tankless+storage tank+ recirc= ?

[Vermontchill]

Hi,

I have a bit of a strange setup that I am considering and has hoping to get some feedback and ideas.

My wife and I run a mid sized B&B in vermont with 19 guest rooms. Up until a few weeks ago, our heat was provided by 2- 250,000 BTU boilers. The boilers fed a hydronic loop in the attic that fed fan coil units for each room. The boilers also heated 2 119gallon storage tanks for domestic hot water, with a recirc loop so the guest rooms would get fast hot water.


We were flooded out last week and our boiler room was under 5 feet of water. All of the technicians that have looked at the boilers have basically said its not worth repairing due to the extent of damage.


Our heat is no big deal. We already have electric baseboards in the room for backup heat. The old boiler and fan coil system was so ineffecient (I estimate less that 50% efficient), and with oil prices north of $3/gallon, the electric baseboards aren't such a bad option.

Our domestic hot water is another story. Ideally I would just get 1 big 100 gallon, 300,000BTU hot water heater, but I really don't have $10,000 sitting around to spend on one. I was considering trying a 199,000 BTU Rinnai tankless with a recirc line controlled by an aquastat, to keep my existing 119gallon storage tanks hot, When I have a bit more money, I would by 3 or 4 more tankless heaters and eliminate the storage tanks, or at least just switch to 1 small storage tank.

I have a recirc line going out to the guest rooms. I was planning on trying to control it with an flow switch on the cold water fill line for the tanks, that would turn on a 1/25hp circulator, that way when a guest turns on a shower, it gets hot water out to them in a reasonable amount of time (not instananeous, but better than waiting 10 minutes to drain out a 1 1/2 inch pipe). An aquastat on the return line would shut off the circulator when hot water makes its way back to the boiler room.

I am estimating peak demand to be around 24gpm. The showers all have 1.5gpm shower heads, and I can't imagine more that 16 people showering at once. Peak demand would be a rare occurrence though and I think the tankless should help the system recover pretty fast.



Does this system seem like it would work? I was thinking of using Rinnai tankless heaters, since they make it really easy to add a 2nd tankless if one is not meeting peak demand.

I appreciate your opinions.

 

[Caduceus]

I have never seen such a set up and I'm not sure if it will work out as planned. Do your holding tanks have thermostats that are connected to the boiler? If so, these thermostats would be connected the same way to a pump that would come on to circulate the water through the holding tanks. The water usually has to meet a minimum draw to turn on the tankless unit, so the pump has to be adequate. You will then have to circulate the water in a loop back through the tankless and the water has to be hot enough to raise the holding tank temperature to an acceptable temp in a reasonable amount of time. The max temp setting on the Rennai is 140 degrees F. I'm not sure that it will be very efficient to have that low of a temperature trying to heat two 119 gallon holding tanks. When not in use and the water cools, the tankless will also have to run to maintain holding tank temperature. You will also have to check the specs with the manufacturer to see if the unit will shut down if you feed the already heated water through the unit again as it recircs.
It sounds as if this would situation would be create a taxing demand on the tankless unit.
I'm not a specialist with these set ups, some guys deal with tankless hot water on a weekly basis and could probably give a lot of insight. But, when in doubt I would contact the manufacturer and simply ask if the tankless can be used to replace a boiler for heating the indirect holding tanks. They may have all of the info for the design or they might tell you the reason why you shouldn't do it.