Tankless Garage Hot Water Heater Questions

[James Glenn]

I've designed a hot water heater in my garage that works well, but I'm wondering if the pump is under-powered for the application and if what I've got set up is safe.

I have 23 feet of 3/4 in copper pipe (inclusive of elbows using a conversion chart). I have 3 feet of galvanized pipe (all junctions between copper and galvanized have dielectric couplings on them). The connectors to the tankless HWH are 1/2in. I was able to bring the system to a balance by adding two 15 fin cast iron radiators in series (bought them from a guy who was tearing out the old hot water heating system in an old home). The system will run at 150 degrees at the HW outflow and 65 degrees on the inflow. I have a watts 500800 pum[p on it right now and it seems to do fine for moving the water (I can't find the GPM or head rating for this pump).

The tankless HWH has a mechanical temp adjustment on it. If I keep the temp and gas adjustments on the lowest settings after 25 minutes the system stabilizes at 155 degrees outputting a lot of heat from the ceiling mount blower and both floor mount radiators. My concern is one of safety. Is this safe? It's all either galvanized pipe or copper. I've been advised on another forum to ditch the tankless heater and get a boiler. Honestly, I can't justify the cost when I'm currently getting excellent heat out of my setup.

Here are a few questions:

1) The Takagi T-H3M-DV-N High Efficiency Indoor Tankless Condensing Water Heater (NG) is rated for recirculating systems. At least that's what the documentation says. At 672$ it appears to be the best option for replacing my current hot water supply if that's needed. I'm not ready to replace the hot water heater in my home and quite frankly, I can't justify much more cost than that for a garage heater (3 car). It has PVC exhaust which is a plus. I'm going straight through the exterior wall for venting.

2) Currently the system is set up with the taco air scoop and expansion tank on the hot water side right after it exits the heater and before the watts pump. I've heard that it would be better on the cold water side. Is that the best design? Is it worth changing the position? I'm assuming the expansion tank and air scoop still should be placed before the pump regardless.

2) With 21 feet of 3/4 inch pipe and 5 feet of 1/2 in. galvanized pipe, a 50,000 BTU rated ceiling radiator with fan and the two fifteen fin cast iron radiators on the floor, what size pump would work best? In looking at the charts it looks like the Taco 007 0r 008 would work. Is the Taco 008 the best option? I'm not sure what the BTU rating is for each of the cast iron radiators or what they add in terms of additional pipe length for the GPM or head loss calculations.

3) If the system is producing lots of heat with the current Watts pump, which as I mentioned, seems to run nicely at around 155 degrees at the hot side outflow and 65-70 on the inflow, is it a good idea to redesign? Is what I have potentially unsafe? Can it work more efficiently with a more powerful Taco pump or should I just start over with a better design (e.g., new Takagi unit and a new Taco pump to support it.

Thanks for your input. I live in Utah and the winters are cold.

 

[Fitter30]

Your watts pump has a max temp of 140*. watts pump is way to small for a boiler it's not moving enough water with a 90* difference between in and out. Taco 007 pump and expansion tank have a max temp of 230* your fine with pump being where it is. Takagi boiler will be grossly oversized what btu heater are you using now?

 

[Sponsor]

 

[Bannerman]

I'm not clear on what you are referring when asking about it being 'safe'.

The Tagaki web page specifies that model maybe utilized for hydronic heating, radiant floor systems and heating applications so it appears they consider it to be suitable to function as a boiler.

The appropriate flow rate appears to be between 6.5 GPM which will permit only 30℉ temperature rise max, to as low as 1.75 GPM to allow up to 140℉ temp rise through the heat exchanger. These ratings are while the unit is firing at the maximum rate (120,000 BTU input). Because it utilizes a modulating burner, the gas input will be automatically constantly variable to as low as 15,000 BTU as appropriate to deliver the desired temperature for the outflowing water.

Since you are utilizing cast iron radiators and iron piping, the water should not be used directly for DWH purposes. Fresh incoming water contains dissolved oxygen which will typically cause corrosion and breakdown of iron components so it would be best to maintain the system as a sealed closed loop and also utilize a scale and rust inhibitor.

Because the radiators and unit heater are within only a few feet of vertical rise/fall from the unit, the system pressure could be reduced to reduce pressure stress in the old radiators and unit heater. A hydronic heating system is often configured between 15-30 psi pressure and the Tagaki installation manual specifies the appropriate water pressure range is 15-150 psi so 20 psi should be sufficient.

 

[James Glenn]

Your watts pump has a max temp of 140*. watts pump is way to small for a boiler it's not moving enough water with a 90* difference between in and out. Taco 007 pump and expansion tank have a max temp of 230* your fine with pump being where it is. Takagi boiler will be grossly oversized what btu heater are you using now?

Thanks for the help. I went ahead and moved the expansion tank and location of the circulation pump. I ordered a Taco 008 pump and removed the watts pump. I've got the expansion tank and air scoop on the suction side of the circulation pump on the cold water inlet. I have a solid run of copper 3/4 pipe from the hot water outlet to the ceiling mounted radiator. The BTU rating on the tankless HWH I have now is rated 15,000 to 75,000 BTUs and appears to be more than adequate for the system.

 

[James Glenn]

I'm not clear on what you are referring when asking about it being 'safe'.

The Tagaki web page specifies that model maybe utilized for hydronic heating, radiant floor systems and heating applications so it appears they consider it to be suitable to function as a boiler.

The appropriate flow rate appears to be between 6.5 GPM which will permit only 30℉ temperature rise max, to as low as 1.75 GPM to allow up to 140℉ temp rise through the heat exchanger. These ratings are while the unit is firing at the maximum rate (120,000 BTU input). Because it utilizes a modulating burner, the gas input will be automatically constantly variable to as low as 15,000 BTU as appropriate to deliver the desired temperature for the outflowing water.

Since you are utilizing cast iron radiators and iron piping, the water should not be used directly for DWH purposes. Fresh incoming water contains dissolved oxygen which will typically cause corrosion and breakdown of iron components so it would be best to maintain the system as a sealed closed loop and also utilize a scale and rust inhibitor.

Because the radiators and unit heater are within only a few feet of vertical rise/fall from the unit, the system pressure could be reduced to reduce pressure stress in the old radiators and unit heater. A hydronic heating system is often configured between 15-30 psi pressure and the Tagaki installation manual specifies the appropriate water pressure range is 15-150 psi so 20 psi should be sufficient.

Thanks, that's helpful. I really do want to get this right.

 

[James Glenn]

Thanks for the help. I went ahead and moved the expansion tank and location of the circulation pump. I ordered a Taco 008 pump and removed the watts pump. I've got the expansion tank and air scoop on the suction side of the circulation pump on the cold water inlet. I have a solid run of copper 3/4 pipe from the hot water outlet to the ceiling mounted radiator. The BTU rating on the tankless HWH I have now is rated 15,000 to 75,000 BTUs and appears to be more than adequate for the system.

I had another expert recommend the 008. Is this going to be a mistake? I can probably contact SupplyHouse and get the 007 as it is the weekend and the other one probably won't ship until Monday.

 

[Fitter30]

Taco 008 pump runs at a higher head and a less gpm 14 verses 23 that 008 should be ok. Most condensing boilers are designed to run at 20* - 35* temp difference between supply and return high fire.

 

[James Glenn]

Taco 008 pump runs at a higher head and a less gpm 14 verses 23 that 008 should be ok. Most condensing boilers are designed to run at 20* - 35* temp difference between supply and return high fire.

Thanks. I'll let them know that the 008 is fine to ship. Thanks for the advice.

 

[James Glenn]

Thanks. I'll let them know that the 008 is fine to ship.


Thanks for the advice.

I just wanted to say thanks to all of you who gave me such great advice. I redesigned the flow and added the Taco 008. I tested and fired up the system yesterday and was amazed. It runs incredibly efficiently and I now have only a 50 degree drop between outflow and inflow. It may be smaller than that, but that's what the inline gauges are showing. I've got an infrared gauge and I'm going to test it gain today . It also holds the temperature setting perfectly allowing the system to run with the outflow water temp at 155 degrees and outputting a lot of wonderful heat. I can boost my garage from 40 degrees to 65 degrees in about 30 minutes. I have the system PSI at 30 just because the THWH needs 22 psi to fire. Once the system is warmed up it takes almost no time for it to get back to pumping out a lot of heat. The Taco recirculation pump is amazing--very quiet and efficient. Thanks for all the help.

 

[James Glenn]

Taco 008 pump runs at a higher head and a less gpm 14 verses 23 that 008 should be ok. Most condensing boilers are designed to run at 20* - 35* temp difference between supply and return high fire.

I just wanted to say thanks to all of you who gave me such great advice. I redesigned the flow and added the Taco 008. I tested and fired up the system yesterday and was amazed. It runs incredibly efficiently and I now have only a 50 degree drop between outflow and inflow. It may be smaller than that, but that's what the inline gauges are showing. I've got an infrared gauge and I'm going to test it gain today . It also holds the temperature setting perfectly allowing the system to run with the outflow water temp at 155 degrees and outputting a lot of wonderful heat. I can boost my garage from 40 degrees to 65 degrees in about 30 minutes. I have the system PSI at 30 just because the THWH needs 22 psi to fire. Once the system is warmed up it takes almost no time for it to get back to pumping out a lot of heat. The Taco recirculation pump is amazing--very quiet and efficient. Thanks for all the help.

 

[James Glenn]

I'm not clear on what you are referring when asking about it being 'safe'.

The Tagaki web page specifies that model maybe utilized for hydronic heating, radiant floor systems and heating applications so it appears they consider it to be suitable to function as a boiler.

The appropriate flow rate appears to be between 6.5 GPM which will permit only 30℉ temperature rise max, to as low as 1.75 GPM to allow up to 140℉ temp rise through the heat exchanger. These ratings are while the unit is firing at the maximum rate (120,000 BTU input). Because it utilizes a modulating burner, the gas input will be automatically constantly variable to as low as 15,000 BTU as appropriate to deliver the desired temperature for the outflowing water.

Since you are utilizing cast iron radiators and iron piping, the water should not be used directly for DWH purposes. Fresh incoming water contains dissolved oxygen which will typically cause corrosion and breakdown of iron components so it would be best to maintain the system as a sealed closed loop and also utilize a scale and rust inhibitor.

Because the radiators and unit heater are within only a few feet of vertical rise/fall from the unit, the system pressure could be reduced to reduce pressure stress in the old radiators and unit heater. A hydronic heating system is often configured between 15-30 psi pressure and the Tagaki installation manual specifies the appropriate water pressure range is 15-150 psi so 20 psi should be sufficient.

Thanks for all the info. I was able to make all the needed adjustments and the system is functioning very well. Nice to have it warm while I was working on my son's car yesterday.