Installing an Automatic Shutoff for Boilers
BACKGROUND
I’m renovating a 150-year-old 3-story 5,000 sq ft building in Rutland, Vermont. It was constructed as a single-family home and then was split into 4 units.
The building currently has 4 boilers—one subdivided for each unit—that provide hot water for both domestic use and piped radiators. That is what I currently have but I’m on the verge of doing a heat pump retrofit and putting in heat pump hot water heaters.
I need to set up a temperature-controlled automatic shutoff for my boilers. My plan is to retrofit the building with air-source cold climate heat pumps and they are rated to be effective down to 0° Fahrenheit. My ideal solution would be for the heat pumps and boilers to communicate through a single thermostat and for the heat pumps to be used exclusively until the temperature drops so low that only the boilers can be effective. I don’t know of any way to network my heat pumps and boilers together like that—so I’m looking for a simple temperature-based shut-off system for my boilers that I can set (lower than 30° F).
QUESTION
What kind of simple temperature-based shutoff can I install for the boilers?
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Replies
A simple mechanical thermostat with a remote bulb should work. Just make sure the range is within your desired setpoint. Something like this:
https://www.grainger.com/product/JOHNSON-CONTROLS-Line-Volt-Mechanical-Tstat-38Y058
Another option would be to get a thermostat interface module for the mini splits and use a standard three stage thermostat. Wire two of the stages to the mini split and the third to the boiler. This way the boiler only runs if the minisplit can't keep up.
A good cold climate hyper heat minisplit will be able to carry the whole place even in Vermont winter. A ducted mini split in a well designed system with good distribution would let you eliminate the boiler completely and reduce the complexity by a fair bit.
This is great, thank you! I like the idea of eliminating the need for boilers altogether. That would require serious improvements to the thermal envelope. I worry about not having a backup heat source during power outages.
Using three-stage thermostats sounds ideal for my current plans. I thought there are no ductless systems on the market where all of the heads network together so they can be controlled by a single thermostat.
Are there mini-split air handlers that network together to run off of a three-stage thermostat?
As a backup, I can talk with my project’s electrician about your suggestion of using a mechanical thermostat with a remote bulb.
If your plan is a multi split setup with heads in living space plus each bedroom, stop.
In a multiplex setup, even the smallest 3 zone unit would be way oversized for the load, chances are the units will be cycling excessively creating comfort and efficiency issues. It will provide heat, but you might only hit a COP of 1.5 to 2. Better than resistance heat but not by much. For a multi split setup to work well it needs to be sized close to actual loads.
In most cases where you have a lot of rooms, the best setup is a single slim ducted unit. These can be mounted in a hallway with short runs to each room (see attached).
A single wall mount in the living space backed up by some baseboards in the bedrooms and bathroom would work.
In either case, most mini splits require a thermostat module for each unit. Some do have BACnet support to network the units but that will require custom supervisory control. Best to keep things simple, stick to standard thermostats.
Boiler pumps and modcons won't run in a power outage situation, so not much different than a heat pump. Also with a heat pump there is no chance of rads or boiler freezing, much less damage if there is an extended power outage.
There are many 3 ton hyper heat slim ducted unit that will deliver full rated power down to 5F such as this:
https://ashp.neep.org/#!/product/25352
I doubt even an uninsulated 4 plex would have a 12 ton heat load, you can definitely carry the whole place as is with a well designed setup. If you have natural gas or propane consumption data for each unit, you can do a bit of math and figure you actual heating needs:
https://www.greenbuildingadvisor.com/article/out-with-the-old-in-with-the-new