Cold water rushing past heating element. Will the element feel hot?

bobbo7410

Curious question, figured what better place to ask than the physics forum.

So I just picked up a used hot tub. Was running great when I bought it. Filled it up with water, turned it on, started its cycle. It can take over 24 hours for the tub to reach 102 degrees.

There's a 12 inch heating tube in which water rushes past, "heating" the water which gets pumped back into the tub. Now I when I touch the heating tube, it doesn't feel hot to the touch. Though perhaps it won't feel warm/hot until the water equalizes in temp? Perhaps a strong flow of freezing water would essentially cool the heating element just as much as it would be heating it up, so I wouldn't immediately feel the tube get hot?

Anyone have any thoughts on this? (or perhaps it should get piping hot immediately and my heater is busted) I'm thinking I should feel some sort of heat....

russ_watters

Do you know how much heat the heating element is supposed to dissipate? Can you measure the amperage of the element or hot tub?

Yes, it is possible for the pipe to still be cool to the touch just downstream of a heating element, depending on the particular geometry.

bobbo7410

I'm not sure how much heat it's supposed to dissipate nor can I readily measure the amperage. I know it is a 1kw heater...

Here is the actual diagram. The section labeled 1 represents the 12" long heating tube, that is the heating element itself. Water rushes through it. That tube is cold to the touch. The water currently rushing through it is approx 50 degrees or less.

DaveC426913

I've had a couple of hot tubs.

The outflow won't be that much warmer than the inflow. No, it should not feel hot to the touch, not until the water's temp is higher.

Stick a thermometer in the tub. It should be rising a degree or two an hour. (You don't need to wait hours & hours. How long has the tub been running? How cold was the initial water? At ~2 degrees her hour, is it already several degrees warmer?)

DaveC426913

What you want to be worried about with a used tub is leaks.

bobbo7410

Thanks dave! Yeah I popped a thermometer in it when I first turned it on. The initial water temp was 51 degrees. It is now showing 53ish degrees and has been exactly 1 hour. The outside temp is 33 degrees. There are quite a few variables that could influence only a few degrees of change, yet nonetheless it looks promising.

I think I was initially concerned because I figured I would most definitely would feel some heat from the heating element. Though had to step back and think from the other end of the spectrum, such as flushing cold water to cool a hot element.

russ_watters

1kW is the heat it is supposed to dissipate.

That's not a lot of heat - enough to heat a 1m radius by 1m deep tub 0.87C per hour. Is it cold and dry where you are? Can you cover the hot tub? What's the volume of the tub?

DaveC426913

It will surely come with a lid. Not much use without one.
They are typically 200-500gal corresponding to 2-6 person tubs.

[EDIT] Yep. The pump diagram is for a TX which, according to Solana's website (http://www.solanaspas.com/) is a 130gal 2-person tub.

As for cold and dry, well, he says it's 33 degrees. (Presumably, that's F).


This is just like my situation. And yes, it takes just about 24 hours to come up to temp.

bobbo7410

Chicago IL here, constant 33 deg F. The tub is indeed 130gal with a nice insulated top. It's been rising steadily every hour. 51-53-55-56-59

Thanks for all the helpful info!