If your home hydronic heat boiler is dual-mode and heats your household water, I’d recommend replacing it with a heat-only unit and a separate on-demand tankless water heater. The combined units are less energy efficient for year-round use (basically, you have to run an oversized burner to keep the water tank hot for household water, and you can’t shut it down for summer), and like any dual-job system, one or both just don’t work as well as they should.
Hot water to an AC unit? Do you mean some kind of combined AC/fuel-fired-heat unit?
At the office the boiler provides the hot water that is pushed up to the AC unit in the attic space. Boiler in the basement, heating AC unit in the attic, pipe in between to run the water loop up and back. Therefore the boiler provides the hot water to the coils in the AC unit.
This was the setup when I purchased the office condo 15 years ago. Has worked for me so far. However the boiler is old, leaking, and the burner tech told me yesterday it is time get a new one as I am wasting fuel and money on repairs.
It’s a reasonably efficient and cost-effective solution, especially in regions where non-heating days are a minority (thus getting use of the boiler reservoir that’s kept hot 24/7/365). But separate units are even more efficient and let you turn off the heating fuel for at least a couple of months each year.
Electric is hands-down the most expensive way to heat water, short of burning your house down around a kettle. It’s the only solution sometimes, and can be almost competitive in very small-use situations, but overall, electric water heating is at the top of the cost charts, by a significant margin. (Propane/oil is about half that, and pellet or wood about half that - and yes, people do have modern wood-fired boilers.)
We have an oil-fueled dual unit, and it’s on the list to switch the water heat to a tankless propane model. Won’t save a ton, but it will cut oil usage by some and give us much more reliable and steady hot water.
That is what I do. The oil-fired boiler is on 6 months out of the year for heat, the rest of the time we get hot water from an electric heater. I don’t have a gas line, and the cost of setting up propane didn’t seem worth it for what would have been a small decrease in monthly bills.
“Hot water to the coils in the AC unit” and “heating AC unit” don’t seem to compute. There are units that provide both heating and cooling, but they use different parts of the unit to accomplish each task, and there has to be coolant somewhere in the equation.
We have propane (generator and cooktop), so it would be a minor update. My biggest question is whether or not I can slave a propane water heater’s flue to the furnace’s - otherwise it will be harder to install and require some messy exterior vent or flue, big downchecks. Electric isn’t an option in the cost range for here.
I would probably go to a 250 gal tank further out in the woods, from the 100 that’s next to the house. That would be a plus on a couple of fronts.
You’ll have to check this out, but I think you can use a small auxiliary flue like that used for small gas heaters, no more than a 2 or 3 inch pipe that you run through the wall and up the side of the house. I considered switching to propane a few years ago when I replaced the oil burner but the numbers didn’t crunch at the time. Since I’d like a propane generator I should just consider setting up for propane for everything but the furnace.
I’d just rather not - such installations are ugly. The current furnace sits at the base of the chimney, which is a dual-flue design (one for the furnace, one for the fireplace) and thus is cosmetically far more attractive than a 'luminum pipe up the wall. I haven’t gotten around to asking anyone authoritative (like the town inspector, an acquaintance) if a propane wall unit could just use the same flue as the furnace. One of those “seems perfectly sensible, probably has some valid code objection” things.
I understand, it’s complications of that sort that kept me from switching over. I think if my house was more amenable to duct work I would have switched completely from oil hydronic to propane forced air. Anyway it’s done it’s a major overhaul.