Piping radiant using a water heater

Radiant23

Hello. Saw this today and not sure if it would actually work well. It’s a multiple zone (12) system using 2 manifolds and multiple actuators to zone. The returns are tied together and tap into the low sidewall (recirc line) tap on the water heater. The guy put a pump on the cold and the hot, both pulling out of the tank. I’m pretty sure the dip tube was not pulled out being when both pumps ran (all zones calling) the pump pulling out of the cold was about 15 degrees cooler then the pump pulling out of the hot. Verified water running through all the loops with thermal flir camera, so I know it’s not air bound. 

Wanted to give info before question so you guys know I’m just not a random hack trying to figure it out. I just am genuinely curious if this actually works. I almost think it would being the tank is a large tank of water and pulling from the top at the cold and hot should not really matter. Also assuming the dip tube is removed from the cold. 

bburd

It makes no sense to me, and I think it would work only if the dip tube was removed. Otherwise the supply line pumping out of the cold side would short circuit from the return.

kcopp

is the return on the base of the tank?

Radiant23

It’s on the low sidewall. Pretty sure where the recirculating line would tie in if it were being used for a typical water heater install. That is right in my opinion. 

hot_rod

The dip tube would need to be removed.

Most water heaters have the dip tube built into the dielectric nipple that comes in the tank, these days. It looks like those factory nipples were removed and brass nipples used, so the dip tube is probably gone. Only one way to know for sure

12 loops on a standard 37,000 water heater? Is this a concrete slab radiant?
Whats the heat load of the building?

Radiant23

Do not know for sure but based on radiant plans there should be roughly 2,600’ of 1/2” alumapex on the manifolds total. That with roughly 50’ of 3/4 copper and 10’ of 1”. I would think the system could handle around 65,000 btu but only getting that 37,000 BRU an hour. 

This was installed roughly 12 years ago, I just walked on first time for service. Issues with a few loops not heating and client wanting thermostats not battery operated. Found a few of the plungers under actuators stuck so freed them up and everything working. No real complaint with performance or heat keeping up at this point that they told me, I did ask. 

It is above subfloor in Warmboard panels. I really was fixated on the two pumps out of the top of the water heater. Thought it was not a great idea, but then realized if dip tube is out of water heater, with two equal pumps, what would really hurt it. 

hot_rod

What does the label on the tank show for input btu? Take that times 70% to get the actual amount you have available

A common 37,000 water heater would give you around 21,000 btu/ hr

There are some higher output water heaters available, what size do you have?

two pumps on top are not an issue. Even if the dip tube is in one, it still circulates whatever temperature the bottom if the tank is at, or gets up to

Radiant23

Tank is 40 gallon, 40,000 BTU burner. Thanks for the note on the pumps on top. 

Radiant23

It is a Rheem power vented water heater. RHLNQ041340991

EdTheHeaterMan

I did one of those for a customer one time. After Hurricane Sandy, homes in Brigantine NJ were getting raised as much as 11 feet higher than they before the storm. That created a bunch of 11' ceiling "unusable" crawlspaces in town. This one customer wanted a low cost boiler to connect to the radiant tubing he was installing in the concrete floor of the "unusable" crawlspace. Somehow I believe his Mother-in-Law was going to live there because she could not handle the steps.

Perhaps he was just preparing his mother-in-law a place in hopes of another storm.

Turns out that a power vent water heater makes 110° water quite nicely, perfect temperature ...and it is much less expensive than a dedicated boiler.

Just so you know. The water heater was dedicated to the floor heat only, No potable water went into that appliance. No water from that tank was used for human consumption, and no animals were harmed in creating this post.

hot_rod

70- 75% is typical efficiency for a tank type heater, power venters may be an the higher end.,

40,000 X 75% = 30,000 btu/hr as a realistic output.

2600 feet of pex tubing on 12" centers would cover about 2600 square feet of space.

If the load is 11 btu/ sq ft or lower you are covered.

To know for sure you need a heat load calculation.

Usually what you find is the system will heat adequately until you get colder outdoor conditions. Then you may run of of output from the heater. If it runs non stop and doesn't heat the space, it is noit a pump issue, you don't have enough heater capacity.

If tank type water heaters they run below 100F for long periods they tend to start corroding from constant condensation inside, keep an eye on that.

Radiant23

Good info on the interior corrosion. I did not know that. I agree 100% the water heater size does not cover the load on coldest days. Being I was only there for a couple hours on first visit, they probably did not want to offer all the issues they have. I did notice they have supplemental wall electric heaters in a few rooms so for sure it is not enough.