indirect deltaT?

Ken_40

If you have a condensing boiler, the higher the dT the better. If not, the reverse may be so.

The ultimate "limit" is in the performance of what the bather(s) experiences! Boiler output in BTU's, limit settings on both the boiler and I/W/H, as well as storage capacity in gallons of the indirect all play a part in the answer; as does the number of people in the household.

Showering vs. baths also is a major factor.

In general however, sentence one is the "basic answer."

Let us know what happens.

Rob_32

indirect deltaT?

What should it be? Have a plain-Jane indirect; manuf can't tell me what the pressure drop is through the coil "we don't make it any more." Manual just says "supply 4-10 gpm through the heater" but it's unclear to me whether that is 4-10 gpm when it gets to the coil (i.e., figure up the resistance curve using everything but the coil itself and pump appropriately), or actually measure the flow rate as installed. Coil appears to be about 8" dia of 3/4" tubing, maybe 24-30" tall? (Going by parts diagrams, etc.).

I'll put a temp gauge on the return, but the hand-test suggests ~160, maybe a little less coming back. Supply sensor says 180 supply.

Thanks.

J.C.A._3

Maybe...

A name of the tank will help a bit more.

Some coils are finned, while others are smooth. Water conditions may make the finned coil get "plugged" with debris while sitting idle, (Bring in goo from the city/well, add heat....makes its own life forms!)and lessen its tranferability to the water being heated.

Some brands recommend cleaning the coil with Ice machine cleaner, while others will go YEARS with no problems at all, depending on the water conditions.

The more we know, the more we can help. I don't want you to throw a company under the bus, but there is quite a considerable knowledge base here that I can bet, have dealt with any company out there. Chris

Brad White_150

It varies all the time....

For any indirect I have seen in action it seems to work like this with a low-mass condensing boiler:

From an acclimated cold start both boiler and tank, the incoming boiler water will be near room temperature as will be the tank, presumably.

Boiler supply will get ahead of the tank temperature shortly and rise to near boiler setpoint limit, say 160-170 degrees. Here is where the delta-T is the widest, as much by that time as 90-100 degrees tank-to-boiler and 60 to 80 degrees across the boiler.

The delta-T's continually narrow until the tank is satisfied then she shuts down.

From a "topping off" start, with the tank just below setpoint, the first slug of boiler water (boiler from cold start) can be 70 degrees or whatever the last heating setpoint was; wide variation here.

An odd but not surprising thing happens during a topping-off sequence with a cold-start boiler. The boiler return goes back at near tank temperature at first (that HEX water sitting there in a 130 degree tank), then goes cold as the system volume slug passes through, then the hotter water comes on in earnest. A bit of a dip then production.

Often the boiler (mine in this case) goes to about 150 before the tank hits the 140 degree setpoint during a topping-off or when keeping up with a constant demand of shower, laundry, dishwasher together. Narrowest Delta-T at this time.

In short, I would not worry too much about Delta-T so long as your boiler can support the load you are imposing. If the boiler is on the shy side of full capability, the delta-T will be wider for longer and it will just take more time to reach tank setpoint. But condensing will occur for longer periods and that is a good thing too.

My $0.02

Brad

hr

The limiting factor

in your tank, or any other, is the coil size. I'd say 10 gpm would be a high side number for the amount of flow if indeed you have a 3/4" coil. Probably more like 6-8gpm.

Velocity noise will indicate excessive flow, which of course will lessen the life of that coil.

Hot, hot water, say 180, and high flow rates really degrades copper quickly. Throw in some high TDS, or less than perfect ph boiler water and it gets worse.

The delta t will be highest when the tank is coldest. Same as in a radiant floor system. As the delta t closes you will hear more boiler cycling and slower heat exchange.

If you really wanted to know, measure the id of the coil tube and fill the coil with water then air pressure to get the actual capacity to determine the coil length. With that info you could get a handle on how much HX is realistic.

This type of coil (my least favorite due to lime up potential) is common in many indirect, tankless, and solar tanks. More like a 5/8" coil diameter. This happens to be a Vaughn.

I remember seeing HTP wind this copper during a factory tour. I believe that is how HTP started out building copper finned coils, mainly for tankless coils.

hot rod

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J.C.A._3

I get that Brad,,,,

But what about a cold start, cast iron boiler, with a "built in" differential ?(12-15° in most cases)

I know that the process CAN be reversed considerably for the first few moments...but what will make it react faster?

The answer is the obvious..bumping up the tank temp. and mixing it down for the end receivers. Let the boiler have enough time to respond to the call for heat, and the faucet user/bath-taker be the ultimate decider.

Tempering and anti-scald is the trick...Making it work is our job, and one that should NOT be taken lightly. To make it perform properly is the goal...The knowledge of HOW to make it happen properly, is ours.

Warren Buffett put it the best... Lead, Follow...or get out of the way. Those of us that KNOW, are moving forward, the rest should just be getting out of the way.

Ask Mr. Etherton about keeping the water temp. too low, and I'll bet he'll agree with me.

Lifes lessons take a while to set in....but once they do, you won't forget! Chris

J.C.A._3

H.R.

Didn't HTP switch to a copper/nickle combo to keep the goo from staying in the "fins" of the Super Stor after a couple of years?

I know they make coils for a multitude of boiler companies, and have used them extensively, but believe they are mostly copper.

I could go on with the "subtle changes" that all makers have made over the time I've been in the field....but that would take a day or more. JCA

Rob_32

more info

Thanks for the insight, all. Indirect is a State Systems Escort, vintage 2003. I believe State is part of A.O. Smith. I didn't intend to disparage them. Their tech support has actually been excellent and they were totally upstanding about a new stat under warranty; the flow rate question is the only one they haven't been able to answer. It's a 40 gal. Smooth coil. Attached is a pic from the manual. Water quality here is quite good (no significant TDS, ph or carbonate issues). The boiler is a single-stage, cold-start, non-condenser. Recovery is actually quite good. It's run at 140F and tempered down with a Sparco/Honeywell.

This may be just an academic question, performancewise. My main concerns were a high dT suggesting potentially inadequate flow, and potentially slower recovery. Sounds like it's better to be on the low end of the flow rate than the high.

Thanks.

Steamhead (in transit)

Your present delta-T

tells us that 20 BTUs per pound of boiler water are being transferred to the water in the tank. That sounds OK to me. Are you getting enough hot water at your faucets/showers?

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Rob_32

yes. neat conversion. hadn't though about that. thanks.

Unknown

Hey HR, you know any tricks to actually get the square feet of exchanger (and type) out of the MFGs when you can't physically have the units present to rip open or pump up? I keep trying to get installers to use larger tanks instead of upsizing boilers, but my hands are tied until I can really run numbers and demostrate to them how it works.. which I can do, right up to that nagging little piece of missing info, HE area and type. and the MFGs don't seem to be eager to divulge that sort of info.

hr

If you dig

most all manufactures have that data listed. Often times it is listed in the engineering sheets, not the glossy sales pieces.

Weil/ Triangle Tube list 24 sq ft for the Smart 60.

SuperStor has their spec listed for the Ultra SSU 60. It has 20 sq ft.

Burnham/ Crown MS53 shows 9.7 sq ft.

E 34 ErgoMax shows 32.7.

Buderus and Viessmann have their spec listed.

Easy to see how that tank in tank Triangle Tube/ Weil tank performs so well As well as that tank full of copper ErgoMax design.

What you will find is the finned coil types drop performance REALLY fast with hard water conditions. The hotter the boiler water the faster minerals fall from suspension and cling to the HX surface.

Think of a electric water heater element.

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Brad White_150

Super-Stors

To expand upon HR's data, the Super-Stor SSU-20, 30 and 30 Low-Boy models have 15 SF heat exhangers with a recommended flow rate of 8 GPM.

The SSU-45 and 60 have the 20 SF heat exchanger with a recommended flow rate of 10 GPM.

The SSU-80 and 119 have the 34 SF heat exchanger with a recommended flow rate of 12 GPM for the 80 and 14 GPM for the 119.

You can see some proportionality here. The exchangers are CuNi (cupro-nickel) and I am not sure of the composition. 85% Cu and 15% Ni is common in industry.

Their DW models all use a 20 SF exchanger but vary the flow rates from 10 to 14 GPM.

The commercial line 45 and 60C's have 40 SF exchangers and the 80 and 119C's have 68 SF exchangers. Flow rates from 20 to 28 GPM. Yikes.

hr

Another indirect trick

if you need a lot of DHW and have the fizz (hp) to drive it, Buy an indirect with dual coils and feed both. It usually is a lot cheaper than dual tanks, especially in the larger capacity tanks.

Stick a 399 Munch on a dual coil 120 tank and behold the power of two!

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Unknown

you guys rock, thank you!

Rob_35

wonderment

Always fascinating how some of these threads evolve. Learn something new and interesting every time down the road. Thanks.