Transitioning Away From Hydronic Heat
Decision time. Zone 5 Eastern MA.
Project is a two story gambrel w/ finished attic. Prior to rehab, all three floors were heated with hydronic cast baseboards or radiators. Zero insulation. Various issues lead to a near complete interior and exterior gut. All that remained was the field stone foundation, structure, and the board sheathing.
Rebuild. Much improved exterior water and air barrier using adhered WRB. R40 gambrel attic/roof insulation with the remaining first and second exterior 2×4 and 2×6 walls dense packed cellulose.
Ducted AC was a requirement, after rebates this resulted in ducted Mistsubishi Hyper Heat systems for all three floors.
Finished attic will use the heat pump for all heating and cooling. The question is floors 1 and 2. I’m tempted to forgo a restoration of the first and second floor hydronics. The boiler, indirect HW tank, and all emitter plumbing is end of life. Substantial cost to replace. But the emitters just need a refinishing.
I am told we will really miss the hydronic radiant heat November-February (versus blown hot air). And it is unclear to me where the OPEX cross over is heat pump versus the gas fired hydronics.
How would you approach these choices?
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When I get a project where removing an existing hydronic system is on the table, it usually involves weighing the same considerations that you are laying out. The client wants whole house air conditioning and the house is getting refreshed so there's a chance to put in proper duct work, so there isn't any heating load reason requiring the hydronic system to remain.
I am a proponent of keeping hydronic systems. It's a nice back-up system for cold weather that could really damage your house, it's silent and comfortable heat, it has a connection to the history of the home, etc.
So far, out of half a dozen clients in this situation none of them have kept the hydronic. If the decision is just about dollars and cents, people usually just rip it out.
Typical hyper heat cold climate heat pumps run around a COP of 3 on yearly average. Say if your power is $0.15/kWh, with a heat pump you get 1 therm of heat for $1.47. If your natural gas cost
and efficiency of boiler is more than that, than the operating cost will be more.
I'm in the land of cheap gas and expensive hydro and the cost is pretty close (gas is %15 cheaper). The item that adds a fair bit in there is resistance hot water, but if you are going with heat pump water heater already, the cost would be even closer. If I had to replace boiler/indirect/piping, the return on cost just on operating savings will be near forever.
Right sized modulating heat pumps are much more comfortable than typical forced air. They are significantly quieter and maintain a very even temperature without cycling.
We went from hydronic to ASHP's last fall in Connecticut, and the comfort has been fine. Since they modulate, you don't have the on-full-blast cycling that a "regular" ducted air furnace would. We just set them on 68 and let them idle along... It took a while to get used to not doing setbacks at night, though. Also, we kept the hydronic for now, less work than ripping it out! We still have the separate oil fired hot water heater. When that goes, a hybrid ASHP hot water heater will go in, and I might want to re-gain some basement space and remove the oil boiler.
There are air-to-water heat pumps. The issue is that their output and efficiency varies depending on the delta between the outside temperature and the water temperature. So while old oil or gas boilers would typically have a leaving water temperature of 180F, an ASHP might be at 105F or so. So just hooking it up to existing radiators you're going to find that there isn't nearly enough radiation capacity -- assuming that the existing radiators were properly sized, and the building envelope hasn't been improved.
We also went ASHP last year in western NJ. There is a small difference - and it is fan noise!
It's not that bad though unless you want the true silence of hydronics.
The air blowing is gentle enough that you won't notice that. Energy cost is about the same.
I started the year last year with the hydronic system but the ASHP was so much more effective in maintaining temperature that the hydronics never kicked in and so I eventually disabled it. And the app based controls make it easier to operate.
If you've cut the pipes I don't think there is a good justification to re-plumb. However, if you have a single compressor system, you probably should keep around a couple electric space heaters in case something breaks on your coldest days!
Anyone have experience mating hydro coils to ducted Mitsubishi Hyper Heat systems?
I just got mine working yesterday.
What would you like to know?
Thanks Steve. What brand coil? Aquecoil? Does that integrate with Mits thermostats or is that a second TSTAT? I don't understand the process when you decide to heat via coil versus the heat pump.
You'd use a dual-fuel thermostat, like https://www.honeywellhome.com/us/en/products/air/thermostats/wifi-thermostats/prestige-iaq-kit-with-redlink-eim-and-sensors-ythx9421r5101ww-u/
It has an outdoor temp sensor, and you configure a temperature to switch between the heat pump and the hydro coil. We switch from heat pump to the coil (natural gas boiler) at 15F.
Thanks. Now at least I know what questions to ask of my installer. The Mits air handlers are already installed. I will ask that they accommodate future hydro coil and plan to run the supply/return water lines for the coils.
Also, we use the Mitsubishi Thermostat interface to connect to a standard thermostat: https://www.mitsubishicomfort.com/sites/default/files/thermostat_interface_pr.pdf
I have a Delhi coil like this one:
https://www.canarm.com/residential/hc-heating-coils-and-cw-cooling-coils.html
Do not use an external thermostat. You can use the external heat control relay:
https://nonul.mylinkdrive.com/files/Application_Note_1024_-_Supplemental_Heat_Control_-_Rev_20120810.pdf
This also allows the heat to run during error conditions, and during defrost.
Yes we have hydro heat as backup for our ducted air handlers. We used the Aquecoil kits:
https://www.aquecoil.com/specs/hhuspecs/files/AQUECOIL_HHU_MI-Series.pdf
We use a honeywell thermostat that treats it as a heat pump with backup heat. We switch to the hydro coil at 15F (our economic balance point).
The bulk of our space heating needs are met by two ductless mini-splits, but we also have an oil-fired boiler that lends a hand whenever the weather turns sharply colder (our home is reasonably energy efficient, but the exterior shell still feels uncomfortably cool as temperatures fall below -10°C). Our hot water baseboards eliminate the chill that we would otherwise experience at these times, and they ensure greater room-by-room uniformity.