New Buffer-Centric Design, CI boiler + 2-way HX + large DHW/Buffer tank

[Buffalobillpatrick]

As I don't know how to post pics. I will try to describe a new design I'm working on.

Burnham Revolution RV3 boiler, heats 120 gal. DHW/Buffer storage tank (called Tank from here on) via flat plate HX.

30* delta aquastat on Tank controls boiler fire. (say 140*-170*) DHW output is tempered down with 3-way thermostatic valve.

On Boiler side of HX (Taco 007) pumps into (HX-side-A-in), through HX, (HX-side-A-out) & back to boiler. A purge delay relay will take left over heat out of boiler at end of fire.

On Tank side of HX, (Taco bronze 006, runs continuously) pumps from bottom of Tank into (HX-side-B-in), heat is gained in HX,
(HX-side-B-out) back into top of Tank.
(Taco 006 is enough pump for 50K BTU heat transfer @ 6 GPM + keeps tank destratified)

System can draw heat during the above Tank heat storage cycle or long after, from the Tank, where it performs the Buffer function of preventing short cycling in shoulder weather.

If the boiler is off, the HX transfers heat in the reverse direction from side-B to side-A & draws down the Tank temp. The water flow in the HX does not change direction on either side.

When hydronic slab system thermostat calls for heat, its (Taco 007) pumps cool system return water (say 80*) ALSO into (HX-side-A-in), heated in HX, (HX-side-A-out) (say 110*) to a 3-way therostatic valve which tempers it down (say 100*) then provided to slab.

BBP

 

[Buffalobillpatrick]

This design should provide long infrequent burns with minimal components.
BBP

 

[Sponsor]

 

[Buffalobillpatrick]

Finaly I feel justified, by none other than: John Siegenthaler, P.E.

http://www.pmmag.com/articles/96765-alternate-methods-to-pipe-a-buffer-tank

The system that I designed & installed in my house is much like fig4, except that I have a flat plate HX just before the tank. There is a pump on the tank side of HX. The buffer tank is also my DHW. System can heat during boiler burn or between burns by pulling heat out of tank via the HX in opposite direction.

I can barely understand my description in my 1st post.
Tank is a 4-port Lochnivar Sun-Saver 80 gal.

Works great!

 

[Dana]

At 80 gallons and a delta-T of 30F on the aquastat it's WAY more buffer than is actually needed to keep a 3-plate cast iron beastie with 55KBTU/hr from short-cycling, but I'm sure it works just fine!

Compare that configuration to the sketch of an ErgoMax "reverse-indirect" hot water heater/buffer topology:

The difference between a reverse-indirect and the SunSaver approach is that the stored water is heating system water, not potable water which reduces the number of pumps on the system since there is no need for the plate heat exchanger. (The potable coil heat exchanger is coils of water immersed in the heating system water, and the input port from the boiler is designed to create turbulence over those copper coils when the boiler loop is running.) The tees off to the radiation in Ziggy's pretty picture are just ports on the ErgoMax, rather than exterior plumbing, the supply & returns port being at the top of the tank, with the heating return directed to the bottom of the tank with a dip-tube. I assume you used the solar ports on the SunSaver for the (HX isloate) boiler loop?

To get reasonable hot water performance out of a reverse-indirect requires storage temps of 130F or higher, but that would not a problem for a Revolution RV3. With a mod-con driving a reverse-indirect storage temps higher than 130F start to cut into operational efficiency, and by 140F it limits the efficiency to about 90%. But if you need 140F+ average water temps for your radiation the Revolution RV3 runs about the same efficiency as the more expensive mod-con. The aquastat that comes mounted on the Ergomax has only about a 7-8F delta-T, but that's easily changed by additional control, such as a strap-on aquastat uses as the high-limit. They start at 26 gallons (costs about a grand at full-retail), which would work just fine with a Revolution RV3 at 30F of hysteresis unless you have a soaker tub to fill, but their 48 gallon unit (about 2 grand retail) would be better for filling tubs, given the limited burner output.

 

[Buffalobillpatrick]

Dana, I agree 100% ,
except:
Erogmax are hard to find & $$$
I found my 80 gal. Lochnivar Sun-Saver tank, new in box, at Habitat for Humanity for $400
Flat plate HX cost $150 on Fleebay

My boiler burns are about 30 minutes (very high altitude) & I never run short of DHW.
My radiation is high temperature in this house, staple-up under subfloor, so a good match to CI boiler.
I have 3 Grundfos 15-42 pumps. All are powered by SSR's

 

[Dana]

Ergomax & Thermomax (another reverse indirect vendor) units show up on that online auction site from time to time too, steeply discounted. (I nabbed a scratch & dent 48 gallon Ergomax that way for about $600 several years ago, $700 after shipping and minor repairs.)

A $400 SunSaver is unbeatable though- I suspect they're north of $2K through distributors! At 80 gallons I'm sure you'll never run out of hot water.