Building a hydronic heating system using a tankless hot water heater

Newbie62

I got the wild idea off an internet site to build a hydronic heater for my garage. It is a sealed system using a radiator in the ceiling, tankless Ecotemp FV112 4 gpm on the wall, taco scoop w/valve, expansion tank and filling valves. I pressurize it with my garden hose and then close the fill valve. It's currently pressurized at 60 psi. I'm using a Watts hot water recirculation pump. There's about 3 gallons of water in the entire system. I have gas flow to the gas valve. The system turns on and runs normally if I have the bypass valve closed, (garden hose connected, dump valve open (essentially an open system). When it is in this mode the water heater turns on and runs just fine. However, when I close the fill valve, open the bypass valve and run the water as a closed system, it will run one short 4-5 second heating cycle and not come back on. If I close the bypass valve and then open it up immediately, the heater fires and does a 4-5 second cycle and shuts off. I know it is using a diaphragm with differential pressure to turn on. How do I adjust the valve to allow the same pressure on both sides? It has one screw that extends outside the cover inside a brass tube. There are two other screws side by side up on the bottom of the valve (LR). The attached PDF shows the valve. I have friend who thought I need a higher flow pump (the watts pump is a simple hot water recirculation pump) and to add a hot water storage tank with 5-10 gallons. What do you think is the answer? When it is running, it puts out a lot of hot air. Any ideas?

Solid_Fuel_Man

This is why boilers and water heaters are two completly different things. 

On demand water heaters have high head (read a lot more) restriction heat exchangers. 

The diaphragm flow switch may not be adjustable. 

Please use a boiler, and a relief valve of 30psi. High pressure hot water can be a scary thing...especially if any of the piping isnt iron/steel or copper. 

JohnNY

The only shot you've got at getting that to work is to create a primary-secondary piping configuration and get as much water through the tankless heater as quickly as you can. I'm guessing you'll need a Taco 0014 or similar for the primary loop, then you can use your little pump to bring the water to your heat emitter.

Jamie Hall

Some of those ideas off the internet are just plain terrifying...

JohnNY

Jamie Hall said:

Some of those ideas off the internet are just plain terrifying...

Shocking.

Ironman

If a tankless water heater could replace a boiler, there would be no need to make boilers.

Solid_Fuel_Man

And boilers are generally quite a bit more efficient. 

The money you saved using an on demenad water heater will be wasted by running and purchasing an energy hog pump and the associated piping. And the wasted gas from a lower efficiency.  Water heaters are designed for 50degree inlet temperature. 

jumper

Jamie Hall said:

Some of those ideas off the internet are just plain terrifying...

Water heaters enjoy much bigger delta temperature. You'll need a water heater rated for more btu s for hydronic heating.

motoguy128

Some inexpensive Combi Boilers aren’t that much more than a tankless. Depending what you have for Domestic water heating now, you could use a combi for DHW for the whole house AND heat the garage. We’ve done two installations like this in the last 2 years.

Ironman

I doubt that the OP will respond again after receiving news he didn't wanna hear. That's a shame because what was given was factual.

Having looked at the pics again, I noticed that the circulator is a Watts/Grundfos timed hot water recirc pump. I believe it's a UP15-35 that's used on that.

@Newbie62,
I wish you'd have come here first and gotten proper advice, but please don't let that cause you not to heed what's now been offered. There's no cheap way of getting around doing a radiant job right with the right equipment and design: the laws of physics are not influenced or changed by economics.

We're here if you want help.

Newbie62

True enough. I've been tied up with a lot of other projects and work. I do appreciate the right kind of help and want to make this a safe installation. So, here's where I am at right now. 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. 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. The watts 500800 seems to do fine for moving the water. Yes, I am new to all this, but I am interested in learning how to do it right. Unfortunately I didn't find you guys until I was well on my way into this project. So, I appreciate your patience and I do have a number of 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 me. I'm not ready to replace the hot water heater in my home and quite frankly, I can't justify much more than that. It has PVC exhaust which is a plus. I'm going straight through the exterior wall for venting.
2) Currently the taco air scoop and expansion tank are on the hot water side before the watts pump. One of you indicated that it would be better on the cold water side. Is that the best design? I'm assuming the expansion tank and air scoop still should be placed before the pump. Is that correct?

2) With 26 feet of 3/4 inch 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 be best? 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 producing lots of heat with the current Watts pump, seems to run nicely at around 150 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. I can sell the watts pump or install it in the house. I'm sure I can also sell the tankless heater I currently have. I want this to work right and be safe.

This is all new to me. I have done a fair amount of copper plumbing over the years and I am a capable carpenter, welder and auto mechanic. I like learning how to do things right. I appreciate your patience.

Newbie62

I neglected to mention that the system currently has a hot water pop off valve installed. I'm trying to do this safely.

mattmia2

Look at something like an htp uft or some of the lachinvar mod-con boilers, you will spend the little bit you save in the price of the tankless heater vs the boiler in setting up controls to make the tankles sort of act like a boiler.

mddack

Did you ever make this work with the Takagi? I have a the Takagi T-H3 you mention installed for water heating and would like to add a little bit of heat to a newly added insulated garage. I only need about 20000 btu but since the T-H3 is rated for space heating it seemed like a good option. There is a diagram in the manual for it as well. Does anyone have any recommendations for or against using the Takagi T-H3 for space and water heating?