I'm having trouble with my tankless radiant floor heating system

Hal1

I've been having an issue with my tankless water heater that I use to heat my floor. Recently my thermostat calls for heat for zone 1 or all 3 zones but the heater doesn't turn on. 

Ironman

We really need more info about the details of the system. Can you post some pics?

Are you actually using a tankless water heater as the heat source?

EdTheHeaterMan

If you are using a tankless water heater for the heat source, you have a problem. They are not designed for that. You can add some controls and aquastats to force the system to do space heating, but you are asking for failure on a regular basis.

The circulator pump on a "Closed System" does not make the water move as fast as it would when you open a hot water faucet and let the hot water fill a tub or go down the drain with an "Open System." On most water heaters the entering water temperature is in the 50°to 60° range and comes out at 110° to 120°. That is a 60° temperature difference. On a closed system, the temperature difference between the cold in @ about 95° to the heated water out at about 115° can be only 10° to a maximum of 20°. Two different control systems are employed to operate a space heating system and a potable water system.

You got the wrong one... and I know "It has worked fine until now"

How come all of a sudden it failed? All of a sudden the wrong heater was installed and it caught up to you.

Sorry for being so blunt, but this is the reality. Now if you post some pictures of the heater, the piping, and the controls so we can all see what you have, we may be able to get you up and running. Someone did have it running before so there is no reason why we can't get you running again. The pictures should be from far enough back so we can see the pumps and fittings that are connected to the heater. from several angles. like "ceiling to floor"

Respectfully submitted,
Mr.Ed

Hal1

The system was designed by The Radiant Company aka Warmtoes and had 2 Takagi tankless water heaters, one the TK just for domestic hot water at 199k btu & the other the TK-JR, a 140k btu for the radiant floor system. The JR worked fine for 10 years but needed TLC cleaning and parts but still word fine until 2 years ago when it **** the bed and found out that AO Smith bought Takagi and I got the exact match for the JR and it worked fine until this heating season. The TK is still going strong without issues.

I spoke to my cousin who is a master plumber and retired P&H teacher at a technical school and he said it was probably the controller. He suggested I talk to the manufacturer rep and online Q&A boards, so here I am.

I have an emails in to the Radiant Company & to AO Smith and posting to online boards like this one. I have also swapped out a circulator pump for zone 3 which got that one back on track a couple years ago and swapping another today for zone 1. Even though the Grundfos pumps were designed by my radiant heat company they were not rated for the 167 degree temperature which the system produces but 140 degree max. The identical pump replacement is now rated for 203 degrees. They produce a pressure of 0.8 gpm which the system requires 0.75 to activate which might be part of the problem but it doesn't explain why when all 3 zones call for heat and the pressure going into the heater is 1.1 gpm it still doesn't activate.

As I said, got any ideas what could be wrong with it?

Hal1

TJ-JR on left & TK on right.

Hal1

Ironman

This issue of using a tankless for space heating comes up on here almost weekly. And it’s almost always the same. As Ed explained, a tankless is not designed, controlled or approved for space heating. The resistance to flow through its heat exchanger is way too high for standard hydronic circulators. A hydronic circulator like a Grundfos ups15-58 creates about a 5 psi pressure differential; the tankless is designed for 60 psi coming into it and an open faucet on the other side.

The problem is almost always a flow issue. Have you checked the inlet strainer?

If you keep asking on every forum that you can find, your gonna end up being confused by the multiplicity of conflicting answers you get.

Think about it: if a tankless could take the place of a boiler, there would be no need to make boilers.

Again, if you’ll post some pics of your setup, we could better give you some assistance.

Jamie Hall

You're asking that poor tankless to produce 167 F water? Egad...

Could I ask what the return temperature to it is? The reason for asking is that at least some tankless units have a maximum inlet temperature, and yours may be objecting to your return water. Some also have a maximum outlet temperature -- which is usually 140 F (as required by the newer codes for water heaters -- but not boilers) and that may be part of the problem, too.

Hal1

I replaced the left Zone 1 pump and the pressure kicked up to 1.3 gpm and the heater is back heating my basement slab. Thanks for your help.

Ironman

To help clarify:  gpm is the flow rate, not the pressure. Pressure can, and should, exist even when there’s no flow in a system. It exerts force equally in every direction. Flow is in one direction (normally) and its volume is measured in gpm. Head is the resistance to flow in the system generally from friction as water moves through the system. This is called dynamic head which is not to be confused with static head. Static head has to do with the height in a column of water.

toeknee

Controller says supply temperature is 167 but in the picture of the circulators, the gauge in the bottom right looks like its displaying over 180 degrees, probably 190. And the return is reading 64 on both the controller and the gauge in the picture. Sounds like no flow through the tankless

Hal1

Thanks for all your help but the new pump fixed my issue. The flow rate required to start the burner is now achieved and my issue is over.