where setback works. You dont get called out to those situations.
Still doesnt mean that there arent situations where setback works, particularly with deliberately oversized systems that dont need to use strips to come back off setback.
Bet it does with a bigger system.
The obvious approach there is a heated toilet seat, stupid.
That's logical, since the whole purpose of a heat pump is to be more efficient than resistive heating.
Also logical, but aren't there programmable thermostats that are smart enough to deal with this? I have a cheapo programmable thermostat which is supposed to be able to intelligently determine how much time it takes to recover. A well-designed programmable thermostat for a heat pump would be very conservative about this and would err on the side of recovering way too early (hours early) rather than having to kick in the resistive heating.
I don't know that such thermostats exist, but it seems like it would not be all that hard to make one that is smart enough to avoid having to turn on the resistive heating due to setback. Machine learning techniques are pretty good these days, and the thermostat has good information available to it (the average rate the temperature drops when the system isn't running gives it an idea of the load, and the duty cycle gives it an idea of its capacity relative to the load). The worst problem seems like it would be changing weather (where it's 30F colder one night than the previous), but even that can be solved if the system realizes the temperature is dropping a lot faster over the course of the night than it was the previous night and knows not to expect last night's data to be a good indicator.
- Logan
Or just use the inside and outside temps and the measured performance of the system at that outside temp to know when it should come off the setback to have the inside temp back to normal temp at the time specified. Not a shred of rocket science required at all.
And if the occupants of the house are going to be out for the day, is it really necessary to get back to normal temp for the short time in which they are getting up, showering etc and heading out ? It may make more sense to do that stuff at the setback temp and let the system bring the temp back up to the normal temp in the late afternoon before they are due to return instead, and be able to use the higher outside temp to get back to normal temp without using the strips at all.
Abby Normal pontificates:
I disagree.
Nick
You disagree with almost everything Nick. Have you EVER installed ANY heating or cooling equipment? Have you EVER installed a heat pump? Do you have test equipment and data loggers to measure the performance? Do you have any PRACTICAL experience? Or do you just like to disagree? You obviously have a brain, it is a shame you don't do something more constructive with it.
Stretch
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That is sure economically viable.
To get a heat pump to work with setback, you are oversizing the system. Either a single oversized unit, or multiple units. It takes a rediculous situation to make setback work on a heat pump.
Yes multiple systems, now as high as three, just to prove a point that you can setback a heat pump.
Sure, buy 3 heatpumps to prove set back. Spend extra money on the ductwork involved for three systems, including back draft dampers, lol what a crock
Well I believe you were speaking first of two systems, then realized you needed three, so as not to be short cylcing in the heating mode, and in the summer having too much sensible cooling that the stat would be satisfied in 5 minutes run time.
Three systems sure, that is practical,
No you could have three systems share a common supply and return, just get it to work there einstein.
No not at all just install three heat pumps. Or two heat pumps and a central AC. Hey maybe you could get a two stage one and a central AC. SUre is a lot of tap dancing to prove that you could viably set back a heat pump.
Setback has a value, just not with heat pumps.
A fossil fuel system sized right on the money for the heat load in a high thermal mass home may not be the best system to be setting back either.
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