Morphed Copperfin to Tankless?
Hello HeatingHelp.com community! : )
Would like to understand the operational dynamics (issues) in this hypothetical morph from "Copperfin to Tankless" operation.
Prelude: (Motel environment -140 guest rooms - six large Jacuzzi - six commercial washers)
Current operating system is a natural gas Lochinvar CopperFin water heater (645K btu - w/two stage firing 322.5K btu/322.5K btu) with an external 100 gallon storage tank.
This system has a 1" dedicated "keep alive" circulator return loop.
The total (2") copper hot water distribution piping throughout the entire building (610 ft length) equates to around 100 gallons of hot water residing just within this pipe network at all times.
My thoughts are this:
How would this system function if I were to eliminate the "physical" 100 gallon real storage tank and let the distribution pipe network emulate the storage tank?
I'm thinking it should function closely as a "Tankless" hot water system with its inherited efficacies (ie: gas savings, etc).
…… your thoughts?
Comments
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Hi Mad Dog! : )
……. greater heat loss from where?
The heat loss from the keep alive loop and feed are a sacrificial heat loss in this or a tankless system with a return loop (ie: to provide the desired instant hot at any tap)
……… also, I'm not constantly heating the 100 gallons of stored water in the boiler room tank.
I would think that would cut the idle heat demand in halve already!
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Absolutely not good. First you would get so much condensation in the boiler which would lead to a quick failure of the hot surface igniters. The location of the first stage hot surface igniter are close to the inlet header and due to incorrect piping of storage tanks with copper fins I replace them on a consistent bases.
Second is just like the first in that the condensation would start to cause the heat exchangers to plug. copper fins are design to not have less than 110 degree water entering. this will lead to hardening of the copper fins. you would have ignition failure because you will eventually lose your pressure differential and not close pressure switch
Then even if you could get by these issues you would have a really high delta T due to low domestic water temperature entering the boiler. Without looking at the specs I believe the max design delta T is 40 degrees for most copper fin boilers.
in short, buy something designed to handle low temps
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Hi Pedmec! : )
Question,
Since this motel has two of these copperfin units (one as an off-line back up only) ……. couldn't I just series these two units together?
First unit brings up the cold street temp up 40 degrees (ie: 65 street temp to 105 degrees) ….. then the second unit 105 to 140 degrees?
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The problem with any large tankless setup is minimum load. You are no doubt going to have wide temp swings if say a few lav faucets are running. The load from 2 lav faucets isn't enough to fire one boiler and you have 2 and I am assuming these are single stage units that don't modulate? (see now they are two stage but low fire is still too high for your min load) They make commercial tankless heaters in your BTU range but they require a storage tank to buffer the load or else comfort will be greatly reduced. If you want a system that can do the whole building with no storage you really need a multi tankless setup, sized to handle the minimum load (a single lav faucet) and the maximum load continuous.
to clarify your thought process works in theory when you have a continuous load though, but since you do not you would need a storage tank to buffer the load, or else your boilers will short cycle and will not maintain temperature accurately (not to mention the additional concerns @pedmec stated )
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Boiler will short cycle without enough water volume in the system.
Eliminate the tank and put 320k btu low fire output?? into a 100 gallon system example:
100 gallons of water=about 800lbs of water
800lbs of water x a 75 degree rise=60,000BTU. 1 boiler output is 320000/60 min=5333 BTU/ min.
From a cold start (if my math is right) you will get 11 min run time from 1 boiler on low fire with out any HW draw
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Thanks guys! …… starting to pick up on all this you say! : )
As Pedmec pointed out , a wide delta T (over 40 degrees) on the in and out pipes on the boiler will cause heavy condensate to form on the copper fins (heat exchanger) resulting in blocked exhausts vent air flow and water dripping onto the igniter glow bar (ouch!!)
Observation:
A tankless water heater can take cold street water (65 degrees) and bring it up to 180 degrees all day, …… a 115 degree delta T !!
I assume they can do this because they keep the burners, igniters and vent air flow paths clear of the guaranteed heavy production of condensate. (ie: keep all this stuff above the condensate drip flow ….. out of harms way).
Observation:
As GGross pointed out, the major problem is dealing with very low demand conditions. You may only need the maintain the desired temperature with, lets say 400 btu, but your boiler's lowest btu stage is 322.5K!! Hence the boiler fast cycles into a death spiral : (
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tankless water heaters are typical rated with a 77 degree temperature rise that is the number most use to get the gpm rating
I notice when I check into the typical new Holiday In Express type of hotel you see anywhere from 4- 8 vents out the side wall for a battery of tankless.
Rinnai and others make hotel specific tankless that hang on a tank, the Duo. This way they have some dump, and also micro load capability
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream2 -
…….. pondering! (or giving this dead horse one more good beating!) : )
OK! ……. so trying to morph a conventional boiler to emulate a tankless unit …. I'm confronted with two major issues.
(1) The extreme delta "T" into and out of the boiler. (this causes it to rain condensation down onto the gas flame, glow bar ignitor, hot surfaces and plugs exhaust vent air flow).
(2) As the boiler is un-modulated (other than its two firing stages) a call for additional heat is way overwhelming for low flow demands (this causes severe short cycling as this excess heat has no place to "sink" or "buffer" into … ie: a hot water storage tank!).
My emulated or virtual hot water tank (pictured above), the hot water distribution piping equates in volume to a 120 gallon storage tank of hot water, as its part of the "keep alive return loop" (This loop is maintained at 125 Degrees F.)
The problem is, I don't have access to this pre-heated, 120 gallons of water, for buffering (storing) my boilers very large excess heat production on a firing cycle.
My thoughts are:
Instead of using a circulator (that simply flows the distribution pipe water back slowly thru the boiler during a no-use condition to off-set the normal heat loss of the loop) …….. but to replace it with a variable speed "vortex" pump instead.
The variable speed vortex pump (running both in no-use and full use 24/7) would now afford me the use of the "stored" pipe water (aka "storage tank") for the proper delta "T" into the boiler and extend firing cycles …. same as if I had a real "physical" hot water storage tank".
…….thoughts on this?
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copper tube boilers are sensitive to flow. The manual usually tells you the flow required and acceptable delta across the boiler.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0 -
what is the difference between a variable speed circulator and a vortex type pump?
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream0 -
……. much larger developed differential pressure across the pump? (and I don't think a circulator functions in an "open loop" high head environment.
Note: "centrifugal" pump is probably the more appropriate term.
I may have lets say just one shower head (2.5 gpm) drawing hot water for 20 mins …….. I don't want the the boiler to fire as I all ready have plenty of hot water to meet this demand in "the pipes".
I have to be able to "pump" (circulate) this pre-heated water out to an "open loop" while heating (w/stored heat only!) the incoming cold water replacing it.
I'm assuming that at some "Flow Demand" verses "Stored Heat Reserve" I will be pumping (circulating) water at a much faster flow rate than the slower in coming cold street flow.
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My thinking:
At "no hot water use demand" = Pump circulates at only a slow speed, only enough to off-set the heat loss in the feed and return loop , loop maintained at 125 degrees F.
At a "low flow" demand (ie: one shower head) = Pump starts slow and slowly ramps up as it delivers or exchanges its stored BTUs to the incoming cold water as needed to maintain set temp. Once this "stored water" drops below an acceptable temperature level (Hysteresis) …. the boiler fires up and tops out the "stored" 100 gallons in the pipe loop to set temp (Buffered BTUs!).
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……. more thoughts:
In a hot water storage tank, where your storing a large volume of water in one large containment vessel (tank), there the corrosive minerals within the water have time to come out of suspension and accumulate to the bottom of the tank, out of reach of a turbulent "flushing" flow.
In a "distribution feed pipe w/ return loop" mass storage environment ….. these minerals (scale) are constantly "flushed" out of the hot water system.
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part of the reason those low mass copper tube boilers have a storage is so you can also recirc through it. The recirc pump only moves enough gpm to overcome the heat loss of the DHW loop, out and back. The city water pressure allows water to flow from the faucets,of course. Using the boiler pump as the recirc pump also, sounds challenging.
The first calculation I would do is what is the required gpm through the DHW loop. That doesn’t change generally, unless you have dynamic balancing valves on various DHW recirc loops. Then a delta p circulator responds to the balancing valves modulating.
This issues goes through the steps to size the recirc, and how the Caleffi ThermoSetters automatically balance a DHW recirc system. This valve gets used to solve problematic recirc systems, like pinholing from excessive recirc velocity. Also as an anti legionella protection device.
Also in new construction to provide an efficient recirc system.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream1
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