Help with Hydronic system

[rmorey]

We built a home on the side of a lake. Power is 4 km away (3 miles). Solar was our only option, so we installed a 4kW system to handle our electrical requirements. We installed a wood fired 125,000 Btu boiler in the workshop to feed a 60,000 Btu/hr load. (+\-30,000 Btu/hr house, 30,000 Btu/hr workshop).

Installed 1/2" Pex Al Pex, loops are 12" apart. Each floor has 3 - 300' loops, fed from a single manifold.

Heatloss Calculations

GRUNDFOS 2699 FC pump feeds a 1" Pex main loop from boiler to house to a sidearm exchanger and back to workshop with 6 GPM/13.6' head loss. After the sidearm, secondary loop (close "T's) feeds a GRUNDFOS Alpha pump with 2 GPM/ 9.5' head loss.

Back at workshop, another secondary loop same as house. 2GPM/9.5' head loss.

Boiler manufacturer made me put a 140 degree thermic "T" valve in return line at the boiler, to keep temps above 140 degrees.

When no call for heat, or just one load calls, everything works fine. When all loads call, the cold water from floors, return to boiler and floors begin to heat. Cold water begins to cool boiler. Eventually, boiler temp drops below 140 degrees and valve closes, sending main pump output to return of boiler. Temps go up, valve opens, hot water goes to load, and cycle continues. Stove never shuts off and load never gets satisfied. Can't figure out what I've done wrong. Could use your ideas.

 

[Tractor Seabee]

Is this a DIY design or a mechanical design effort? There are a lot of things to consider when designing a Hydronic system that even the professional engineers screw up on and service techs have to straighten out. I was one of those techs years ago and the industry has changed a lot since I gave it up. No way could I diagnose your system in a forum like this or even on the phone. First off the reason for what is happening is to keep water from condensing in the flue section of the boiler and short cycling under low load. You probably need an auxiliary surge tank set up as a thermal separator. Recommend you get professional help and a proper diagnosis or you can spend a lot of money and time trying things.

Ron

 

[roadhunter]

This ^. I was a PM on a USFS job that had to be changed after installation to make work when the engineers plan failed and a tech ended up correcting thier mistakes.

Might be helpful to provide a layout of the system and some pics.

 

[dadster4]

The boiler should have a mixing valve installed on the hot supply line to mix 140 degree boiler water with the cool return line water from your loops. The tempered "mixed" water should only be about 105 degrees and go to your supply manifolds, the third port of the mixing valve is piped to the boiler return. This will provide the proper temp water for your in-floor loops AND proper return water temp to protect your boiler from condensing.
Check your "thermic t valve", does it modulate to maintain a set temperature, or is it strictly open/close?
Is your pex run under your subfloor, encased in a lightweight concrete slab, or under finished tile or wood flooring?
I have designed and installed many in-floor heating systems, but more information is needed before your system can be fixed. Pics, t valve specs, layout of piping, manifolds, main loop, and secondary loops.

 

[rmorey]

Here is a functional diagram that I did before I started. I had the HVAC guy we bought the boiler from look at it and we figured everything would work fine.

The trouble started when the thermic valve was installed. The boiler formerly had a snap disk which powered the pump. At 140 degrees the disk opened and power to the pump was shut off. The stove heated up and the pump was repowered.

The company had a problem with people plugging the pump directly into the wall to bypass the snap disk. As a result, they had cold water returning to the boiler and therefore had a rust problem. The thermic valve eliminated that issue for them. Now I can't get heat to the load for an extended period.

 

[teejk]

I have a friend in the commercial boiler business and also proficient in in-floor radiant (he designed ours although I don't have the wood boiler...but I know he has installed/maintained a few of those also). I can ask him to look at your diagram. In the meantime it seems to be a problem when both locations are calling for heat. Your manifolds have supply and return temp gauges. Are they balanced to show T Delta of around 15 degrees F? I'm hardly an expert but you might have to increase the flow rate so warmer water returns to the boiler (less absorbed into your floors but that gets made up with time). Again I'm not an expert. Complete Radiant Floor Heat Information Site is pretty informative (although my boiler friend doesn't agree with some of his ideas). As for rust, you have a "closed loop" system. My boiler is a Weil McLain "on-demand" and they require a corrosion inhibitor be added to the system.

 

[teejk]

The boiler should have a mixing valve installed on the hot supply line to mix 140 degree boiler water with the cool return line water from your loops. The tempered "mixed" water should only be about 105 degrees and go to your supply manifolds, the third port of the mixing valve is piped to the boiler return. This will provide the proper temp water for your in-floor loops AND proper return water temp to protect your boiler from condensing.
Check your "thermic t valve", does it modulate to maintain a set temperature, or is it strictly open/close?
Is your pex run under your subfloor, encased in a lightweight concrete slab, or under finished tile or wood flooring?
I have designed and installed many in-floor heating systems, but more information is needed before your system can be fixed. Pics, t valve specs, layout of piping, manifolds, main loop, and secondary loops.

The more I think about it, I think you nailed it. 3 300' loops and that valve still expects to receive 140F water? In the house I think I only push 120F water (a flooring thing in an under-floor tube system). I set the manifolds to maintain 15F differential...In the shop (embedded tube in real concrete) I run an LP water heater as the source but for some reason I have to keep it at around 120 otherwise the pressure tends to build too high. Hydronic heat is a delicate balancing act I think. Now the one thing I can't opine on is the OP is using wood (HOT). That complicates the balancing act a lot I think depending on his floors.

 

[Tractor Seabee]

You have a fairly complex system. As I stated before you need a buffer tank for boiler flywheel effect and as a thermal separator. This establishes a a primary loop to keep the boiler happy and then the secondary loop feeds the down stream zones. The secondary loop feeds the zones using what in the trade is called closely spaced tees. These are design issues that cannot be redesigned via this forum. You need professional help instead of DIY ideas. As designed; IMHO, your system will never work right w/o serious rework. It takes an on site analysis of what there is and how it is assembled.

Ron

 

[rmorey]

I should mention, this is an open system, with the boiler operating at 175 degrees with an 8 degree differential between on and off.

So the theory is, 6 GPM is sent down the main loop at 175 deg. Close spaced T's at the secondary loop feed hot side of 120 degree mixing valve. To pump @ 2 GPM to floor. Back to cold side of mixing valve and to main loop again. 120 degree return water @ 2 GPM mixes with the 4 remaining GPM @ 175 degree restore 6 GPM and to lower temp by 10 degrees. Thru sidearm and return to workshop floor close spaced T's and same deal... Back to boiler at 155 degrees. 20 degree delta T.

 

[dadster4]

After looking at your piping layout, I have a couple more comments. The hot water tank should be piped in series on the PRIMARY loop, not the secondary house loop as installed. I would tap in after the potable hot water heat exchanger, to the "cold" side of the hot water tank, out the "hot" side, and back to the primary loop. This will create a "thermal buffer" volume of high-temp water, which your system does not have. The boiler and primary loop will remain hot all the time, more potable hot water will be available, and the secondary loops will have a larger volume of hot water to pull from when they need it (this will stop the short cycling of the boiler) Also check that the mixing valves are properly installed with the primary loop's first closely spaced tee feeding the hot port, the radiant loop return and cold port are piped to the second closely spaced tee, and the mixed supplies the radiant loops.