Takagi tk4 no hot water issues

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Guyh

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I have recently installed a tk4-in-ng
Worked great for two weeks and then ended up shutting down and giving the error code 991 for abnormal combustion
I reset it and it does ok a for a couple weeks then does the same thing
I have adjusted the back draft damper because I noticed an unusal response with the burner blower- after opening additional hot water valves the blower slowed down as if water was only trickling through the line
Any ideas?

Thank you in advance
Guy
 

DonL

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Where is your back draft damper located ?

That unit should have a backdraft Preventer. Not a adjustable Damper.


Good Luck on your project.
 

Guyh

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It is the first section of stainless exhaust vent coming out of the unit, back draft damper and condensate catcher. (nova vent 2nvvd4)
 

DonL

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It is the first section of stainless exhaust vent coming out of the unit, back draft damper and condensate catcher. (nova vent 2nvvd4)


Can you take pictures ?

That may give someone on the Forum a better chance to help you.

There are many reasons that it may not be working properly, and a lot of rules to install the vent properly.


Good Luck.
 

Guyh

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Will take pictures later, attaching a drawing of how its installed.
What are some of the issues that could cause the problem I have described?
Why would the blower slow down when I open more hot water points?
Venting installed is pretty straight forward, I also left the old pvc exhaust vent from the old hot water heater open to the utility room as it provided fresh air coupled with the wall openings.
The furnace in the utility room has a seperate fresh air intake and exhaust.
 

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Guyh

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The gas supply is 3/4", and only the water heater and furnace use gas. It started this befor the furnace was turned on for the winter
I have checked for obstructions and see nothing
 

DonL

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It is the first section of stainless exhaust vent coming out of the unit, back draft damper and condensate catcher. (nova vent 2nvvd4)


How did you adjust the damper ?

The Fan speed should not be changing, and that model you posted has no adjustment, that I can tell.

Do you get any water out of it, The condensate catcher ?

Is it mounted Vertical or Horizontal ?
 
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Dana

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Most tankless units will need a 1" or 1.25" dedicated home-run gas line to very near the regulator/meter, with no side branches to operate perfectly. It's possible that this installation was marginal on that from the get-go, and slipped from the "mostly working" to the "spittin' out error codes" side, but it's hard to say for sure. What is for sure is that you only have about 15 "equivalent feet" (including the equivalent impedance of the ells & tees) before you've blown past the 190,000 BTU input of the TK4, just on raw BTUs, but a modulating burner like that will start mis-behaving well before it's at full fire if the gas plumbing is marginal:


Natural_Piping_Chart.png


Just a guess, but unless that thing is mounted right where the gas line enters the house you will probably need fatter pipe to get reliable service out of it. The 3/4" connection to the unit doesn't mean it can sip fuel through a twisted mile of 3/4" plumbing and work correctly.
 

Guyh

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Most tankless units will need a 1" or 1.25" dedicated home-run gas line to very near the regulator/meter, with no side branches to operate perfectly. It's possible that this installation was marginal on that from the get-go, and slipped from the "mostly working" to the "spittin' out error codes" side, but it's hard to say for sure. What is for sure is that you only have about 15 "equivalent feet" (including the equivalent impedance of the ells & tees) before you've blown past the 190,000 BTU input of the TK4, just on raw BTUs, but a modulating burner like that will start mis-behaving well before it's at full fire if the gas plumbing is marginal:


Natural_Piping_Chart.png


Just a guess, but unless that thing is mounted right where the gas line enters the house you will probably need fatter pipe to get reliable service out of it. The 3/4" connection to the unit doesn't mean it can sip fuel through a twisted mile of 3/4" plumbing and work correctly.

Thank you, i will swap the 3/4 rigid with csst to eliminate elbows
 

Dana

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According to Table A.2.2 in the gas code, every 3/4" ell is worth 2.06 feet, every 1" ell is worth 2.62'. So those four ells of 3/4"add up to another 8.25', putting you at 21 equivalent feet for the run which already blows it even if it were a home-run to the regulator. But that's equivalent-feet after it tees off another 1" run (or is it a simple reduction, not a tee?) with another 20' of 1" with 2 ells, worth 2.62' each for an equivalent feet total of 25.25' of 1".

Even if it were a perfectly straight run of 1" with no tees or ells, 32' of pipe would be some marginal for a 190,000 BTU/hr modulating burner, but would sort-of make it.

To do this right, make a dedicated run (with no branches or tees to other appliances) of 1.25" gas pipe that reduces down to 3/4"only within 2' of the tankless, tying in as close as possible to the meter/regulator. If it's still misbehaving, there may be something else going on either with the unit or the regulator.

What you currently have is woefully undersized for the tankless- it's surprising that it works as well as it does, only shutting down & spitting codes every week or two as opposed to daily.
 

DonL

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Good Info Dana. That very well could be the problems.

I am still trying to think of why the vent blower would change speed.

It looks like it should be ON or OFF, when looking at the service manual.
 

Dana

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I've never dealt with the TK4, but I've monkeyed with a few earlier Takagi models (including the older KD20 which is heating my house.) Takagi's vent speed control algorithms change speed all the time based on the sensed conditions, and aren't purely a function of firing rate. There is usually a flue-purge at the beginning of the firing at a higher blower speed, which it backs off once it's burning. While in general the blower speed is higher at higher firing rates, it's not possible to tell what the firing rate or flow rate is (or much else) based purely on blower speed, since water temperature and exhaust temperature feedback can cause the blower to step up or down within a very narrow firing range. There is also a post-purge at a higher blower rate once it has stopped firing.

Until the fuel supply issues are solidly ruled out any blower speed changes are something of a red herring.
 
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