Setbacks?
I am an energy consultant working with homeowners of existing homes. Could use a little input on a recent question. Customer has a second home, upstate NY, ZONE 5, and resides only weekends. Asks what should I set back my t-stadt to during the 5 days a week we are not here? 1998 home, oil boiler, 3 zones, indirect hot water heater. Typical conventional (ranch) construction. I have come to believe too much setback short-term can be both an energy and comfort penealty in an average to poor enclosure home. When you get into multiple days setback I’m thinking common sense is the more the better and vacation mode on the dhw. Anybody have any insights would be greatly appreciated.
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Replies
Mike,
It depends on how vulnerable the plumbing pipes are to freezing. Air leaks in the basement or air leaks in an isolated bathroom can lead to frozen pipes, even when the living room is at 42 degrees F.
I would say that a thermostat setting of 50 degrees F should be relatively safe, but you need to know the house and use some judgment.
The homeowner might also consider installing an alarm that notifies the homeowner when the indoor temperature drops too low (as might happen if the boiler malfunctioned).
50 degrees, water main shut, outside wall sink cabinets opened.
Water drained homes I set at 45 degrees.
Install a Nest tstat. Customers adjust heat remotely (via internet) ahead of visit and will know temp at all times in case of loss of heat or power.
Martin is right. I've seen pipes in a basement freeze at a 45-50F setting due to a leaky sill and uninsulated box sills. In many homes some of the plumbing runs right along these locations, and a large amount of cold air infiltration here can freeze pipes, despite the fact that you have a temperature setting well above freezing. Understand the rest of the building in terms of air leakage and thermal performance to roughly gauge what may or may not be okay.
>>I have come to believe too much setback short-term can be both an energy and comfort penealty
How would there be an energy penalty from setback?
David, What I was getting at with the energy penalty is in heating, and especially in hot water heating, there is a radiation factor that is important to think about. It takes a while for all the elements in your space to come up to temperature, right? Heat goes to cold, right? So if short term you set back too far then you must call for heating longer to overcome this. To gain efficiency there is a line somewhere between maintaining temperature and the cycle time to replenish. I think from the response most everyone is mostly worried about too much can cause freezing so I'm left thinking as much a set back as possible before the posiblity of freezing. Please correct my thinking if I'm off base.
The heat loss out of the house goes down the lower you set the temp, since there's a lower delta-T across the insulting materials. With setbacks as deep as 45F for an interior temp those savings are substantial. It will ALWAYS use less fuel overall to run with a lower temperature, independent of how long, or at what temperature the radiation takes to bring the house back up to temp.
The amount of time it takes to heat the place back up depends on the output BTUs of the boiler, the thermal mass of the house and the outdoor temps turing the recovery ramp. With a boiler tightly sized to the actual design heat load (very rare with oil-fired boilers- 2-3x oversizing is more typical), that can take several hours, and that is where using in internet-attached thermostat such as the Nest (recommended by AJ BUILDER) or Honeywell Redlink or similar comes in: If the homeowner knows when they expect to arrive, they can use the internet to check the temp at the house and start the recovery ramp sufficiently in advance that it will have reached the setpoint by the time they arrive, for zero shivering in the cold waiting for the house to heat up.
If the foundation walls (but not the slab) are insulated to R10 or better and all of the plumbing is indoors you can get away with fairly low indoor temps without much risk of freeze-up, since the basement would be the warmest spot in the house when the setback temp is below the deep subsoil temps. If the basement isn't insulated, leaks tons of air, and the basement temperature is not controlled as a heating zone, it could be an issue even at 50F. If the foundation isn't insulated, it SHOULD be, and would be quite cost effective at the current price of heating oil.
Thanks, Dana and everyone!