View Full Version : Best GPM for tankless hot water heater? UPDATED: We have hot water!

Watermark Farm
Dec. 21, 2010, 06:40 PM
I've got a bid from a contractor to install one in my barn but am not sure how big or small to go with the unit.

Barn is private with 5 horses. Mostly my need for hot water is the occasional bath, washing poopy butts in winter, and making hot mash feed. Pretty light use. I'm in California.

Would 1.5 GPM at 115 degrees be sufficient? Contractor is suggesting this size unit for me.

Tom King
Dec. 21, 2010, 08:16 PM
Watersaving showerheads are 1.85 to 2.5 gallons per minute. Temperature out the spray end is simple combination of cold and hot water. If you have 1.5 at 115 and you want 100 degree out with 55 degree cold, your total flow will be 2 gallons a minute. Warmer cold will of course mean more flow at 100, but it would take 85 degree cold to get to 3 gallons a minute.

I don't like water saving showerheads to use for me, and I sure don't want to wash a horse with one. Our wash stall flows 7 1/2 gpm. That's a 3/4" hose with a high flow sprayer.

Time might be money for some of us, but time is life used up for all of us.

Watermark Farm
Dec. 22, 2010, 02:00 AM
Thank you Tom King. Pretty much what I thought.....not big enough.


Watermark Farm
Dec. 22, 2010, 06:53 PM
Thanks again, I went back to contractor and we will be going with a much larger unit that should net me about 8 GPM of warm water at the wash stall. Thanks again for the analogy about low flow shower heads. I have them here in my CA home and HATE them. Showers wind up taking 3x longer due to low flow!

Dec. 22, 2010, 09:01 PM
Oh dang, mine does a delta-T of 50-55 degrees at 2-3 gpm and it's PLENTY for bathing horses!

Unless you're running the hot water wide open, no cold at all, the gpm from the water heater is only part of your total flow. In summer the hot water put out by mine is much warmer than I use for bathing, so I mix hot and cold and there is plenty for very vigorous hosing.

Tom King
Dec. 23, 2010, 12:41 AM
8 gpm is a lot, which is great if you have a system that will provide that much. I need to use a 25' 3/4" hose in our wash stall to be able to get to 7 1/2. Make sure you check that too or you may end up with a unit larger than you need. Just time how long it takes to fill up a 5 gallon bucket and do a little simple algebra.

Watermark Farm
Dec. 23, 2010, 10:23 PM
Our water pressure is very strong, about 18 gpm at the barn as I have a 2" water main going there. Do you need a 25' hose in order to lower the flow rate? Now I'm worried I'm going too big with this.

Basically contractor is now suggesting a unit that would output about 4.3 gpm of hot water after I explained to him what 'bathing a horse' entails. He said this is what he'd put on a 2 bedroom cottage. I might at some point install a washer in the tack room as well.


Dec. 23, 2010, 10:38 PM
My guess is that Tom has to use a short, fat hose in order to not LOSE flow. A longer, narrower hose would impede flow.

If by 115 degrees you mean that's the temperature of the water coming out of your hot pipe, that's HOT and much more than you need for bathing. If you could get 2 gpm from that source and turned on the cold as well, at say 65 degrees/1-1.5 gpm you could have water at around 90-plus degrees at 2.5-3 gpm, which IME is fine unless you like fire-hose horse bathing. :D

If your ground water is very cold or you like to bathe with shower-type temperatures and/or very high water pressure, then you need a step up. But probably not a very big one. :)

Watermark Farm
Dec. 26, 2010, 04:04 PM
The contractor will come Tuesday to start installation of my hot water system. I'll let you all know how my $2100 gamble pays off! Somebody bitch slap me for spending this kind of money on something so frivolous!

I do like good pressure when bathing. We have a mule with really coarse hair and he is shown a ton and it's hard to get him rinsed, even when clipped.

Tom King
Dec. 26, 2010, 05:49 PM
Pressure and flow rate are two separate things. You can have any combination of the two. A 3/4" hose flows roughly twice the volume (gpm)as a 5/8" hose. A shorter hose, as well as the fewest turns possible, flows more than a longer hose.

I use the 25' 3/4" hose for two purposes. The first is to get enough volume to operate the fire hose type pistol grip nozzle we like for washing horses. http://www.google.com/products/catalog?hl=en&expIds=17259,18167&sugexp=ldymls&xhr=t&q=bosch+fire+hose+nozzle&cp=22&rlz=1W1ADRA_en&wrapid=tljp1293400775906036&um=1&ie=UTF-8&cid=2009157345630003700&ei=4boXTbjUHMP48AbSrNSiDg&sa=X&oi=product_catalog_result&ct=result&resnum=4&sqi=2&ved=0CCUQ8wIwAw#That nozzle needs at least 7 1/2 gpm to operate which is the maximum we can get from our system on a deep well that only supplies 4 1/2 gpm. Obviously, we can't run it all day at that rate. We don't use the nozzle at a small hard stream, but rather a medium width spray that still has enough pressure to get down to the skin but not too much that it bounces all back on you-the horses actually seem to enjoy it. No matter how much soap you use, it will rinse it right off. That nozzle has a twist for the spray pattern, and the loop handle on top controls the volume, so you can get the spray just like you want it.

The other reason for the short hose is that it's also used to fill the watering bins right outside the wash stall. The short hose drains by hand in a few seconds in this freezing weather, and also fills them quickly.

The girls used to like the curly-que hoses to wash their horses. I had the big hose and would always switch it out to wash mine. I'd wash mine in a couple of minutes. Finally, for some reason the little curly-que hose got left on the trailer, so Pam started using my hose. She always thought it was too big. Her comment after about 3 seconds-"This is amazing".

She had a 5/8" hose to rinse down the dog porch. I had to hook up a 3/4" to run my pressure washer around there and she used that while it was hooked up with a fire hose nozzle. She liked it so much that she asked me if it would increase flow even more if I upsized the supply pipe under the house to the faucet that the dog porch hose is hooked up to. :D:winkgrin::cool:

The curly-que hose got tossed somewhere in that process and nothing has been in the wash stall since, but the 3/4" x 25' hose.

Dec. 26, 2010, 06:40 PM
Oh, I HATE those curly little skimpy hoses! A system is no better than its weakest link.

Watermark Farm
Jan. 10, 2011, 12:54 AM
The hot water heater is in and it's GREAT! The horses are all in shock. I go to rinse mud from their legs and they LIKE it. I am making them hot hay pellet mashes nightly. I wash my hands so often in the new tack room sink I am starting to think I'm developing OCD. Life has never been so good.

Went with a 177,000 btu unit from Bosch. Wowza. No shortage of hot water or pressure. Definitely not a cheap project!

Thanks again for all the info and advice.

Now I have to figure out the right hose.

Jan. 10, 2011, 09:10 AM
I think my vet loves coming to my dinky little barn because I always have HOT WATER for him to wash up. :)


Tom King
Jan. 10, 2011, 09:28 AM
Here's the best hose for real water flow. The Flexogen 3/4"

Bailey's has the 25 footer for 18.95 right now:


I'll try one of these if the Flexogen ever wears out:

It should be a lot more flexible than the Flexogen.

Tom King
Jan. 10, 2011, 09:29 AM
This is also the nozzle we like:


Watermark Farm
Jan. 11, 2011, 01:58 AM
Thanks for the links Tom King. I have that Gilmour Flexogen hose all over my farm. I currently have a 50' one attached to the hot water spigot. I've been disappointed in the performance of mine. One is just 2 years old and already cracked in places and kinking a lot. What am I doing wrong? I coil them when not in use and they are never left under pressure. And I'm in CA so it's not like it's 40 below around here.

And Deltawave, the big joke around here is that our rather clean vet will think we've done the hot water sink for him. He is famous for doing this hand and face washing ritual post-horse-visit (much to the giggling delight of the teens at my barn; the vet is quite handsome) involving flowery scented products from Bath & Body Works that we are always invited to smell and sample. I will be laying in a supply of lavender hand lotion and other essentials for his next visit....hoping to impress the vet with my taste.

Tom King
Jan. 11, 2011, 09:32 AM
This one looks pretty interesting too:


Jan. 11, 2011, 11:08 AM
watermark, did you get gas or electric? You will like knowing that I have a Bosch gas water heater in my house that was installed in 2001. It has given me 10 years of great service. I have had do simple maintenance a couple of times, but nothing a field technician couldn't easily handle. I highly recommend it. It is still going strong.

Watermark Farm
Jan. 13, 2011, 11:33 AM
Nines, I went with propane. Good to know you've had decent service from yours. Contractor installs them a lot and says they seem to last really well.