Morphed Copperfin to Tankless?

RickDelta

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?

Mad Dog_2

Much greater heatloss...Mad Dog

RickDelta

@Mad Dog_2

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!

pedmec

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

RickDelta

@pedmec

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?

GGross

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 )

EBEBRATT-Ed

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

RickDelta

@GGross

@pedmec

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 : (

RickDelta

@EBEBRATT-Ed

@GGross

@pedmec

I've learned a lot today after talking with you guys! : )

Thank-you!

hot_rod

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

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