Running Out of Hot Water with IWH

DrVetur

Hi guys. New home owner and new to New England and therefore new to oil, boilers, radiators, and indirect water heaters.

I am running out of hot water with only one person showering. I have a SuperStor Ultra IWH with its Honeywell aquastat at 130. This connects to a Taco SR501 single zone relay. Heating it is an oil-fired Buderus G115 85,000 net BTU/hr with it's Honeywell L8124a set to 190/170/10. The space heating for the house is 4 zones with Taco zone valves (also need to make a separate post about why zone 3 is triggering as well as 4 when 4 is the only one calling for heat), hence the IWH being on a single zone relay, I think. I have witnessed the SR501 LED turn on, calling for heat and turning on the boiler.



(One of the wires from the bottom left of the L8124a runs to the SR501 but connects to nothing)

Any suggestions? Settings, flow rate problem, old equipment? It seems to occur even with my efficient shower head from Mass Save. Thanks in advance.

GW

Can you grab a jug or bucket and calculate how many gallons of hot water you’re getting (not at the shower. At a sink- something without cold being mixed in), AND time how long it takes?

Too much GPM will lessen the tanks capacity

You may or may not have a problem. What size water heater?

You may have a shower valve issue. When it happens next, stick you hand under the sinks hot water and see if that is also getting cooled off. If the lav is still hot then you have a shower valve problem

What’s the temp of the hot water at a sink? 120 to 130 is the rule in Massachusetts. Just because the control says 130 doesn’t mean a lot. The hotter it is the more showering you can do. But the more dangerous it is if you have kids or old folks and the like. You may need to jack up the temp some. If it’s truly 130 and you’re running out of hw, not making sense, unless we are talking long or multiple showers

Also the sensor may be acting too slowly to kick on the boiler. If you pull out the sensor it should have a heat conductive grease on it (don’t touch it. It’s messy)

Boiler relay- your boiler is maintaining 170 minimum? Wow you should be cruising with lots of HW (zero lag time to get the boiler heater up) I don’t understand your general concern with the boiler controls. Doesn’t seem related.

HVACNUT

How quickly do you run out of hot water?
Almost immediately or after 5 minutes or so?
Broken or split dip tube in the water heater maybe.

DrVetur

Thanks GW. Posted the boiler settings for completeness (not sure if they are optimal in general either). I should have clarified too that I take longer showers. When it does happen, the bathroom sink tap is also only tepid to warm at best (like the shower water) with only the hot water tap open, so doesn't seem to be shower valve. 45 gallon tank.

Will measure water temp, how many gallons, and time. And I just checked that that IWH's relay LED is on and activating the boiler currently with the shower running. Despite this, my wife (who takes shorter showers) says she had to turn the shower towards more hot near then end of her shower. I've made sure the shower valve regulator is not interfering. So, hot water recovery not keeping up with shower's GPM? No dip tube tube, cold enters tank externally from bottom.

pecmsg

Id like to see pics of the piping from a distence

mikeg2015

What’s the tank size?

When the Indirect is running, does the boiler run continuously? Means is all of the 72,000 or so BTU being “consumed”.

If it is, it should produce 2 gpm with a 75f temp rise so nearly continuous output for 1 standard shower head.


For reference I have a 40 gallon WM indirect and 80-100k is all I can extract from my steam boiler and we can fill a small whirlpool tub or my daughter can take 20 minute showers and not run out. Boiler actually cycles occasionally since it’s 200k input.


If not. I wonder if the boiler is under fired, needs to be tuned or something is going on there. Maybe indirect coil is fouled and not transferring heat. Water out East seems to be notoriously hard and mineral deposits are a problem.

GW

Yes many indirects don’t have dip tubes. The Superstor doesn’t.

You’re GPM may be too high, may need to bust out some simple math and some bucket/ stopwatch

When the water goes cool have the other spouse grab the piping at the water heater. That would pretty much confirm your notions

Another idea is maybe a mixing valve (for a toilet) is acting up. This is very rare

Mike you’re getting the boiler to heat the indirect at a constant pace? Never heard of that one, extremely unlikely you can get a normal household system to transfer that much heat (72000 btu) on the fly

DrVetur

Measured 2 GPM from the shower head. After allowing time for recovery, kitchen sink water temp at all hot was 133-135 on a Thermapen insta read, 130 from the bath tub tap.

Jamie Hall

Basic math here... if you are really getting all 72,000 BTUh output from the boiler into the SuperStor, at 2 gpm you maximum temperature rise will be 72 F. If your incoming water is at 50 -- not uncommon -- you'd get a maximum temperature out of 122. If it's 40, 112. Those are maximum figures, and factoring in even minor losses you are going to get a cold shower eventually... and not all that long into the shower, unless you are ex-Navy.

Intplm.

@DrVetur You say. "with only one person showering."
Check and double check to see if this "running out of hot water" is happening at other places/other faucets, and not just the shower. Run the hot water, and ONLY the hot water at another point. If its only happening at the shower then it is at the shower, and that can be a more common and fixable problem.
Also do what @GW suggests.
A little investigative work will surely get to the bottom of this.

pecmsg

DrVetur said:

Measured 2 GPM from the shower head. After allowing time for recovery, kitchen sink water temp at all hot was 133-135 on a Thermapen insta read, 130 from the bath tub tap.

Water heaters need 140°F minimum temperature. (Legionnaires Disease) Then a mixing valve limiting point of use supply no higher then 120°F. (Scalding Temperature)

DrVetur

Measured kitchen tap at 1.7 GPM. Using this, started with tap temp 137. At 14 minutes (thus 23 gal) kitchen tap temp was down to 115. Boiler running and IWH relay LED on. Tank is 45 gal. Boiler pipe running into tank very hot to touch. Hot water pipe coming out of tank warm-hot. At 33 min, tap temp back to 130.

STEVEusaPA

Most likely the aquastat settings aren't accurate. Plus, you really should not need (or use) a low limit if you have a storage tank. But you do want protection from low temperature water returning to the boiler.
After confirming with math that you're not exceeding capacity (as @GW suggested), maybe it's as simple as confirming you have your indirect set at maybe 145° and mix it down coming out of the tank. You should never run out of hot water.
Also like @Intplm. suggested, make sure it's all fixtures.
You could (should?) switch to a modern aquastat like the Hydrostat 3250 with the new well, and take advantage of it's many newer features like circulator hold off, more accurate temperature readings, low water cut-off. Wire it up as a cold start, wire the indirect as another zone utilizing ZC/ZR

GW

Yes we have different topics going on here, BTU transfer isn’t really the matter. A tank is a tank. Even with the boiler off, you should get 80% of the 45 gallons, probably more with that low of flow

137 to 115 with half the tank’s capacity isn’t normal, particularly at that small gpm/flow.

You may have to keep the tests going- at what point did the 137 start dropping? How many gallons in? It should stay working 2 or 3 degrees for more than half the capacity, I’d say 70% or more

If it drops soon you have some piping issue going on (cross connection with cold)

Jamie Hall

OK. Some more basic math. You had 45 gallons of hot -- 140? -- water to start. You've pulled about half of that out -- if the incoming water is at 40, with perfect mixing and no heat input, that would be a water temperature of 90. But you are still getting 117 -- basically then a 27 degree rise. That's a heating rate of only 22,000 BTUh.

Very roughly.

At that flow rate, the full output of the boiler should have raised the incoming water to about 120, give or take. Now... does the temperature continue to drop? I'm thinking a combination of some scale on the heating element -- could be inside or outside or both -- plus poor mixing in the tank.

DrVetur

Thanks GW and Jamie. With the 1.7 GPM kitchen tap, temp dropped from 137 to 115 at around 14 min then rose back to 130 by ~ 30 min and stayed there (stopped test at 40 min). The boiler says 85,000 BTU/hr NET. I think all space heating zones were off. Both boiler and tank are ~ 15 years old if I remember correctly.

As for the 2 GPM shower experiences, not sure if space heating zones were running, and didn't wait long enough to see if water temp recovered. Also subjectively seemed colder than 115

GW

Ok that’s odd, You’re saying the water heater temperature went down, then went back up?

I would let the water heater fully heat up, and do the test again, this time with the boiler off.

pecmsg

How old is this system?

Still waiting on pics.

GW

> @pecmsg said:
> How old is this system?
>
> Still waiting on pics.

G115 with SSU, it’s a modern system.

DrVetur

What's the safest way to turn the boiler off for that test @GW?

DrVetur

GW

Probably that red thing in your pic, is that a switch? That’s the power switch

pecmsg

GW said:

> @pecmsg said:

> How old is this system?

>

> Still waiting on pics.



G115 with SSU, it’s a modern system.

DrVetur said:

That's a lot of rust for a Modern System!

pecmsg

DrVetur said:

What's the safest way to turn the boiler off for that test @GW?

You should have a switch on the boiler.

GW

Not all systems are good looking 😀

pecmsg

GW said:

Not all systems are good looking 😀

5 maybe 7 years?

What's chances the heat exchanger is scaled up?

DrVetur

I think boiler and tank are ~15 years old. As for the rest of the plumbing... the house is an antique. For example, waste pipes are cast iron.

Boiler off test: Kitchen tap (1.7 GPM) maintained >130 until 18 min, 70 by 20 min, so less than 80% of 45 gal of 130 degree water delivered.

GW

So 30 gallons or so

I’m not extremely positive those indirect’s are truly 45 gallons. They were 40 gallons way back in the day, then they switched the model to 45 gallons. But whether they actually changed the tank itself, I’ve always wondered about that. Plus, the very bottom of the tank won’t get hot.

Depending on the GPM of your showerhead, and the temperature of the shower water, with a little bit more math, you could determine if this test jives

I thought most showerheads are more like three or 3 1/2 GPM.

Still seems odd, I have installed a couple hundred of those same water heaters, I never get any complaints. And your water heater is running hotter water than what I’m normally setting them up. Normally 120 to 125

Is this well water or city water?

DrVetur

City water. Shower head filled 5 gallon bucket in 2 min 30 sec. I'll have to time the shower with space heating zones off and see what happens. Seems like since the kitchen tap dipped to 115 but then recovered that they're may be a delay but that the boiler should be able to keep up. Maybe if space heating zones have been running it's not enough BTU/hr to the water heater. I'm interested in the temp rise equation and what should be needed from the Buderus to heat the new water entering the SSU from 40-50 to >120 and maintain at 2 GPM.

Larry Weingarten

Hello, Is there a hot water recirculation line?

Yours, Larry

Jamie Hall

BTUh = 60 minutes/hour times 2 gallons/minute times 8.3 pounds/gallon times 1 BTU/pound - degree F times (120-40) degree F = 79,860 BTUh.

Which is more than your boiler can provide.

GW

(8.33 x 60 = 500)

500 is the constant with this formula

500 x 2 (gpm) x 70 (temp rise) = 70,000 BTU


Now for the boiler math-
If you can get all 70k intro the water heater it would be a small miracle. The little Taco pump may be running 5 or so, maybe 6. But the harder part is dropping the full 28 degrees. The super stor hx isn’t “that good”

70000/500 is 140, 140/5 is 28

A bigger pump may be better if you truly want to pull this off. Get that baby to 10 gpm and you’ll be closer

Turning up the boiler temp will allow more BTUs to jump into the tank as well.

If this was a cold start boiler it would be a no go

Jamie he says it’s 85net, so maybe be can pull it off?

If you’re handy with wiring, you could run your zone valve circulator through the SuperStor Taco SR501, causing the heating pump to drop out when the water heater is firing

More experiments for you. Good thing you’re at home with nothing else to do (unless you’re medical, grocery or truck driver)

DrVetur

Thanks @Jamie Hall . Other than using <2 GPM shower heads, would upgrading to a higher heat output boiler at some point make sense, or am I being too picky?

GW

Simplest is jacking the water temp and install a mixing valve at the tank.

Next stop is bigger water heater

Or fiddle with the stuff I just typed.

DrVetur

Thanks @GW

Brewbeer

Is the indirect tank set up as priority?

hot_rod

GW said:

So 30 gallons or so



I’m not extremely positive those indirect’s are truly 45 gallons. They were 40 gallons way back in the day, then they switched the model to 45 gallons. But whether they actually changed the tank itself, I’ve always wondered about that. Plus, the very bottom of the tank won’t get hot.



Depending on the GPM of your showerhead, and the temperature of the shower water, with a little bit more math, you could determine if this test jives



I thought most showerheads are more like three or 3 1/2 GPM.



Still seems odd, I have installed a couple hundred of those same water heaters, I never get any complaints. And your water heater is running hotter water than what I’m normally setting them up. Normally 120 to 125



Is this well water or city water?

Plus you have to subtract out the coils to determine the actual tank volume.

EBEBRATT-Ed

@DrVetur

If the hot water temp is initially ok then dropped and then went up then the control that runs the burner likely has too much differential.

So picture this that the tank is up to temp and the burner is off temp control satisfied. You start drawing hot water and it's good for a while then the temp drops, either the burner has not come on yet or it did come on but not soon enough.

On a continuous draw the boiler kept up and you did not run out of hot water

EdTheHeaterMan

I can tell by the wiring picture that the burner does not operate on a call for hot water from the IWH. The boiler is maintaining 160° by way of the L8124A. The temperature drops below 160° then the burner operates to 170°, then stops.

On a call for heat, the burner can operate to 190° but it may take some time to get there since the space heat circulator is operational.

Give me some time to get a better wiring diagram using the existing controls. This may also solve your zone valve design problem on your other post.

EdTheHeaterMan

I'm hoping that my colleagues online will agree with this sequence of operation logic in this diagram.



On a call for heat, the end switch of any zone valve will operate the burner and the circulator for the heating zones. If the heating zones cause the boiler water temperature to fall below 160° (based on your 170° low setting and 10° diff setting) the space heating circulator will stop and the burner will continue to operate until the boiler water temp rises to 170°, allowing the circulator to operate again. this cycling of the circulator will continue until the return water from the heating zone(s) is high enough to keep the boiler temperature above 160° until the call for heat is satisfied.

This wiring logic will also bring on the burner on a call for hot water by way of the Taco Relay 4NO Terminal supplying power to the burner through the ZR terminal in the L8124A. This will operate the burner safely through the high limit control in the L8124a control. (currently, your wiring does not bring on the burner on a call for heat from the DHW tank.)

This may give reduce the DHW temperature droop you experienced in one of your previous tests.

This rewiring will get you in a better place and you don't need to purchase any new controls. If you are not comfortable doing the wiring, have a pro electrician or oil burner or HVAC tech do it.

Ed the Heater Man

P.S.
I would purchase a taco ZVC -4 or 5 to make zone value operation and troubleshooting easier in the future.

mattmia2

I've though about putting a plate heat exchanger on the inlet of the cold water and outlet of the boiler water to pull more of the heat out of the boiler water. you would have to be careful about not causing the boiler to condense if it is a conventional boiler and be careful about the HX scaling on the dhw side.

If you are losing heat in a zone because of some ghost flow that is going to make your problem worse.

Because of where the probe is on an SSU there is probably 5-10 gallons of cold water in the tank before it starts heating, so you aren't just instentaniously heating what your draw is, you are also recovering from that.

Here is the first draw table for an SSU-45 from the manual. It is at 90 degrees f delta t with 141,000 btu/hr boiler output;

Your boiler output is under half that so you really should have a larger tank or larger boiler according to the manual.