Smart buffer tank?

Karl_Northwind

I'm designing up a panel radiator system.  I'd like to do radiators with TRV's on each, very small zones, and don't want to be short cycling the boiler.   the system will be running in the 120-170 degree range thru the heating season.



My thought was this:  is there any reason I can't/shouldn't use the Smart 50 gallon indirect tank as a buffer tank as well as DHW?  it'd be a buffer tank as well as air separator and hydraulic separator.



 I'd use the outdoor reset to control the DHW tank temp between 120 and 170F, and pull heat off that to the space heating zones?  I'll most likely be using a grunfos alpha and Uponor EP manifold to serve 2 radiators in series per loop, about 10 radiators, 50,000 BTU peak heat loss. 



assuming that's not a bad idea, would it be better to run the boiler hot to the top port on the smart and then pull the heating loop supply off the bottom, as well as the boiler return off the bottom port, or to parallel both the boiler and the heating loops with Tees at the top and bottom boiler ports on the DHW tank, allowing either the boiler or heating loops to move heat thru the tank. 



seems almost too good to be true for these micro-zoned systems, or small ultra-low heat loss houses we deal with. 



thoughts? 



thanks

karl

Zman

Reverse indirect

What you are describing is a reverse indirect. You cannot do it with a TT Smart. The outer (boiler side) jacket is steel and cannot be exposed to domestic water.A coil design exchanger like heat-flo and others would work well.



I think what you are trying to do would work well with a non-condensing boiler and injection mixing. A mod con boiler would have trouble keeping up with the domestic load when it it is firing at the lower temp range.

Karl_Northwind

not reverse,

The outer shell on the indirect would still be heating water, but would rely on the ability of the inner tank to conduct heat both ways. 



I might use a non condensing, but would most likely use a mod con, with the indirect circuit (aquastat at 120F) firing the boiler at DHW rate (180 F) and a separate heat aquastat (tekmar 256? ) controlling the DHW tank's temp during heating season, at varying boiler rates to keep the DHW tank between 120 and 170F depending on the outdoor temp. 



some of the zones I'd be heating would be as little as 5000 btu, which would seem to me to lead to short cycling. 



k

Zman

I see

I think you are the right track. You are going to have a heck of a time generating domestic water when you boiler loop is at 120. I think from a control point of view you are making work for yourself. Mod cons come with everything you need to do outdoor reset and domestic priority.

I think it would go with a smaller domestic tank (smart 30 or 40) and use the money you save to buy a buffer tank. for the heating side.

Carl

Karl_Northwind

buffer

Gotta think thru the ODR control, but if the DHW temp dropped below 125 or wherever the DHW aquastat is set, the boiler would go into DHW mode, 180 F. 

got a suggestion for small buffer tanks?  carbon steel?



karl

Zman

Boiler Buddy

Boiler buddy is one. It provides hydrolic separation as well.There are plenty of folks using electric water heaters as buffer tanks.

Carl

Mark Eatherton

Issues with your idea...

As has been pointed out, if a load on the DHW pops up out of no where and the tanks and driving fluid is at 120 degrees F, you WILL run out of DHW as teh system reacts.



Maintaining the tank at 120 degrees F is good for propogating Legionella...



I have done this before, but used a true reverse indirect and maintained a minimum tank temperature of 130 degrees F. Also don't forget anti-scald requirements on potable side.



Look at Turbomax. They make them as small as 30 gallons, and at that size, can take like 200K btuH input for GREAT output on DHW loads.



I see WIN WIN WIN scenario here doing this. If you wanted to make the system REAL smart, you could incorporate a 3 way bypass around the tank such that when you are at design condition, the "buffer" function is bypassed, thereby eliminating un-necessary increased standby loss and higher DHW temperatures...



Thanks for thinking out of the box.



ME

Karl_Northwind

whee

as always, I value your input Mark, The turbomax looks like a nice tank.

and I like the idea of the diverting valve for high load conditions where short cycling won't be a concern.



of course we could maintain the DHW at a higher temp, and as a solar installer, I'm used to putting mixing on DHW systems.  It's the exception where I don't need it. 



It gets boring in the box



karl