Can I use pool water to radiant heat concrete deck?

ricksmarine

I am mid construction on a 18x40 pool (10' deep end) with 8x10 hot tub incorporated within pool - 2 zone 400k btu gas heater. Auto cover and salt system. I intend to run the pool/spa year round. I'd like to construct a valved manifold off output side of pool heater to supply 250' runs of 3/4" pex, 8"oc under concrete pool deck 400sf +/- and simply return the radiant heat water back into one of the pool skimmers. I do understand that if the pump fails the pool brine in the concrete will freeze. (An adjacent compressor can easily blow out the lines when the system is not in use.) Is this feasable and can it work? Has it been done before?

HVACNUT

Not sure of the effect to the pex with the chemical makeup of the pool water. Plus, what if someone pee's in the pool?

hot_rod

Get a heat exchanger that is designed for salt water. Got enough boiler output for both.

pecmsg

ricksmarine said:

I am mid construction on a 18x40 pool (10' deep end) with 8x10 hot tub incorporated within pool - 2 zone 400k btu gas heater. Auto cover and salt system. I intend to run the pool/spa year round. I'd like to construct a valved manifold off output side of pool heater to supply 250' runs of 3/4" pex, 8"oc under concrete pool deck 400sf +/- and simply return the radiant heat water back into one of the pool skimmers. I do understand that if the pump fails the pool brine in the concrete will freeze. (An adjacent compressor can easily blow out the lines when the system is not in use.) Is this feasable and can it work? Has it been done before?

Where is this pool located?

ricksmarine

The pool is located in Sandy, Utah so we have pretty good winters/snow here at 5,000ft elevation. The local contractors are pricing out a complete separate radiant heat system at "telephone numbers" with high output wall packs.

I considered building a heat exchanger (and a closed loop glycol circulated system) to avoid the potential for system freezing below concrete but don't believe that I would get enough/efficient heat transfer to make it work cost effectively.

With a covered 37,000 gallons(+/-) of pool water at 80 degrees I was hoping to easily "valve control" the system flow through my manifold (or completely shut it off) by measuring the output temperature of my heating water circuit lines as they visibly dump back into the pool via the skimmer. I have a decent sized air compressor that can be permanently installed adjacent to the pool equipment that can be dedicated to blowing out the radiant lines by simply closing the supply valve and opening an air valve. This will also keep the lines free of "mung" in the summer.

(Now that I'm thinking about it, since it gets so damn hot here in the summer, keeping the water flowing through the hot (stamped concrete) decking may assist in heating the pool as well?)

The salt water system for the pool (vs an old school chlorine system) will not affect the integrity of the pex - nor will somebody's pee...(I don't believe) - I am thinking 3/4" pex is the night size with the lines 8" oc with 250' max circuits. I might be closer to 300sf of concrete decking that needs to be heated - still need to calculate that and then determine how many loops i'll need.

GroundUp

A stainless tube and shell HX to split the radiant from the saltwater and allowing glycol in the radiant side would be probably the best option, IMO. I like your out of the box thinking

pecmsg

cupro-nickel heat exchanger needed in salt water. 

Or Titanium. 

ricksmarine

I have a marina and saltwater boating background and I'm concerned about dealing with electrolysis issue with the use of any dis-similar metals. The low salt content in the pool system will not affect PEX or PVC. I'm trying to avoid a separate pump system as well to circulate the glycol. I have a pretty damn big reservoir of hot pool water and only have to get the entry way from the house to the pool deck to about 35-40 degrees to melt snow....

ricksmarine

Here's a pic of the project today. Just want to get from house to pool w/ my ski bunny next winter...

hot_rod

Use the pool pump to flow a bit thru the loops. It should work. At what temperature does that pool water freeze. I live in Draper theneighbors want to dump heat in these 110F temperatures, use the deck as a heat dump in the summer.

ricksmarine

Thee's not a lot of salt in the water so I would say it would probably freeze at 30-32. Again, I would blow out the radiant heat manifold when the system is not in use (ie - as you know we haven't had snow in the last 3-4 weeks and I'd blow it out from safely say April 1 - November 15.) Blowing it out would involve simply closing the manifold valve and opening the air valve.

I imagine that the pool deck will be pretty damn hot just from the sun no matter what color I choose for the stamped concrete deck - I guess I could try to use the radiant as a "heat dump" as you say if needed.

The major questions I need to answer is how much "line (heat) loss" will I have over 250 feet of tubing, how fast should the water be going through the line(s), do I really need insulation (2" foam) under the PEX (i'm concerned about concrete integrity/cracking) and would prefer a "full slab".

Thanks.

pecmsg

To play the devils advocate 
if your not there
if it starts freezing early

whos blowing out the pipes?

Ironman

Here are the issues that I can see:
1. Any metallic pex fittings, manifolds, or other components are not gonna hold up under the salt. I have first hand experience dealing with this where someone didn’t use the right components.
2. The pool pump is designed for an extremely high volume of water, like 60 gpm +. You can’t force that through 3/4” pex lines.

The proper approach is a titanium heat exchanger with the necessary ancillary components on the hydronic side.

Zman

The 80-degree pool water is not going to make the slab feel noticeably warm.
If this were my project I would set a condensing boiler sized big enough for the pool and a snowmelt load and run the boiler loop with 50% glycol. I would then set a titanium heat exchanger sized big enough for the boiler to run at low boiler side water temps so it will run in condensing mode most of the time. If you pipe it correctly, you could use the slab to heat the pool when conditions allow.

Most pool boilers are just copper fin boilers that have been made to withstand (sort of) the harsh pool chemicals and environment. My experience has been that they don't last very long and they are not very efficient.

ricksmarine

Thanks for the "devil's" take. My plan is to integrate a permanent and simple "blow out" system. After initial testing ( to determine water flow/how long system needs to run before snowfall to be effective, etc) the lines would be cleared when the radiant system is not operating. 24kw Generac will be providing back-up power to the pool and pump equipment.

I'm not that concerned about the slab being "noticeably warm" underfoot - just warm enough to melt snow over time, Not as critical as a pedestrian walkway installation at an airport IMO....

My guess is that if I have it running 15-20 days during the winter its going to be a lot....

hot_rod

Your plan is certainly worth a try if you or your plumber is willing 
The pool would need to be circulating and a 3 way valve could divert a portion to the slab. Short 3/4” loops as you mentioned would keep pressure drop low. You can buy or build composite manifolds and fittings.

A separate boiler could be added later 

The insulation would limit heat loss downward and speed up the start up melt. And save some $$. It is common even under high load commercial slabs 

If course heating the great outdoors doesn’t come cheap

ricksmarine

Bob,

I spent some time learning from Calefi's 7/21 online technical manual last night and found it extremely informational for me.

First, it reinforced my opinion that I am personally unqualified to correctly estimate and assemble the proper equipment needed for a "traditional industry standard" heat exchange system and (other than removing/replacing failed components) I am less qualified to maintain/adjust the system as needed over time to make it work properly and efficiently. (ie quote: "HEAT EXCHANGERS SUPPLYING ANTIFREEZE-PROTECTED CIRCUITS When a heat exchanger is installed as the “heat source” for a portion of the overall system, that portion is fully isolated from the remainder of the system. As such, it must include components used in all closed-loop hydronic systems. These include a pressure-relief valve, expansion tank, pressure gauge, provisions for filling and purging, and one or more separators to remove air, dirt and magnetic particles. Optional components include a flow meter and thermometers or temperature sensor wells to monitor the inlet and outlet temperatures of the heat exchanger. In some applications, specifically district heating systems, heat meters are also installed on the closed circuit served through the heat exchanger."

That's a lot of expensive "gear" sitting outside of my pool house. There's also a section in there about warming a 32 degree garage floor that warns against potential for freezing up the fresh water side of exchange plates upon start-up "within a minute". I can easily see this happening in my case more often than not...

Second, I have to think that the "right" components are going to cost me upwards of $20,000.00 + the cost of a plumber who may (or not....) be a pro. I'm butting heads with a few stubborn "experts" who are, perhaps rightly so, encouraging me to do it "right" or not waste my time and reminds me that my 10 year old son can run my small Toro snowblower pretty damn well.

Here's a quote from your manual that caught my attention:

"Swimming pools have very high thermal mass. Without proper detailing, this mass can “dominate” the water temperature in the system, possibly preventing other subsystems that may be operating at the same time from meeting their respective loads. This condition must be avoided."

Well, this "condition" appears to be exactly what I am looking to exploit - at least on a very limited and TEMPORARY basis. My buddies are telling me "You don't have enough btu's; There's not enough recovery; You're going to get frustrated at having to purge you system every time you start it (I don't get that..); Pool heaters are not designed fo radiant heat systems and have way too much flow - you need a conduction wall pack - it's going to cost you a lot of $$ to buy and run though; You're going to forget to blow it out and the pool brine is going to freeze and destroy the lines and damage the concrete (not likely if you knew me); your idea is too far out of the box and I've never seen it so it probably won't work...if it was that simple somebody would have done it already.....

I guess what it comes down to is this: my swimming pool is not the size of a 275 gallon drum - if it was the size of the 80 degree "Caribbean Sea" there would be no doubt I would have enough heating water to thaw snow off of my pool deck - since I'm already biting the bullet on heating cost for the pool water who can I consult with that's qualified to tell me if I have enough of it to thaw snow on 300 sf of slab and for how long before the pool water is too cold to skinny dip in without "shrinkage"?

hot_rod

Doing some quick math, 400 sq ft of slab at 150 btu/ ft output for a quick responding melt would need 60,000 BTU/ hr. That needs to come from somewhere.
Your 400,000 pool heater at this altitude and derate would maybe give you 340,000 btu/ hr to cover both loads. If the pool is uncovered in winter, you will need all that input to keep the two pools from losing temperature quickly.

If you use just the hot tub portion in the winter you have enough btu for the melt.

Realistically a small mod con for the melt would be more efficient and sized to do the job correctly. I doubt many plumbers would want to be attached to that pool HX idea without engineering it properly and sizing the heater, pumps and HX for the loads.