switching from heat pump water heater to indirect (seasonaly)

hotwired

My zombie apocalypse mentality led me to setup a dual hot water heating system. #1 is the good ol' boiler mate, running off our oil fired ancient cast iron boiler, while #2 is a new 60 gal hybrid (electric, heat pump, hybrid), with valves so I can use em' both, or one at a time. My understanding is that indirect is about the BEST way to heat water in winter, as the boiler is running anyway and the WORST in the summer where (excuse the layman's language), this "great big behemoth oil sucking monster" is coming to life just to warm up a bit of water. Now I use common sense as to when to switch them but is there any deeper thinking on the matter? I mean, I'm guessing as soon as the nights don't get any colder than 40-ish, I'm good to use heat pump? Some other facts:

1. We use a wood stove so the boiler really just keeps the house from getting lower than 65, while we use the wood stove to push it above.
2. We have 2 mini splits we ocassionally use for quick heat in LR or BR, but not often, and use em' more for keeping house dry in summer
3. We live in Central Maine
4. Boiler and water heater(s) are in walk out basement. I spend most of my day in basement, particularly in finished part where I have my office, so keeping humidity out as long as possible is preferable for papers, fabric of carpet, furniture padding, etc. (musty smell).

Thank you in advance.

Hot_water_fan

If the heat pump is providing adequate hot water, you should use it all winter too - it’ll crush the oil burner in costs. So keep the indirect but don’t use it. If the heat pump is in the same room as the boiler, it technically is scavenging heat from the boiler. 

EdTheHeaterMan

OK, so you have a "great big behemoth oil sucking monster" that heats your home. but it is too inefficient to heat the DHW? If you had a dedicated Oil Fired water heater tank, How big would it need to be to provide sufficient DHW for peak demand?

Then during the 20+ hours that the water heater is not firing because everyone is at work or school, and only one or two people might do some dishes at lunchtime, That oil fired or gas fired tank is connected to a vent (usually a chimney) that is acting like a vacuum cleaner, sucking the boiler room air past the 120+° water tank. That boiler room air is coming from outside and infiltrating into the boiler room in some way or another. In the summer that air is 80° or 90° and that air will exchange some of it's lower temperature to the hotter water in the tank. Some of that heat in the tank will go into the air passing thru the boiler up the chimney.

Now consider that the water heater gets cooler even if no one is using DHW. So the burner must relight 2, 3 or 4 times a day to recover that heat that was lost up the chimney.

Compare that to the tank that is not connected to the chimney, so the heat stays in the tank longer, up to 5 times longer. That is 5 times less oil burner operating because the boiler is a cold start boiler (not maintaining temperature like a water heater does). After the "great big behemoth oil sucking monster" boiler takes less than 10 minutes to make that tank of DHW, then stays off for up to 20 hours (if no hot water is used) or only operates when there is need for hot water by a tenant or two. The "great big behemoth oil sucking monster" has so much capacity that the indirect water heater recovers much faster than the oil fired or gas fired water heater that is connected to a chimney... so you can use a smaller storage tank for DHW and reduce the standby loss even more.

And with the Heat Pump DHW, what happens when the HP can not keep up with the demand, Is there a standard electric resistance element set in there to increase the recovery capacity? Will you need 2 tanks or more to have sufficient storage to offset the slower recovery that is provided by the electric HP tank that you decide to install? If you do go with summertime HP water heaters, let us know how that works out, Enquiring minds want to know!

Hot_water_fan

@EdTheHeaterMan I read it as both are already installed. If that’s not the case, hopefully @hotwired will let us know. I see no downside to running the heat pump - should it not suffice, the indirect is there. I suppose they could even be run in series, with the heat pump upstream of the indirect but we’re rapidly reaching diminishing returns here. 

Larry Weingarten

Hi, I like the idea of seeing just what the heat pump can do. Also it's a very efficient dehumidifier ... when its running. I know I'd want to be able to use either tank by itself, but it's a question to me of if piped in series, which would be upstream, and I think that depends on how you use the equipment. Throw enough valves at it and you can make it flow either way!

Yours, Larry

fentonc

I would definitely just try to leave the heat pump in 'hybrid' mode and see how it performs (and report back!) - as @Hot_water_fan mentioned, if it's next to the boiler it will just be indirectly pulling heat from it from it anyway (and dehumidifying your basement in the process).

EdTheHeaterMan

Larry Weingarten said:

Hi, I like the idea of seeing just what the heat pump can do. Also it's a very efficient dehumidifier ... when its running. I know I'd want to be able to use either tank by itself, but it's a question to me of if piped in series, which would be upstream, and I think that depends on how you use the equipment. Throw enough valves at it and you can make it flow either way!
Yours, Larry

Larry, I have installed two water heaters this way so the Primary Operating heater is always downstream. You use the dormant water heater to keep the water from getting stagnant and developing microorganism growth. It also acts to preheat the cold water to room temperature so the operating heater needs to work less. When the season changes and the other fuel is a better choice, close 3 and open 3 valves and you are back to preheating and operating in the opposite direction.