|
You are not authorized to post a reply.
|
 Prev Next
|
| Author |
Messages |
|
jbaron
Registered Users
New Member
Posts:43
 |
| 10/08/2008 12:55 PM |
|
Greetings.
Has anyone here ever investigated the use of their hot tub as a heat dump during hot or sunny weather? This is new construction, and a new, redwood, always filled hot tub that will be heated (even without this feature) via a boiler or hot water heater/heat-exchanger system that will have a solar heat water storage tank.
It seems that if I keep the hot tub at 100 degrees throughout the summer (the house is on the CA coast, and the ambient temperature will be between 60 and 75 degrees during most of the critical time), that I can get almost 100,000 BTU of daily heat dump capacity through radiation of the 100 degree water and, more importantly, the evaporation of the water into the atmosphere, which is exothermic to the remaining water in the tank, simply by having a non-solid cover on the hot tub that allows good access to the atmosphere.
Thanks in advance,
Jeff
|
|
|
|
|
|
|
Brock
Registered Users
Basic Member
Posts:444
|
| 10/08/2008 5:48 PM |
|
| Oh ya! Not an outside hot tub, but I used our kids 800 gallon pool as a heat dump. Our system is set up to heat and indoor pool / hot tub and cool the house in summer. The house got warmer and we couldn't dump all the heat in the pool and rather than dump the heat to the field I started heating up the kids outside pool. On the hottest days I even ran a small pond pump acting as a fountain which dumped the heat even faster, it was odd to have it warm, but nice to sit in at night under the stars. I thought it was a better way to use the waste heat ;) |
|
Green Bay, WI. - 4 ton horizontal, 16k gallon indoor pool, 1.8kw solar PV setup, 3400 sq ft |
|
|
jbaron
Registered Users
New Member
Posts:43
|
| 10/14/2008 5:27 PM |
|
Brock,
Do you have this automated, or do you do this manually?
Jeff |
|
|
|
|
Brock
Registered Users
Basic Member
Posts:444
|
| 10/15/2008 10:35 AM |
|
Sort of automated. I ran additional 1 ½” lines in and out to the pool. But I had to manually switch between the indoor hot tub and the kids outdoor pool. If I had to do it again I would have put in automated valves and I would have run the 1 ½” inch lines under the cement and outside.
I do have the heat pump set to turn on if it gets a heating call from the pool control panel. Then I broke that heat call line with a cooling call from the house T stat. In other words the pool has to be calling for heat AND the house has to call for cooling to turn on (summer setting switch). The only time that was really an issue was when you wanted to warm up the indoor hot tub you had to turn the house thermostat down (although this wasn't a problem in summer). Or the larger problem was before I added the outside pool the indoor hot tub was hot and the pool was as warm as it needed to be so we didn't get cooling anymore. Once we added the outside kids pool we could dump at least twice as much heat in to it.
It was neat to sit in it and think it was all "waste" heat that in a traditional A/C situation would be blown out in to the air. |
|
Green Bay, WI. - 4 ton horizontal, 16k gallon indoor pool, 1.8kw solar PV setup, 3400 sq ft |
|
|
jbaron
Registered Users
New Member
Posts:43
|
| 10/15/2008 1:13 PM |
|
This all started running around my brain because we need to find a way to economically dump heat. Yes, we could dump heat through a coil into the ground, I suppose, but using cold water to lower the temperature is not practical from a cost perspective: the cost of 100 cubic feet of water is about equal to the natural-gas cost of actually heating the water via solar those 80 or so degrees, about $6 for 500,000 BTU's, or 100 cubic feet of water.
We have a few other alternatives that are also in the running - automatically heating 550 square feet of outdoor concrete/stone patio and walkway when the heat is available, and being able to radiate 20 degrees F of heat, would cost about 100,000 BTU's for each cycle - an excellent choice as well. We'd just need to run the tubing before the patio was installed.
The only problem is the controller - I've yet to find a controller that is sophisticated enough the make these decisions on its own, but I am quite computer literate, and have found a piece of software (MisterHouse) that I could use to perform these complicated tasks.
Jeff
|
|
|
|
|
gregj
Registered Users
Basic Member
Posts:218
|
| 10/15/2008 5:06 PM |
|
Posted By jbaron on 10/15/2008 1:13 PM
We have a few other alternatives that are also in the running - automatically heating 550 square feet of outdoor concrete/stone patio and walkway when the heat is available, and being able to radiate 20 degrees F of heat, would cost about 100,000 BTU's for each cycle - an excellent choice as well. We'd just need to run the tubing before the patio was installed.
Is a concrete slab radiator more efficient as a heat dump than the traditional air source heat pump coil?? Seems like on a hot sunny day the chance that a solar heated slab will efficiently dump heat is slim to none.
I'm missing something here. |
|
|
|
|
jbaron
Registered Users
New Member
Posts:43
|
| 10/15/2008 6:07 PM |
|
The house is on the CA coast, and the nighttime temperatures are most often in the 60's during the summer months. In fact, June, July, and August are often plagued by cloudy, 62 degree daytime weather, and a warm, radiant patio during the evening is something that would be rather nice. In fact, we talked about plumbing the patio before the heat dump idea came into being, but when I calculated the amount of heat it would take - $2-$3 worth, we decided that it was a bit wasteful.
Jeff |
|
|
|
|
Brock
Registered Users
Basic Member
Posts:444
|
| 10/15/2008 7:16 PM |
|
One of the other power users in an air source heat pump or traditional AC is the big fan. In our last house I calculated that to be using just over 450w alone, it moved a LOT of air. Using GSHP you only have 100w to 200w of pumps circulating the water in and out.
I would think the patio would work, but keep it on the north side of the house, or if it is on the south side put a sun shade over it and paint it white :) You might run in high ground or patio temps although you would get that as well with a hot tub at about 100F |
|
Green Bay, WI. - 4 ton horizontal, 16k gallon indoor pool, 1.8kw solar PV setup, 3400 sq ft |
|
|
Brock
Registered Users
Basic Member
Posts:444
|
| 10/15/2008 7:21 PM |
|
I am confused on the controller side, if it is just a slab let the house call for cooling and your done, just dump the heat as needed to the slab.
Or do you mean if you use a hot tub as the dump? One thing I toyed with was setting the small fountain pump (spraying up in to the air falling back in the pool) to come on at 100F. It is amazing how much heat that helps get rid of. I think it’s a combination of the evaporation of the water in the fountain and it sort of stirs the surface of the water to which also helps let the heat out. |
|
Green Bay, WI. - 4 ton horizontal, 16k gallon indoor pool, 1.8kw solar PV setup, 3400 sq ft |
|
|
jbaron
Registered Users
New Member
Posts:43
|
| 10/15/2008 7:42 PM |
|
Brock,
The "slab" is not the house slab (which will be radiantly heated and will call for heat as needed) but rather, the slab under the outdoor patio and stone walkway, probably 4 or so inches of concrete and then 2 more inches of stone etc. The slab/patio/walkway and the hot tub are the two heat dump possibilities that I've mentioned.
When I calculated the amount of heat that a 100 degree hot tub would lose in a 60 degree environment, it was about 20,000 BTU/day via radiation, just as a slab radiated heat. However, when the hot tub is uncovered, water evaporates, and that is VERY endothermic. I calculated that an uncovered tub in that environment would lose about 5 gallons of water per day! That process would consume 80,000 BTU / day, most of which would come from the water.
If I were going to do another experiment in addition to the fountain experiement that you have done, I'd try a fan blowing across the water. The equations to calculate water evaporation are very sensitive to airspeed on the water. I believe that a speed of 1.3 meters / second across the water would double the amount of water that evaporates. I'd guess that your ad-hoc fountain achieves much of this benefit as well. Did you ever measure the amount of water that you lost in a given day?
By "controller", I mean something that can, for example, dump heat into the patio but only if the patio is less than 80 degrees, and only before midnight. Something more sophiticated than the one or two temperature thermostats that I've seen.
Jeff
|
|
|
|
|
Brock
Registered Users
Basic Member
Posts:444
|
| 10/16/2008 1:12 PM |
|
Ahhh, I get it, yes I figured the slab was outside and being used as the heat dump. I don't know how much water was lost from evaporation because the kids played in it almost every day and I had to put between 25 and 50 gallons back in to top it off. They splashed / dumped out a LOT of water, kids :)
Yes I would think a fan blowing across the surface would do a very similar thing to the fountain.
On the controller side I don't know for sure but you can set a time of day and switch between two things with a pool control system. The trick would be having it switch back to the hot tub if the slab was up to temp. Honestly I have though that would be something built in to the regular pool controls. So if you hit the "hot tub" (or whatever button) button, it switches over to the hot tub until it reaches its set point, then switches back to the pool. Seems like this would be a common heating call, but none that I know of do this. They just switch over for a set period of time, or by the time set in the schedule. |
|
Green Bay, WI. - 4 ton horizontal, 16k gallon indoor pool, 1.8kw solar PV setup, 3400 sq ft |
|
|
Wobbly
Registered Users
New Member
Posts:6
|
| 11/23/2008 9:22 AM |
|
Posted By jbaron on 10/08/2008 12:55 PM
Greetings.
Has anyone here ever investigated the use of their hot tub as a heat dump during hot or sunny weather? This is new construction, and a new, redwood, always filled hot tub that will be heated (even without this feature) via a boiler or hot water heater/heat-exchanger system that will have a solar heat water storage tank.
It seems that if I keep the hot tub at 100 degrees throughout the summer (the house is on the CA coast, and the ambient temperature will be between 60 and 75 degrees during most of the critical time), that I can get almost 100,000 BTU of daily heat dump capacity through radiation of the 100 degree water and, more importantly, the evaporation of the water into the atmosphere, which is exothermic to the remaining water in the tank, simply by having a non-solid cover on the hot tub that allows good access to the atmosphere.
Thanks in advance,
Jeff
[/quote]Hot tub water chemistry must be balanced within certain limits. Adding large amounts of make up water required due to evaporation would require LOTS of effort to maintain water chemistry balance. To keep the tub at 100F would require 110F or more leaving temperature from the heat pump.
|
|
|
|
|
|
| You are not authorized to post a reply. |
|
|
|
ActiveForums 3.6
|
Professionals Serving
Your Location:
|