tankless flow control

icy78

Does anyone know, if there is a tankless water heater that has an automatic flow control on the domestic side? What I mean is if you exceed its GPM rating. Say design is 2 gpm max at a 70 degree rise. Then you increase flow thru it. Will it automatically adjust the flow thru itself, to maintain your desired outlet set point?

gschallert

Takagi has flow control, where they throttle their own flow to try and maintain temp set point. There might be others as well.

bob eck

all tankless gas water heaters work with an incoming sensor and an outgoing sensor.
The control knows you set it for 120F degrees out going water temp.
The out going temp sensor stays at 120F and when the incoming water temp is cold and the unit has to raise the water temp higher the GPM output will go down and when the water temp is warmer the GPM output will go up.
The tankless gas water heater will also adjust the amount of BTU needed to be supplied to the burner. When incoming water temp is low the BTU input will be high and when incoming water temp is warmer the unit will reduce the BTU needed to supply the hot water.
Purepro tankless gas water heater 199,000 max BTU input minimum 19,900 BTU input
Also with tankless gas water heaters there is a minimum flow rate to get the unit to turn on.
Purepro TGWH has an minimum flow rate of .5 GPM if not flowing more than .5 GPM unit will not turn on.

gschallert

bob eck said:

all tankless gas water heaters work with an incoming sensor and an outgoing sensor.
The control knows you set it for 120F degrees out going water temp.
The out going temp sensor stays at 120F and when the incoming water temp is cold and the unit has to raise the water temp higher the GPM output will go down and when the water temp is warmer the GPM output will go up. ..

Perhaps I was mistaken but I read icy78's question as "is there a tankless that will throttle back it's own flow when demand exceeds GPM capacity in order to maintain set temp rather than deliver the higher flow rate of cooler *undesirable* water?" Last I was aware not all tankless HWH's had that feature but Takagi did.

icy78

> @gschallert said:
> all tankless gas water heaters work with an incoming sensor and an outgoing sensor.
> The control knows you set it for 120F degrees out going water temp.
> The out going temp sensor stays at 120F and when the incoming water temp is cold and the unit has to raise the water temp higher the GPM output will go down and when the water temp is warmer the GPM output will go up. ..
>
> Perhaps I was mistaken but I read icy78's question as "is there a tankless that will throttle back it's own flow when demand exceeds GPM capacity in order to maintain set temp rather than deliver the higher flow rate of cooler *undesirable* water?" Last I was aware not all tankless HWH's had that feature but Takagi did.

Yes, that's what I'm asking. Today I addressed a low flow issue on an WM Aqua Balance. It was in my head that they would modulate domestic hot water based on set point. That is not the case with these per the factory. I cleaned things up and got their well side adjusted right, and went from 1/2 gpm to 3.5 at a fixture. This gave me 85f water. So I guess they have to manually adjust the flow, or....i go back and pack a little dirt back in the inlet screen LOL.

gschallert

icy78 said:

Yes, that's what I'm asking. Today I addressed a low flow issue on an WM Aqua Balance. It was in my head that they would modulate domestic hot water based on set point. That is not the case with these per the factory. I cleaned things up and got their well side adjusted right, and went from 1/2 gpm to 3.5 at a fixture. This gave me 85f water. So I guess they have to manually adjust the flow, or....i go back and pack a little dirt back in the inlet screen LOL.

No, I think most tankless designs will still deliver maximum flow at whatever output temp they can manage when exceeding GPM@setpoint. Hence the not so hot shower. The model you mention is rated for 4 GPM @ 70 degree rise, and you're only getting 85F water at one fixture? Seems like something's still wrong with that scenario. They should be able to get two concurrent showers (with low flow shower heads) even at 40F inlet well temps. What is the inlet temp from the pressure tank? And what's the pressure tank set to for PSI?

icy78

> @gschallert said:
> Yes, that's what I'm asking. Today I addressed a low flow issue on an WM Aqua Balance. It was in my head that they would modulate domestic hot water based on set point. That is not the case with these per the factory. I cleaned things up and got their well side adjusted right, and went from 1/2 gpm to 3.5 at a fixture. This gave me 85f water. So I guess they have to manually adjust the flow, or....i go back and pack a little dirt back in the inlet screen LOL.
>
> No, I think most tankless designs will still deliver maximum flow at whatever output temp they can manage when exceeding GPM@setpoint. Hence the not so hot shower. The model you mention is rated for 4 GPM @ 70 degree rise, and you're only getting 85F water at one fixture? Seems like something's still wrong with that scenario. They should be able to get two concurrent showers (with low flow shower heads) even at 40F inlet well temps. What is the inlet temp from the pressure tank? And what's the pressure tank set to for PSI?

This one is an 80. It's only rated for 2 GPM. Sales had talked him out of a bigger unit, and rightfully so.I think you'd have to go to the 150 to get 4 gpm.
I glanced in the shower and saw one of those shower heads like you might wash an elephant with.

icy78

......like you might wash an elephant with. I'm suggesting addding an indirect.

gschallert

Sounds like sales didn't know what they were talking about. Combi boilers should be sized to DHW demand not heat load since the min fire BTU diff is negligible with turndowns but the max BTU is crucial. They should have gone with the 120 at a minimum. 2 GPM is adequate for 1 bath 2 person household, barely.

Just another example of measure twice cut once, or in this case measure first install once. You can stick an indirect in using the combi to heat the tank but someone should do the demand calcs so the tank is sized right and customer doesn't get screwed second time. A very expensive lesson.

icy78

> @gschallert said:
> Sounds like sales didn't know what they were talking about. Combi boilers should be sized to DHW demand not heat load since the min fire BTU diff is negligible with turndowns but the max BTU is crucial. They should have gone with the 120 at a minimum. 2 GPM is adequate for 1 bath 2 person household, barely.
>
> Just another example of measure twice cut once, or in this case measure first install once. You can stick an indirect in using the combi to heat the tank but someone should do the demand calcs so the tank is sized right and customer doesn't get screwed second time. A very expensive lesson.

Well the load in this house is about 50k if you leave the doors open! Lol
Most days the 80 will idle along. 120 would be WAY too big even with 10:1 turndown. 3 zones. Running about 155 water on 0f days and 125 most often he tells me. He had a 80% 70kbtu in before. That's 56k on a good day. Cycled a lot in even really cold weather.
How is 2 gpm at 130f not enough? 2 people one bath.

I'm sure I've read numerous times on this forum, to size a combi on heat loss, not domestic. Too easy to oversize the heating side otherwise.

I believe he wanted the 155 because of the 4 gpm domestic. Too big for many days. 20ft of fintube at 125f, that boiler would be constantly on/off.

gschallert

icy78 said:

Well the load in this house is about 50k if you leave the doors open! Lol

Most days the 80 will idle along. 120 would be WAY too big even with 10:1 turndown. 3 zones. Running about 155 water on 0f days and 125 most often he tells me.

I have to disagree that the 12K min fire would be "WAY" too big as compared to the 8K they ended up with, that 4K is negligible when compared to the difference between 80K & 120K high fire.

icy78 said:

He had a 80% 70kbtu in before. That's 56k on a good day. Cycled a lot in even really cold weather.

I've always had to chuckle at the number of people who fall into the Goldilocks Principle trap when converting to HE modcons. Somehow it's perfectly acceptable to have a huge mismatch in min output to heat load when it's CI standard efficiency but the modcon replacement can't be too small or too big, it has to be exactly right sized. LOL!

icy78 said:

How is 2 gpm at 130f not enough? 2 people one bath.

Two person household presumes two fixtures should be able to draw concurrently. Shower & dishwasher, shower & sink, shower & laundry etc. 2 GPM just squeaks by if you run the numbers.

icy78 said:

I'm sure I've read numerous times on this forum, to size a combi on heat loss, not domestic. Too easy to oversize the heating side otherwise.

I haven't read that myself and if I had I would certainly point out the error in that logic. What I have seen pointed out numerous times is to size a boiler paired with indirect on the heat loss rather than the indirect specs, which is correct. It's always better to oversize the heating by a few K than underfire the DHW by 40K+ with a combi.

icy78 said:

I believe he wanted the 155 because of the 4 gpm domestic. Too big for many days. 20ft of fintube at 125f, that boiler would be constantly on/off.

There are other means of finessing micro-zoned installs and/or lower end heat demand during shoulder seasons to offset short cycling. In this case the customer was right to want to size to DHW. As I said, an expensive & painful lesson but one I'm sure the customer will never forget. ;-)

icy78

My first reaction to reading your last response was , " what a jerk" BUT, after thinking about it a while I humbly have to reluctantly agree with most of it. DANG
I could very well have mistakenly thought that 155 was too big,( altho that wasn't my call, I'm out here to try to fix a low flow issue.)
I know he was told that he woul have to go to 155 to get 4 gpm on the combi.

Anyhow, say a 155 had been installed. Lots of DHW now.
Let's say there's often a 6-8k btu load on some zone.
I understand the goal is, to get that modcon to run on lowfire as long as possible.
Are you talking buffer tank or are you saying don't worry about that bottom end so much, a few cycles is not worth sweating about.
Or mixing valve?

I mean no disrespect @gschallert.
I'm open to learning and ready for it.

FWIW the customer did the install. He did get advice from our sales guys tho.

And I'm sure I've often read to size the boiler for the heatload when also using it for DHW. Wouldnt that mean an indirect then?

Just saw his wife. She's tripping over how well it works now. "I can run shower and kitchen sink!"

Well, that's hot news!☺
I'm betting the strainer was restricted after the first hour of running that combi, what with disturbing all the piping and dirt in it.

So maybe that's o.k. but it can't happen again on another install.

I'm going to study and let the sales people know. They only know what the reps tell them.

gschallert

icy78 said:

Anyhow, say a 155 had been installed. Lots of DHW now. Let's say there's often a 6-8k btu load on some zone. I understand the goal is, to get that modcon to run on lowfire as long as possible.

The goal is to keep it running continuously on low fire during heaviest demand, aka design day(s) and utilizing the heating curve with ODR to maximize burn times even as demand lowers. As you move into the shoulder seasons where heat loss is less than the lowest firing rate you need to finesse it. Most modern HE boilers come with some type of anti-short cycling technology built in plus you can play with heating curve SWT, fan speeds and even thermostats that allow you to control CPH or make sure a microzone isn't calling alone. 8K, 10K, 12K, 15K are all so close at that end the difference is insignificant.

icy78 said:

Are you talking buffer tank or are you saying don't worry about that bottom end so much, a few cycles is not worth sweating about.
Or mixing valve?

I'm saying don't sweat a few cycles. Or even a few dozen. I seriously wouldn't give a second thought to it unless you were seeing 4+ CPH and even then you could probably tweak software or t-stats to get it down to 2. I setup my in-laws Vitodens 100 combi shoulder season profile to cycle no more than twice per hour even in May/June when heat loss is well below the min fire rate of 37K. I don't think a buffer tank is necessary until you're dealing with excessive existing micro-zoning or not enough total emitter mass, both which tend to be the result of poor system design from the beginning. Combi boilers and tankless DHW units are a little different animal than stand alone boilers and are designed for a different duty cycle. The wear and tear aspect of cycling on combis gets a bit overblown IMO.

icy78 said:

I mean no disrespect @gschallert.
I'm open to learning and ready for it.

LOL I didn't perceive any disrespect. Thinking I'm a jerk doesn't bother me at all, you're free to think what you will and words can't harm me. Was it the Goldilocks reference? :P Fwiw, that is a real thing and never ceases to amuse the crap out of me when I see it.

I have to run but can post the DHW demand formula that I believe should be used to size appropriate combi/tankless equipment this weekend. Any salespeople and even installers should be asking prospective customers basic questions about their needs before recommending something. Measure first, install once.

gschallert

Okay, here's the DHW demand formula that can be used to determine minimum GPM needed by combis/tankless.

P=(Tm-Tc)/(Th-Tc)

P= hot water ratio
Th= Supply hot water temp
Tc= Cold water inlet temp
Tm= Desired mix temp at fixture

Example: Scenario #1 dead of winter Portland OR <- determined inlet temp used

P=(110-40)/(140-40)
P=70/100
P=.7

.7 x 2.5 GPM shower head = 1.75 gpm of 140 degree hot water needed at fixture. If you have low flow shower head, say 2 GPM then the result is:

.7 x 2 GPM shower head = 1.4 gpm of hot water needed at fixture.

What if you want to lower SWT from 140 to 125, then you get
P=(110-40)/(125-40)
P=70/85
P=.82

.82 x 2 GPM shower head = 1.6 gpm of hot water needed at fixture.

Fixture flow rates, cold water inlet temps, desired mixing temp at fixture & what you decide to set supply temp at all go into determining how many GPM of hot water are needed at any given fixture. This should be calculated for every hot water fixture in a home and the results added up to combine fixtures that need to draw concurrently to determine the correctly sized combi or tankless for DHW.

While it's crucial to identify GPM needed for hot water on demand installs, the same information can be used to calculate the best fit tank to install as well.

HTH,
Grace

p.s. You asked earlier about how is 2 GPM for two persons not enough and I said it might be, "barely" - consider the following scenario at my in-laws house using the formula above:

P=(105-52)/(135-52)
P=53/83
P=.64

They have a 2 GPM flow shower & 2 GPM flow kitchen sink. Both drawing concurrently would require 2.5 GPM of hot water. The Aquabalance 80 combi wouldn't cut it but their Vitodens can deliver 3+ GPM@83 degree rise so it's all good. I bump up their supply temp in the winter to offset the lower inlet temps and then drop the supply back down to 125 in the warm seasons.

icy78

Above and beyond!
Thankyou!

EPeters281

This thread has me considering an ab155c over the ab120c I've been considering for my home next year. I was drawn to the ab120 because it's a small home (33k heat loss) but the DWH performance doesn't seem to fit the bill. The tub has a 4gpm fixture and the inlet temperature will be around 52°f. I noticed the performance curve of the ab120c showed a 55°f increase at 4gpm - if I'm following this correctly, I'll see about 107°f at the tub for the kids - under the desired 110° max I'm planning around.

Am I over thinking here or should I just plan on the ab155c and not worry about short cycling?

Jamie Hall

All of which is why I don't like combis...

paul_somlo

icy78 said:

Does anyone know, if there is a tankless water heater that has an automatic flow control on the domestic side? What I mean is if you exceed its GPM rating. Say design is 2 gpm max at a 70 degree rise. Then you increase flow thru it. Will it automatically adjust the flow thru itself, to maintain your desired outlet set point?

My Takagi TK-Jr has a flow control valve.

Came across this thread while fixing an issue with the flow sensor. Noticed something strange, though - with the tankless idle, I can hear a ticking noise about every 3 seconds that I've narrowed down to the flow control valve. I probed the two wires that lead to the valve with a digital scope and sure enough, the valve is being pulsed multiple times about every 3 seconds. This is with no hot water running. I called Takagi tech support, they have no idea why this would be.

I'm pretty sure this is controlled by a microprocessor on the controller board. The only way this could unintentionally happen, is if the firmware went bad. And that doesn't happen very often. My best guess? In some households, the usage pattern is such that the valve may never get actuated, so it ends up sticking. In an effort to mitigate that, they actuate the valve on a regular basis.

Anyone have any other theories?

Paul