Side loop tap low temperature for hydronic heat

[Kluger]

I have an older A.O. Smith GDHE-50 water heater that I've just recently hooked up to a hydronic heating system using the side-loop taps as part of a combination space heating/water heating application. This is in a very well insulated barn. Overall, it is working, but I've got a few questions about the performance of the water heater. I'd like to run the heater at a lower temperature with a wider differential, but I've got it cranked up at the moment to keep the building warm enough - it's 3°F outside right now!

1. With the heater set at 150°F, water temperature at a nearby tap is up to 145°F, as expected. Water temperature at the upper side loop fitting seems to range between 70°F and 85°F as the water heater cycles. I would expect it to be a bit lower than the top of the tank due to stratification, but this seems like an excessive drop. What temperature difference between the top outlet and the side loop is typically expected?

2. The differential is set at 5°F (I'd prefer higher). Measuring water at the tank top adjacent to the factory temperature probe (not the side loop) it ranges from ~145°F to ~169°F. Again, this seems excessive. Any ideas why this is near 25°F swing when the differential is set to 5°F? I don't need 5°F, but ~10°F would be much more reasonable. The 'kick-in' temperature is correct, but it seems to have a lot of overshoot. Or is this normal?

3. If higher temperatures are required (Prefer ~100°F) by the hydronic system does the system need to be fed from the top of the tank rather than the side loop?

Thanks for your thoughts!

 

[Fitter30]

Side coil piped in the bottom out the top? Can't find any info on line for btu output of side coil? Or at a certain water temp. Side coil want to see 10-15° diff. Service manual don't know if its the same programming page 49.

https://www.google.com/url?q=https://assets.hotwater.com/damroot/Original/10004/318556-000.pdf&sa=U&ved=2ahUKEwjXpOLb-4eLAxU0hIkEHbuONrsQFnoECAcQAg&usg=AOvVaw2WEKVnTTQzZTy8zAvonTu-

 

[Sponsor]

 

[Kluger]

Side coil piped in the bottom out the top? Can't find any info on line for btu output of side coil? Or at a certain water temp. Side coil want to see 10-15° diff. Service manual don't know if its the same programming page 49.

https://www.google.com/url?q=https://assets.hotwater.com/damroot/Original/10004/318556-000.pdf&sa=U&ved=2ahUKEwjXpOLb-4eLAxU0hIkEHbuONrsQFnoECAcQAg&usg=AOvVaw2WEKVnTTQzZTy8zAvonTu-

Thanks for the reply. The side loop is definitely using the top as source and the bottom as return. The water heater is rated at 100,000 Btu/hr but I've never found a rating specifically for the side loop and I've really crawled through their available documentation. I'll double-check calculations based on their stated recovery capacity to see if turns up anything interesting, but I'll be surprised. (Won't be the first time!) 10-15° drop from the top to the side outlet was around what I was expecting but it's way beyond that. What's bugging me is that it doesn't appear to be heating much of the volume of the tank anywhere near the setpoint. Still pondering. Is the 20° overshoot of the setpoint typical? Cheers, Dan

 

[Fitter30]

Thanks for the reply. The side loop is definitely using the top as source and the bottom as return. The water heater is rated at 100,000 Btu/hr but I've never found a rating specifically for the side loop and I've really crawled through their available documentation. I'll double-check calculations based on their stated recovery capacity to see if turns up anything interesting, but I'll be surprised. (Won't be the first time!) 10-15° drop from the top to the side outlet was around what I was expecting but it's way beyond that. What's bugging me is that it doesn't appear to be heating much of the volume of the tank anywhere near the setpoint. Still pondering. Is the 20° overshoot of the setpoint typical? Cheers, Dan

Here's a service manual page 19 under mixing valves. That 20° rise is pointed out when using small amount of water. Regular gas heaters can have the same problem. Mixing valves are not instantaneous especially with small amounts of water.

https://www.google.com/url?q=https://www.pexuniverse.com/uploads/docs/pdf/aos-gdhe-installation-instructions.pdf%3Fsrsltid%3DAfmBOorPxCfRxk8F8V4eEezG1SCcbqDn7ensNybF4fzuYdnHTVHwPCNB&sa=U&ved=2ahUKEwjso-zy6JiLAxVtMDQIHay7EqUQFnoECAEQAg&usg=AOvVaw1ZVZiWpo9YO8TRw3w1BKy9

 

[Kluger]

Interesting. I guess the overshoot is normal and it really isn't a big deal, I'd just prefer to get higher temps at the side tap. I wouldn't have thought of this application as "small hot water uses" as the pump supplying the hydronic system is often running for hours. There's no mixing valve on this setup as it wasn't expected to be necessary to run the heater so hot - and it's in a barn where I'm usually the only one around.

Part of all this is my learning how this all works in the real world so I can apply that information when I set up the basement system one of these years. Looks like it's better to take the hot water for the hydronic system from the top tap and return it to the bottom side tap.

Thanks for looking that up, I have read the manuals & I missed it. Wish they said something about the temperatures & btus available at the side tap.

Cheers, Dan

 

[Fitter30]

Never seen the btu rating or piping. Call the factory ask for tech with complete model and serial for rating. The higher the water temp the more minerals are driven out of water. Flush the tank at least twice a year might also want to change out the anode rod.

 

[John Gayewski]

Some of the terminology is getting messing me up here. Is the water that heats your floor the same water that is in the potable water or is there a heat exchanger?

 

[Reach4]

Some of the terminology is getting messing me up here. Is the water that heats your floor the same water that is in the potable water or is there a heat exchanger?

While I had been thinking potable water was being used for heat, https://www.hotwater.com/products/u...ct-vent-proline-xe/gdhe-50-300/100305404.html says

Helical coil internal heat exchanger lengthens heat transfer cycle and reduces lime & scale build-up to maintain high efficiency operation

 

[Kluger]

Some of the terminology is getting messing me up here. Is the water that heats your floor the same water that is in the potable water or is there a heat exchanger?

There is a double-wall SS heat exchanger between the water heater and the hydronic system. Only potable water in water heater.

When I return from upcoming trip I will try getting some specs from the factory. I probably should have started there.

Thanks

 

[Reach4]

There is a double-wall SS heat exchanger between the water heater and the hydronic system. Only potable water in water heater.

When I return from upcoming trip I will try getting some specs from the factory. I probably should have started there.

Are you saying there is an additional heat exchanger external to the water heater?

 

[John Gayewski]

A heat exchanger will not transfer all of the heat, but your 80 degree heating water seems low to me. If the tank is 145 then it seems to me you should be getting higher temps from fire heating loop. I would think 100 minimum.

Heat exchanger sizing is based on surface area, delta t, and gpm. There should be a table somewhere that has some info for sizing.

Do you have a guess at the heading load your shooting for in the garage?