Gas Boiler Backup for Thermal Tank

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toddmcc

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Hi all,
I have a radiant floor system that is set up as an open/closed system. On the closed side, a wood gasification boiler runs for about a 6-8 hour burn producing about 100-120k BTU/hr and dumps the heat into a 1000 gal water tank. The open side then circulates the heated water on the radiant floor side according to heat calls. I want to add a gas backup to this system on the closed side and am looking at something like a Slant Victory VSPH 180. Since I don't regulate the boiler according to heat loads, I just need a direct vent boiler that will be able to come on and dump all its heat load into the holding tank for some extended burn time. I can set the backup to come on based on the tank temp and will probably run it for at least a 10 degree increase. Any comments on whether this is a good candidate for such a system?

Thanks,
Todd
 

Dana

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Most wood boilers are running at atmospheric pressure, which means the system water is prone to having oxygen in it, which will corrode the water side of the heat exchangers in a cast iron boiler. You may have to install an isolating heat exchanger to give the boiler a reasonably long service life. The boiler also needs to be running at least 10 psi pressure to avoid macro-boil on the water side of the boiler's heat exchanger plates

The minimum entering water temperature of a cast iron boiler necessary to prevent corrosive condensation on the fire-side of the heat exchangers is about 130F, and many manufacturers void the warranty if chronically operated with entering water under 140F. (Don't know what SlantFin's take on this is.) The maximum water temperature necessary for most slab-radiant systems is under 100F. Storing water more than 10F hotter than it needs to be on design day incurs much higher standby losses in the buffer tank. That may be fine if you're burning scrap wood at near-zero cost, but it probably isn't what you want to do when using the gas-burner as back-up.

A boiler with 154,000 BTU/hr of DOE capacity is ridiculously oversized for 97 out of 100 houses out there. Size the backup for the size of the heat load of the house, since it doesn't really need to be any bigger than that to do it's job, and will result in fewer, much longer burn cycles, which is good for longevity.

If your maximum water temp requirements are under 140F (probably are) you're probably better off using a direct-vented Burnham ESC series boiler, which is internally compensated to be able to tolerate 110F return water. Alternatively, a finned copper or stainless water-tube heat exchanger type boiler like Laars or RayPak swimming pool heater type boiler may make even more sense, since it's tolerant of low entering water temps and water that may have a higher oxygen content.

Taking it a step further, cheap condensing Takagi T-H3 hot water heater is just fine for use as a backup unit, and you can set the firing rate by the output temp and flow rate. Keep the flow between 2-4 gpm, set the output temp to something like 20F above the temp you're setting the backup storage temp (or whatever it needs to be to deliver your design-day BTU/hr rate requirement at 2-3 gpm.) IIRC the Takagis need at least 12-13psi system pressure to operate correctly without flash-boil, so it too would need to be set up with an isolating heat exchanger if the wood boiler is operating at atmospheric pressure.
 

toddmcc

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Hi Dana,
Thanks for the in-depth reply. There is some good food for thought in there.

Just to clarify a few things, the wood boiler does not run at atmospheric pressure but runs in a closed loop system (so with expansion tank, air scrubbers, etc) and transfers heat via a heat exchanger with the holding tank. I think it typically runs about 40 psi but that will fluctuate with the water temp. I use a three way valve to protect the boiler against cold water return so the boiler sees temps greater than 140 most of the time. The radiant floor runs under 1" thick wood floors so it needs higher temps as I have found anything less than ~140 and it can't keep up with heat demand on a typical winter day (this is a 200 year old farmhouse). Some of my early boiler tracking figures were showing demands of about 30-35k BTU/hr.

So the original design was based on the gasification boiler's needs which was it needed to dump all its output somewhere as the efficiency is greatly reduced if you try and control the burn rate of these things (like via a damper). I have found that the buffer tank suffers about 60-80k BTU standby losses in a 24 hour period which was acceptable when burning the wood as it was free (well my labor) but now that would be different.

So my initial thinking was to mimic the wood boiler and be able blast the thermal tank but obviously that is not an efficient solution using gas. So it sounds like I either have to reduce the gas boiler size to just keep up with heat demand via the buffer tank or use something like a tankless heater, possibly to supply the floor directly and bypass the tank itself. Hmmmm, I'll have to give it some more thought.
 

Dana

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If you're running the system at 40psi and it's fluctuating you may have to build some expansion tanks to use a cast-iron boiler. Most cast iron boilers are set up with 30psi relief valves, though the rating on the boiler is typically 50psi, and 50psi relief valves can be retrofitted (or ordered that way.)

A tankless water heater can handle 100 psi, and would be fine. Setting it up as the backstop low-limit for the tank temperature is going to be easier and better than setting it up to run the radiant floors directly. To run the floors directly you may have to set it up primary/secondary in order to not over-pump the tankless, whereas if it's slaved to an aquastat on the buffer tank you can just set the flow to something that won't wreck the tankless, and just the output temp. If it's a 140F storage temp there's almost no point to using a condensing tankless, but it might still be cheaper to use a condensing tankless depending on the length of the exhaust vent. With the output set to 160F (the T-H3 can be set that high. as can the non-condensing T-D2U version) and the return slurping 135F water off the bottom of the buffer tank, a 3 gpm flow through the tankless delivers 37,500 BTU/hr, and a 4 gpm flow delivers 50K.

Anything over ~4 gpm puts wear & tear on the units, and usually involves a pretty hefty pump. The pumping head on a tankless is much higher than on a cast-iron boiler, but still not crazy-high. Folks who set them up to pump 6-10 gpm with a low delta-T directly into a radiant floor system are usually disappointed with both the efficiency and longevity.

Other tankless vendors' units can be set up to work- I'm only referencing Takagi models since unlike most tankless vendors, they don't automatically void the warranty when use in this type of application.
 

toddmcc

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Dana,
"With the output set to 160F (the T-H3 can be set that high. as can the non-condensing T-D2U version) and the return slurping 135F water off the bottom of the buffer tank, a 3 gpm flow through the tankless delivers 37,500 BTU/hr, and a 4 gpm flow delivers 50K. "

That is a brilliant and elegant solution, I love it! It keeps me from having to try and tie into the closed system and mux all the control for the pumps, etc. The return water will be high enough that a condensing unit doesn't make sense so something like the T-D2U makes perfect sense. Looks like at 4 GPM the pressure loss is about 10 psi so I just need to size my pump (SS since it is an open system) and tie in an aquastat to control it and should be good to go. Thanks for all your help, I appreciate your time and insight.

Todd
 
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