Converting Hottub 5.5 kW electric heat to Natural Gas, tankless heater versus boiler?

[mikestub]

I'm visiting the site to inquire of other's input on a hot tub heating conversion approach. See signature or profile for other details.

Working on converting a 425 gallon CalSpa hottub 84x84x48 from electric heat (5.5 kW) to tankless water heater natural gas fired on recirculating closed loop to heat exchanger.

I'm hoping to find others that would understand the difficulty of setting up a typical condensing tankless water heater, to do a closed loop at about 6.5 GPM hot side flow. The target would be to use the tankless to manage a closed loop on demand at 145 deg. F. This should allow the target of 2 degrees per minute heating on the hottub side of the circuit (bath temp NTE 104 deg. F.).

Thanks to any of the experienced members who might have some input to the issues involved.

The problem appears to be making sure to include an expansion tank and pressure regulator to insure the heater isn't over-pressured or will shut down heater on a hot loop side rupture.

Additionally, there is concern of the minimum temperature delta on the heater side of the circuit. It seems tankless heaters don't want to see high temperature input water, (assumes well or municipal cold water coming in). The heater side of the loop would potentially see nearly a full set- point output temp coming back in the water inlet returning from the heat exchanger as the cold (pool) side exchanger temperature rises, (depending on the exchanger flow and efficiency). Most tankless want something like 20-34 degree F. delta in that respect. I assume this has to do with protecting the combustion exposure on the heater's internal heating circuit, from over-heating.

Tankless heaters typically have a self contained computer controller with limited user inputs. The design above likely requires the ability to deter unwanted shutdowns (such as inlet overtemp), but also create emergency or cycling shut downs to control the entire system if there is a tub or re-circulation issue.

Hopefully some of the contributors here have some experience or advisement on the approach.
Otherwise I might be stuck with using a more commercial "boiler" design intended for recirculated hot side loops, as in rapid hydronic heating or embedded floor heating.

Thank you again for any input. I'll gladly document the results here for the benefit of others.

 

[wireform]

You're probably better off to heat via a heat exchanger keeping the water separate.

 

[mikestub]

You're probably better off to heat via a heat exchanger keeping the water separate.

Hi, if you read my original posting again you'll notice that is exactly what I'm trying to accomplish.

 

[JoyfulNoise]

A gas fired pool heater is the correct appliance for your application as it is designed to be a recirculating heater. What you’re trying to do is force a single pass heater (tankless water heater) to do something it was not designed or intended to do.

A 100kBTU/hr Hayward gas heater costs around $1,000. Im assuming you must have the tankless heater already or got it for cheap/free and are trying to figure out how to shoe-horn it into being a hot tub heater?

 

[JoyfulNoise]

Also … @RDspaguy might have some insights ….

 

[mikestub]

Also … @RDspaguy might have some insights ….

Thank you JoyfullNoise,

You are correct that I was hoping to modify a single pass tankless to avoid the cost of a "boiler" design. the BTU load I'm wanting however pushes me into $700.00 category for a tankless, plus the add-ons to make it work. A $1000.00 re-circulation / boiler system is likely more cost effective with a longer service life. I'll look into your references and see if I can manage that option. I had not found a decent priced resource that could define the problem but as you explained it, the light went on, making more sense from a long term practical approach. I'm targetting 120,000 BTU output to the exchanger, just from rough estimates of Michigan winter demands on an outside insulated tub for 1.5 degrees per minute rise with a 70 degree F. standby temperature to 104 degrees usage.

Thanks again,

Mike

 

[JoyfulNoise]

Most of the major pool equipment brands have 125kBTU/hr units. Raypak makes the AVIA forced air heater that has independent WiFi controls. All of them include a basic Fireman’s switch control for requesting heat. I think you’ll spend more upfront using a standard pool heater but you will save time not trying to Jerry-rig up a custom solution that may not have all the safety controls in place.

 

[RDspaguy]

a basic Fireman’s switch

This.
I use a cheap tankless in my bbq trailer. It is made to get cold water in and shower-hot water out at full flow. If I so much as turn down the flow it boils. I have to shut off the heater minutes before the water or it boils. I've blown out pipes, heaters, faucets, and even melted my grey water holding tank. I can't see a way to make it work for this application, it barely works for what it was made for.

 

[mikestub]

Thanks again, both...

Portable tankless have sensors that are supposed to modify the burner on / off when flow rates drop, to maintain steady set point temperature despite flow, typically within 1.5 to 4.0 gpm, depending on size. Low flow will quickly overheat if the down stream sensor has aged or is not connected securely, which can cause the overheat condition if the flame is not shut off approaching set point.

It does seem that a more commercial boiler type system for small pools or hottubs will eliminate most of the secondary equipment and design I would have to do to Jerry-rig a cheaper portable unit in place. At that point there's several safety hazards that remain for lack of development / testing. Better to avoid the hack if possible.

The question then is if I can find a small retrofit natural gas boiler for hottubs at a price that's less than hacking a portable tankless at risk.

At this point I'll be researching the supplier listed above and some other resources. If anyone knows of such a unit small enough to be cost effective, that's probably where I'm headed. Any further references would be appreciated.

Mike

 

[RDspaguy]

Portable tankless have sensors that are supposed to modify the burner on / off

Not mine.
In any case, you cannot heat up a chunk of metal (heat exchanger) and expect it to instantly cool just because the burner is off. Stop flow and it WILL boil. That's what a firemans switch does, keeps the pump running after control shutdown to dissipate residual heat from the heat exchanger.
That said, very few, and I mean VERY few, portable spa control systems can operate a firemans switch. Most are designed to work specifically and only with the heater they make for it and would require additional "jerry-rigging" of control components to function properly, if it's even possible. In some controls the firemans switch itself would cause flow errors and shut down the tub because the pump is running (or rather the flow switch/pressure switch is closed, which tells the computer it is) when it shouldn't be.
I do not recommend this course of action. Use the heater it came with.