Your thoughts on my OWB performance

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MVrana

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I'd appreciate hearing your thoughts on what I'm seeing with my OWB. For background, it is plumbed to a water/air heat exchanger in my gas furnace. Water jacket setting is at 190 F. Temp ouside is in the mid 30's. Area being heated by this unit is about 2000 sq. ft, built in 1924, moderate insulation, many windows (some new, some old), 10 ft. ceilings.

When the furnace first kicks on in the morning (t-stat setting change from 62 to 68), the air coming from the vents is a toasty 136 degrees. After running continuously for about 30 minutes, house temperature has risen to about 66 degrees. But, now the air temperature coming out of the vents has dropped to about 103 F. What does this say about my system design? Does this suggest water capacity of the boiler is too small? Or, is this just a typcial response curve as the boiler fires and tries to heat back up? Note that since we keep the thermosat set low at night (10 pm to 5:30 am), the boiler probably hasn't fired very much during the night and it may take a little time for the fire to build up (bed of coals may not be very hot/active). The OWB does have a fan for combustion air.

Thanks for your input.
Mark
 
Something doesn't sound right, but there are so many variables I need more info to help. My first thought is you have some type of water flow restriction. With 190 degree water your initial plenum temperature should be higher. Where and how are you getting the register air temp? What size is your hx? What is the low set point for your owb? What kind of pump and how far is the run? What did you use for underground pipe? If you are only gaining 4 degree's in 30 minutes and still can't maintain hot air something is up. Give us more details. Also what type of boiler do you have?
 
What OWB do you have?

How cold does the water get in your OWB before the draft blower kicks on?

Did you get the air out of your water lines?
 
Thanks applefarmer and windwalker. The system does indeed have some flow restrictions. It was installed with only 5/8" pex-al-pex--that is a story for another day. I plan to replace the lines with 1" pex-al-pex next summer. The distance from the boiler to the heat exchanger is about 125 feet. I am running two taco 009's in series to compensate for the high head. However, if flow was the main problem, then why would the temperature drop with time? It would seem that under a low flow scenario, the water temp coming from the boiler would remain fairly constant? since the air temperature is dropping with time, I can only assume the boiler water temperature is dropping as well.

Temperature loss from the boiler to the heat exchanger is about 2 F. Temperature reduction through the heat exchanger is about 32 F, give or take. The vent temperatures I posted are not in the plenum, but at the air registers in the house. I used a thermocouple from our lab to measure the temps--very accurate. The particular vent I take measurements from is about 15-20 feet away from the furnace.

The boiler is a Hardy H4. Temperature differential at the boiler is 10 degrees (on at 180, off at 190). I'm fairly certain that I got all the air out of the lines.

Thanks again for your input.
 
i don't think you'll see any thing better till you up size your lines to 1"......i have replaced 5 5/8" lined systems that hardy dealers put in with 1" lines....the water temp can't keep up with that small of flow.....not enough gpm...

i didn't see you had 2 009's....are they pushin' and pullin'?......boy thats a lot of pump for 5/8" line......how high is the head?
 
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The problem is your small pex lines. You can't move enough volume of water to your hx when your furnace blower fan is running. I don't know of an easy cure other than to replace the lines.
 
ghitch and applefarmer, I understand and agree with what you are saying about the 5/8" lines. They are definitely too small and will be replaced later. But why the temperature change over time? Seems to me that if I have low flow from the boiler, then the boiler tempature should remain fairly constant. How is it that I can get 136 degree air initally, but then it falls to 103 later?? Something isn't adding up. I'm puzzled.
 
ghitch and applefarmer, I understand and agree with what you are saying about the 5/8" lines. They are definitely too small and will be replaced later. But why the temperature change over time? Seems to me that if I have low flow from the boiler, then the boiler tempature should remain fairly constant. How is it that I can get 136 degree air initally, but then it falls to 103 later?? Something isn't adding up. I'm puzzled.


there's not enough volume of water to keep up with the H/E....so as the water moves though the H/E it cools off before it leaves it.....there is where your gettin' your 36f drop out of the H/E....should be only a 10f to 15f drop....more volume will take care of it.....

btw the H/E lines are 1" correct?...then it goes to 3/8" x how many 3/8" lines in header.....if it's over 5 with 5/8" line your starvin' the H/E.....do your pumps run all the time or on demand?
 
ghitch, I can't remember exactly, but I think the heat exchanger main lines are 3/4" with around five 3/8" lines. The pumps run on demand. Right now I have the two Taco 009's in series (6" apart) pushing water to the heat exchanger. They are located on the back of the OWB.
 
ghitch, I can't remember exactly, but I think the heat exchanger main lines are 3/4" with around five 3/8" lines. The pumps run on demand. Right now I have the two Taco 009's in series (6" apart) pushing water to the heat exchanger. They are located on the back of the OWB.

how big is the H/E?....16" x 18" 2 row,3row , 4 row,ect?

in series won't move any more water than just 1 pump.....need to put the other 1 in the return pushin' to the boiler.....1 pump should be plenty....i have pushed over 40ft head to 5 different H/E"s that where 24"x 24" 4 row coils with that same pump.....it's a wonder the lead pump hasn't burnt up....
 
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I don't think that changing your house temp 4* in 30 minutes is bad. I can never see that. I'll get 2* in 30 min, with a 18 X 20 HX. I plan on changing that to a 18 x 26 with 1" connections, so I have a complete 1" run from the boiler to HX and back.

That's why I don't drop my house temp too much only about a 2* swing from night (68*) to morning (70*)

Central 5036, 1" pex underground, (1) Taco 009
 
What is the water temp inside the boiler when this happens? Comparing the boiler water temp when the furnace 1st kicks on to boiler water temp after you notice the outlet temp drop should give you the answer .Is your boiler water actually dropping 30 degrees? If it is, its not a flow problem, your boiler just cant recover fast enough. (possibly normal)if your boiler temp isn't dropping 30 degrees you have a flow problem....at least this makes sense in my pea brain:)

I found last year I was better off just leaving my thermostat the same.By trying to raise the house temp rather than maintain it, it would drop the boiler water temp to much and it takes to long for the fire to catch back up to demand.
 
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Why is that? Pumps in series increase pressure differential. Higher pressure differential, more flow...


1 pump pushin' into the other pump you won't gain anything.....now you parallel the 2 in the same line then you have something....
 
Is you wood seasoned? Last year I only burnt unseasoned wood. I fought my boiler all winter. I would put a heaping wheel borrow load in, in the morning and at night. My furnace ran constantly. This year I have seasoned wood. 6 sticks in the morning and 6 sticks at night and that is about all it takes. My furnace still runs about every 30 minutes or so but not as long. The boiler water temp recovers much faster due to the seasoned wood. I have 3/4 pex ran for around 125 ft. 80 ft outside and 45 ft in my crawl space. My HX is 24 x 24 x 4. It has 2 rows. My duct work is not insulated and neither is my crawl space. So I have a significant heat loss I'm sure. My termostat stays on 72. The thermostat calls for heat at 71 degrees and kicks off at right before 73 degrees. I still have a lot of work to do under my house but this year seems to be alot better cause of the seasoned wood.
 
1 pump pushin' into the other pump you won't gain anything.....now you parallel the 2 in the same line then you have something....

Au contraire, you'll gain flow in both scenarios. To determine the pump curve for two pumps in series, you add head values of the two pump curves. To determine the pump curve for two pumps in parallel, add the flow for every head value.

In either scenario you wind up with more flow. To determine which will net a higher flow you need to graph the performance curve of the system against the pump curves of each of the two configurations.
 
Thanks everyone for all the good input.

Marc has it right. For two pumps in series, the head is additive at the same flow (gpm). For two pumps in parallel, the flow (gpm) is additive at the same head. Since my system was improperly installed with 5/8" kitec, the head is extremely high. This extremely high head limits the flow to something on the order of 3 gpm even though the pump is capable of 8 gpm. Adding a second pump in series helps overcome the high head somewhat, and the flow then increase a little (to about 5 gpm). Since the system head increases as a function of flow rate, it wouldn't do me any good to install the pumps in parallel--head is the limiting factor with the small lines.

Using two pumps is just a band-aid fix until I can get the lines switched out.
 
Au contraire, you'll gain flow in both scenarios. To determine the pump curve for two pumps in series, you add head values of the two pump curves. To determine the pump curve for two pumps in parallel, add the flow for every head value.

In either scenario you wind up with more flow. To determine which will net a higher flow you need to graph the performance curve of the system against the pump curves of each of the two configurations.

I've done the graph using data from Taco. It predicts the modest gain (from about 3 gpm to 5 gpm) I mentioned in my last post. I haven't verified the real gain through any actual measurements yet.
 
I've done the graph using data from Taco. It predicts the modest gain (from about 3 gpm to 5 gpm) I mentioned in my last post. I haven't verified the real gain through any actual measurements yet.


i have been doin' HVAC and hydronic work for over 30 years and i have never seen any gain on pumps in series......maybe a gallon or 2 per min....correct line size and pump size wins out everytime.....

i just know what works and won't don't work......i have been know to be wrong too..

just my 2 cents
 
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