Carwash glycol floor and hot water system

Garfield

Hello, I'm trying to design and then install a system for a Carwash. I'll tell you what I'm thinking and then you can get out the red pen! 3 Navien 240 a tankless with circulating pump and a heat exchanger with glycol for the floor heat in the bays. So softened water would run through the Naviens and circulate for floor heat and probably an air exchanger for heating the equipment room. I want to add pics but the option is grayed out.

kcopp

For starters.....

Do NOT use a tankless water heater to heat the radiant floor where a boiler is what you need.... Use a boiler. Triangle tube, Burnham, Lochinvar, Smith etc.... The head loss through the tankless is to high and the wrong application.... I don't care what the tankless Mfg says. HX is a good idea.

meplumber

kind of agree

While this application falls under process water and not DHW, there are still equipment longevity issues to consider.



We had a contractor up here that was installing Tankless water heaters on radiant system a few years back. They wouldn't last 5 years. They just aren't designed for it.



Save yourself a bunch of headaches and put in boilers. Use the HX to make the hot water for the Process. then glycol the entire heat loop.

Garfield

1/3 service?

I do see your point. What I'm wondering is if I have 3 of them that rotate service on a cascading system won't this help the longevity issue. How do I post pics?

SWEI

Big Picture Time

Post pictures as attachments - look for the [Browse] button just below where you typed your reply.



Best to look at the entire load profile -- there may well be some efficiencies to be gained with the right design.

HDE

Don't do it

That would be a disaster.

First tankless fire on a combination of temp rise and flow combined. Since tankless manufactures limit pump flow rate through their heater to 4 GPM max and your heating system is designed for 20-30 degree delta T, the fire rates would be 40,000 to 50,000 Btu max.



What's the car wash domestic demand, rarely will three tankless do a car wash much less snowmelt too.



The only successful heating application for tankless is a small combi application with air handler only IMO.

Garfield

More info

I have only about 1700 sf to provide floor heat for. (other bays are shut down in winter.) From reading online it seems that about 100 btu per sf. I was thinking with a properly sized pump on a vfd to control return temperature I could get the units to fire properly... Though with 3 I wouldn't need full fire. Another wash owner has run floor heat with glycol directly in the tankless for 9 years. It would probably run half the time for 3 months per year and the rest of the time be just providing wand water.

Garfield

Load

I believe 9 gpm considering derating for rinsing, foam, waiting etc. and 1700sf of floor heat.

kcopp

So if you.....

only have 1700 sq ft it would make even more sense to do it right and use 1 boiler instead of 3 tankless water heaters.  Why do you still want to consider the tankless water heaters after 3 or 4 people have stated that this is a poor choice? In the long run you will look better (if you are the contractor) and there will be less headaches.....Its a win- win! Furthermore the gas bill stands to be Much higher w/ a tankless as they are around 84% afue vs. a Mod con that is upwards of 96%.

Garfield

Fair enough

Yes I see the point. There's currently a 900000 btu raypak for glycol and a 1100000 btu raypak for the floor. Gas bills ran 3800 in the high months. The Navien I looked at was 98% efficient and was rated for the use with a 10 yr warranty on exchanger. I just bought the Carwash.

SWEI

heat loss

Any idea how what kind of cycle times the floor heat runs for?

hot_rod

check the snowmelt load

depending on your location 100 btu/ sq ft seems a bit light? Is it inside and outside? The exit ramps and inside the automatic bays are a concern for icing anytime the temperature is below freezing, regardless of snowfall.



I'd agree with a mod con, maybe two and stage them to keep them condensing. Then a a HX or indirect for DHW.



Modern carwashes really don't use all that much hot water. Last one I worked on had a medium sized tankless to generate DHW which was just for mixing the soaps and chemicals. Everything else including the rinse was cold water. The equipment suppliers usually have a very exact number for DHW required.

Garfield

Cycle times

I really don't know. There are no slab heat sensors. Both boilers are direct vent so I'm sure that's a substantial waste. They also weep hot water and I'm hoping cold will do the trick.

Zman

What are your loads?

Ok,

First the hot water. How much hot water do you need in the summer? How much in the winter. What temp does it start at and what do you heat it to? If you are reclaiming some of the water, this may be tricky to figure out and you may need to check with the manufacture.If you are just heating water and putting it down the drain you could just observe the water meter during peak times and see how many gallons per hour you are using. It's all getting heated right? Get that info and we can size the water load.



What do you do with the snow melt? The car wash is covered right? It sounds like you have a bunch of covered self service bays. Are you just trying to keep the slab warm enough to prevent icing or are you melting snow? Is the system on all winter with no controls? Do you know if the slab has insulation under it? What part of the country?



It sounds like a condensing boiler with an indirect hot water heater would save you tons of money. You need to spend some time figuring the loads and some money on the right equipment and controls.Take a long look at the upfront costs and the savings down the road.



Keep in mind you don't need to add the 2 loads together. How often do people wash cars when it is 20 degrees out?



Carl

Garfield

loads

9 gpm for the wands and 1700 square feet for the glycol floor heat.  Incoming water is about 60 degrees and from what I have learned floor heat is 1-200 btu per square foot.

Garfield

pics of system

This is the glycol boiler, circulation pump tag and solenoid valves that let the hot water in before the pumps.  

Zman

Design

The hot water load looks like 9gpm x 500 x 60 delta T = 270,000 BTUhr. You will also have to derate for glycol and altitude if needed.

I would consider a Triangle Tube 399 with 330,000 output. I would use a smart 120 heat exchanger. The low head boiler design will allow the glycol to be piped directly without a heat exchanger.Pipe it primary secondary just like the manual says

For controls I think you need a slab sensor with a setpoint controller. You could use the boilers outdoor reset so that you are only heating the slab as much as you need.

Carl

Garfield

Thank you!

I guess the exchanger setup you suggest is the opposite of what I was thinking.  I was going to run city water through and to the wands and let the glycol be on the heat exchanger loop.  As I understand it the glycol is less efficient at heat transfer so I'm losing my efficiency gained with a condensing boiler.

Zman

There are advantages

The advantage to not running the city water through the boiler is longevity. The smart does a nice job of shedding the calcium and other deposits that you don't want in your boiler. You don't lose the glycol deration btu's the fluid simply cannot carry as many btu's as water. With a good sized tank you will get nice long boiler cycles as opposed to the the constant start stop of the instantaneous. The boiler will heat the domestic water at high temp during you domestic water priority then turn down to the ultra efficient low temp for slab heating.

Carl

Garfield

size exchanger

Assuming I'm going from potable water through the boiler and then to the glycol how do I size the heat exchanger.  I think the glycol would be 45 degrees and the water 135 then coming back as cool as possible to help with the condensing.  Thanks, John

Zman

Hmm

I have yet to find a condensing water heater design that I am comfortable with especially in a commercial application.

That being said, the easiest way to size you exchanger is to cheat. Once you know your designed gpm and delta t for both sides as well as your glycol percentage, just have the manufacture do the math.

Someone on here probably has a quick formula for this.You are basically trying to figure out how much surface area you need.

Carl

Zman

Costs??

Are you thinking about going with a direct water heater in order to save some costs?

This makes sense only if you are planning to sell the business soon. You will install 2 of the systems you are describing or 1 "real boiler" in the same time period. Think long term if you plan to own long term.

Just a thought,

Carl

Garfield

Thanks for your help

Just looking at these posts and realized I never followed up.  I did install a triangle tube prestige 250 with 2 heat exchangers.  In all but the busiest times a 175 would have worked.  That being said the floor heat does need the 250 when it's been off and I'm wanting to open the next morning. Thank you for all for the advice.

SWEI

Thanks for checking back in

always good to hear how things worked out.  By heat exchangers do you mean indirect water heaters or flat plate HXes?  What size?



Also curious how the gas bills are looking?

Garfield

gas bills and exchangers

Gas bills were down only a little in the summer months while making hot water but on the floor heat there were substantial savings.   Heat exchangers were 100 plate 5x12 i bought off of ebay.