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OWB: Heat Loss from pump to pex?

Discussion in 'Firewood, Heating and Wood Burning Equipment' started by sw18x, Nov 8, 2012.

  1. sw18x

    sw18x ArboristSite Operative

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    I'm trying to maximize the efficiency of our OWB, because we've got a very inefficient old farmhouse. It's our second winter in the house, and even after blowing insulation, replacing a couple windows, and chasing down every possible draft I could find, the OWB still can't quite keep up with all three zones. I guess it's a matter of principle at this point, but even though we're only going through about a therm and a half of gas a day (gas boiler is wired to kick on when the OWB starts to fall behind), I'm not going to quit 'til that gas meter stops moving.

    Bottom line, I think I need about 10 more degrees of heat entering the house through the heat exchanger. With the OWB set at 180, at the top of the cycle when the blower shuts off the pex line in the house reads around 161. So I traced my heat loss backwards, and found most of it occurs right at the stove: the pipe leaving the back of the boiler reads 180, this pipe leads to a Taco pump and that reads 180 too, but then right as the pex leaves the pump, the pex reads 166 degrees. In the house this translates to about 160 or 161, so I'm only losing about 6 degrees between the stove and house - wish it was zero but I wasn't the one who installed the lines and I doubt they were insulated. So here's my question: why am I losing so much heat between the pump and the pex? Is that even possible, or is it a distortion of the infrared gun? I tried wrapping the pipe and pump with insulation, with no effect. Funny thing is, when I check the temp of the pex line re-entering the back of the furnace, the difference in the reading between the pex and pipe isn't nearly so extreme, which leads me to believe that the heat loss is real.

    Here's a pic - red arrows indicate 180 degree reading, blue arrow = 166 max. The pic was taken pre-refab, I've since replaced these pumps and added insulation around the pipes and lines. View attachment 261493
     
  2. projectsho89

    projectsho89 ArboristSite Operative

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    Normal readings for an IR thermometer. The PEX conducts heat slower than does metal, so the outside surface temp will be lower. I discovered this a few years ago.

    If you want a more accurate temp reading off PEX, you need a contact heat probe attached to the PEX surface with thermal grease, then insulate the heck out of it so the heat cannot escape to the air.
     
  3. rob206

    rob206 ArboristSite Operative

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    Maybe try a larger hx? My first couple years I used a 100,00btu hx, then bought a 125,000btu hx and it drastically cut down on the amount of time the furnace blower had to run. We now have no problem heating the house in even the coldest of temps.
     
  4. projectsho89

    projectsho89 ArboristSite Operative

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    In order to evaluate the HX, we need to know the inlet and outlet temps and the pump flow rate.
     
  5. Butch(OH)

    Butch(OH) Addicted to ArboristSite

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    Agreed, otherwise pretty much just guessing.

    Having enough (A) BTUs at the source or (B). not being able to deliver them to the interior of the home or (C) not being able to extract them at the home is three very distinctly differant problems and you have not identified which you have. Answering this gets you pointed the right direction.

    Symtoms of each are
    A, How hard is the OWB working when the gas burner has to kick on? Easily maintaining temps or draft open all the time and not keeping up?
    B This shows up as large temp drops measured across the inlet and outlets of the exchangers
    C This would be likely if the loop temps are OK and the temp drops across the exchangers are very low. Happens when the interiors get clogged or coated with minerals and heat cannot transferr.

    That being said two things did stick out to me so far
    1 Photo looks like 3/4: pex going to house and 1" return? If so red flag goes up. 3/4" is just over 1/2 the capacity of 1" and is closer to zone size pipe, not main feed. If this or other flow difficulties is indeed the problem it will show as a large drop in the loop temp across the exchanger(s) or more simply just watch how your OWB is doing. Does it work hard under high demands? Or is it kind of coasting, easily maintaining temp while the gas burner has to kick on to heat the house?

    2. Other red flag so far was 6 degree stated drop from furnace to house. This is not small, it is a huge loss. My run from furnace to the basement is 75' and mine drops 1 degree or less. This does not as much affect delivered BTUs as it affects how much wood your going to burn to heat mother earth.
     
    Last edited: Nov 9, 2012
    kyle1! likes this.
  6. Ohio066

    Ohio066 ArboristSite Lurker

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    Just EAST of BFE
    if you dont keep the pipe dry water will suck the heat right out of it. ran mine in 4" drain pipe.
     
  7. sw18x

    sw18x ArboristSite Operative

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    Sorry for the delay in responding, I got busy trying to catch up on last minute outdoor projects with all this nice weather we've been having in western NY. Here's what I can tell you so far:

    I used a digital food thermometer, wrapped in insulation, to check the temp of the pex at the stove and in the house right before it enters the heat exchanger: 173 outside, 164 inside, so the drop is 9 degrees or so. All my lines run through a buried pvc pipe, and I'm 99% sure the previous owner who did the install didn't insulate. However, I've never seen any sign of water seeping out of the pvc so I'm hoping they're dry and not sitting in water. Right now I'm working on some ideas of how to insulate without dragging the lines out, but that's another project.

    The heat exchanger is actually a SuperStor Ultra indirect fired hot water tank. I don't have any specs on it but I'll try to look it up, I know I have the old manual that was left in the basement when we bought the house. The water from the OWB flows through the exchanger in the Superstor tank, then the water in the tank flows into a gas boiler, from gas boiler to the house, and then back into the Superstor tank.

    So, when the burner sits near the low end of the cycle, this gives me inlet temps (and now I'm going to revert to my infrared gun numbers, which based on the above numbers run about 5 degrees cooler than the contact probe) of around 156 - 160 degrees. At that temp, the outlet pex reads somewhere around 143 + / - a couple degrees. I have my gas boiler set to kick on when the temp drops below 139 in the tank, so my outlet temps will never be below that. Funny thing is, the infrared reading on the outlet seems to be a lot more accurate than when I try to take a reading on the inlet (I can tell what the actual water temp is by looking at the temp gage plumbed into the outlet of the gas boiler). So when the OWB and the baseboard heat is really working on a cold day, there's about a 15 - 18 degree difference between outlet and inlet temps.

    The OWB doesn't necessarily "coast", but neither is it firing all the time. I'd have to say it's working just the right amount: I usually top it off at 8pm, then again at 7am, and 5pm when I come home. If it worked harder, it wouldn't make it through 11 - 12 hour burns. The gas boiler only pops on occasionally, and only for a short burst when multiple zones have been firing off in a row and the tank temp begins to fall below that 139 mark. I should note that I have a very "creative" wiring system: I've looped the aquastat for the Superstor tank with a strap-on aquastat on the outlet for the gas boiler. It may sound strange but it works remarkably well: when the thermostat in the house trips, the aquastat in the Superstor tank (= heat exchanger) prevents the boiler from prematurely firing while it's waiting for the hot water to flow in from the tank. But at the same time, if the water in the tank falls below the tank aquastat setting, wiring the strap-on aquastat into the loop means I can set the boiler to stop firing based on outlet temp, not the temp of the water in the tank. I tried just using the tank aquastat, and it took over 10 minutes of continuous firing before the gas boiler could raise the temp of the water in the Superstor tank from 139 to 149 (the aquastat on the Superstor has a differential setting of 10 degrees). With my current setup, if the water from the tank falls below 139 the gas boiler fires for less than a minute during the cycle, this allows the system to catch back up and usually the boiler doesn't fire again for several more cycles.
     
  8. Encore

    Encore ArboristSite Operative

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    Time to pull those lines up and insulate them man. You're losing a ton of heat right into the ground.

    Get those lines insulated like they should be and you'll be able to go much longer between fills on that stove.

    You've got all that stuff to have it setup to where you shouldn't even need the gas, why not just do it right and get it out of the way?

    Depending on how deep the lines are it shouldn't take you too terribly long. If it were me, that'd be my main job until it was done before it gets really cold out and the ground is frozen.
     
  9. hondaracer2oo4

    hondaracer2oo4 ArboristSite Operative

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    You are losing a significant amount into the ground unfortunately. I too have a very old home, built in 1795 two story colonial which I have been working to better weatherize. I have a Hardy H4 which I am on my second year running, I am running a FHA system though which makes a difference in how things work. I installed everything myself and did a lot of research on what I needed to size everything with, I didn't want to do things twice. If I remember everything correctly my water to air HX is a 200k btu exchanger which matched the size of my Oil FHA system. What is your boiler size in the house? I assume that when you were running the boiler that it could always keep up? If so match everything in your system to that. Also if I remember correctly a 1 inch pex will carry about 85k btu effieciently. Since I was shooting for a 200k btu exchanger with a 20 degree heat drop I ran two 1 inch pex lines each way. 2 going into the house, 2 returning. I also needed a pump that was going to move about 13 gpm. ( I know I am throwing alot of numbers around here but I did alot of calculations based on heat drop across the heat exchanger, friction loss in the pipes etc that I can't remember at the moment) I ended up with a Bell and Gossett three speed NRF 36 pump(HUGE PUMP!) I ended up having a spray foam guy come and spray foam the lines right in the trench surrounding the pipes from the boiler to the house. The run underground is 75 feet. I lose 1 degree from the outgoing boiler to the inlet side of the heat exchanger. My suggestion is to rip up the lines next spring and carefully calculate everything out and do it right. It cost me $400 for the 400 feet of 1 inch pex, $500 for the guy to come out and spray foam 75 feet of lines in the ground and spray foam my boiler and $200 for the pump. For an investment of about $1100 you should be much more comfortable next year. Let me know if you need some help with all of the calculations, Id be happy to help.
     
  10. mtfallsmikey

    mtfallsmikey Addicted to ArboristSite

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  11. sw18x

    sw18x ArboristSite Operative

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    You're right, the best thing to do is dig it all up, but right now it would be tough to justify the time and cost involved with excavating, especially given the fact I've got to dodge a septic tank and leech field for the garage (installed at the same time as the OWB). I'm a little burned out from all the other work I've done to fix the system to get it running right, and even if the dollars per month savings are 30 or 40 in the peak of winter, that works out to maybe $100/year, so for the time being I guess I'm going to live with what I got.

    I was toying with the idea of trying to blow insulation up the pvc pipe where it enters the basement, or drag the tube-style insulation along the lines using a wire fishing tool, but I doubt either plan will work. If anybody has some clever ideas for how to insulate that pipe without digging it up, I'm all ears.
     
  12. hondaracer2oo4

    hondaracer2oo4 ArboristSite Operative

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    Get a spray foam guy with a flexible wand for retrofitting walls with spray foam. Snake it to the end of the pipe from the basement. Turn on foam, pull spray foam line back out as foaming. Just thinking out loud though. I doubt you can find a spray foam guy willing to do it.
     
  13. dave_dj1

    dave_dj1 Addicted to ArboristSite

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    How would you insure the foam would be getting under the two pipes? I'm with the masses and say dig 'er up and replace it with something better. I know, no one wants to hear it or do it but unless you don't mind feeding the smoke dragon and still burning gas, you will have to do it.
     

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