Converted Old Optima To Natural Gas On Hot Water Heater And Heat Exchanger

[mws]

Had lurked on this forum a bit in the last few years and learned a few things. But a year ago when I was looking, saw little on natural gas conversions.

All the people in the industry I talked to described huge 150K-200K BTU/hr pool heaters, requiring 2" gas tubing and extensive surgery to control the gas heater. The application they could picture in their minds, was to re-heat 60*F water to operating temp every time you wanted a dip. I wanted the hot tub to act like a hot tub and simply be hot all the time, year round, without using electricity to heat it. I wanted both standard mode, economy mode and the filter cycles with heat, to act like they do normally. All the pros scratched their heads and basically told me to take a walk. They wanted me to do an expensive solar rig and use electricity in the winter. No one I talked to or looked at online had a good gas solution. Plus, I was quoted at least $4000 per spa (I have a swim spa as well) to do it. I would be able to do both, for around $4000 and only because I had to spend more than $2000 trenching under concrete for a gas line. Right now, I figure I spend almost $3000 per year heating the two spas using electricity. A PV system to try to support it and my home usage would be in the 10-11 KW range, About 50 grand.

I successfully converted my 1998 Sundance Optima to cng- heated water with a Bradford White 75 Gallon 76K BTU/hr and a stainless 55K BTU/hr tubular heat exchanger. The relay that turns on the old electric heater element now sends 220v voltage to special a 220v signal relay that switches on my 110v Grundfos pump. Mcmaster Carr has the relays for about $18.

very simple and it works. Hot water, year round, on natural gas, which in California, is a little more than $1 per therm. Plus, with the water tank I could use solar thermal as a supplement, if I wanted.

I am editing a video which clearly shows the theory of operation and how long it took to heat from 63*F to 101*F (from 4:30PM to 11:06 PM, basically. not bad). I'll post it soon.

[mws]

The video which shows how I did it.

[Roger]

Good old DYI ingenuity.

[mws]

Thanks, coming from a tech, that is a nice compliment. I appreciate it. Have it running since Saturday, (It's much more insulated now) and as of today, Wednesday, running flawlessly. Now, to plum in the big 3000-gallon H2X, and try to schedule their heat cycles to not overlap. The H2X is 88*F all year long. Will be REAL interesting to see how long it takes to heat from 60*F.

[Dr. Spa]

The nice thing about DYI ingenuity is there's rarely any consideration for code violations or liability ;-)

I've seen this type of heating done somewhat frequently on older wooden hot tubs. It works extremely well, though the normal chemical use in a hot tub tend to greatly shorten the life of the DHW water heater.

[mws]

why don' they use a heat exchanger? then they only need to replace that--not the whole hot water heater.

btw: these stainless exchangers seem to have become a commodity that is being produced in large numbers and sold by a bunch of vendors on Ebay. That's where I got mine.

http://www.ebay.com/sch/i.html?_trksid=p2050601.m570.l1311.R1.TR4.TRC2.A0.Xpool+heat+ex&_nkw=pool+heat+exchanger&_sacat=0&_from=R40

this was the store I used, they have a great selection of pumps as well. Good phone service.

http://stores.ebay.com/Outdoor-Furnace-Supply/Shell-Tube-Heat-Exchangers-/_i.html?_fsub=2233445014&_sid=20243294&_trksid=p4634.c0.m322

[mws]

good point about code. would be interesting to see if I could even come close to doing anything like this per code in my county.

[Roger]

A 150,000 BTU pool heater is not that expensive though. And with your swim spa also I wonder how that water heater and x will keep up? If your in a fairly moderate climate probably no big deal.

[Dr. Spa]

With the DHW heater of 76k BTU's, you wouldn't need a heat exchanger rated at over that.

76k BTU's heating 3000 gallons of water should do about a 2-1/2 degree per hour temperature rise (my chart shows 100k BTU's doing 5 degrees per hour on 2000 gallons).

[mws]

 

A 150,000 BTU pool heater is not that expensive though. And with your swim spa also I wonder how that water heater and x will keep up? If your in a fairly moderate climate probably no big deal.

 

I appreciate the comments. These are great.

the local spa techs I talked to never spent any real time figuring this out, this was all just 15 minute phone calls where they assured me they could do it, but most wanted to heat the water from complete scratch and were not that interested in trying to figure out the logic involved with allowing the spars to cycle on and off naturally. One guy did think he could make the heaters cycle, but then we get into the fact they wanted two heaters...

They wanted to install one on the hot tub and one on the swim spa since they could not imagine a central source for the heat. For the swim spa I was getting quotes of 200K BTU/hr at least. And they said the gas pipe would need to be large enough to accommodate the larger of the two. So the quotes they came up with off the top of their head were around $4K installed per spa. So around $8K total. I did the whole job for about $4K and mostly because the pipe install from my house to my barn was expensive, had to burrow under 16' of concrete driveway. We went with 1" ID for the DWH. Less expensive than the 1.5" to 2" ID or so pipe they wanted for the larger pool heater.

The added advantage of using a DWH is: if I ever wanted to, I could supplement with SWH. If I had a pool-type heater I'd have to add a tank. CNG will have to quadruple in price, at least, to force me to do SWH. Plus, I would prefer to not have to add diverter valves, blow-off valves, extra logic, freeze protection, etc. Not unless I have to.

In fact, this whole natural gas with water heater tank solution came to fruition because I spent months looking into solar water heating and was confounded by the fact the affordable ($1000 or so) SWH tanks were all electric back up. So, I would be back to spending about $200 a month to heat the spas in the winter. And the systems were super complex and for any sort of winter performance, I wanted to go with vacuum solar tubes. The designers I talked to recommended two 75Gal tanks, and at least two 30-tube arrays. It got into-- again-- $8K total or more, to try to tinker it together--it may or may not work--and then, only to be forced to use electric heating in the winter.

as for complexity, I love the passive-ness of my design, even over using a direct pool heater with no tank. There's no extra logic, except an $18 relay. And the spa thinks nothing has changed. The tank heats water when it needs to, all by itself, doesn't even need electricity, of course. The only more elegant thing I could have done was to probe the back of the board and look for 24v control signals to the relays and use that instead. I guess I got lazy :-P

Even if I add the swim spa, there will be no special diverter valves or other electronic logic. The pump just grabs hot water from an available source and pumps it. I just need to buy one more $18 relay and a pump and an exchanger.

[mws]

 

With the DHW heater of 76k BTU's, you wouldn't need a heat exchanger rated at over that.76k BTU's heating 3000 gallons of water should do about a 2-1/2 degree per hour temperature rise (my chart shows 100k BTU's doing 5 degrees per hour on 2000 gallons).

 

interesting. good to know thanks!. I bought a 55K BTU for the hot tub, without doing any calcs, but also, because that's the smallest they make in that configuration. So, I can easily bump up to the 75K model, which is in good supply.

In the summer, when I measured this, I got these figures, you can double check my math.

Big spa has 1*F drop per 24 hours in temp which should be 25000 BTU @ 3000 gallons

Hot tub at 450 gallons and 4*F drop per 24 hours is 15000 BTU to re-heat it

the ratio is 1.6, which would have led me to buy an 88K BTU exchanger, but who is to say my 55K one is not maybe a bit big for the hot tub. I'll try the 75K BTU.

technically if there were no heat loss at all, I would need only 40,000 BTU per day for both. But the DWH already seems to be consuming about a therm (100,000 BTU) a day, according to PGE smart meter. It was on max, and pumping out water at 142*F, according to my thermometers. I am trying to turn it down a skosh to see if it can still keep up with the tub, just as an experiment.

lucky for us, the weather has been so consistent in northern California that I can tell when more gas is being used, just by tracking several previous weekdays when we are not home --which all look exactly the same for gas consumption--and then adding the DWH being turned on during weekdays and comparing the previous weekdays.

[mws]

Thanks for all the great thoughtful comments, so far. I could use some guidance on thermal shielding for the pipes running to and from the hot tub to the heater. I would really like to use a reflective, radiant thermal shield tape or cover of some kind. And foam insulation.

currently I am only using the standard foam tube insulation available at hardware stores for freeze protection of pipes. I am actually getting the same temp into the pump as I do at the heat exchanger's "tube in" but... I am probably also soaking the entire pipe system with heat before I get those equal numbers.

question: reflective shield on tube first, then foam? or foam first, then reflective? Or only one or the other?

Also, how to thermally cover the pump? Right now it's just a room-heater for my barn.

[Dr. Spa]

Your calcs are accurate.

My understanding is that reflective insulators ONLY work if there's an air space in front of the reflectiveness (they "reflect" back "radiant heat", which only occurs through empty/air space.)

Keep in mind, PVC starts to soften and melt at 140.

[mws]

 

Your calcs are accurate.My understanding is that reflective insulators ONLY work if there's an air space in front of the reflectiveness (they "reflect" back "radiant heat", which only occurs through empty/air space.)Keep in mind, PVC starts to soften and melt at 140.

 

yup. I looked up the PVC temps. That's why I only used it on the "shell side" the spa side. That hot tub had "better" not ever reach anywhere near 140*F. I won't insulate PVC and galvanized steel pipe together, either. So I should be OK.

anybody got a reflective tape they like? the self-adhesive padded stuff at Home Depot kinda sucks.

[mws]

finally did some googling, found some interesting high tech stuff.

so THAT's what the orange pex is used for:

http://ecomfort.com/radiantpex-al-34-pex-al-pex-tubing-300-length-orange-2486.html

I thought the aluminum might help for thermal, but it's to help it retain it's form and not change size as it heats up and cools off.

But for thermal protection, here you go, $15 a foot and your worries are over.

http://ecomfort.com/r-flex-254mm-1-insulated-pex-dual-pipe-328-3950.html?utm_source=adwordsfroogle&utm_medium=product_search&utm_campaign=adwordsfroogle&utm_content=3950&gclid=CJ-_v_PZg7wCFcKDQgodJBgAuA

[Roger]

I think you could find a happy medium between that 15 buck a foot stuff and the cheap pipe foam from the box store. Go to Grainger and have a look. Or an insulation supplier like API supply in MPLS/StPaul not sure if they will sell to you but I am sure there are places that sell their products near you. Where are you?

[mws]

Roger I think you're right. I found this stuff on Amazon. Should get it this weekend. I called all the local hardware stores and they did not have it on the shelf.

http://www.amazon.com/Covalence-Adhesives-880930-Reflective-Insulation/dp/B005MK3WQ2/ref=lh_ni_t?ie=UTF8&psc=1&smid=A2QI0BKCYX1BAG

[mws]

Have not tried the heavy duty pipe insulation yet. Spent the last coupla weekends plumbing the 3000 gallon H2X swim spa to the same DWH. The H2X has it's own pump and heat exchanger. The Bradford White DWH is keeping up just fine with one hot tub at 103*F and one swim spa at 88*F.

for the techs: the only larger exchanger available was 88 BTU/hr sizing, not in the 70K range as we discussed. I went with 88K and it's working just fine. Pump is a 3-speed 26 series Grundfos. I thought a 3-speed 15 series was possibly too limiting in GPM.

I started with the DWH heater at full blast to get it heated quickly. Therm usage has gone up over the few days its been running on gas, I have just now trimmed the DWH temp down a bit and will know soon an approximation of extra gas usage per day. Meanwhile, kwh use of electricity for the swim spa alone has gone down about 20 kwh per day, at an average of 23 cents per kwh. A savings of at least $138/month in electricity for the swim spa alone. Of course, gas use will go up, but I don;t think the swim spa will use more than 1 therm a day at about $1.10 per therm.

The video clips are in the swim spa forum.

[mws]

yippee. I remember our first bill heating the tub using electricity

twice in one month from cold to hot (we had to drain it and re-heat

it) and heating the the swim spa for the first time, was like $750!!

I finally have the results of heating both the hot tub and swim spa on gas alone for one entire month of billing. Also, some home improvements to help with insulation and lower gas usage were in place during the entire cycle..

total bill $287. previous month $425 (and the hot tub was on gas for half that month, but swim spa was on electricity) the $425 bill was less than the month before that, even. (probably colder outside + both spas on electricity)

electric usage with 2 spas was 1700 kWh last year in Feb-March, this year, no spas, 700 kWh from Feb-March. 1000 kWH less!

gas usage is about same dollar amount from previous month to last month, even though we added spas (warmer month--less home heating). Last year we used only 96 therms in the same period, this year same period=105 therms, so 10 more therms per month this year, to heat the spas. I estimated 15-20 therms extra for the spas in my calculations, so we are likely saving 5-10 therms or so on home heating.

_MWS

[Roger]

Nice. I wish I had your drive.

[Robb235]

I know this thread is old... But what are thoughts on using a tankless hot water heater as opposed to a 75 gallon domestic water heater?