Do smart thermostats make sense in a high-performance home with heat pumps?
NormanWB
| Posted in General Questions on
The idea of using significant setbacks to reduce energy usage or even smart/learning thermostats seems at odds in a high performance home using heat pumps or am I missing something?
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Replies
I tend to agree you should set your heat pump thermostat and forget it for heating.
Now we are getting into cooling season and you will be away from home from 7:30 AM to 5:30 PM 10 hours should you set back your thermostat?
Walta
I don't think "smart" thermostats are a good idea in any situation. Having a machine or second party privy to controlling appliances in your home is kinda scary don't you think? More smart users are what we need.
I would generally agree with you about smart gizmos in general having downsides, but many “smart” thermostats can operate independently of a network connection and still do a lot of things (basic setback, vacation mode, etc), but with a much easier to use user interface.
Setback and vacation functions can save significant amounts of money if set appropriately. I run a lower heating temperature at night for sleeping, for example. In terms of just holding a fixed setpoint though, there isn’t much difference. I’m not so sure the occupancy sensing features of some smart thermostats would be good with modulating-type devices either since those devices try to run continuously at low levels to be in equilibrium with the space.
Bill
To date I have not found any significant savings if any with smart thermostats. On furnaces and boilers, the internal limit will cycle the equipment during the long call for heat when playing catch up.
On cooling and heat pump (heating) the compressor will run every second they system is trying to achieve set point.
As I joke on my blog from time to time in posts, you can pay for your BTU over the course of the day or lump sum before you come home.
Other factors come in to play but that's a separate post.
From a standpoint of cooling or heating g the home up before arriving when away for a few days prior (weekend homes) the wifi stat could be considered a comfort luxury.
There are smarts built in, you don't need a third party stat. For mini split heat pumps to run with top efficiency they don't just need an on/off heating call, they need to know the space temperature so they can modulate accordingly. A regular thermostat simply closes a relay to call for heat/fan/cooling/etc. but doesn't actually communicate the space temp or error from set point. Some units have occupancy sensing, which may or may not be useful.
But trying to get a Nest or Ecobee to run a modulating heat pump won't make it work any more efficiently aside from being good at holding it off when you're not around.
I don't know if Martin has amended his cut on smart thermostats since this blog:
https://www.greenbuildingadvisor.com/article/martins-useless-products-list
I have not found a "smart" thermostat that is easy to override, which is very frustrating. Setback strategies and their impact vary quite a bit by climate. I am all for effective controls but I want to be their ultimate "adult supervision."
Peter
>"...using significant setbacks to reduce energy usage or even smart/learning thermostats seems at odds in a high performance home using heat pumps ..."
SIGNIFICANT setbacks almost always increase energy use with 1-2 stage heat pumps unless you lock out the heat strips to avoid applying resistance heat to the recovery ramp.
With modulating heat pumps (even those without heat strip, such as ductless mini-splits) it usually increases energy use too.
>"I have not found a "smart" thermostat that is easy to override, which is very frustrating. "
The Honeywell Lyric T5 at my house (my kid's idea, not mine) can be manually overridden for temperature at any time as long as the "Screen Lock" function isn't in "Fully Locked" mode.
https://www.honeywellstore.com/store/images/pdf/RCHT8612WF-lyric-t5-wifi-thermostat.pdf
Mi esposa uses it's geofencing mode to set the temperature back whenever she is some (programmed) distance away, but that is often overridden by other family members who come home (or are already home) when she is out of range. The potential security issues of a hacker being able to figure out when you're more than X miles away from the house is of some concern when using that function. I don't know (or particularly care) how easy it is to set the temperature via phone app (It's only one zone), but others might.
I'll assume the OP is thinking "superinsulated" when he says "high performance," and that the heat pump is sized properly for the design heating load, with at most a very small difference between that load and maximum output of the heat pump. There is then almost nothing to be gained by a night-time (or daytime at-work) setback. Sure, you could set the thermostat, manually or by program control back to say 62 F in the evening, but the heat loss of such a house is so low that the interior temperature would never drop to that reduced setpoint. Savings in energy is integrated reduction in delta-T times time. The savings would be a very small fraction of something already very low. Furthermore, with little extra heating capacity over the rate of heat loss, a heat pump could take a few hours to recover whatever drop occurred in the interior temperature, creating a comfort issue. "Set and forget" is realistic for such a house.
I meant a home meeting or exceeding the specifications laid out here (see page 10): https://buildingscience.com/sites/default/files/migrate/pdf/BA-1005_High%20R-Value_Walls_Case_Study.pdf