I check on this every year, and I’m due to check again, but I have never been able to find any HPWH solution that can handle reasonable demand with an indirect system. Do any exist today?
I’m running an Amtrol Boilermate on my oil-fired boiler, and I’ve asked a few HVAC contractors about adding indirect HPWH to the system, but they all say my system is about the best thing they see, and they woudn’t change a thing. What they are not acknowledging is the secondary factors, beyond direct operating cost:
1. The cooling and dehumidification benefits. My basement is not air-conditioned, but it is finished space that we use. It's fine thru the first half of the summer, but it can start to get a little warm down there by August.
2. I’m also burning almost $200/mo in running dehumidifiers down there thru summer, something that would be mostly or entirely offset by the HPWH’s dehumidification. Those dehumidifiers also pump a lot of unwanted heat into the basement.
3. My boiler is under our kitchen, and the floor between is not insulated. The kitchen is already the hottest room in the house in July, between having lots of glass, poorer insulation that adjacent areas, and cooking. Shutting the boiler down would help keep the kitchen from being warmer than the rest of the house.
Our constraints are primarily very-occasional high usage and long delivery distances. We have more than ten sinks, four toilets, three showers, two tubs, two dishwashers, and two clothes washers. I’d bet there are times (holidays/guests) when both dishwashers, a clothes washer, and three showers or a tub are all running simultaneously. We also have to deliver that water more than 100 feet, in some cases, thru mostly un-insulated copper tubing.
Because of the distance, we have the thermostat set higher than any HPWH can deliver, but that’s mostly a winter thing. I’d be happy with cooler water in summer, which is when those secondary advantages mentioned above can also benefit me most. This is why I only consider indirect an option, as I’d be decommissioning it every winter. Alternatively, if there were one that could be set up in tandem with the boiler to run all winter, that could work... but again, we’re then talking two tanks?
The only indirect systems I found go into the DHW loop on a boilermate, via coaxial dip tube. This won’t work, as the restriction of going coaxial on the dip tube would kill our delivery volume. We need at least 3/4” thru the whole system, given the number of simultaneous appliances. I’d be happy going with a commercial unit, as long as we could find one that’s single-phase on the mains supply.