Help with GPM, maybe? (newbie/ DIY)

Craig_Skip

Hello, I have a single zone 4 loop under slab closed loop system in my pole barn. Each loop is between 270&285 feet of 1/2 inch pex.  My set temp for the pole barn is 54 degrees. I am using 1 single grundfos 1/25 HP 3 speed pump (currently set at highest speed) that is located just before the supply side manifold. My Takagi tankless water heater says I’m getting 1.7 gpm and therefore each line of the loops are getting around 0.45 ish gpm.(flow gauges confirm this). I have the water temp set at 105F.  My inlet temp starts at 55-56 degrees.  I do not have a mixing valve installed. 

My question is would a second pump after the return side manifold increase my flow rate to the tankless and thereby heat the slab faster and also the shop? Right now it takes about an hour of the tankless running before the inlet temps reach 65/69 degrees.   

Yesterday the outside temp was 30 degrees and my system ran for 3.5 hours total for 24 hours.  I have closed cell foam insulation on the walls of the 24x48 pole barn.  I also have 2” rigid foam under the entire slab, 20” blown in insulation above the 12’ ceiling.  

Thanks for any advice, and I live in lower Michigan area for reference.

mattmia2

There is an enormous amount of mass in that slab. A radiant system like that isn't something that works well to try to set back then recover, because of the mass of the slab it could take days to heat that mass with the output of a boiler sized to match the heat loss. A higher flow circulator might make it heat more evenly but it will still take hours to days to heat the slab, all the heat that the boiler can put in the water is gone by the time reaches the return if the temp is dropping in to the 60's.

Jamie Hall

I think you are probably getting as much out of that water heater as you can expect to get. Give it time -- lots of it.

Ironman

That tankless water heater is not designed to take the place of a boiler: its heat exchanger has way too much resistance to flow for a hydronic circulator to overcome.

You’ll get some improvement by piping it primary/secondary with a larger circulator like a ups26-99 on the heater, but it’s still never gonna be right until you put the right appliance in which is a boiler.

hot_rod

If you are sure of the data, this formula will show you what the heater/circulator is actually putting into the slab.

500 X flow X (SWT- RWT)

If you use 1.7 for the flow

Now measure the temperature at the supply manifold, SWT, and at the return manifold RWT. This is called the delta T. ∆T

So 500 X 1.7 (69° supply manifold- 54° return manifold) = a mere 12, 750 btu/hr

Two things, you could use more flow ,65- .75 would help.
Also more temperature, if the boiler could supply 80-54 you get 22, 000 into the slab.

Ideally you want to move as much as the heater has available.


The low flow through the heater could be preventing it from ramping up to full output.

Bottom line you need more flow. Bumping to a 26- 99 may be enough. Or a repipe with 2 circs, one for the heater, the correct one for the floor loops.

Accurate data help see exactly what you are getting.

But 4 loop at .75 gpm each could allow you to move 3 gpm. If the ∆T was 20, now you could move 30,000 btu/hr.

There may be an inlet strainer on the heater, if it is partially plugged that too reduces flow.

Your heater manual may have a chart like this showing the performance o the heater.

Craig_Skip

So based on the comments I’m to understand that the tankless heater running for 3.5 hours in a 24 hour period is inefficient?  Instead of running set point at 105F would it be more efficient to run at 120F?  I can go up to 140F. I was under the impression that concrete can only absorb so many but’s over a certain time so weather your running 105F or 120F it would be more efficient using less ng over time.  

Craig_Skip

I should of mentioned that the tankless heater is outputting at what ever temp I set, if I set to 120 it’s coming out at 120 or 105…etc…it’s not having any issues with keeping up with demand

mattmia2

What is the actual measured inlet and outlet temp of the water heater at some point? near start up and after it has been running an hour or 2 would be good points to measure.

@hot_rod did the math right but I don't think we have both inlet and outlet temps to use to find the delta t.

Your setpoint is 105, but is it getting to that temp?

Hot_water_fan

So based on the comments I’m to understand that the tankless heater running for 3.5 hours in a 24 hour period is inefficient?  Instead of running set point at 105F would it be more efficient to run at 120F?

There’s no “most efficient” amount of time to run. It could run 24/7 at 105 or less at 120 and efficiently would be about the same. It’ll run to satisfy demand, so the BTUs per day will even out. 

Craig_Skip

With the room temperature being set at 54F and the thermostat calls for heat the inlet water temp is usually 55 to 57F at that start up.  Within seconds the outlet temp is at 105 which is what my set point is currently.   After 15 minutes give or take the inlet rises to 63-65F, after 30 minutes inlet temp is around 68-70F. I have not checked beyond that time frame for inlet temp.  The longest the system has run is about 2.5 hours but usually runs around 90 minutes.  Example pictured below

GroundUp

If anything, you could benefit from turning the water temp down further. Ideally, the unit would run 24/7 on the coldest day of the year at whatever water temp and firing rate is necessary to maintain the indoor setpoint. That's very difficult to actually achieve, especially since you're using a water heater to do a boiler's job, but the closer you can get to that point the more efficient the system will be. Do not be concerned with a 3.5 hour run time, but just know that efficiency and comfort could be improved upon by reducing the supply temperature. Do NOT raise it, however.

Craig_Skip

Thanks for all the help…. One last thing, if I add that second pump and could achieve 2.5 to 3 gallons per minute from my current 1.7 gpm would that get my slab temperature up any faster?  In other words, the water may be warmer coming back to the unit by not allowing all the heat to be sucked out at the beginning of each loop.  Thanks

GroundUp

Why do you want it to heat up faster? A slab is a huge thermal mass- you should be setting it at a given temp and leaving it alone for the most part. If you want to change temp regularly, a different heating method is a much better option than trying to fluctuate the whole slab. 105-65= 40 degree delta with a 1.7 GPM flow rate so you're getting approximately 34,000 BTU/hr into the space as-is, which should be more than enough to keep it heated on a design day (approximately 30 BTU/sq ft). You could get more BTU in by raising the average water temp, which can often be achieved by raising the flow rate as you've stated. The question of "why?" remains though.