EcoRenovator - View Single Post - What temperature do you keep your house in the winter?

MN Renovator · 02-07-14, 08:01 PM

There are multiple factors to using setbacks.

If you have a heating system based on an inverter heat pump, you'll be at a lower efficiency when it is running at full capacity versus coasting along. If using a standard heat pump and not using heat strips its a bit of a wash. If you've got heat strips you are really losing out.

If your heat capacity closely matches design load and you are close to the design load you need to balance the setback depth to the time that you set the thermostat to restore your temperature prior to getting home. If you are heating with natural gas or propane you are most likely a great candidate for setback because these systems are almost always oversized because of their size increments being in 20k differences and the smallest 95+% condensing furnace is a 40k unit and even that is enough for a fairly deep setback. I have a furnace with a 57k output and I can recover 12 degrees in an hour even when it is -10 degrees F outside, my heat load at -13f is 17k and my house is 2100 square feet, I'm at a definite oversize. I'd be comfortable with a 20 degree setback set to a 2 hour recovery with a 40k furnace with a 2 hour recovery and I'd be at within 5 degrees of my setpoint and I'm okay with that. I'd use a 2.5-3 hour recovery if I wasn't. I work 12 hour shifts and am away from home for 13 hours at a time so I'm willing to drop the temp and my furnace doesn't run when I'm not home at all as I don't lose heat down to the 'safe against freezing setpoint' that I use. With a heat pump of any kind, if sized very closely to the heat load of the house, your setback potential is reduced.

Thermal mass is tricky and if you are using in slab heat through the a high mass concrete floor, you are limited to your setback options but if you are well insulated and use solar water heat, you get the benefit of not losing too much temperature overnight and supplemental heat needs on a well designed solar gain house are far reduced. If you don't have a high solar gain design, I think you are better off not going with high thermal mass, I'm just not seeing the advantage to it without trying to capture external heat.

"On a customer's recent house analysis, I got almost a 25 F decay in 12 hours. Virtually no thermal mass in the home."

Don't forget that the 25f decay in 12 hours is also largely a factor of insulation and infiltration. Even if you have very low mass you can reduce the heat loss and cut this down. A house that loses 25f in 12 hours isn't going to save energy being high mass but at least if they are off to work for 8-12 hours a day they can slip in a setback to reduce the temperature difference to the outside and inside to not lose as many valuable BTUs to the outside.

02-07-14, 08:01 PM	#129
MN Renovator Less usage=Cheaper bills Join Date: Nov 2010 Location: Minneapolis, MN Posts: 939 Thanks: 41 Thanked 116 Times in 90 Posts	There are multiple factors to using setbacks. If you have a heating system based on an inverter heat pump, you'll be at a lower efficiency when it is running at full capacity versus coasting along. If using a standard heat pump and not using heat strips its a bit of a wash. If you've got heat strips you are really losing out. If your heat capacity closely matches design load and you are close to the design load you need to balance the setback depth to the time that you set the thermostat to restore your temperature prior to getting home. If you are heating with natural gas or propane you are most likely a great candidate for setback because these systems are almost always oversized because of their size increments being in 20k differences and the smallest 95+% condensing furnace is a 40k unit and even that is enough for a fairly deep setback. I have a furnace with a 57k output and I can recover 12 degrees in an hour even when it is -10 degrees F outside, my heat load at -13f is 17k and my house is 2100 square feet, I'm at a definite oversize. I'd be comfortable with a 20 degree setback set to a 2 hour recovery with a 40k furnace with a 2 hour recovery and I'd be at within 5 degrees of my setpoint and I'm okay with that. I'd use a 2.5-3 hour recovery if I wasn't. I work 12 hour shifts and am away from home for 13 hours at a time so I'm willing to drop the temp and my furnace doesn't run when I'm not home at all as I don't lose heat down to the 'safe against freezing setpoint' that I use. With a heat pump of any kind, if sized very closely to the heat load of the house, your setback potential is reduced. Thermal mass is tricky and if you are using in slab heat through the a high mass concrete floor, you are limited to your setback options but if you are well insulated and use solar water heat, you get the benefit of not losing too much temperature overnight and supplemental heat needs on a well designed solar gain house are far reduced. If you don't have a high solar gain design, I think you are better off not going with high thermal mass, I'm just not seeing the advantage to it without trying to capture external heat. "On a customer's recent house analysis, I got almost a 25 F decay in 12 hours. Virtually no thermal mass in the home." Don't forget that the 25f decay in 12 hours is also largely a factor of insulation and infiltration. Even if you have very low mass you can reduce the heat loss and cut this down. A house that loses 25f in 12 hours isn't going to save energy being high mass but at least if they are off to work for 8-12 hours a day they can slip in a setback to reduce the temperature difference to the outside and inside to not lose as many valuable BTUs to the outside. Last edited by MN Renovator; 02-07-14 at 08:04 PM..