Another tankless water heater replacing boiler thread

[OurTown]

85 return is too much with the units set at 120F. I bumped it back down to 80 and lowered the on temp to 60. Also I lowered the unit output temp to 110 so that they run longer.

 

[Roz]

How old is your system?

Have you tested your glycol to see the rating?

You might benefit from new glycol and perhaps a stronger dilution. We test our glycol and change it if necessary every few years.

Our mixture is good to zero deg perhaps slightly below zero as the extra cost for the glycol is nothing in comparison to having to close or having ice issues. Glycol costs have tripled since 2020 pandemic but are now starting to come down slightly.

 

[2Biz]

85 return is too much with the units set at 120F. I bumped it back down to 80 and lowered the on temp to 60. Also I lowered the unit output temp to 110 so that they run longer.

I set mine up at 32° or there abouts...At 32° after it normalizes and heater set to 105°, my cycle times are 15 minutes on/15 minutes off with a 15° differential from 55° to 70°. Glycol going to the bays is 90°. At 15° OAT, the system automatically adjusts and on time is about 18 minutes/off time is 12 minutes. At zero OAT, cycle time is about 22 minutes on/8 minutes off or close to that...At 15° bgelow zero it still has capacity and cycles off. Just can't remember how long.

Maybe you could also lower your heater temp so it stays on longer...I think you need at least 15-20 minutes on cycle (at these temps) to soak the slab to keep it from freezing. And then again, maybe your tubing is buried deeper than mine like I mentioned dearlier. There are a ton of varaibles that youll just have to figure out what works best for your installation. The big variable betwee nyour and mine is the zone pump...You are running a lot bigger pump, and also you have two heaters giving you double the volumn....Its heating a smaller thickness too quickly causing it to cycle off too soon....

 

[OurTown]

I set mine up at 32° or there abouts...At 32° after it normalizes and heater set to 105°, my cycle times are 15 minutes on/15 minutes off with a 15° differential from 55° to 70°. Glycol going to the bays is 90°. At 15° OAT, the system automatically adjusts and on time is about 18 minutes/off time is 12 minutes. At zero OAT, cycle time is about 22 minutes on/8 minutes off or close to that...At 15° bgelow zero it still has capacity and cycles off. Just can't remember how long.

Maybe you could also lower your heater temp so it stays on longer...I think you need at least 15-20 minutes on cycle (at these temps) to soak the slab to keep it from freezing. And then again, maybe your tubing is buried deeper than mine like I mentioned dearlier. There are a ton of varaibles that youll just have to figure out what works best for your installation. The big variable betwee nyour and mine is the zone pump...You are running a lot bigger pump, and also you have two heaters giving you double the volumn....Its heating a smaller thickness too quickly causing it to cycle off too soon....

I adjusted the temps so that the next cycle took one hour and 40 minutes ON and 15 minutes off. Scrubbing about 22 degrees off at the time I looked at the gauges. After that, I didn't look at what it normalized to. It seemed to work much better so I opened the SS bays. The sun came out and warmed up outside about 10 degrees. Then about 5 o'clock I noticed the slabs freezing so I shut the bays back down. I'm pretty sure we wouldn't have this issue on days like this with our old 627K boiler if I turned up the slab stat enough. One thing I noticed is that the heaters' GPM is only about 11 total no matter what the rise is, so I will clean the inlet screens again. (probably for the 10th time)

 

[OurTown]

How old is your system?

Have you tested your glycol to see the rating?

You might benefit from new glycol and perhaps a stronger dilution. We test our glycol and change it if necessary every few years.

Our mixture is good to zero deg perhaps slightly below zero as the extra cost for the glycol is nothing in comparison to having to close or having ice issues. Glycol costs have tripled since 2020 pandemic but are now starting to come down slightly.

The heating units and pumps are new and half of the in-slab tubing is new. The ethylene glycol (not propylene glycol) is old and tests to -45F freeze temp and about 9.5 pH.

 

[2Biz]

I adjusted the temps so that the next cycle took one hour and 40 minutes ON and 15 minutes off. Scrubbing about 22 degrees off at the time I looked at the gauges. After that, I didn't look at what it normalized to. It seemed to work much better so I opened the SS bays. The sun came out and warmed up outside about 10 degrees. Then about 5 o'clock I noticed the slabs freezing so I shut the bays back down. I'm pretty sure we wouldn't have this issue on days like this with our old 627K boiler if I turned up the slab stat enough. One thing I noticed is that the heaters' GPM is only about 11 total no matter what the rise is, so I will clean the inlet screens again. (probably for the 10th time)

Something is not making sense...At the beginning of the cycle, mine scrubs off 35°, from 90° out to 55° return...I get right at 5.9-6.0gpm with one heater. Its been that way for 10 years without touching a thing or cleaning the tiny filter. I'm out of suggestions for you except keep raising the on/off temp to where the slabs don't freeze. Basically doing the same thing as what you did with the boiler.

 

[OurTown]

Something is not making sense...At the beginning of the cycle, mine scrubs off 35°, from 90° out to 55° return...I get right at 5.9-6.0gpm with one heater. Its been that way for 10 years without touching a thing or cleaning the tiny filter. I'm out of suggestions for you except keep raising the on/off temp to where the slabs don't freeze. Basically doing the same thing as what you did with the boiler.

I bumped the heater output to 120 and the stat to 65/85. It's about 10 or so degrees warmer than yesterday so I'm sure that will help. It seemed to work better weeks ago with that cold snap so something has changed. Just opened my gas bill and it is $750. That was Dec 7-Jan 8. Can't wait for the next one.

 

[2Biz]

Did you ever run a 5 micron filter inline to get all the crud out? My current duke bill is half yours, but understandable! I'll take it. I couldn't afford to run the old boiler that was in there. $150.89 is just additional rider fee's! Last month total was $268.34...

 

[OurTown]

Did you ever run a 5 micron filter inline to get all the crud out? My current duke bill is half yours, but understandable! I'll take it. I couldn't afford to run the old boiler that was in there. $150.89 is just additional rider fee's! Last month total was $268.34...

View attachment 12581

I ran the glycol through a 5 micron whole house filter when filling. Then I circulated it a little longer but I think you ran yours for a day or two IIRC. I needed to get it up and running right away so I didn't run it that long but probably should have.