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Old 01-14-2012, 12:45 PM   #1
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DHW coil not producing enough hot water when one zone is running


I need some advice on my hot water situation.

I've got a circa 2004 Burnham V7 oil fired boiler with a DHW coil. We have two zones in the house for heating. First floor is large (~2") iron pipe in a loop with mono-T's that go to each convector. Second floor is 3/4" copper with modern hot water baseboard. Each zone has its own circulator pump.

The boiler maintains itself at 180-210*F all year... and even with both heating zones running the temp gauge never dips below 180*F. We get good heat from all the heaters.

The problem is w/ the DHW:
- If no heating zones are calling for heat the hot water is OK.

- If the first floor is calling for heat the hot water is OK.

- If the second floor is calling for heat, we have tepid (~100*) water from the taps, and the shower (with it's own mixing valve) might as well be ice cold.

The mixing valve at the output of the DHW coil had its guts replaced by a plumber last year who had replaced nearly all the plumbing in the house following major renovation.

When in the "broken" state (2nd floor calling for heat and shower/hot taps open full), if I use my non-contact thermometer on the copper pipe leading in to the mixing valve I get a reading of ~100*F, and ~85*F out of the mixer.. so it seems like the water just isn't that hot exiting the boiler. I would expect to see 130-140* entering the mixing valve (though I know the IR thermometer probably isn't great way to do it)

My Q's:

Any ideas why the second floor zone calling for heat would cause this problem, but the first floor apparently does not?

Does it seem reasonable that my DHW coil could be failing after only 5~7 years?

Should I just bite the bullet and install a 40 or 50gal electric tank water heater and bypass the boiler DHW coil? (no gas available)

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Old 01-14-2012, 01:21 PM   #2
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DHW coil not producing enough hot water when one zone is running


Does the second floor pipe join with the first floor pipes in a fat pipe (called a manifold) and just the manifold is connected to the boiler?

Are thgere arrows engraved on the circulator pumps showing which direction the water is flowing? Are you sure the flow is not backwards?

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Old 01-14-2012, 01:43 PM   #3
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DHW coil not producing enough hot water when one zone is running


Quote:
Originally Posted by AllanJ View Post
Does the second floor pipe join with the first floor pipes in a fat pipe (called a manifold) and just the manifold is connected to the boiler?

Are thgere arrows engraved on the circulator pumps showing which direction the water is flowing? Are you sure the flow is not backwards?
It's a manifold--a large diameter pipe enters/exits the boiler and splits off for each circulator. The arrows on both circulators point in the same direction. The "input" to the circulators comes from the top of the boiler. The other side goes into the bottom of the boiler.

I was thinking that maybe the 2nd floor system is able to extract more BTUs than the first floor or something? That's the only thing I would expect to cause this... Then again the second floor only has two rooms with heat in it.....

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Old 01-14-2012, 02:46 PM   #4
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DHW coil not producing enough hot water when one zone is running


Quote:
Originally Posted by bubbler View Post
The boiler maintains itself at 180-210*F all year...
and even with both heating zones running the temp gauge never dips below 180*F. We get good heat from all the heaters.

The problem is w/ the DHW:
- If no heating zones are calling for heat the hot water is OK.
- If the first floor is calling for heat the hot water is OK.
- If the second floor is (also?) calling for heat, we have tepid (~100*) water from the taps, and the shower (with it's own mixing valve) might as well be ice cold.

My Q's:
-could the coil be failing after only 5~7 years? probably not
-electric (or oil) water heater and bypass the boiler DHW coil? eventually

Any ideas why the second floor zone calling for heat would cause this problem, but the first floor apparently does not?
I'm seeing it is a controls issue.
Do you have a schematic of how the control circuit is supposed to flow?

My gut... says that WHEN the DHW calls for heated water...
that *something* should be making/allowing that to happen.

Ideally that would be done without affecting the heating...
but yeah, one or the other of the heating zones may need to shut down 100% for the duration of that HW need and logic says that be the 2nd zone.

Quote:
If the second floor is (also?) calling for heat, we have tepid (~100*) water from the taps, and the shower (with it's own mixing valve) might as well be ice cold.
My gut again... says that the A/B control device which is supposed to choose between that second heat zone and the DHW... has gotten cross wired, or is internally faulty, or the (solenoid?) valve it controls isn't doing it's job of splitting/redirecting the flow.

Controls. It's almost always the controls.

hth

Last edited by TarheelTerp; 01-14-2012 at 02:49 PM.
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Old 01-14-2012, 03:19 PM   #5
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DHW coil not producing enough hot water when one zone is running


Quote:
Originally Posted by TarheelTerp View Post
I'm seeing it is a controls issue.
There are two sets of 'relays' in the system. The first floor t-stat connects to the aquastat and the other seems to be on its own.

As far as I can tell this how the system works:

- Aquastat keeps the boiler between 180-210*F
- When a t-stat closes it triggers the relay which immediately starts the circulator for that zone
- If the boiler is below a certain temp, 190*F?, the burner immediately starts... if the boiler is at a higher temp the circulator may run for a few minutes before the burner starts
- If JUST the hot water is being used then the boiler temperature is apparently the sole control for activating the burner... for example, when we start the hot water taps, w/o any heat being called for, it may be a minute or up to 5-10 mins before the burner will click on.

The reason I think it's not a controls issue is because the boiler temp is always above 180*, it never drops below that... we also have limited HW even while the burning is actively running--example if we're having heat called for the 2nd floor the burner will be running, if we run the hot taps we'll get to tepid water within a minute (the slug of water in the coil is hot, but after that it's basically tepid).

What I'm thinking is that maybe this issue also happens with the first floor running and I've just never notice it as much... so saying it doesn't happen with the first floor might be a mistake...

Is there some way test whether the coil is not functioning correctly? Does the boiler have to be drained (and air purged from the heating zones) to replace it?
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Last edited by bubbler; 01-14-2012 at 03:22 PM.
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Old 01-14-2012, 03:28 PM   #6
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DHW coil not producing enough hot water when one zone is running


Quote:
Originally Posted by bubbler View Post
There are two sets of 'relays' in the system. The first floor t-stat connects to the aquastat and the other seems to be on its own.
Rather than verbally... do you have that written down schematically?
Something to go through sequentially as you determine what is or isn't?
I'm seeing jumping around and assuming...
---

Clearly the boiler is heating fine...
Clearly proper levels of H&C water work some of the time...
This leaves what?

What directs water to the DHW or away from the 2nd heat zone?
Both physically (valves) but also electrically (controls)?

The other things you've brought up merit investigation too...
but that still doesn't change the need to KNOW what you have is right...
and working right.
Quote:
What I'm thinking is that maybe this issue also happens with the first floor running and I've just never notice it as much... same troubleshooting

The mixing valve at the output of the DHW coil had its guts replaced by a plumber last year who had replaced nearly all the plumbing in the house following major renovation. and what else did he do by doing this?

Is there some way test whether the coil is not functioning correctly? Dunno
hth

Last edited by TarheelTerp; 01-14-2012 at 03:33 PM.
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Old 01-14-2012, 03:34 PM   #7
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DHW coil not producing enough hot water when one zone is running


Quote:
Originally Posted by TarheelTerp View Post
Rather than verbally... do you have that written down schematically?
Something to go through sequentially as you determine what is or isn't?
I'm seeing jumping around and assuming...
---

Clearly the boiler is heating fine...
Clearly proper levels of H&C water work some of the time...
This leaves what?

What directs water to the DHW or away from the 2nd heat zone?
Both physically (valves) but also electrically (controls)?

hth
I don't really have anything written down, I could put something together but it would be a guess.

As far as I understand it, the boiler is always hot. If heat is called for the circulators turn on and force the water around... there are no valves or gates to stop it. The DHW is just a coil sitting inside the boiler... when I open a hot tap, water from the water main flows into the DHW coil and there is gets heated... when it exits the coil it passes through a mixing valve designed to mix that very hot water with some cold water to keep the hot tap temps below 140* or so (particularly important when you first start drawing hot water). Again, no gates, valves or relays are changing position at any time.
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Old 01-14-2012, 03:50 PM   #8
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DHW coil not producing enough hot water when one zone is running


Quote:
Originally Posted by bubbler View Post
I don't really have anything written down, I could put something together but it would be a guess.
That's the point though...
without such a diagram... what you have NOW is a guess.

The underlying problem is probably something very elemental...
maybe a control wired NO that should be NC, or a bad solenoid valve coil, or the plumber who recently was in there reversed something or neglected to replace a certain check valve... SOMETHING.

You can make your own diagram.
The pipes and valves and then the controls on top of that.

Go from there.
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Old 01-14-2012, 04:30 PM   #9
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DHW coil not producing enough hot water when one zone is running


Quote:
Originally Posted by bubbler View Post
What I'm thinking is that maybe this issue also happens with the first floor running and I've just never notice it as much... so saying it doesn't happen with the first floor might be a mistake...
Check this.

As the others said, a schematic of your system will help. More pictures, of the tankless coil and controls, will also help.

Is the 2nd floor also a monoflo setup?

I wouldn't even consider an electric water heater. You have a functioning boiler, dump the tankless and put an indirect on it. Set it up as it's own zone. It'll save you money by not having to stay "hot" all the time and can put out a lot more water than any electric.
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Old 01-14-2012, 07:23 PM   #10
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DHW coil not producing enough hot water when one zone is running


My guess is that the water in the boiler is being cooled when the second floor circulator kicks in--notwithstanding the fact that the thermometer continues to read 180 so perhaps the thermometer isn't working correctly. (The Temp/Pressure gage on my boiler nearly always reads a fixed temp but I have a second thermometer on the boiler output pipe at that varies significantly with load, etc.)

Secondly, it is very possible that the DHW coil in the boiler has some mineral build up so the heat transfer has dropped from the new condition. If this is the case the resolution is to either acid clean the coil or replace it.

Last edited by Bob999; 01-14-2012 at 07:26 PM.
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Old 01-14-2012, 07:41 PM   #11
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DHW coil not producing enough hot water when one zone is running


Quote:
Originally Posted by AandPDan View Post
As the others said, a schematic of your system will help. More pictures, of the tankless coil and controls, will also help.

Is the 2nd floor also a monoflo setup?
The first floor has the mono-flow's, and the second floor does not.

First floor has in-wall convectors. My understanding is that the mono-flows divert some of the water in the loop up into the convector, and some continues along the loop.

For the second floor as far as I can tell it's 3/4" copper plumbed directly in a loop with about 25' of hot water baseboard in the middle.

I drew up this schematic to represent the two zones a DHW coil. There are no electric gates or valves anywhere. As far as I can tell the only thing linked to the thermostat or aquastat are the circulators and the burner itself in the boiler. The aquastat is a Honeywell high/low with differential. The high is set to 210* and the low is set to 180*, the differential is set to 15*. There is no problem with the heat in either zone, and there is no problem with the burner starting or the temperature of the boiler (according to the gauge on the boiler anyway... If I turn on the hot taps and not the heating zones I can watch it fluctuate from ~210* down to ~190* and then the burner will fire itself and raise back up...)


Quote:
Originally Posted by AandPDan View Post
I wouldn't even consider an electric water heater. You have a functioning boiler, dump the tankless and put an indirect on it. Set it up as it's own zone. It'll save you money by not having to stay "hot" all the time and can put out a lot more water than any electric.
I'd love to do indirect, but the cost is too high--I was quoted $3300...

Replacing my current tankless coil seems like it's a $500-700 job, though I have yet to get quotes on it.

Putting in an electric tank would probably be ~$500 in materials and I could do the work myself.

I know the electric is of course expensive to run, but honestly i'm not sure that it's all that much more expensive than my current setup that keeps the boiler at 200*F all day every day... from what I've read myself is terribly wasteful, and I know I use at least a tank of oil over the late spring/summer/fall which is basically all for hot water... that's about $450, that's probably well over $50/mo spread out...
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Old 01-14-2012, 07:50 PM   #12
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DHW coil not producing enough hot water when one zone is running


Quote:
Originally Posted by Bob999 View Post
My guess is that the water in the boiler is being cooled when the second floor circulator kicks in...
When this happens... is the 2nd circulator wired in with any other controls?
I'm thinking of something like a flow switch in the HW pipe...
that if open (w/flow) wouldn't let the circulator run until after it closes.

Bubbler: Is there nothing between the HW pipes and the circulator at all?
Your not showing anything at all there...

Quote:
Secondly, it is very possible that the DHW coil in the boiler has some mineral build up so the heat transfer has dropped from the new condition. If this is the case the resolution is to either acid clean the coil or replace it.
How frequently does that need to be done?
Once every year or two? Five?

Last edited by TarheelTerp; 01-14-2012 at 07:53 PM.
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Old 01-14-2012, 08:01 PM   #13
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DHW coil not producing enough hot water when one zone is running


Quote:
Originally Posted by TarheelTerp View Post
When this happens... is the 2nd circulator wired in with any other controls?
I'm thinking of something like a flow switch in the HW pipe...
that if open (w/flow) wouldn't let the circulator run until after it closes.

Bubbler: Is there nothing between the HW pipes and the circulator at all?
Your not showing anything at all there...

How frequently does that need to be done?
Once every year or two? Five?
A big fat pipe exits the top of the boiler and goes into the circulators, there are no valves other than manual ball valves...

Aside from the DHW coil being physically installed in the boiler, there is absolutely no other connection to the heating system pipes. There are no valves, other other devices. I've taken a pretty hard look a the wiring, etc, and there is nothing that occurs when the zones call for heat that changes on the DHW side.

I'm leaning towards this being a scale up of my DHW coil combined with a faster rate of heat transfer from the 2nd floor (vs. the first floor). My suspicion is that because the first floor is made up of mono-flows the temperature of the water going back into the boiler (from the loop) is much higher, due to the mono-flows keeping some fraction of the water bypasing convectors), than the the temperature of the water going back in the second floor where 100% of the flow goes through the hot water baseboards...

There is a mixing valve as shown on my diagram which mixes cold water.

Not shown are the T/P relief valves, but they are all separate from each other--there is one for the boiler autofill, one for the boiler, and one for the DHW coil. I also didn't show the autofill valve for the boiler, or the thermal expansion tank on the boiler water lines. But those are all sep. from the DHW parts. I also just realized that my diagram is terribly inaccurate for the first floor--it ought to show the line moving under the first floor heaters block with little lines going up to it... to reflect that the water is diverted into the convectors, but may also pass by it (due to the mono flows).
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Old 01-14-2012, 08:58 PM   #14
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DHW coil not producing enough hot water when one zone is running


Quote:
Originally Posted by TarheelTerp View Post
How frequently does that need to be done?
Once every year or two? Five?
Really depends on the water quality. With soft water (less than 1 grain per gallon hardness) the coil will probably never need cleaning. The more hardness in the water the more frequently it needs to be done.
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Old 01-14-2012, 09:04 PM   #15
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DHW coil not producing enough hot water when one zone is running


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Originally Posted by bubbler View Post
Should I just bite the bullet and install a 40 or 50gal electric tank water heater and bypass the boiler DHW coil? (no gas available)
One approach would be to leave the coil in the boiler but install the electric heater in the hot water line after it leaves the boiler. This would provide most hot water from the boiler during the heating season--but deal with the cool down that I think is caused by the second floor. But for this to work I think you will need to have the DHW coil in the boiler cleaned/replaced because I suspect is is operating a reduced efficiency because of mineral buildup.

During the summer you could shut down the boiler and do all hot water with the electric heater.

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