Tube Shell Exchanger Output Temps

Hello again all,

Biomass 40 is running top notch now, thanks to tips and tricks of everyone here. It's cycling nice, clean, and I coulnd't be happier.

My next problem is a whole other can of worms (and has lead me down the rabbit hole already trying to diagnose).

My Heating/Cooling guy and I are trying to figure out why I've got low output temps on my tube shell exchanger. Pic follows with current readings after about 48 hours of continuous burn. Bottom line is, I can't crack around 150F. I have 3 water heaters used as storage/buffer, so only about 150 gallons. I have a 16x20 air/water HX in the furnace and a solitary 5 foot slat-fin rad in the upstairs bedroom. 3 speed pumps for each (storage, furnace, rad). Have experimented with all speeds, found that has little impact.

Now all this to say, at around 10F outside, I can keep the 1800 sqft farm house at around 68F, but based on the output numbers, I know there's something amiss. There's got to be more heat there.

Last piece of the puzzle is the tube shell - it's 85,000 but/h. Is it possibly undersized?

Needless to say, I'm happy that the 80,000 btu propane furnace has been silent for over 30 days.

Sorry, by pressurized, you mean closed essentially yes? Boiler loop is closed, contains gylcol at 15%. The other side can be open or closed. The idea being a cold supply line and hot output line could offset domestic hot water to the second story. These are currently closed.

Would a plate style exchanger have been a better solution?

Bob Rohr said:

How much surface area in that HX? I've never had much luck with that type of HX for close temperature approach. Maybe not enough surface area or the design it too wide open?

Small plate style give much better performance, from my experience.

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Got nothing on the spec sheet. However, a 155K has 3.6 sqft and a 300K is 6.8 sqft. So I would estimate around the 1.9 sqft mark.

Right now, system is closed on both sides. Do not want to drag the temps down further by trying to supplement domestic hot water.

Sifting through all your replies, amazing insight. While searching for HX specification details on the limited instruction sheet, I too noticed that the flow should have been reversed. Thankfully it's easy to reverse a pump even with my plumbing knowledge. Mission accomplished in about 5 mins. I'll grab the infrared and take additional readings upon my return home today. However, I had 149F water at the intake of the Furnace based HX, but I believe that is simply because it is the first stop now, instead of the storage tanks. This made a drastic difference in house temperature though. 12F outside last night, 74F inside. Furnace fan running at 1500CFM.

Indeed I have 15-58s all around. Have them all running on the "hi" setting. This includes the boiler side, it's sitting approx. 70 feet away in the wood shed and it's 1" line. Temp readings were taken 6" down/up stream from the inputs and outputs of the tube/shell HX using an infrared thermometer. I could try the food/meat thermometer if this is believed to be more accurate.

To answer the previous question the boiler side is 15% glycol because I do a decent amount of traveling and my wife is on shifts - affords me a some peace of mind. In retrospect a constantly on circ going would have meant skipping the tube/shell HX all together as suggested. It's not too late I suppose, could delete this once I shut down for the season.

I too am leaning toward the HX being undersized, though installer leaning toward a blockage in the tanks (dip tubes), causing lack of flow.

Added a storage tank bypass before work this morning. The intent this evening is to cycle a load heating the lines to the rad and furnace exclusively to measure any changes to output temperatures.

I have requested HX specifications from the manufacturer.

Thank you all for your replies thus far, never ceases to amaze me.

jebatty said:

This identifies your fundamental underlying problem. 1" pex at 140 feet round trip from/to the boiler is undersized and there is no practical circulator or appropriate design that will move the volume of water needed to handle the 140,000 btuh output of your boiler or even the 85,000 btuh you desire at your hx for your pool.

Under these circumstances it makes no sense to me to discuss a different heat exchanger because that will not solve your underlying problem. If your system can provide needed heat with high delta-T and related low flow rates, your system may be able to work, but I don't see any way to solve your pool heating problem with your current design.

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Thanks for the insight. 2x1" insulated lines were recommended by the local wood boiler retailer/supply given my 70 foot run. It was this or 1 1/4". Would you suggest the deletion of the HX all together as Woodmaster suggested? As at this point, I'm stuck with the 2x1" pex for financial reasons.

Also, minor point of clarification, no pool at any point in the system. It's a series of three 50 gallon water heater tanks as storage/buffer which now be isolated/excluded individually (manually), a water to air HX in the furnace and a solitary 5' slatfin rad.

When tanks are excluded, less than 3 gallons needs to be heated for the furnace HX and the Rad. Damn 9 volt died last night in the infrared, however upon bypassing the tanks, my trusty meat thermometer showed about 160F leaving the tube/shell HX after a 1hr burn. Will grab a fresh 9 tonight and get some better readings.

Thank you Jebatty, no pool in the system now or likely ever. That's what the pond is for I think the confusion lies in our choice of a tube/shell exchanger. Our heating/cooling guy happened to have one, yadda yadda yadda, it was a freebie.

To answer your questions cpeltier -

In terms of controls, the circ pump for the furnace HX is a thermostat on the mainfloor. Furnace fan controls are manual. For about 6 hours in the evening, I typically run the fan at 1500 CFM, and back it down to the default re-circulation speed of 500 CFM overnight and during the day while at work. At high speed, it achieves 74F, at recirc speed it maintains a constant 70F. So I think your 'lower, longer' theory is absolutely at work here.

Now for the rad, it's correctly sized for 180F, but certainly not for lower temperatures. I should have around 15'. Having said that... the bedroom it is located in seems to be maintaining 71F so long as the door stays closed. The second story has no duct work, and is heated by electric baseboards - 1500W per room. Going for proof of concept by replacing one with a slatfin hydronic, seems to be a good use of $100 so far. Same setup here, thermostat controls a circ pump.

The 50 minute scenario you describe is about what was expected from the get-go. My original intent was to use these as buffer/tempering tanks (hook one water heater to a 220v and maintain 120F at all times) and afford me an hour so once the boiler had shut down - little wiggle room should my spouse be late from her shift.

I am keeping my eyes open for larger storage, but it has to be of a specific size/shape to get down the staircase of this old farm house. If my repurposed water heater tank theory worked, I simply envisioned a small army of them down there - they are very easy to obtain, all under $50, easy to move, and are already somewhat well insulated. However, I know that each connection adds 2 elbows, restriction, etc. And needless to say, I have to get the above mentioned issues in check before moving on to bigger storage.

All that to say, I'm open to ditching the tube/shell or upgrading it. In either case, perhaps it could be used as a sidearm for DHW?

In the interim, I'm limping along just fine, I just want to make the best of what's there. And I'll join the club perhaps of folks who wish they'd gone 1 1/4"

woodsmaster said:

Only downfall to not having the hx is that you would have to have glycol in all your storage and house plumbing or eliminate the glycol altogether. If your not gone to much I'd insulate the boiler room, install an electric heater in shed for backup and get rid of the glycol. If your gone a lot the I'd get an over sized flat plat, larger pump and hope for the best,

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Way I see it, I could well be out the $200 in glycol or the money for an oversized plate. Shed insulation is an option, it's getting re-sided anyway this summer (good bye 'nail on brick'!), but a back up battery would be an option too. I guess it's a cost-benefit-headache analysis from here on in. Existing HX was a freebie, so I can't complain!

cpeltier said:

At 3 GPM you should be able to make it work with a little wiggle room if you find a much larger HX. The load isn't that high but you need a close approach HX to meet your heat emitter requirements. Pay attention to the head loss in the new HX if you go that route.
They generally have high head at rated output especially the flat plate type. If you go much larger the head loss will be much less at the lower flow rate and the delta T will be much tighter.
I'm thinking something along these lines and your existing pump may work:
View attachment 156364
I would definitely calculate your HX before you buy one as they are not inexpensive!

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Excuse me as I nerd out on this sweet calculator . Thank you for this, picture is worth a thousand and could save a few hundred The above would allow me to keep my existing setup and simply swap out the tube/shell if I understand correctly?

On the plate exchanger front, I understand not all are created equally. If I go this route (which is likely, as it retains all the logic/appeal of original config), I will start digging on here for recommended brands.

Probably inconsequential now, but I said. 15-58s all around, but the boiler loop has a 26-99.