Is There A Way To Calculate Recovery From Setback Time?

 
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Old 02-24-2018, 04:51 PM   #1
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Is There A Way To Calculate Recovery From Setback Time?


I'm working on my new shop and sizing the mini-split I'll use to heat it. It only has a heat load of about 18,000 BTU/hour for 70 degrees on a design day. However, I will likely keep heat at about 50 most of the time (which means heat off much of the winter here), but it would be nice to not have to wait a day to heat it up to 70+ for painting or finishing a project.

I don't really want to spend about $1000 extra for a 3 ton unit, but quicker recovery is also appealing. A bigger unit can also save money. 24 hours of warm up time costs more than 4 hours at twice the power usage.


Any way to calculate this recovery time?

Option 2 is a small mini split plus a wood stove.
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Old 02-24-2018, 06:04 PM   #2
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Re: Is There A Way To Calculate Recovery From Setback Time?


Excellent question, one that bugs me occasionally too, albeit with a slightly different slant.

The wife and I have thermostat wars every winter. She wants to crank the heat way down while we're away from the house, and then crank it up when someone gets home. Aside from the thermal mass deal, which can be relatively easily overcome if you throw enough energy at it, there has to be a way to calculate the best balance in terms of overall economy.

I just haven't been quite cheapskate enough to really go nuts looking for that formula.

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Old 02-24-2018, 06:09 PM   #3
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Re: Is There A Way To Calculate Recovery From Setback Time?


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Originally Posted by Tinstaafl View Post
Excellent question, one that bugs me occasionally too, albeit with a slightly different slant.

The wife and I have thermostat wars every winter. She wants to crank the heat way down while we're away from the house, and then crank it up when someone gets home. Aside from the thermal mass deal, which can be relatively easily overcome if you throw enough energy at it, there has to be a way to calculate the best balance in terms of overall economy.

I just haven't been quite cheapskate enough to really go nuts looking for that formula.
I hate to say this, but I do know with decent confidence that turning down while you're away will save energy. The lower the average temperature, the lower your heat bill.
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Old 02-24-2018, 08:40 PM   #4
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Re: Is There A Way To Calculate Recovery From Setback Time?


Contact the manufacturer who makes the unit you're considering...
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Old 02-25-2018, 05:09 AM   #5
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Re: Is There A Way To Calculate Recovery From Setback Time?


Quote:
Originally Posted by Golden view View Post
I'm working on my new shop and sizing the mini-split I'll use to heat it. It only has a heat load of about 18,000 BTU/hour for 70 degrees on a design day. However, I will likely keep heat at about 50 most of the time (which means heat off much of the winter here), but it would be nice to not have to wait a day to heat it up to 70+ for painting or finishing a project.

I don't really want to spend about $1000 extra for a 3 ton unit, but quicker recovery is also appealing. A bigger unit can also save money. 24 hours of warm up time costs more than 4 hours at twice the power usage.


Any way to calculate this recovery time?

Option 2 is a small mini split plus a wood stove.
If a 18,000 BTU takes 24 hours to recover the area. A 36,000 would take 12 hours, and not use any less energy.

Next, must mini splits won't let you set the heating temp lower then 30 or 64F. So you would have to turn it off if you don't want it to maintain that high of a temp.

As to figuring out recovery time. A load calc to know how many BTUs you need to maintain/reach 70 indoor temp at you outdoor design temp. can be graphed to show recovery time for any outdoor temp down to design temp.
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Old 02-25-2018, 08:47 AM   #6
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Re: Is There A Way To Calculate Recovery From Setback Time?


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If a 18,000 BTU takes 24 hours to recover the area. A 36,000 would take 12 hours, and not use any less energy.
I think that would be too simple. If it's losing 17,000 BTUs per hour, an 18k unit only has 1k available to raise the temperature. 36k should raise the temp 19 times as fast.

Now that I think about it, I believe I'd need to know the thermal mass to calculate this.

I looked at some units that have "vacation mode" of 46 degrees. I'll probably get one of these. Odds are it'll be a 3 weeks off, 1 week on kind of schedule. If that wasn't the case I'd just leave it at 64 all the time.
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Old 02-25-2018, 09:20 AM   #7
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Re: Is There A Way To Calculate Recovery From Setback Time?


the wood stove option is what we use in the house, thermastat set to 66, first one home starts the wood stove, house is usually above 70 in two hours
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Old 02-25-2018, 11:22 AM   #8
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Re: Is There A Way To Calculate Recovery From Setback Time?


Quote:
Originally Posted by Golden view View Post

Now that I think about it, I believe I'd need to know the thermal mass to calculate this.

You would. Keep in mind, when it's at 50F, you aren't losing 17k BTU. It gets even more complicated the manual J, because you have to include an air gap between the heated air and the cold walls. Maybe an R-1 or R-1/2 insulating layer.
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Old 02-25-2018, 12:37 PM   #9
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Re: Is There A Way To Calculate Recovery From Setback Time?


Quote:
Originally Posted by Golden view View Post
I think that would be too simple. If it's losing 17,000 BTUs per hour, an 18k unit only has 1k available to raise the temperature. 36k should raise the temp 19 times as fast.
No, doesn't work like that. If a space needs 17,000 BTUs to maintain 70F when its 0F outside. And is allowed to drop to 50F inside. Then it only takes 11,780 BTUs to maintain temp. Giving a 18,000 BTU unit 6,220 BTUs to raise the temp back up to 70, with the amount of reserve BTU decreasing as the temp rises.

While a 36,000 BTU unit will raise the air temp twice as fast. It will not raise the temp of the walls or objects as fast. So it will short cycle on and off several times after the area's air temp has reached 70. Using more electric to do the same job.

Also, unless you buy a non inverter model. As the rooms temp gets closer to set temp, it will modulate down in capacity. So a 36,000 wouldn't heat the area up much quicker then a 18,000 BTU.
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Old 02-25-2018, 12:48 PM   #10
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Re: Is There A Way To Calculate Recovery From Setback Time?


Quote:
Originally Posted by beenthere View Post
No, doesn't work like that. If a space needs 17,000 BTUs to maintain 70F when its 0F outside. And is allowed to drop to 50F inside. Then it only takes 11,780 BTUs to maintain temp. Giving a 18,000 BTU unit 6,220 BTUs to raise the temp back up to 70, with the amount of reserve BTU decreasing as the temp rises.

While a 36,000 BTU unit will raise the air temp twice as fast. It will not raise the temp of the walls or objects as fast. So it will short cycle on and off several times after the area's air temp has reached 70. Using more electric to do the same job.

Also, unless you buy a non inverter model. As the rooms temp gets closer to set temp, it will modulate down in capacity. So a 36,000 wouldn't heat the area up much quicker then a 18,000 BTU.
I think we're in agreement, except with short cycling. Mini-splits modulate so far down in output so they almost never do this.

Practically speaking, a 36K unit will heat it back up much more than twice as fast as an 18K.

It just occurred to me I build a project for a client that I can use as a test example. It has a 1 ton unit but a 6k design load. Slab floor.
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Old 02-25-2018, 01:03 PM   #11
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Re: Is There A Way To Calculate Recovery From Setback Time?


Quote:
Originally Posted by Golden view View Post
I think we're in agreement, except with short cycling. Mini-splits modulate so far down in output so they almost never do this.

If it modulates down, it won't short cycle as much.

Practically speaking, a 36K unit will heat it back up much more than twice as fast as an 18K.
Modulating down, it won't heat it up twice as fast. If you get a modulating 3 ton, you can't have your cake and eat it too.
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Old 02-25-2018, 01:13 PM   #12
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Re: Is There A Way To Calculate Recovery From Setback Time?


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Modulating down, it won't heat it up twice as fast. If you get a modulating 3 ton, you can't have your cake and eat it too.
Yes you can. They modulate down only as they approach they approach the set point. Often right at it, since the reduction in output is almost instant. Many even read how quick the temperature was rising and plan the ramp-down accordingly.

Basically the opposite the modulating furnaces I've encountered. They start on low and then ramp up if set point isn't hit within a certain amount of time.
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Old 02-25-2018, 02:02 PM   #13
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Re: Is There A Way To Calculate Recovery From Setback Time?


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Yes you can. They modulate down only as they approach they approach the set point. Often right at it, since the reduction in output is almost instant. Many even read how quick the temperature was rising and plan the ramp-down accordingly.

Which means if the temp is rising quickly like you want,
it will modulate down to slow the temp rise.


Basically the opposite the modulating furnaces I've encountered. They start on low and then ramp up if set point isn't hit within a certain amount of time.
A Modulating furnace controlled by the thermostat will go to a higher input coming out of setback.
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Old 02-25-2018, 08:42 PM   #14
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Re: Is There A Way To Calculate Recovery From Setback Time?


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A Modulating furnace controlled by the thermostat will go to a higher input coming out of setback.
That's good to know. My experience is with units with poor control. Maybe I'm thinking two-stage, not modulating.

Still doesn't help me here though.
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Old 02-27-2018, 01:13 AM   #15
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Re: Is There A Way To Calculate Recovery From Setback Time?


A programmable thermostat would lessen this issue, or an InterNet connect Tstat....

It is easy and quick to heat the AIR in your new shop 20 degrees F or ~11 degrees C for the Nort landers, air weighs ~0.075 lbs / cubic foot? so a 14' tall x 50' x 40' = 28000 Cubic feet of air at 2100 lbs, BUT now your 2000' squared of SOG that weighs 40-50 tons and the 30-40 tons of damp subgrade if U cheaped out on underfloor insulation.......

Then there is the internal sheathing/ drywall, ceiling materials, and EVERY thing that you store inside the heated space, all them 50 lbs boxes of mixed hardware left over from that job in 1997........ bags of cement, old model Ts whatever.

I'd consider some floor to ceiling sliding curtain walls to "shrink" the heated area to just the work area.....
Plan b start on the lean too to store all that heavy 'craaapp' that you collect in unconditioned dry (insulated?) storage.

If you haven't poured the SOG, I'd stare at a N.G. or L.P. fueled hot water heater as a affordable 'boiler' for an in floor radiant heat supplement/booster to the forced Hot Air of the mini heat pump .

Its awesome for car service.
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Old 02-27-2018, 07:46 AM   #16
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Re: Is There A Way To Calculate Recovery From Setback Time?


I've built many buildings with in floor radiant. The recovery from opening a shop door (even hangar door) is almost instant.

However, everyone seems to fail to realize that even though air heats fast, you won't be heating the air fast because the surrounding thermal mass takes the heat back out of the air.

Slab already poured 9 years ago before I bought the place, so I won't be adding radiant.

If I was building a shop from scratch, I still wouldn't do radiant in this mild state because the recovery from setback is so slow. My last shop had it and it was great, but served a different purpose. The building could lose heat for 2 days at -40 degrees and not freeze up.
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Old 02-28-2018, 01:15 AM   #17
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Re: Is There A Way To Calculate Recovery From Setback Time?


Use the programmable tstat to turn on the heat at midnight if needed to recover, or whenever to get it warm enough for your start time, if you're really cheap have it set back an hour or two prior to quiting time. Ceiling fans on a switched power that only is hot when the furnace is running.

If your ceilings are tall enough you could scab on a radiant heat layer...

If you have a few extra Sq. ft. you could install a sand filled CMU "trombe" heat sink in the spare space, leave most of the head joints open for more air flow/heat transfer

I'd look at cheaper energy sources, N.Gas if available or wood waste maybe.. even on the commie West Coast your talking maybe 5-7 $ a day? and that sounds high to me.

A door blower test might show you where your air is sneaking in and out?

Digging in a layer of blue board around the SOG would pay in a couple of years, tiling around the shop, with gutters drops that run away from the footers, would dry the subgrade, thus getting you a higher R value for the subgrade, damp is cheaper to heat then wet earth....
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Old 02-28-2018, 08:01 AM   #18
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Re: Is There A Way To Calculate Recovery From Setback Time?


I do have gas but I'd have to trench 150 feet in a difficult spot. Trying to keep cost down. Mini split and gas is about the same cost per BTU, and mini split comes with free air conditioning. Mini split heats roughly 3 watts for every watt you put in. You could call it 300% efficient.

Heat load calc tells me $646 a year for a mini split and $545 for gas.

Having an insulated slab (both under and perimeter) lowers this to $562 and $468.

We simply don't have high heating costs here.

Wood waste though is appealing. I probably make enough to heat the structure year round.
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Old 02-28-2018, 12:45 PM   #19
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Re: Is There A Way To Calculate Recovery From Setback Time?


I'm struggling with a similar debate for a small shop I'm looking to build. The air source heat pump is very appealing given that it heats and cools in one unit which is nice for trying to keep wood at a stable RH, but is pretty limiting when it gets really cold. Seems almost sacrilege to not have a wood stove in a wood shop though.

There a tons of research papers proving that thermostat setback does save on energy costs, within reason. Although I personally don't mind if the shop is really cold in the morning and have to start a fire to warm it up, I do worry about some of the hygrothermal problems that could arise in the cavity if I let the temperature get too cold. This paper is what got me thinking about that. Maybe I'll just do continuous exterior insulation....
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Old 03-01-2018, 02:57 PM   #20
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Re: Is There A Way To Calculate Recovery From Setback Time?


Used Carpet/wood flooring on the shop floor would insulate it lowering the BTU load per hour greatly....just throw it away every year and replace

"Horse barn mats"? much easier to sweep and Vacumn.

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