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

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Discussion Overview

The discussion revolves around the heating performance of a hot tub's heating element, specifically whether the heating tube feels hot to the touch while cold water is rushing past it. Participants explore the dynamics of heat transfer, the expected temperature changes, and the implications of water flow on the heating element's temperature.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant notes that the heating tube does not feel hot to the touch, suggesting that the cold water flow may cool the element as much as it heats it.
  • Another participant questions the amount of heat the element is supposed to dissipate and suggests measuring the amperage, indicating that the geometry of the setup could affect temperature perception downstream of the heating element.
  • A participant mentions that the outflow temperature should not be significantly warmer than the inflow until the water temperature rises, implying that the heating element may not feel hot initially.
  • One contributor provides a rough estimate of the heating capacity of a 1kW heater, suggesting it would raise the temperature of a certain volume of water by a small amount per hour.
  • Another participant shares their experience with hot tubs, noting that it typically takes a long time to reach the desired temperature and that various factors could influence the heating rate.
  • Participants discuss the importance of covering the hot tub to retain heat and the potential impact of external temperatures on heating efficiency.

Areas of Agreement / Disagreement

Participants generally agree that the heating element may not feel hot immediately due to the cold water flow and that it takes time for the water temperature to rise. However, there are differing views on the expected temperature changes and the factors influencing the heating process, indicating that multiple competing views remain.

Contextual Notes

There are limitations in the discussion regarding the specific measurements of heat dissipation and the exact conditions affecting the heating process, such as the initial water temperature and external environmental factors.

Who May Find This Useful

This discussion may be useful for individuals interested in hot tub maintenance, heating systems, and the physics of heat transfer in fluid dynamics.

bobbo7410
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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...
 
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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.
 
russ_watters said:
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.

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.

[PLAIN]http://img440.imageshack.us/img440/6126/asdfsdaf.png
 
Last edited by a moderator:
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?)
 
What you want to be worried about with a used tub is leaks.
 
DaveC426913 said:
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?)

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.
 
bobbo7410 said:
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...
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?
 
russ_watters said:
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?
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.
 
Last edited by a moderator:
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!
 

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