btu's on the bus go round and round

Larry (from OSHA)

how this stuff works. About flow rates and getting heat from the hx to the water in the dhw tank. If the hot water tank has a rating of "whatever" recovery at X btu's boiler output, why doesn't the burner keep firing until the tank is satisfied? Seems like the btu's can't get off the bus. In this instance, the flow is about 4 1/2 gal per minute (3/4" pipe total length supply and return of about 13 feet). The tank requires a minimum of 84,000 btuh for the rated recovery and the output of the boiler is 109,000 btuh. This phenomemon also occurs on those rare design days when all 3 zones are calling for heat, and the burner is not running all the time (or even close). I'm assuming that the boiler is over sized for the load. The total fin tube capacity is about 70,000 btuh @ 180* assuming 4 gallons per minute through the 3/4" pipes.

So, am I correct in assuming that there is no way to get more heat into the dhw tank without exceeding the recommended flow rate, and that anything over 70k output is wasted due to the ability of the baseboards to give up their heat?

Any responses are certainly appreciated.

Larry (from Minnesota)

Mike T., Swampeast MO

"If the hot water tank has a rating of "whatever" recovery at X btu's boiler output, why doesn't the burner keep firing until the tank is satisfied?"

Are you certain that the controls are set up to allow the boiler to fire ONLY on a call for DHW? and to KEEP firing until the DHW is satisfied?

"This phenomemon also occurs on those rare design days when all 3 zones are calling for heat, and the burner is not running all the time (or even close)."

It almost sounds to me as if the DHW is set up such that it is in series with the space heating load. If so, it won't recover during periods of low space heat loss because the burner is firing infrequently and it can't recover during periods of high space heat loss because the B/B is more efficient than the DHW HX at removing heat from the water.

Larry (from OSHA)

a little more detail

the boiler is controlled by a honeywell aq475a outdoor reset control (which works pretty good, by the way). Hi limit is set at about 175* and reaches it in a fairly short time. The dhw is on a priority setting. Some more specifics include: the boiler is a weil mclain HE Series 2 and has 2.7 gallons water content. The dhw tank is a Bradford White power cor 65 gallon which has a high head hx that is about 80 feet in length of .55 inch id pex covered aluminum tube. About 1 gallon capacity in the hx. dhw circulator is a taco 009. The system circ is a taco 007 and the 3 zones are controlled by honeywell zone valves. It is not piped p/s. All in all, it works pretty well, but I'm sure that there are some piping changes that could be made.

I'm just curious about the transfer of heat from source to load and could use a little help understanding the variables.

All the information that I gain helps me determine whether it makes economic sense to go the Munchkin route or similar. Gas prices here in MN are not apparantly as high as other places. Currently about $0.80 per therm. My usage over the last 12 months was 1430 therms. By the way, I used about 10% less gas with the addition of the reset control and the new indirect tank.

Sorry for getting a little long winded. Again, I appreciate any perspectives.

Larry

hr

Not enough flow

at 4 gpm on a 109,000 output boiler. I've never been able to get a Combi or Powercor to perform to the output spec.

Maybe they stuff more of that small diameter coil into them then they tell you.

At any rate, a 109,000 boiler would need to flow more like 10 gpm at 20° delta t to move that many btus. I'm not sure you will get that through that hx. And 10 gpm through your 3/4" supply and return lines would be a bit excessive, anyway

hot rod

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