Vacation property and deciding on effecient use of tankless propane water heater
So I am building a vacation house in SC that will have 6 bathrooms. We (2 people) will live in the house year round but will have people visit periodically. The worse case senario is that all 6 bathrooms will be in use at the same time. Think golfers getting ready for an 8am tee time. Our builder suggests 2 large flow rate tankless propane heaters with a recirculating system. This seems the most cost effective efficient system except for the propane.
I am really having a hard time depending on propane for all the hot water in the house. I have looked at HPWH, electric point of use WH, combinations of both with propane. The only option we have is electric or propane. I even went down the rabbit hole with the minisplit with a water heater (kind of like a HPWH where the cold air exhaust cools the room). However these are not really available yet in the US. AND probably impossible to find someone to service them in SC.
Would it make sense to have 1 large flow rate tankless propane system for 2 people in the house, laundry, DW, sink and then connect to the recirculating system with electric point of use tankless heaters? Is this just over kill trying to stay away from propane?
Another suggestions other than having these 2 enormous systems for the house?
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Minutes/Shower: 10
Showers : 6
GPM/Shower: 2
Shower Temp: 104F
Incoming Temp: 50F
Stored Temp: 140F
Cold % 40%
Hot % 60%
Mixed Temp 104
Required Gallons 72
I think these requirements aren't as hard as the contractors are making them out to be. A large electric tank would do fine here if mixed down to a lower temp.
Other easy and cheap options:
1. lower GPM shower heads
2. Drain Water heat recovery (expands capacity by preheating the incoming cold water for both the shower and/or heater)
If propane is chosen - you could get a large volume, large burner tank too. The storage is what really helps with large, varied loads. A tankless heater is great if you're running showers continuously. But if you're running continuous showers AND use large, sudden loads (like a washing machine), a tankless heater might not work. The buffer of a tank absorbs the short, large draws.
Absolutely NOT a single tankless propane heater. It is important to look at the minimum flowrate as well as the maximums. For 6 bathrooms at 2 gpm, you need a tankless unit that can heat 12 gpm at a 50 degree temperature rise. This requires a BIG tankless unit. I had a case where the tankless unit would not activate at a flowrate less than about 4 gpm, so when a single shower was running, the tankless would never activate. To get hot water you needed to run two faucets and the shower at the same time. Really dumb and also wasteful. Even if it does have a lower activation threshold you still have to consider whether it short cycles when supplying a single shower, causing the dreaded "cold water sandwich."
If it works for you, I would choose a HPWH to serve your primary bathroom for its efficiency and low operating cost, especially compared to propane. For the other baths, a tankless might work, but also consider a 50 gallon propane tank-style water heater or an 80 gallon if you really think it's necessary. You could turn it off most of the time, only turning it on when company is expected and running it at 140 degrees with a mixing valve down to 110. Cheapest install and low operating cost because of the minimum run times for guests.
Maybe the smart move would be a heat pump water heater HPWH that would serve the master, kitchen and laundry and the gas tankless would cover the guest rooms. Or put the tankless after the HPWH and set the thermostat so the tankless only kicked in after the HPWH has been depleted.
Walta
I didn't think of using the HPWH as the main source of water heat. I have called a contractor here in the area and he said the AOSmith HPWH had problems with the electrical board in the unit needing to be replaced. That is the one repair he has encountered with these units.
Anyone have any experience with these?
Cynthia