Your thoughts on my OWB performance

[12vdzl]

The problem is *NOT* flow rate, it is the combination of the lack of adequate stored thermal capacity plus a recovery rate that cannot keep up with the rate of demand.

If it were insufficient flow rate, the temperature of the boiler wouldn't drop so quickly. The BTUs are being moved from the boiler to the house quickly enough, but they are not being replaced at the rate they are moved.

I must have missed where he posted what the boiler water temp actually is, at the boiler itself.

He needs more volume of water going the HX in order to keep the inlet and outlet temp pretty close to the same temp. If the HX is pulling too much heat out of the water before it leaves the boiler will not have a prayer of keeping up. Now if you add more volume of water to the HX you won't lose as much heat accross the HX so the boiler has a better chance at keeping up, while deliverying MORE BTU's to the plenum, at a more sustained rate.

 

[anthony.d]

This is kind of the problem I'm faced with. I replaced my dad's homemade OWB (20 yr old) with a new CB 5036. I own my folks place now.

I have 1" pex to the house with a taco 009, then the HX is a 18 x 20 with 3/4" inlet and outlet lines. (stuff Central Boiler rep sold me)

When the furnace calls for heat I have a 22* drop in temp accross the HX. (I think way too much) I will drain the boiler temp 10* where it opens the damper. (within 30 min or so). I have no problem when there is some wood in the boiler to bring the temp back up. (While furnace is still calling for heat).

Next year I'm replacing the HX with a 18 x 26 which will fit, and has 1" supply and return lines. This should help my flow and not drain the water temp in the boiler so quickly.

 

[John D]

Next year I'm replacing the HX with a 18 x 26 which will fit, and has 1" supply and return lines. This should help my flow and not drain the water temp in the boiler so quickly.

I think your new setup will drain boiler temps faster...bigger exchanger,and lines.So more btu's can be moved faster from the OWB to the air in the home.If your currentsetup is heating the home adequetely,Id stay with it.If the hx cools off a lot after a few minutes,id upsize the lines first,it may be all you need to do.What is the problem with drain owb temps,it does recover right.

 

[mtfallsmikey]

1. Not enough water capacity: Running at, let's say, 8 gpm for 30 min.=240 gal. of water. Out of the 175 gal. capacity, maybe you can get 1/3 or 1/2 of that capacity at 190 deg. OWB's are slow to recover.

2. Lines too small, too much head. You need 3/4 or 1" to get 4-8 gpm. Another series-mounted pump may/may not work. I will certainly work with properly sized lines.

 

[ghitch75]

I must have missed where he posted what the boiler water temp actually is, at the boiler itself.

He needs more volume of water going the HX in order to keep the inlet and outlet temp pretty close to the same temp. If the HX is pulling too much heat out of the water before it leaves the boiler will not have a prayer of keeping up. Now if you add more volume of water to the HX you won't lose as much heat accross the HX so the boiler has a better chance at keeping up, while deliverying MORE BTU's to the plenum, at a more sustained rate.

:agree2:......seen it many times....

 

[mtfallsmikey]

:agree2:......seen it many times....

Just another SWAG install....

 

[rob206]

I must have missed where he posted what the boiler water temp actually is, at the boiler itself.

He needs more volume of water going the HX in order to keep the inlet and outlet temp pretty close to the same temp. If the HX is pulling too much heat out of the water before it leaves the boiler will not have a prayer of keeping up. Now if you add more volume of water to the HX you won't lose as much heat accross the HX so the boiler has a better chance at keeping up, while deliverying MORE BTU's to the plenum, at a more sustained rate.

I think this nails it. It shouldn't matter what your storage capacity is, your boiler should be able to recover much more quickly than 30-60min. With your current install, if you were to replace just the boiler with one that has a lager water jacket and kept everything else the same, you would still have the recovery problem. The only thing that would change is the frequency of firings.

 

[mtfallsmikey]

This is kind of the problem I'm faced with. I replaced my dad's homemade OWB (20 yr old) with a new CB 5036. I own my folks place now.

I have 1" pex to the house with a taco 009, then the HX is a 18 x 20 with 3/4" inlet and outlet lines. (stuff Central Boiler rep sold me)

When the furnace calls for heat I have a 22* drop in temp accross the HX. (I think way too much) I will drain the boiler temp 10* where it opens the damper. (within 30 min or so). I have no problem when there is some wood in the boiler to bring the temp back up. (While furnace is still calling for heat).

Next year I'm replacing the HX with a 18 x 26 which will fit, and has 1" supply and return lines. This should help my flow and not drain the water temp in the boiler so quickly.

Something to keep in mind folks....his 22 deg. drop (Delta T)across the HX is not uncommon. I still think IMHO the major problems I've seen in OWB system performance is the concept of stringing all of the HX's in series. Each HX down the line from the first one will see progressively lower supply temps, and it adds to the head pressure, cuts fl;ow rates...that's why I like primary/secondary piping, one primary loop, separate loops and circs. with each HX.

 

[anthony.d]

I only have the boiler hooked up to one HX which is a 18" x 20" in the furnace plenum. I don't have it hooked up for domestic water right now. (This gives me a 22* drop across the HX)

I'm changing everything next year. And I plan on setting up a 1" manifold so I can supply each thing separately. ( the HX, kick space heaters, domestic water) Plus replace the 3/4" fittings.

My central boiler dealer sold me 3/4" flanges for the pump and 3/4" ball valves and other fittings. I wasn't sure why because I have 1" pex from boiler to house. But I was assuming they knew what they were selling.

Now thinking back to my hydraulic class I took years ago, I see flow problems with using 3/4" fittings mixed in with the 1" pex.

 

[MVrana]

I think this nails it. It shouldn't matter what your storage capacity is, your boiler should be able to recover much more quickly than 30-60min. With your current install, if you were to replace just the boiler with one that has a lager water jacket and kept everything else the same, you would still have the recovery problem. The only thing that would change is the frequency of firings.

You guys will have to help me understand this (I only have a Ph.D. in Engineering). I don't see why the temp change across the heat exchanger really matters. In my case, the large change in temperature is caused by low flow. So, although the water returning to the boiler is colder, there is not as much of it. If the system were set up to give a smaller temperature change, say 10 degrees vs. 30 degrees, that would require much higher flow/volume, and therefore you would have more slightly cooled water returning to the boiler . In simple thermodynamic calculations, you should end up with the same effect on the temperature of the water in the boiler. It is a function of both temperature and volume (the volumes of hot and cold water being mixed), not just temperature.

MtFallsMikey, I am running two independent loops (separate pumps) from the boiler--two separate pumps to two separate heat exchangers. Nothing in series.

My system seems to be working better with the two pumps in series, as well as with spreading the heat demand in the morning over a longer period of time. Programmable T-stat increases 1 degree at 3:30, 1 degree at 4:30, and two degrees at 5:30 to go from 64 to 68 degrees. The air from the vents is always very hot by doing this, and the blower needs to only run 10-15 minutes. When I tried to get the full 4 degrees at once, the air from the vents cooled down and the blower would run and run and run. Too much demand at one time.

Thanks again for the input everyone. This has been fun.

 

[John D]

I think your system is fine,you fixed your problem by stepping up the temps slower...In my case,my OWB needs more time to recover from heavy loads than my oil fired boiler,much like yours,I have learned to not make more than 3 degree changes to the tstat,and all is fine.If i make a bigger change,and my tenants heat kicks on or the DHW kicks on,it will run them all,but may take 45 minutes to satisfy everything.

 

[KsWoodsMan]

You guys will have to help me understand this (I only have a Ph.D. in Engineering). I don't see why the temp change across the heat exchanger really matters. In my case, the large change in temperature is caused by low flow. So, although the water returning to the boiler is colder, there is not as much of it. If the system were set up to give a smaller temperature change, say 10 degrees vs. 30 degrees, that would require much higher flow/volume, and therefore you would have more slightly cooled water returning to the boiler . In simple thermodynamic calculations, you should end up with the same effect on the temperature of the water in the boiler. It is a function of both temperature and volume (the volumes of hot and cold water being mixed), not just temperature.

MtFallsMikey, I am running two independent loops (separate pumps) from the boiler--two separate pumps to two separate heat exchangers. Nothing in series.

My system seems to be working better with the two pumps in series, as well as with spreading the heat demand in the morning over a longer period of time. Programmable T-stat increases 1 degree at 3:30, 1 degree at 4:30, and two degrees at 5:30 to go from 64 to 68 degrees. The air from the vents is always very hot by doing this, and the blower needs to only run 10-15 minutes. When I tried to get the full 4 degrees at once, the air from the vents cooled down and the blower would run and run and run. Too much demand at one time.

Thanks again for the input everyone. This has been fun.

I'm still curious about where your supply line for the HX is plumbed into the boiler, Top or Bottom ?

If it is a bottom feeder then you are drawing the cold water back out of the boiler before it gets reheated.

It has been mentioned in the past that the manuals say to connect it this way but experience on the forum has shown this to be incorrect. Plumbing the supply line to the top of the boiler provides better results.

I would want the hottest water from the boiler, not the coldest, to heat my house.

 

[MVrana]

I'm still curious about where your supply line for the HX is plumbed into the boiler, Top or Bottom ?

KSWoodsman, sorry I didn't reply to your question. The feed is plumbed into the top of the boiler, so I am getting the hottest water. Return goes into the opposite corner, bottom of the tank. Good suggestion, though--thanks.

 

[12vdzl]

MVrana, glad you got it figured out.

Are 1" lines still your future ?

 

[MVrana]

MVrana, glad you got it figured out.

Are 1" lines still your future ?

12vdzl, Thanks for all your input. At this point, I'm not convinced 1" lines will help me out. My system seems to be limited by the boiler capacity, since I am able to cool the entire boiler volume fairly fast. That said, I completely understand and agree that using 1" lines is the correct way to set up the system (correct flow velocity, flow volume, lower head, etc.). Using 1" lines would also allow me to remove the 2nd pump and keep it as a backup. In terms of costs, new piping vs. maintaining a second pump, it is about a wash.
I'm going to see how my current setup runs through this winter, then make a decision about new lines next spring/summer. So far we've only gotten down to about 30 degrees at night, and everything is working fine. I'll be curious to see how it does when it gets into the teens or single digits (hopefully no time soon).
Thanks again for your help.

 

[ghitch75]

12vdzl, Thanks for all your input. At this point, I'm not convinced 1" lines will help me out. My system seems to be limited by the boiler capacity, since I am able to cool the entire boiler volume fairly fast. That said, I completely understand and agree that using 1" lines is the correct way to set up the system (correct flow velocity, flow volume, lower head, etc.). Using 1" lines would also allow me to remove the 2nd pump and keep it as a backup. In terms of costs, new piping vs. maintaining a second pump, it is about a wash.
I'm going to see how my current setup runs through this winter, then make a decision about new lines next spring/summer. So far we've only gotten down to about 30 degrees at night, and everything is working fine. I'll be curious to see how it does when it gets into the teens or single digits (hopefully no time soon).
Thanks again for your help.

it won't suck the life out of the boiler with the right line seize because your return temp will be higher.....and your air out up temp. will stay higher....

 

[John D]

it won't suck the life out of the boiler with the right line seize because your return temp will be higher.....and your air out up temp. will stay higher....

Boiler temps won't be any higher ,they may be slightly lower,as the higher flow will allow the hx to stay hottter pulling more heat in less time from the OWB....return water may be hotter,but there will be a lot more water returning,so overall not much change.

 

[MVrana]

Thanks John D. I'm seeing this the same way you are. By the way, what is the water capacity of your Shaver 250?

 

[John D]

Thanks John D. I'm seeing this the same way you are. By the way, what is the water capacity of your Shaver 250?

It says 230 gallons.....I wish i went with the 290....,but thank god I changed my order from a 165,I dont think it could have handled my home with feeding 3x a day in the cold,and still would have fell behind...