Indirect-fired water heater w/ electric backup (GARN system)

[varadhammo]

Hi all,

I'm in the process of designing a heating/DHW system around a WHS2000. The GARN will be providing DHW as well as space heating for our monastery's community bath house (4 full bathrooms, but only used to full capacity sporadically), as well as radiant floor for the meeting hall (~2000sf). We want electric backup DHW, to serve two purposes: 1) the option to not fire the GARN during the summer months if DHW demand is very low and 2) as a simple backup in case the GARN is ever down.

In looking around, I see there are a few indirect water heaters with integrated electric backup available, mostly marketed for solar applications, but it seems like they would function in wood boiler systems as well. My question is, how does this compare to other options in terms of price & functionality or suitability for our application?

For example, one idea I have is using a regular 80 gal. indirect WH in series before an electric 50 gal (just rough numbers for capacities). There could be a difference setpoint controller w/ circulator to basically make the electric into an expansion tank for the indirect, in which case the electric would pretty much be idle when the GARN is hot, except perhaps under high loads when the indirect can't keep up, right? Or, simplify and forget the circulator/controller and just let the electric make up for its own standby losses, perhaps keeping the setpoint of the indirect higher than the setpoint of the electric?

Of course, another disadvantage is the space an additional WH takes up (there's no basement, we'll basically build a utility room around the mech. stuff).

What approaches have people here used?

Thanks and much appreciation for everyone here's generosity,
Jacob

 

[maple1]

I have an ordinary 80 gallon electric hot water heater, that is heated by my system water via a flat plate heat exchanger.

Circuit breaker to the electric heater has been turned off for over a year now as I burned all summer for DHW (fired once a week).

The domestic side of the FPHX is circulated by a B&G Ecocirc that I think is mainly used in a hot water recirculation setup. The system side of the FPHX has a zone valve in it that is controlled by a Johnson A419 controller with the probe surface mounted at the bottom outlet of the hot water heater in close to the tank, which is also where the domestic side of the FPHX draws from via a T. The end switch in the zone valve controls my main load pump (a Grundfoss Alpha) the same way the rest of my zones do, and it also controls the Ecocirc through an RIB relay. So when the Johnson sees water that is below setpoint at the bottom of the hot water heater (think I have it set at 110°), it opens the zone valve on the system side of the FPHX, which then starts the circ on the system side and the circ on the DHW side.

Works great and gives fully automatic control when the electric heater breaker is on. I just leave it switched off as I would rather have a brief period of lukewarm water than the elements coming on if the system water gets a bit low on temp.

I started out looking for an indirect tank that had elements in it, but gave up after not finding many choices. Or at least easily obtainable ones. Then again I had to special order my Alpha pump (and the FPHX and the Ecorcirc) so I'm in a backwoods spot for getting this stuff.

EDIT: Forgot to add, I also have a sidearm exchanger in series with the FPHX. I started out with just that, circulated by convection. It also worked fine by itself with the hotter boiler water when burning in the heating season, but I found it inadequate when trying to heat DHW from storage in the non-heating season. Rather than pull it out all together, I just left it there & plumbed the FPHX into it. Made for a bit of an easier re-plumbing job at the time.

And to add again, I also pre-heat the DHW with a couple of rolls of pex buried inside my storage enclosure.

 

[varadhammo]

The electric unit with a sidearm HX sounds good, but our water is hard, hard well water and I don't really want to deal with the cleaning and maintenance of the HX. As an alternative to the idea in my original post of an indirect tank and an electric tank in series, how about an indirect tank with a tankless electric unit & circulator loop for the electric backup/boost?