Oil Fired Boiler & Mini Splits

Ted22

I have a 2 story brick house with a full basement and an oil fired boiler with a mono flow system. I also have a mini split system. I would like to utilize the mini splits more in the spring and fall (25 degrees and above). The mini splits keep the living area warm. My fear is pipes in the basement or in the walls will freeze if I don't use the oil furnace. I'm trying to figure out the best way to use the mini splits and have the boiler run just enough to keep the pipes from freezing.

Jamie Hall

More of a control problem than anything else. Is the system piped as primary/secondary, or is it just one loop around?

What you want to do is to run the heating pipe loop -- whether it's the secondary or goes through the boiler -- at around 90 to 100 at all times. Won't take much energy to do that. Then use a low temperature aquastat to fire the boiler just enough to keep that temperature.

Ted22

I also need to fire the boiler for domestic hot water.

ethicalpaul

I'm ignorant of a lot of things, but it seems to me if you are firing the boiler for DHW the following two things might be true:

1. The basement will be warm enough that the pipes don't freeze (at 25 outdoor temp, I think even without running the boiler for DHW they wouldn't freeze).

2. It isn't going to be much of a savings at all, if any since you are keeping the boiler running anyway.

Extra free personal opinion: Get your DHW out of your boiler so you can shut it off in the other 3 seasons!

STEVEusaPA

Why not just use the mini splits down to 40°?

EBEBRATT-Ed

As @ethicalpaul mentioned if its 25 deg out you basement pipes will not freeze. In fact your basement pipes would be ok down to 20 degrees unless you basement walls are full of holes. Running the boiler for dhw will help heat the basement

HVACNUT

I think the concern would be the piping in the exterior walls going to the 2nd floor. With a monoflo system I assume it's an older home. Do you know the insulation R value in the walls? A glycol mix for 0°, accounting for wind chill, would do the trick.
If it were me, I'd go by humidity as well as temperature with the changeover from ductless to hydronic.

hot_rod

Piping in or near an outside wall or any un-insulated space certainly may freeze. I have seen HW baseboard freeze in a heated house due to infiltration around the walls.

Although not my first choice PG boiler antifreeze can be used and is safe with DHW generation via indirect or tankless coil.

Ted22

It is an older home, brick/block and plaster. My concern is more the pipes in the walls than the basement. There is a bathroom on the second floor too. I'm thinking about putting a thermostat on both floors. Then the circulator would run off whichever one was calling for heat. We pretty much live on the first floor unless somebody is staying over. Per my service guy I put tiles on the fins upstairs so the heat goes up and comes back down, except for the bathroom. That keeps the rooms upstairs 10-15 degrees colder than downstairs with the doors shut when it's cold. I could put a second thermostat in the upstairs bathroom, set the temperature low and see how that works when I use the mini splits downstairs.

HVACNUT

@Ted22
Adding a separate thermostat to the 2nd floor bath wont do much unless you make it a separate zone. A parallel thermostat will overshoot the 1st floor. And I wouldn't wire it in series. The brick exterior will help with the wind but doesn't do much to block the cold.
A fairly simple project would be to make the whole 2nd floor its own zone. From the basement, disconnect the risers to the 2nd floor rads. Pipe all supplies and returns separately back to the boiler and add manifolds, a circulator and a switching relay.
There are also wireless thermostats if it's hard to get a wire to upstairs.
Note: The pipes that are removed from the monoflo loop cannot just get capped. The Tee's must get piped back into each other.
The easiest way. Just use the ductless heat for the shoulder seasons.

jacobsond

I have run my mini split heat-pumps with temps as low or lower than 10deg. My hot water heating pipes have never had any issues. Even the ones in the outside wall that I foolishly put there. Dont try to overthink. If your only going to use the minis down to 25deg is your insulation that bad you have to be concerned? At 10deg outside running minis my unfinished basement might get 60deg. We try to keep the house 68deg so even the pipes on the outside wall just have 1 layer of drywall covering them so its unlikely to freeze. You might want to tighten up your house. That would save you more.

rsilvers

Add CryoTek anti-freeze to the heating system. Its $134 per 5 gallons. Pump in one of those and then use a refractometer to see what the new freeze temp is. If you are happy with that, you are all set. If not, pump in another 5 gallons. Main downside is that you lose some of it if you bleed your zones.