Not enough DHW - 1 year old thankless coil

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Old 03-12-14, 08:33 AM
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Not enough DHW - 1 year old thankless coil

Hi all -

I have a problem with DHW production through my tankless coil. Last year I added some new rainfall showerheads, but I have since added some restrictors to try and keep from overrunning the tankless coil, unfortunately it doesn't seem to be working. Whats more, it seems that even while doing the dishes, I need to keep moving the knob hotter and hotter. And forget about filling a bath tub.

I recently replaced all the controls on the old peerless oil fired boiler in preparation for adding a third zone, as well as eliminating the two line volt thermostats that directly powered the circulators. The old controls used to have a dumb honeywell L4081B aquastat which had no low side relay, only high limit, and circulator cutoff. This means it would run between the high limit and hi minus 10 all year long. It had been set at 190 for hi.

I have now replaced the the aquastat with an L6081A which is a proper triple acting aquastat with hi, lo and diff. Its coupled to a Taco SR504 switching relay to control the circulators. The heat functions well, although the pellet stove does the bulk of the work on the lower level. The aquastat is currently set for 190 hi, 170 low with a 20 diff.

Not having the boiler constantly sit at 190 has made an existing DHW production issue all the worse at the lower temp.

The tankless coil is only a year and a half old, as the old owner replaced it when we bought the house. We had the burner tuned up last year, and this year I replaced the electrodes and nozzle. The boiler is an older peerless, so old I cant trace the date by serial number. Its rated at 107K BTU, and has a remanufactured beckett gun, replaced in 2002 as per the tags hanging on the pipes. The nozzle is .75. There is no tempering valve anywhere.

We have had a number of plumbing leaks which have required us to drain the pipes ten or so times over the last year. I have bled all the air out each time, and there is an air elimination doohickie on the top of the boiler (not an inline air scoop), and no, the cap is not screwed on tight.

I know that the tankless coil is a horrible way to generate hot water, but can it really be so bad you cant fill a bath tub with the low side set to 170? I do plan on eventually moving to an indirect water heater which is why I went with teh SR504 over the SR503, or possibly a propane rinnai. Natural gas is currently 5 houses away, but they want 40K to trench it to me. I'm hoping to limp along on this old oil fed boiler until all the neighbors switch to gas. With the addition of the pellet stove, I'm down to about 300 gallons a year, instead of the 1000 last year. I hope she makes it.

Thanks for any suggestions.
 
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Old 03-12-14, 03:45 PM
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GB,
Your vent or any heating parts other than the aquastat have any bearing on how your hot water performs.

The fresh water enters the coil and gets heated by the boiler water as it goes through the coil to deliver hot water.
Tubs are the biggest draw so that's usually the first place you will have trouble. In your case where you're cooling off in the sink and there's no mixing valve, which means you're getting straight hot water whatever the coil can produce would seem to be a limed up coil.

If you have high water pressure, the water may be going through the coil too fast to get heated. Did the coil ever produce enough hot water.
You may have very bad water. As fresh water enters the coil it leaves mineral deposits that stick to the coil walls and as they build it acts as insulation and doesn't allow the boiler to penetrate and heat the water coming through.
The worst the water the worst the liming.

Two things can be done to correct this. There is an acid that is run through the coil to deli me it or you can replace the coil again.

This is start anyway, I hope this helped a little.

Good Luck,
 
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Old 03-12-14, 03:56 PM
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Thanks spott. I don't suspect that the coil is crudded up, as it's only 18 months old. We don't have a lot of mineral deposits in our water. Very little buildup on showerheads and glass doors, etc. From what I remember, the DHW was fine last year before the larger rainfall showerheads, at least I didnt notice anything out of the ordinary. The water seemed to be fine all summer long, but there is a very large difference in input water temp, which is surely hurting things. This winter, Im amazed the cold water comes out in liquid form.

As I mentioned, I added rubber washers with very small holes as makeshift flow reducers to the shower heads, in the hopes that will help us out. This morning was better but still not where we would like it.

Please pardon the ignorance, but there's no way the tankless coil could be partially submerged, right? I don't know what the inside of my boiler looks like. I suspect any air pockets would be eliminated in short order by the air purge at the top of the boiler. I do hear it working whenever I have to bleed the hydronic lines.

Also, while we never ran our tank under 1/8, I did not change the fuel filter this year, so maybe there is a fueling issue with the burner? I have not noticed anything out of the ordinary, and the heat seems fine from the baseboard rads, though we are pretty much only using the top floor zone, as the pellet stove heats the first floor nicely.
 
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Old 03-12-14, 05:33 PM
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GB,
There are no dumb questions.
Hot water heat is a closed system, which means it is completely full of water, so the coil is submerged in water.
With no mixing valve, your hot water should come out almost as hot as your boiler water, which would scold you.

What you can try is turn on your tub or shower and feel the pipe right out of the tankless to see if its hot.
It should be extremely hot at the beginning and as it runs if it gets cooler it's a sign it may be the coil.
Is it possible to take some pics of the piping.

Do have excessively high pressure?

Time is irrelevant when it comes to liming up the coil.
As far as the oil filter goes. If your boiler is operating you should get hot water. If your boiler heats the house it is working properly to supply you with hot water, as least as far as the boiler goes.

Although this is the age of lower temps, tankless is old school and doesn't respond well to low boiler temps. They are designed to sit in 180° water to get the desired rating. You most likely have a coil rated @ 3.5 GPM, which is why it cannot supply the big draw fixtures.

See how hot it is coming right out of the coil and how long it takes to cool down and that will tell us what direction to go in.
 
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Old 03-12-14, 05:49 PM
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Thanks again spott. I understand hydronic heat s a closed loop system. However after draining it ten times, I didnt know if it was possible I had air trapped in the boiler, or even if the dissolved air might come out of suspension within the boiler, making it so the coil was not fully submerged.

The boiler flame seems as strong as its ever been, and I reseated the aquastat bulb in the well to make sure its getting good contact. It seems to cycle on and off at the correct temps, even faster than the temp gauge.

I replaced the fill valve regulator, boiler gauge and pressure relief valve this past fall after finding the boiler pressure above 70 psi one day. The old pressure valve was caked shut, and looked to be older than time itself.

As for the fixtures, I believe the shower heads were rated at 2.5 GPM, but we have 75 to 80 psi at the hose bib, so it might have been a little more than that before i installed the flow restrictors. My completely uneducated guess puts these shower heads around the 2.0 GPM mark.

The aquastat controls are set at 190 hi, 170 low and 20 diff, so the boiler keeps DHW between 160 and 180 at idle. That should be hot enough, no?

I am concerned about upping the a'stat to 200/180 since I have some PEX runs, which will do 180* at 100 psi, but I dont know how quickly it falls off after that. In general, its probably not a good idea to run pex at 200+ degrees.

Thanks again
 
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Old 03-12-14, 06:16 PM
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GB,
I asked about the pressure because that determines how fast the water comes through the coil. The coil is rated at that GPM with a pressure of roughly 50psi. If you have a shutoff valve on the cold water going into the coil try closing it a little, slowing down the flow.

If you think there's air in the boiler, if you feed into the bottom of the boiler, isolate the supply & return if you can, increase the pressure to about 25psi and open up the relief valve to bleed. If there's any air that will take of it.
Let the boiler cool down first though and don't forget the bucket.
As I said, check the temp of the hot water supply right out of the coil.
 
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Old 03-12-14, 09:01 PM
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How many shower heads running at one time? A standard shower is rated at about 2.5 gallons per minute and if you have just one rain shower in addition, even if only flowing 2 gpm that adds up to 4.5 gpm. If the coil is rated at 3.5 gpm flow rate you can easily see the problem. Add in the really cold winter water and there is another factor to your problem.
 
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Old 03-13-14, 07:33 AM
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Thanks guys.

I have a shutoff valve that feeds both the DHW and fill valve. I may try shutting this, but my concern is that we will have to then put the mixer on the shower to full hot - or nearly so.

I didn't think about purging from the relief valve. I gave it a shot, but I don't think there was any air. Makes me feel a little better, taking one more thing out of the equation.

I couldn't find my infrared thermometer in time this morning to take temps from the pipe, but it seems plenty hot to the touch (very very quickly).

As for the shower heads, the rainfall head replaced the standard shower head. It is not in addition to. Also, only one shower is going at a time.

I further restricted one of the shower heads, and that seems to have helped considerably now, at the expense of anything resembling pressure. We may switch one shower head to a 1.6GPM standard water saver head to see if that maintains the heat better. We will then decide if we prefer the low pressure rainfall, or the high pressure water saver head. Unfortunately none of this really helps us fill the bathtub. And with a kid on the way, it's gonna be a pain filling the tub at half throttle all winter long.

One other thing, I noticed that while the aquastat lo side is set for 170, which means the boiler shoud fire for DHW at 160, the gauge was reading closer to 152*. I kicked the lo side up to 175. This puts me at a 15* spread between lo and hi as per the controls, however it would still be a 20*+ spread per the boiler gauge. Do you guys see any cause for concern raising the lo temp setting?
 
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Old 03-13-14, 10:36 AM
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hi GuitsBoy-

One other thing, I noticed that while the aquastat lo side is set for 170, which means the boiler shoud fire for DHW at 160, the gauge was reading closer to 152*.
Just a newbie, but if you have a new gauge and a new aquastat – something there doesn’t sound correct. I didn’t think new gauges could be that inaccurate. But maybe the other guys who really know something could clarify.

(Guess 5% isn't that far off - but I thought they were better than that. Who knows? LOL)
 
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Old 03-13-14, 02:32 PM
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GB,
Your low temp comes on @ 150° because of the 20° differential you set. Set differential @ 10° and the boiler will come on at 160° and set your hi & lo 20° apart again.
How quickly did the water turn cold out of the tankless when you ran the faucet.
There is a Flo control made especially made to restrict the cold water going to the tankless to match the output so only the amount of water that the tankless is capable of producing is allowed.

At 80psi that water is going through too fast to be effectively heated, plus you're wasting a lot of water that is passing through and isn't being heated.
 
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Old 03-13-14, 03:07 PM
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Spott, I dont believe that is correct, at least not with the 4081 and 6081 honeywells. Just like the hi side, the low side re-enabled at lo minus 10. The diff simply dictates how long it burns for. A diff of 20 allows the temp to come up to lo plus 10. A diff of 25 allows it to burn to lo plus 15. Once the diff temp is hit, it re-enables the circ, and cuts the burn unless there is a call for heat. As perboth the honeywell documentation, as well as inspectopedia and even NJ Troppers witeup, the lo start burn is always lo -10 regardless if the diff is 10 or 25.

But yes, I agree that at 80psi, we are probably flowing more than we think.
 
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Old 03-13-14, 03:08 PM
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That's not the way the diff works on the 6081 LowLimit Spott...

The LL has a FIXED negative DIFF of 10F. That's always there. No matter what you set the DIFF dial to, the bottom end of the control range will always be 10F below the setpoint.

The DIFF dial ADDS to the LOW setting MINUS 10.

Bottom of control range or 'cut-in' : LOW - 10F ... always, no matter what the diff setting is.

TOP of control range or 'cut-out' : ( LOW - 10F ) + DIFF

Examples:

LOW setpoint of 160, DIFF of 10

Boiler will fire at 150 and cut off at 160

Move DIFF to 20 and cut in will still be 150, but cut out will be 170

The 6081 uses the same mechanism as the 8124 but without all the other relays and stuff.

The difference you are seeing between the aquastat and the thermometer is probably due to gauge accuracy, aquastat accuracy, AND the fact that the water temperature inside the boiler could well be different between the point that the aquastat is sensing the temp, and the gauge is sensing the temp.

Guits, if your nominal city water pressure is 80 PSI, you might consider having a regulator installed to drop it down to say 60 PSI or so. IMHO, 80 PSI is too high for residential water pressure...
 
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Old 03-13-14, 05:21 PM
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You both are absolutely right. It's called brain freeze. I have an 8124 on mine and it runs the same way.
I'm glad you're both on the ball. Sorry for the wrong info.
 
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Old 03-13-14, 06:47 PM
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No worries at all, the worst you've done is made me double check some research - not a bad thing to do.

Anyway, I the low side may be a few degrees off, so I am narrowing the spread to 15 degrees, because when theory and results don't line up - go with the results. Additionally, the low flow shower head will be installed in the more severely offending shower tomorrow after work. I will use a known flow head as a baseline to see if its legitimately a DHW production issue after all, or simply overrunning the coil with demand.

Thanks again for all the help guys, I appreciate all the feedback, suggestions and ability to bounce ideas off you.
 
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Old 03-14-14, 10:41 AM
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Guits I don’t know whether this helps you, but your thread helped me. I have the “thankless” coil also and like you I find it wanting. LOL

Just started to use the shower stall in my late Mothers bathroom and noticed that the shower is definitely hotter than the shower-over-tub that I use in my bathroom. Scratching my head. So I was just ready to start detective work to see what in the world I could find in the plumbing lines that might explain it.

Then just by luck at the right time I saw this discussion about flow rate through the coil, shower head rates, etc, and the light bulb came on. I had installed a low flow shower head (big one, Rainsoft or something? not sure) in the shower stall a few years ago. Aaahaa- so that’s why I am seeing a big difference.

So I see the big difference also. But I know what you mean about trying to fill a bathtub. hahaha
 
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Old 03-18-14, 07:50 AM
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Swapping shower heads seems to have fixed my issue - or at least functions as a workaround. We have a 2.5 GPM head in the shower with lower pressure, and a 1.5 GPM head in the shower with higher pressure. The 2.5 is selectable and the middle massage jets are disabled, so its only the outer jets. This makes me think the flow is actually closer to 2.0. As long as the wife says its enough water for her, Im fine with it.

BTW, I meant to bring this up before, but my boiler has a DHW pre-heater that heats the input water before it goes into the coil. But even with this, its just not enough.

Anyway, thank you guys for your help.
 
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Old 03-18-14, 10:07 AM
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my boiler has a DHW pre-heater that heats the input water before it goes into the coil
Can you show us a pic of that?
 
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Old 03-18-14, 10:29 AM
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Can you show us a pic of that?
I'm at work right now, but here's the exploded view from the Peerless JO/JOT series manual.

The pre-heater is number 17. It is listed as "Tankless Heater" as opposed to #13 which is listed as "Tankless Coil". The cold water feed goes through number 17 first, and is then routed directly into the coil. During hot water usage, the pipe is warm coming from the pre-heater, and quite hot coming from the coil.
 
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Old 03-18-14, 10:58 AM
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I've always thought that the 'other' coil, #17, was used with steam systems. Being that a steam boiler isn't 'full to the brim' with water, it needs to be mounted lower in the block. Don't think I've ever seen a boiler with both connected in series!
 
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Old 03-18-14, 11:07 AM
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The model number is JOT-3H-W, which from what I can tell, denotes it is indeed a hot water boiler. The series is capable of either/or, however. The placard shows both hot water as well as steam capacities, which I thought was weird. Also, the #17 coil is not mounted down low, its roughly at the same height as the tankless coil. Above the firebox anyway. The boiler is pre-1984, so finding any literature on it is pretty much impossible.
 
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Old 03-19-14, 08:10 AM
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Is the hot water "better" in the winter and when the zones in the house are calling for heat?

Maybe the domestic hot water coil has too small a capacity (gallons per minute) for the desired temperature rise.

Maybe there has to be at least one circulator running to churn the water in the furnace so the water immediately surrounding the DHW coil isn't stagnant and cool.

Is there a tempering system that bleeds cold water into the DHW coil output to help prevent scalding but which may be bleeding in too much cold water? Note: A sink or shower that is turned off using a sprayer hose and not the faucets can cause cross feeding that leads to this kind of problem.

Would you consider installing a passive convection preheater consisting of a few dozen feet of finned pipe (perhaps dissected from FHW baseboard radiators) installed in the cold water line DHW inlet? This would be mounted zig zag on the wall or ceiling out of the way of persons' heads and shoulders.

AFAIK the water pressure does not matter.
 
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Old 03-19-14, 08:17 AM
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Thanks Allan. I believe DHW is actually worse in the winter due to the much colder input water. The water starts out very hot and gets cooler as it flows, so I dont believe it stagnant water. We do not have any tempering or anti scalding device at all, its straight from the coil. Coil capacity seems to be the answer here, since restrictive shower heads have certainly helped. Still, I'm surprised the coil cant keep up with 2.5 GPM.

As for the passive preheater - I was actually considering that, so its funny you mention it. Although I think simply swapping out for low flow shower heads in teh winter is a simpler solution, at least until we can switch to natural gas, or install an indirect tank.

Thanks.
 
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Old 12-07-15, 09:27 AM
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Just figured I'd update this thread. The lower flow (1.6 - 1.75 GPH) shower heads have made a big difference. Giving the boiler a good cleaning helps as well. The water is certainly hotter while one of the heat zones is calling for heat, and the boiler temp is above where the aquastat would normally kick on. The biggest factor I see reducing our DHW production is the input water temp. On cold days, the water coming is is barely above freezing, under 40* F. For whatever reason, the water temp coming from out main changes drastically with the seasons, or even the weather from day to day. On days where the input temps are under 40*, it just takes a very long time t o bring it up to temp. But in the end, the current setup is working well enough for us for the time being. Natural gas has come a couple houses closer, so hopefully we wont be limping along on a 30+ year old boiler for too much longer.
 
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Old 12-09-15, 06:13 PM
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On cold days, the water coming is is barely above freezing, under 40* F. For whatever reason, the water temp coming from out main changes drastically with the seasons, or even the weather from day to day. On days where the input temps are under 40*, it just takes a very long time to bring it up to temp...
Yes, in cold-weather climates such as NY, a water heater has to work much harder during the winter months for exactly that reason...the water coming into the building is much colder and therefore requires more BTUs to heat. Here in my area, the incoming water temp varies from a high of around 70*F (August) to a low of around 35*F (February).
 
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Old 12-10-15, 06:56 AM
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I just noticed that the tankless coil is rated at 5 GPM, and can just barely keep up with 1.7-2.0 GPM shower heads. No idea what the input temp is when they rated it at 5 gpm though.
 
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Old 12-14-15, 03:30 AM
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GB,
Last year I was a similar issue and I have a Superstor indirect. The water coming was just that cold it wasn't heating quick enough. Mine was only happening when 2 people showered at the same time. I still need to rewire my Beckett Aquasmart as per instructions provided on this awesome forum.

I have Taco SR506 with 3 heat zones plus IWH set to priority. First year I tried letting Aquasmart control it but the short run times were driving me nuts. Second year I was on Intellicon HW+ which improved run times but the cold water I had to make adjustments to aquastat.

Shortly I will rewire to control IWH from Aquasmart which bypasses the heat manager settings. I'm still tempted to install a new thankless coil (as a pre heater). The large water pocket above chamber on my Peerless with no coil seems to heat up too fast as compared to full water moving from boiler. Aquastat shows 180 but water to zones is about 10-15 degrees colder (3 thermometers and a Flir).
 
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Old 12-26-15, 08:18 PM
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I just noticed that the tankless coil is rated at 5 GPM, and can just barely keep up with 1.7-2.0 GPM shower heads. No idea what the input temp is when they rated it at 5 gpm though.
FYI, tankless coils have their hot water output rating based on the maximum burn rate the boiler can handle. Said another way, if the boiler isn't at it's maximum firing rate then hot water output will suffer. I've seen many many instances where a boiler has a smaller nozzle for whatever reason, and tankless hot water performance is poor. Just another reason to install an indirect hot water heater
 
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