New Pool / Propane Heat Question

[Dirk]

I found some rough rules of thumb. Until experts come by, a little noodling just for s&g: According to Dr. Google, Ph.D., surface area of the water is the driving factor of pool heat loss, about 80%. So rough rule of thumb for surface heat loss in btu / hr is surface area times delta t (in degrees F) times 5 (yeah, rough rule of thumb lol). So let’s say 8x8 spa and delta t of 58-32 = 26. So about 8,300 btu / hr. about 6 million btu / mo. and using 80% heater efficiency = about 82 gallons of propane per month. Divide by 0.8 to account for the other 20% guestimated loss other than surface and presto, when outdoors is hovering at freezing and the spa is maintained at 58, expect 102 gallons of propane use per month.

Latent heat is ignored — spa is pretty shallow and that also gets too complicated lol. There will also be losses through the plumbing and equipment, etc., etc. Add another % fudge factor and say 120 gallons / mo.

If Dr. Google is correct, it might, in theory, be possible. Still very risky but at least sort of do-able. Since you’ve made the commitment to have the spa, maybe experiment in the Fall and see what your actual heat loss looks like. My bet is I’m way off but who knows lol.

Not seeing how this could be right. Not your math, I mean the 80% loss via surface area. What is the surface? The top surface only? Or all six surfaces? And if Dr. G meant only the top, how does one come to that number without considering the other five surfaces? Is it 80% when the other five surfaces are insulated? Or is still 80% when the other five surfaces are colder than the top? How does the air temp and humidity affect that 80% number? What happens on a sunny winter day, when the air temp is, say 65°, but the pool, still 35° from the night before, is still sucking heat out of the spa?

Again, I don't know nearly as much about this stuff as Dr. G, it's just a matter of logic (to me).

All this speculation aside, you still have a nice design for a pool/spa combo. I really like that you can cover both with one cover. The spa will be a great place to socialize. Kids will love it. And speaking of kids (yours or guests') being able to cover both bodies of water is a fantastic safety feature. Plumbing the spa to work both separately and together with your pool was smart. It probably didn't cost all that much extra, and will potentially open up many possibilities.

You'll either be able to afford to run the spa year 'round or you won't. It's probably not something that any of us can calculate, you'll know soon enough. And even if you aren't able to use your spa in the middle of winter, maybe you will be able to make great use of it in the fringe seasons because of the smart plumbing set up. I had a skimpy swim season this year, maybe my worst ever. Only about four months. Awful. If that happens again next year, I'll be staring at my pool from my deck, in the spring and the fall, and you'll be in your spa!

I mostly just dip in my pool, I don't swim around in it all that much. When my pool is 70° in the Spring, I would love to have a spa that was 85°, just to cool off on a warm day without chilling my bones. The spa will be great to have, even if you don't always run it up to 105°. So don't mind me, you built a smart pool and we're all rooting for you that it'll be everything you wanted!

 

[SoDel]

Not seeing how this could be right. Not your math, I mean the 80% loss via surface area. What is the surface? The top surface only? Or all six surfaces? And if Dr. G meant only the top, how does one come to that number without considering the other five surfaces? Is it 80% when the other five surfaces are insulated? Or is still 80% when the other five surfaces are colder than the top? How does the air temp and humidity affect that 80% number? What happens on a sunny winter day, when the air temp is, say 65°, but the pool, still 35° from the night before, is still sucking heat out of the spa?

Again, I don't know nearly as much about this stuff as Dr. G, it's just a matter of logic (to me).

All this speculation aside, you still have a nice design for a pool/spa combo. I really like that you can cover both with one cover. The spa will be a great place to socialize. Kids will love it. And speaking of kids (yours or guests') being able to cover both bodies of water is a fantastic safety feature. Plumbing the spa to work both separately and together with your pool was smart. It probably didn't cost all that much extra, and will potentially open up many possibilities.

You'll either be able to afford to run the spa year 'round or you won't. It's probably not something that any of us can calculate, you'll know soon enough. And even if you aren't able to use your spa in the middle of winter, maybe you will be able to make great use of it in the fringe seasons because of the smart plumbing set up. I had a skimpy swim season this year, maybe my worst ever. Only about four months. Awful. If that happens again next year, I'll be staring at my pool from my deck, in the spring and the fall, and you'll be in your spa!

I mostly just dip in my pool, I don't swim around in it all that much. When my pool is 70° in the Spring, I would love to have a spa that was 85°, just to cool off on a warm day without chilling my bones. The spa will be great to have, even if you don't always run it up to 105°. So don't mind me, you built a smart pool and we're all rooting for you that it'll be everything you wanted!

What you say makes perfect sense and I can’t explain it through thermal diffusion legerdemain (finished school 40 years or so ago and haven’t really looked back lol). The only explanation I get from what I’ve come across is that the thermal loss from the surface is so great that it dwarfs the (likely high as you point out) loss from the shell. It’s not that it’s not losing a lot of heat from the shell, it’s that it’s just losing a lot more from the surface. Has something to do with convective vs conductive vs radiative vs evaporative cooling. Something like the shell is mostly conductive loss but the surface has all the types of losses plus evaporative cooling. A plastic cover won’t do much to reduce much of that loss, IMHO, because I only see the cover as reducing evaporative cooling. If the water were a lot warmer than the air, a cover might help more.

If looked at as a box say 8x8x3’ filled with water, it takes (according to doc G) about 6,000,000 btu per month to maintain at 26 degrees F above ambient. Stepping back, that is a lot of heat applied to a relatively small box with not a lot of effect. It’s about as much as I would use to heat my entire house at only 10 degrees F warmer (and humble brag mumble, mumble, I have a pretty big house — that is a lot of heat). It is losing plenty no matter how sliced so your logic is not wrong. But I’m probably wrong lol.

One way to find out for certain is for op to enjoy the spa and see how much heat it actually uses. A good way to do it!

 

[Dirk]

One way to find out for certain is for op to enjoy the spa and see how much heat it actually uses. A good way to do it!

Exactly. The shell is in, cost to heat it is going to be what it's going to be.

So I think I was both kind'a right and kind'a wrong. A cover might retain more heat than I had thought, but because of the heat loss through the concrete there won't be much of any to retain for very long. The cover will slow the loss down some amount, but the spa will become cold quickly anyway. Like the difference between cooling off in an hour vs an hour and five minutes. No idea what the actual cooling off period will be, but it will most certainly be faster than the typical time between uses. In other words, the starting temperature of the spa, each spa-use session, will be the same as the pool, whether he covers it or not (and regardless of the season).

So theoretically, the OP (or you!) could roughly calculate the BTUs required to heat that much water for a couple hours, starting at whatever the median pool water temp might be in MD. Or you could build him a chart at 10° increments (30, 40, 50, 60, 70 80), what it would cost to warm the spa from each temp to 100°, then interpolate those costs across typical pool water temps in MD in a given year, to come up with a rough estimate of gas costs (based on local gas prices, accounting for projected gas price increases). No problem. Get on that, will you? When can we expect that? (Prefer color charts, please.)

 

[SoDel]

In other words, the starting temperature of the spa, each spa-use session, will be the same as the pool, whether he covers it or not (and regardless of the season)

I think that’s 100% just based off my own pool / spa. I could spend as much as I want to heat it today and it’s basically gone for all practical purposes tomorrow. I haven’t really tracked the spa but for the pool, it’s something like 2,500,000 btu loss per day if starting at only 20 degrees F above ambient.

While just noodling around with this, I was kinda hoping some of the folks from Southeast or Southwest who run year round and use heat in cool weather often would chime in. I think we’re more both right than not. Wherever the heat goes, it would cost a lot and not last long.

 

[Dirk]

I could spend as much as I want to heat it today and it’s basically gone for all practical purposes tomorrow.

Right. But is that because you're filtering the spa through the pool each day? That, of course, would equalize the temperatures. I guess what I've been shooting down is the notion that the OP isolating his spa's plumbing from his pool's, so that he could run it stand-alone in the winter, would help with the heat loss. The temperatures of his two bodies of water will still equalize quickly because the concrete walls of the spa won't insulate it from the pool's water temperature. It'll happen fast enough that covering it won't matter, practically speaking. My theory is, like you, he'll have to start from scratch each time he uses the spa.

I have a buddy who is building a house for a client entirely with a new "green" technology, using concrete for everything (no stick framing, not even the roof). But he explained to me that the concrete offers almost no insulation on its own. So the "green" tech is combining the concrete walls and ceilings and floors with layers of some type of foam. It's the foam that does the insulating. They decided to build the pool at the same time, in the same way! I'm dying to hear how this comes out, and how well it works. This could be a breakthrough in pool construction, especially considering our climate changes and ever increasing energy costs. Imagine heating a pool for 1/10 the cost.

Of course, with the pool industry's track record of technology adoption, we can expect this "breakthrough" to occur in a few hundred years. But by then, we won't use pools, because we will be spending our time completely submerged in our battery cocoons, supplying power for our AI masters. I suppose the cocoons will be well heated, so there's that to look forward to.

 

[SoDel]

Right. But is that because you're filtering the spa through the pool each day? That, of course, would equalize the temperatures. I guess what I've been shooting down is the notion that the OP isolating his spa's plumbing from his pool's, so that he could run it stand-alone in the winter, would help with the heat loss. The temperatures of his two bodies of water will still equalize quickly because the concrete walls of the spa won't insulate it from the pool's water temperature. It'll happen fast enough that covering it won't matter, practically speaking. My theory is, like you, he'll have to start from scratch each time he uses the spa.

I have a buddy who is building a house for a client entirely with a new "green" technology, using concrete for everything (no stick framing, not even the roof). But he explained to me that the concrete offers almost no insulation on its own. So the "green" tech is combining the concrete walls and ceilings and floors with layers of some type of foam. It's the foam that does the insulating. They decided to build the pool at the same time, in the same way! I'm dying to hear how this comes out, and how well it works. This could be a breakthrough in pool construction, especially considering our climate changes and ever increasing energy costs. Imagine heating a pool for 1/10 the cost.

Of course, with the pool industry's track record of technology adoption, we can expect this "breakthrough" to occur in a few hundred years. But by then, we won't use pools, because we will be spending our time completely submerged in our battery cocoons, supplying power for our AI masters. I suppose the cocoons will be well heated, so there's that to look forward to.

Yes, assuming the spa is completely isolated, I’d expect it to lose all of its heat very quickly and as you say, start from some low baseline each time. I can heat my pool to 85 from 65, and it’s basically back to 65 in 24 hrs. If I heat the spa and then turn the system back to pool mode, it’s basically instantaneous that the spa drops to pool temp. I’m not sure if I’m saying what I mean — I think you are spot on that any body of water will lose all of its added heat very quickly in Winter, so each heat event will be starting from whatever the temp had been maintained at each time. If it’s maintained at 58, it will be 58 the next morning, even if it was heated to 101 the previous afternoon. I think that’s what you’re saying?

 

[Dirk]

Yes, assuming the spa is completely isolated, I’d expect it to lose all of its heat very quickly and as you say, start from some low baseline each time. I can heat my pool to 85 from 65, and it’s basically back to 65 in 24 hrs. If I heat the spa and then turn the system back to pool mode, it’s basically instantaneous that the spa drops to pool temp. I’m not sure if I’m saying what I mean — I think you are spot on that any body of water will lose all of its added heat very quickly in Winter, so each heat event will be starting from whatever the temp had been maintained at each time. If it’s maintained at 58, it will be 58 the next morning, even if it was heated to 101 the previous afternoon. I think that’s what you’re saying?

Sort of.

- Typical pool/spa plumbing has the spa dump into the pool at least daily to maintain its FC and filter. In that setup, the spa's water temp will equalize with the pool's, and it will be near instantaneous when the spa joins the pool. As you mentioned, that setup loses any heating benefit of the previous day.

- Stand-alone spas (typical fiberglass jobs) have a shot at retaining some residual heat from day to day (use to use) because they're insulated, and have thick, insulated covers, and the outside of the spa and its cover are subjected to ambient air temperature, so may benefit from a sunny day.

- The OP's spa, which is plumbed to mimic a stand-alone spa, will unfortunately not have the same heat retention of a true stand-alone for three reasons:
1. the pool cover he's got will not retain heat as well as one of those thick insulated covers that seal to the top of a fiberglass stand-alone spa, and
2. the concrete walls won't insulate the water as well as a fiberglass stand-alone spa, because concrete is a crummy insulator, and
3. in the winter, the outside of his spa will be surrounded by a massive heat sink that will be a near-freezing temperature, regardless of ambient air temp.

I learned from my scuba diving classes: water conducts heat much better (faster) than air. So not only will his spa spill its heat faster into the pool water (right through the concrete), his spa won't benefit from a warm winter day, because it's in essence under water. If his spa had been built above ground, like a typical pool/spa combo, and also plumbed independently, that would have helped a little. The concrete walls would have still been poor insulation, but the spa water would not dump heat into the surrounding air as fast as it would into water. He would likely still have lost all of the previous day's heating regardless.

Now in the summer, or even the fringe months, his pool water will help his spa stay warmer, because while his night air might cool way down, he'll have his spa surrounded by a giant warming blanket (the warmer-than-air pool water). This is when his pool cover will also help, by minimizing evaporation from both the pool and spa.

So his sudo-stand-alone spa setup will actually be more beneficial in the summer than it will be in the winter. And as I mentioned, having a cover that will enclose both pool and spa is a really nice feature. Every pool design always has some number of trade-offs.

 

[SoDel]

Sort of.

- Typical pool/spa plumbing has the spa dump into the pool at least daily to maintain its FC and filter. In that setup, the spa's water temp will equalize with the pool's, and it will be near instantaneous when the spa joins the pool. As you mentioned, that setup loses any heating benefit of the previous day.

- Stand-alone spas (typical fiberglass jobs) have a shot at retaining some residual heat from day to day (use to use) because they're insulated, and have thick, insulated covers, and the outside of the spa and its cover are subjected to ambient air temperature, so may benefit from a sunny day.

- The OP's spa, which is plumbed to mimic a stand-alone spa, will unfortunately not have the same heat retention of a true stand-alone for three reasons:
1. the pool cover he's got will not retain heat as well as one of those thick insulated covers that seal to the top of a fiberglass stand-alone spa, and
2. the concrete walls won't insulate the water as well as a fiberglass stand-alone spa, because concrete is a crummy insulator, and
3. in the winter, the outside of his spa will be surrounded by a massive heat sink that will be a near-freezing temperature, regardless of ambient air temp.

I learned from my scuba diving classes: water conducts heat much better (faster) than air. So not only will his spa spill its heat faster into the pool water (right through the concrete), his spa won't benefit from a warm winter day, because it's in essence under water. If his spa had been built above ground, like a typical pool/spa combo, and also plumbed independently, that would have helped a little. The concrete walls would have still been poor insulation, but the spa water would not dump heat into the surrounding air as fast as it would into water. He would likely still have lost all of the previous day's heating regardless.

Now in the summer, or even the fringe months, his pool water will help his spa stay warmer, because while his night air might cool way down, he'll have his spa surrounded by a giant warming blanket (the warmer-than-air pool water). This is when his pool cover will also help, by minimizing evaporation from both the pool and spa.

So his sudo-stand-alone spa setup will actually be more beneficial in the summer than it will be in the winter. And as I mentioned, having a cover that will enclose both pool and spa is a really nice feature. Every pool design always has some number of trade-offs.

Well said. Only one small quibble — the way my spa is set up (I assume most are similar), when the spa is active, it is isolated from the pool — all spa water flows pump, filter, heater, SWG, back to spa, like the pool doesn’t exist. It can be left in this mode and the pool closed conventionally, lines blown, etc., leaving just the spa open and active. It’s not that pool water will mix with spa water and cool it, it’s what you’ve been saying all along: the heat loss from the spa alone will be extreme, covered or not. The spa can be heated to whatever temp and within a relatively short period of time, it will be just as cold as before any heat was applied (based on heating my pool a couple times when it wasn’t really cold out, my guesstimate would be the spa would cool back down in way less than even 24 hrs when it’s really cold — each time using it would be starting from scratch as you’ve said).

When we bought our house, it had fibreglass indoor spa / hot tub thing sunken in the floor of an enclosed porch. My dear wife insisted I saw it into pieces so it could be removed through the door and turn that room into part of the house, insulation, windows, HVAC, etc., that became her office and art studio. I did as instructed and marital harmony was preserved (plus it got her out of my office lol). To each his own I guess. I thought that spa was pretty nice. We got the concrete spa with pool combo mostly because the PB presented the design that way, it looked really cool, and in a sense we just didn’t know any better. I’d say on balance it has worked out ok; it does look cool and we and our guests enjoy it. It is closed by end of Oct.