Navien NCB 240 extreme cold water sandwhich frequent pressure purges

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carriagehousereno

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Hey all,
So let me describe my problem first and then I'll describe the specifics of my installation.

The Problem:
Over the past 2 weeks we have began experiencing the cold water sandwich for the first time since installation. It doesn't"t happen consistently, but when it does we often go from scalding hot to ice cold, sometimes more than once during a shower for example. It's also happened at the kitchen sink once or twice, but like I said, not consistently enough for me to identify any kind of pattern.

Additionally we've been hearing the pressure relief valve open and release water pressure of the unit sometimes several times within an hour.

Installation:
The home is a two story old brick carriage housebuilt about 100 years ago, 1700' including a 500' apartment on the first floor. It sits on top of a stone foundation crawlspace which isn't insulated and some voids in the foundation where doors etc used to be are just timber framed with plywood and stucco on the outside, while the floor of the crawlspace is concrete with an exception of about 32' that was dug below grade to accomadate the hieght of the mechanicals which is a dirt floor. 75% of the drafty old wood windows have been replaced and the second floor ceiling is modestly insulated but the attic has no additional insulation on the underside of the roof. The home currently has hot water radiators and a small wood stove providing heat in the colder months. When I bought the home 5 years ago, the Bosch Aquastar had been installed previosuly to us ,I assume to save space since it was cramped in the crawlspace.

Back in January we were experiencing an issue with our Bosch Aquastar HG. Incoming water temp was 37F and the aquastar was barely getting it to 104F. The plumber I was consulting with recommended Navien and since I was planning to change out my old Weil-McLain boiler soon, he suggested the combi-boiler. Due to the DHW needs for the small apartment and the remainder of the home being a combination of two full baths, two kitchens with dishwasher and a washing machine, he recommended we go with the largest of the combi-boilers, the NCB-240e.

Since I was leary about disconnecting the working boiler in the middle of the coldest part of winter, he suggested I put in a small loop on the space heating side of the unit and then do the complete retrofit when the weather warmed up. I installed a five foot loop at his recommendation.

We haven't experienced any problems with the DHW at all over the first 3 months of installation. Since the issue started I took some notes on the specs the unit was reading and here is what I found.

Incoming DHW temp - 55F
DHW Target Temp - 120F
Incoming Space Heating water temp - 109F
Space Heating Temp on display - 140F
Water pressure - hovers around 19-20PSI

When there is a call for hot water, the temp of outgoing water spiked to 139F and then the pressure in the system also spiked up above the PRV setting of 30PSI to around 36PSI and the valve began dripping a little and relieving pressure.

The parameters that have changed since installation are that the outside temps are now consistently 20-30 degrees higher than they were in the winter when it was installed and the incoming water temp for the DHW is about 20 degrees higher, I also installed a check valve on the automatic water feed supply line as a back flow preventer about a month ago. Prior to that I had the supply to the auto water feeder turned off and would check and add water whenever I noticed the pressure getting close to the minimum operating pressure.
We have noticed no discernible difference in the water pressure in the home that should affect the operation of the DHW system. I checked the screen on the water inlet for the DHW and is clear.

My Guess:
I think that what's happening is some combination of the outside air temps and incoming water temps being higher are causing both the space heating and DWH systems to heat up more than necessary and increasing the pressure in the system to the point that the PRV opens.

What settings should I change to keep this from happening? and is this the root cause of the cold water sandwich or just the frequent PRV openings we are hearing?

Thoughts...?
 

Jadnashua

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The closed loop heating needs an expansion tank if you don't have one.

For 5' of radiator, the minimum fire on the combi boiler is probably way overkill (and the whole thing may be radically oversized), so it rapidly reaches max temp and shuts down since it cannot dissipate even the minimum heat of the boiler. When shut down, it's not making domestic hot water. It cools enough, then refires. When it was colder, it could dissipate more heat, but as things warmed up, it can no longer.

This is a guess...
 

carriagehousereno

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Thnanks for the response Jim, I was thinking it must be something along those lines. I'm just now designing the heating system so i"m hoping that I'll get it installed before the issue with this becomes too frustrating.
 

Dana

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The NCB 240 is a lousy fit for most houses. The domestic hot water is usually sufficient, but most homes don't have heat loads high enough for the thing to modulate in space heating mode, and insufficient radiation to keep it from short-cycling itself into a reduced lifespan & low efficiency at condensing temperatures. At minmum fire the burner put out over 17,000 BTU/hr in condensing mode. It takes about 85-90' of typical baseboard (PER ZONE!) to emit heat at that rate at condensing temperatures, and not cycle on/off. A 5' stick of baseboard would need to emit 3400 BTU/foot to not cycle, which would take super high temperatures & pressures. At 140F it's emitting only ~1200-1500 BTU/hr so there's more than 15000 BTU/hr (250BTU/minute , 4 BTU/second) more heat going into the zone than is coming out, and less than 5 lbs of water in the boiler + zone plumbing, so it's temperture is slewing something like 250BTU/min/5lb= 50 F per minute, or ~1F per second. For a swing from the min-undershoot where it refires to the max overshoot where it turns off is 10F, that's still a 10 second burn, an extreme short-cycle.

If you have big old fashioned high volume radiators the thermal mass of the water might keep it from short cycling, but even for a 1700' barely indicated antique in PA it's probably way oversized for the heat load.

Do at least the napkin math on both your heat load, and your radiation when sizing modulating condensing boilers.

If the PRV is kicking off on every other space heating burn it's likely that either the system is overfilled, the expansion tank isn't properly pre-charged, or the expansion tank is the wrong size. If the system has an auto-fill valve that is seeping that can overfill the system over time, raising the pressure.

Note, the NCB 240 almost always has to be plumbed primary/secondary to stay within it's safe operating parameters in most systems. Navien sells a pre-engineered manifold to provide adequate hydraulic separation between primary & secondary, but there are other ways to achieve that which can be cheaper. If the system needs more thermal mass to keep from short cycling a high volume hydraulic separator may be a better choice than the Navien thingy.
 
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glorybound

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Hey all,
So let me describe my problem first and then I'll describe the specifics of my installation.

The Problem:
Over the past 2 weeks we have began experiencing the cold water sandwich for the first time since installation. It doesn't"t happen consistently, but when it does we often go from scalding hot to ice cold, sometimes more than once during a shower for example. It's also happened at the kitchen sink once or twice, but like I said, not consistently enough for me to identify any kind of pattern.

Additionally we've been hearing the pressure relief valve open and release water pressure of the unit sometimes several times within an hour.

Installation:
The home is a two story old brick carriage housebuilt about 100 years ago, 1700' including a 500' apartment on the first floor. It sits on top of a stone foundation crawlspace which isn't insulated and some voids in the foundation where doors etc used to be are just timber framed with plywood and stucco on the outside, while the floor of the crawlspace is concrete with an exception of about 32' that was dug below grade to accomadate the hieght of the mechanicals which is a dirt floor. 75% of the drafty old wood windows have been replaced and the second floor ceiling is modestly insulated but the attic has no additional insulation on the underside of the roof. The home currently has hot water radiators and a small wood stove providing heat in the colder months. When I bought the home 5 years ago, the Bosch Aquastar had been installed previosuly to us ,I assume to save space since it was cramped in the crawlspace.

Back in January we were experiencing an issue with our Bosch Aquastar HG. Incoming water temp was 37F and the aquastar was barely getting it to 104F. The plumber I was consulting with recommended Navien and since I was planning to change out my old Weil-McLain boiler soon, he suggested the combi-boiler. Due to the DHW needs for the small apartment and the remainder of the home being a combination of two full baths, two kitchens with dishwasher and a washing machine, he recommended we go with the largest of the combi-boilers, the NCB-240e.

Since I was leary about disconnecting the working boiler in the middle of the coldest part of winter, he suggested I put in a small loop on the space heating side of the unit and then do the complete retrofit when the weather warmed up. I installed a five foot loop at his recommendation.

We haven't experienced any problems with the DHW at all over the first 3 months of installation. Since the issue started I took some notes on the specs the unit was reading and here is what I found.

Incoming DHW temp - 55F
DHW Target Temp - 120F
Incoming Space Heating water temp - 109F
Space Heating Temp on display - 140F
Water pressure - hovers around 19-20PSI

When there is a call for hot water, the temp of outgoing water spiked to 139F and then the pressure in the system also spiked up above the PRV setting of 30PSI to around 36PSI and the valve began dripping a little and relieving pressure.

The parameters that have changed since installation are that the outside temps are now consistently 20-30 degrees higher than they were in the winter when it was installed and the incoming water temp for the DHW is about 20 degrees higher, I also installed a check valve on the automatic water feed supply line as a back flow preventer about a month ago. Prior to that I had the supply to the auto water feeder turned off and would check and add water whenever I noticed the pressure getting close to the minimum operating pressure.
We have noticed no discernible difference in the water pressure in the home that should affect the operation of the DHW system. I checked the screen on the water inlet for the DHW and is clear.

My Guess:
I think that what's happening is some combination of the outside air temps and incoming water temps being higher are causing both the space heating and DWH systems to heat up more than necessary and increasing the pressure in the system to the point that the PRV opens.

What settings should I change to keep this from happening? and is this the root cause of the cold water sandwich or just the frequent PRV openings we are hearing?

Thoughts...?
 

glorybound

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Hey all,
So let me describe my problem first and then I'll describe the specifics of my installation.

The Problem:
Over the past 2 weeks we have began experiencing the cold water sandwich for the first time since installation. It doesn't"t happen consistently, but when it does we often go from scalding hot to ice cold, sometimes more than once during a shower for example. It's also happened at the kitchen sink once or twice, but like I said, not consistently enough for me to identify any kind of pattern.

Additionally we've been hearing the pressure relief valve open and release water pressure of the unit sometimes several times within an hour.

Installation:
The home is a two story old brick carriage housebuilt about 100 years ago, 1700' including a 500' apartment on the first floor. It sits on top of a stone foundation crawlspace which isn't insulated and some voids in the foundation where doors etc used to be are just timber framed with plywood and stucco on the outside, while the floor of the crawlspace is concrete with an exception of about 32' that was dug below grade to accomadate the hieght of the mechanicals which is a dirt floor. 75% of the drafty old wood windows have been replaced and the second floor ceiling is modestly insulated but the attic has no additional insulation on the underside of the roof. The home currently has hot water radiators and a small wood stove providing heat in the colder months. When I bought the home 5 years ago, the Bosch Aquastar had been installed previosuly to us ,I assume to save space since it was cramped in the crawlspace.

Back in January we were experiencing an issue with our Bosch Aquastar HG. Incoming water temp was 37F and the aquastar was barely getting it to 104F. The plumber I was consulting with recommended Navien and since I was planning to change out my old Weil-McLain boiler soon, he suggested the combi-boiler. Due to the DHW needs for the small apartment and the remainder of the home being a combination of two full baths, two kitchens with dishwasher and a washing machine, he recommended we go with the largest of the combi-boilers, the NCB-240e.

Since I was leary about disconnecting the working boiler in the middle of the coldest part of winter, he suggested I put in a small loop on the space heating side of the unit and then do the complete retrofit when the weather warmed up. I installed a five foot loop at his recommendation.

We haven't experienced any problems with the DHW at all over the first 3 months of installation. Since the issue started I took some notes on the specs the unit was reading and here is what I found.

Incoming DHW temp - 55F
DHW Target Temp - 120F
Incoming Space Heating water temp - 109F
Space Heating Temp on display - 140F
Water pressure - hovers around 19-20PSI

When there is a call for hot water, the temp of outgoing water spiked to 139F and then the pressure in the system also spiked up above the PRV setting of 30PSI to around 36PSI and the valve began dripping a little and relieving pressure.

The parameters that have changed since installation are that the outside temps are now consistently 20-30 degrees higher than they were in the winter when it was installed and the incoming water temp for the DHW is about 20 degrees higher, I also installed a check valve on the automatic water feed supply line as a back flow preventer about a month ago. Prior to that I had the supply to the auto water feeder turned off and would check and add water whenever I noticed the pressure getting close to the minimum operating pressure.
We have noticed no discernible difference in the water pressure in the home that should affect the operation of the DHW system. I checked the screen on the water inlet for the DHW and is clear.

My Guess:
I think that what's happening is some combination of the outside air temps and incoming water temps being higher are causing both the space heating and DWH systems to heat up more than necessary and increasing the pressure in the system to the point that the PRV opens.

What settings should I change to keep this from happening? and is this the root cause of the cold water sandwich or just the frequent PRV openings we are hearing?

Thoughts...?
 

glorybound

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Hello did you resolve your issue? I installed a 180 this spring and are experiencing the same problem trying to find a happy balance I have had spikes on top end and bottom I know they will cut off if either one goes to high or low ,Replaced makeup water regulator checked pressure tank andput inline make up water line pressure gauge ,had it running uo to high cutting off .And I adjusted pressure regulator lower but on occasion get low incoming pressure cutoff checked codes ,Now I have it set to incoming at 13 and bounces to 27 psi when running Turned down temp to 140 and it did help cant see leak anywhere getting an error for air too. Piping does run through concrete slabe in closet where old boiler sat ,Wondering if maybe leaking in slab or sucking air? All plumbing is done correctly copper pipe install with slave pump iso valves 30 psi popofff scoop type with high vent 1 foot above rest of piping Any thoughts ? thanks in advance
 

Dana

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Hello did you resolve your issue? I installed a 180 this spring and are experiencing the same problem trying to find a happy balance I have had spikes on top end and bottom I know they will cut off if either one goes to high or low ,Replaced makeup water regulator checked pressure tank andput inline make up water line pressure gauge ,had it running uo to high cutting off .And I adjusted pressure regulator lower but on occasion get low incoming pressure cutoff checked codes ,Now I have it set to incoming at 13 and bounces to 27 psi when running Turned down temp to 140 and it did help cant see leak anywhere getting an error for air too. Piping does run through concrete slabe in closet where old boiler sat ,Wondering if maybe leaking in slab or sucking air? All plumbing is done correctly copper pipe install with slave pump iso valves 30 psi popofff scoop type with high vent 1 foot above rest of piping Any thoughts ? thanks in advance

If it's swinging between 13 psi and 27 psi over the course of a space heating cycle the expansion tank either isn't big enough , or not properly pre-charged.

If "... bounces ..." means it is instantly swinging to 27 psi as soon as the circulator is running there is some restriction on the loop limiting flow.

The domestic hot water temperature rarely needs to be more than 115F to be able to deliver 110F water in a tub-fill, or 105F water to a shower. At low flow through the domistic hot water side of the combi-boiler the temperature regulation is often less precise as the burner doesn't have infinite turn d0wn range. At a given flow there is a minimum temperature rise that it can deliver.

A leak in the pipe under the slab won't suck in air, it will just cause the system pressure to drop if the system is isolated, without an auto-fill valve. If the system pressure isn't high enough to compensate for all the pressure differences created by the pumps, it can pull in air from the system vent. Typically pumping induces less than 5 psi pressure difference.

Ideally the expansion tank and vent would be on the input side of the circulation pump, with straight run of pipe between them. That allows the expansion tank to behave as a shock absorber, damping out any high frequency pressure pulses from the pump impeller, and keeps the pump from cavitating.

0909pm-Sieg-fig1-lg.jpg
 

glorybound

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Thanks alot Dana these combi are relatively new for us ,I checked expansion tank it was 10 psi I will check pump pressures on both ends of the loop . And check Hi-vent I also was going to turn off feeder and see if there is a leak in the system explaining the air I really appreciate your help!
 

Dana

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Thanks alot Dana these combi are relatively new for us ,I checked expansion tank it was 10 psi I will check pump pressures on both ends of the loop . And check Hi-vent I also was going to turn off feeder and see if there is a leak in the system explaining the air I really appreciate your help!

For the record, how much radiation (of what type) do you have on your system (broken down by zone, if zoned), and how big is the expansion tank?

What temperatures are you running on the heating side? Is the temperature under outdoor reset control?

If it's a 1-2 story house, pre-charging the tank to 15 psi (when there is no pressure on the wet side), and setting the system to 13 psi is usually about right. If the tank was at 10 psi and the system set to 13-15psi (cold) there is less space in the tank for accommodating the expansion of the system water as it comes up to temperature. When completely out of space in the tank the pressure spikes quickly.
 

glorybound

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Hey Dana, Sorry so long getting back to you, Holidays and all! You were right on that tank was to small and your right that combi fires so high and fast that the pressures would spike, Replaced all good Thanks a bunch for your expertise! Regards
 

Dana

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Hey Dana, Sorry so long getting back to you, Holidays and all! You were right on that tank was to small and your right that combi fires so high and fast that the pressures would spike, Replaced all good Thanks a bunch for your expertise! Regards


You're welcome!

With the zone radiation sizes and a bit more information it's possible to optimize the system for higher comfort and higher efficiency, if that's of any interest. Too many condensing boilers are operating at an unnecessarily high temperature, too- high for condensing efficiency, and satisfying the thermostats so quickly that the room temperatures fluctuate with hot flashes followed by the long chill. With outdoor reset control (the outdoor temperature sensor is cheap) it's usually possible to find a temperature range that doesn't short-cycle the boiler, and deliver long comfortable heating cycles that get lengthen as the outdoor temperature falls.
 
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