Indirect Water Heater Question

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Old 09-20-12, 04:58 PM
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Indirect Water Heater Question

I have installed a 32 gallon Buderus S120, it has the Honeywell Aquastat which is wired to the priority zone point in the SR504 relay and set to 120F. As a note I also have a 3250 hydrostat.

Im pretty darn sure I correctly sized the hot water heater for the house & occupants. But the problem is Im running out of hot water about 6.5 minutes into a shower and this with no other hot water in use anywhere else in the house. The boiler will fire when called upon and the circ pump operates jsut fine.

At this stage Im assuming the differential, or "cut-in" setting is set too low on the aquastat, calling for heat too late and not allowing for a quick enough recovery - does that sound right?
 
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Old 09-20-12, 06:01 PM
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What is the differential set to? How many BTU is your boiler?

I have a 30 gallon Burnham with a 105k BTU boiler. With one shower going, I have never run out of hot water. I have the indirect set to 130F and I think the diff is set to 10F
 
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Old 09-20-12, 07:28 PM
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cant read the differential dial. it's in the back of side of the aquastat - kind of a peculiar location for it. the manual for it is terrible too
 
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Old 09-20-12, 07:35 PM
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Is there a tempering valve on the indirect domestic outlet? If not I would add one and turn the heater up to 140F. Google "legionella" for part of the reason why.

Boiler firing to 180 on an indirect call?

Sorry to ask this question, but it has to be asked... you did install the 'dip tube' on the inlet side and it's not piped backward, right?
 
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Old 09-20-12, 08:03 PM
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no tempering valve installed - thanks for the legionella tip~
boiler fires right up to 190F on indirect call
IWH is piped up proper

boiler is 100k BTU rated.
 
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Old 09-21-12, 06:51 PM
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Is the piping between the boiler and tank 1" pipe? You would struggle getting 9 gpm through a 3/4" pipe with a 007 pump.
 
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Old 09-22-12, 05:30 AM
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3/4" and as close as possible to source, but the buderus has 3/4" tappings
right now, what im thinking is i may have to adjust the low level temp setting on the hydrostat to 130-140 from 120.

unless a larger capacity pump is needed? need to check tables again, i dont like it when i overlook details like this. thnx for the tip
 
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Old 09-22-12, 07:34 AM
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I'd also go with a mixing valve and crank up the tank temp. like Trooper suggests. You'll get the benefit of safer hot water for longer periods of time, not to mention longer burn on/off times. By experimenting with the temp. and differential settins, you can get it to a point where you will virtually never run out of hot water.

My layman's opinion is that, most of today's indirect tanks under 40 gals are going to need to be kept well above 120* to satisfy most households' hot water demands, and probably will need a mixing (tempering) valve installed to function optimally.
 
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Old 09-22-12, 06:55 PM
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Thors, I believe you missed the point. For 9 gpm which is the flow they require is 1" pipe. A larger pump may not be the answer. As you increase flow with a larger pump you create more resistance to flow. You may even create harmonic noises or pipe erosion.
Don't fall into the trap that a tapping size has anything to do with pipe size. IT doesn't. Today manufacturers are using water velocity to increase heat transfer. When you increase flow you increase transfer. So connecting a 1" pipe to a 3/4" tapping the coil flow is increased thus transferring more heat to the tank water. Increase pipe size keep the same pump.
Most retrofit boilers installed today into systems utilizing an IWH will operate fine with a 20 - 27 gallon tank. Tanks should be operated above 120f. When you are at 120f and a 10f differential that means the boiler won't start until the tank drops to 110f. Now the boiler starts and the tank temp continues to drop during boiler start-up and fires. Now the tank is 105. You feel this.
The use of a tempering valve will in a couple years be code on any water heater. Might as well get used to using them now.

Just like boilers tanks should eb properly sized.
 
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Old 09-22-12, 07:08 PM
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ight now, what im thinking is i may have to adjust the low level temp setting on the hydrostat to 130-140 from 120.
That won't do anything for you.

You need to raise the temp on the indirect aquastat!
 
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Old 09-24-12, 09:46 AM
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OK guys...raised aquastat to 140F

i verified the differential setting at 10 deg. the numbering on that thumb wheel sure is small.

so far...wife took a 20 min shower with no complaints from her - except from me b/c that's too darn long, i frown upon hollywood showers.

spoke to the installing plumber about the thermostatic mix'g valve. we're working on this next.

Mr Beck - next year, when heating will be secured for the season and recoup $$, planning to revamp my heating zone, will modify the IWH zone supply piping then~

thnx for the help, gents!
 
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