Unusual Pressure Drop in Boiler while using hot-on-demand

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Old 03-03-08, 04:17 AM
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Unusual Pressure Drop in Boiler while using hot-on-demand

Hi All.

I get such good advice here, that I thought I'd come back for a weird thing I'm seeing.

My boiler provides both HWBB and hot-on-demand water for my house. The boiler has been working great for the most past and all the parts appear to be in good shape.

What I'm noticing is that whenever I run my clothes washer (which is right next to the boiler) in "warm" cycle, the pressure gauge on the boiler starts to drop from 20psi down to 10 psi. Eventually the boiler kicks on and the pressure struggles to get back up to 20, but it eventually does.

What's interesting about this problem is that it does not happen if I run the "hot" cycle of the washer, which you'd almost expect to happen.

I had a guy come by and he was scratching his head. I guess he didn't know for sure where to even start. I don't even know if I have a problem or not, but if this is a warning sign of a bigger problem I want to tackle it before it's too late.

Does anyone have any ideas on what I can do to diagnose this?

Thanks as always!

DW
 
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Old 03-03-08, 02:25 PM
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I can think of at least one way that this could be somewhat of a 'health' issue... _IF_ the pressure is dropping because water is coming out of the boiler, and into your domestic supply, then that's a problem that I'd want to know about and repair, if I were you... who knows what's in that boiler water !

It's possible that the pressure drops because the boiler is cooling off as you are drawing the water, but I would think the same thing would happen when you run a HOT load of wash ...

Unless we can see all the piping, might not be able to diagnose... so, take a bunch of pictures, set up a free account at www.photobucket.com and upload the pics there. Provide a link here so we can view the pics.

Take at least one wide shot of the whole boiler, then enough shots so we can see all the piping going into and out of the hot water coil in the boiler. Maybe there's something piped wrong that we can spot ...
 
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Old 03-03-08, 02:52 PM
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High pressure goes to low pressure. His domestic system hopefully is above 20 psi. Maybe by running hot the boiler is cooling down to the point where the boiler is firing up before he can notice the difference in pressure. There are more gallons being drawn out in a hot cycle than a warm cycle. The thermal circulation around the coil is going to be different.
 
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Old 03-03-08, 03:03 PM
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I'm wondering though if someone "knuckleheaded" something somewhere along the way ... like maybe tapping the feed for the tankless coil from AFTER the pressure reducing valve ? That would 5uck...

I'm actually down with the cooling off theory...
 
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Old 03-03-08, 04:58 PM
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Trooper, et al.

This is why I love this forum. You guys give great responses!

I've included a photo of my boiler. I hope this helps with further comments.

I find the "cool down" theory interesting, because I forgot to mention that as the pressure was dropping so was the temperature. The moment the temp was back up to normal, so was the pressure.

Is there something I can read to understand the "cool down" better? Is this a bad thing?

Cheers!

DW

 
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Old 03-03-08, 05:50 PM
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Other than the mixing valve being upside down it looks OK. This would not cause the symptoms you are explaining.
 
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Old 03-03-08, 06:31 PM
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I don't see nuthin' neither...

If you are seeing 10 PSI from cool to hot, it's possible that the expansion tank might need a shot of air ... I wouldn't think you would see much more than 5 PSI. You got fin-tube baseboard ? if not, what radiation ?

"Cool Down Theory"

When you heat water from room temp to around 180F it expands about 4% in volume. That expansion needs somewhere to go, and that's what the expansion tank is for. There's a 'cushion' of trapped air in that tank that gets squeezed and makes room for the exanding water.

Basically just physics, water expands when heated, and contracts when cooled ... well, except for when it's ice, but we won't go there...

If there's not enough air in the tank, there won't be as much room for expansion, and you would see wider pressure swings. If the air in the tank gets too low, the pressure swings become wider, and at some point the pressure relief valve would open and spew hot water on the floor because the pressure went over 30 PSI.

I'd check it if it's been more than a few years since it was checked last ...
 
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Old 03-03-08, 07:22 PM
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rbeck, NJ:

Thanks for the replies.

NJ -- yeah I have fin-tube baseboard. standard stuff. When the guy I called came by, I had him check the air pressure in the expansion tank and he said it was fine (I couldn't see the number on his gauge). If I were to check it myself, what should I see?

Here's more info -- I had to have the filler valve (the one right before the expansion tank) replaced last year. Is it possible that this could be a factor?

Thanks as always,

DW
 
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Old 03-03-08, 08:18 PM
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If the tech didn't first drop the boiler pressure to near zero, he did not get an accurate pressure reading on the tank.

You can not measure the air charge in the tank when there is pressure on the water side. You _WILL_ get a reading, but the reading you get will be that of the pressure in the boiler, NOT the pressure of the air charge in the tank.

If you put a gauge on there now, your reading should be close to what shows on the boiler pressure gauge, depending on accuracy of both gauges.

The only way you can get any meaningful tank pressure reading is with no pressure on the water side.

Having the fill valve replaced shouldn't affect the tank, if that's what you mean. that fill valve _might_ be set a little low though, if it drops to 10 PSI or less when cold. Should be a minimum of 12 PSI in your boiler, more if it's taller than a 2 story home.

So, you wanna give it a try ? I'll point ya to some directions if you wanna give it a go... or, use the search function here and see if you can find directions ... if not, lemmee know and I'll run it down for ya.
 

Last edited by NJT; 03-04-08 at 06:01 AM.
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Old 03-03-08, 08:29 PM
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I cut these directions from another post and edited them a little:

Grab a garden hose. Screw it onto any boiler drain valve. Direct the hose to a drain or a bucket.

TURN OFF THE BOILER. Let it cool to 100F or less.

CLOSE the MANUAL water inlet valve. NOTE: It does not appear that your installer saw fit to add a manual fill valve on your system ... he should have installed one when he replaced the auto fill valve. In order to shut off the water to your boiler, you will need to close BOTH the ball valves near the ceiling on both the HOT and COLD water lines to the house.

OPEN the drain valve, and let water out until the PRESSURE GAUGE on the boiler drops to ZERO.

CLOSE the drain valve.

Take a small air compressor, or a bicycle pump, to put air in the tank. You will need between 12-15 PSI in the tank. Use a good quality tire gauge to measure the pressure.

When you have the proper air charge in the tank, OPEN the manual water fill valve and check that the boiler pressure returns to between 12-15 PSI. If it's less than 10 PSI after a few minutes, yell at us and we'll walk you through increasing the pressure a wee bit.

Turn boiler back on.

When the boiler is COLD you should have between 12-15 PSI on the gauge, and when it is HOT, maybe 18-20 PSI MAX.
 

Last edited by NJT; 03-04-08 at 06:00 AM.
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Old 03-03-08, 09:03 PM
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If the bucket is a necessary part of the system I think the trooper pegged the problem.?
 

Last edited by burnerman; 03-03-08 at 09:06 PM. Reason: spelling
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Old 03-04-08, 04:02 AM
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Thanks NJ -- I'll give it a try and report back.

burnerman -- the bucket's there because I heard it was a good idea to always have a bucket under the relief valve. I have never seen water in the bucket in the 4 years I've lived here.
 
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Old 03-04-08, 06:12 AM
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Yeah, not a bad idea to keep a bucket there. Just peek in the bucket every now and then to check no leaks...

I noticed one more thing in the pic... It may be a camera illusion, but it appears that the two brown thermostat wires are laying on the flue pipe ? If so, move them away ...

I'm also not crazy about the way they installed the two purge valves above the zone valves. If they leak, they look like they will drip on the ZVs ... no need to change anything, just look at them now and then to make sure.
 
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Old 03-04-08, 06:34 AM
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dpsw------Did you say pee(k) in the bucket to check for leaks?
 
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