Flow direction - baseboard heating

dpenny

I have a 12 yr old Rinnai tankless used for baseboard heating ONLY. Mild climate, 2 weeks of 25F weather. Did not work well this season. Am having hard time getting anyone to come to service it or replace.

So, I am first trying to understand what I have. Plse see attached diagram. How do I figure the flow direction in the ONE pipe? And is pump placed correctly?

_Rinnai Piping.pdf

jesmed1

Can you post some photos of the boiler and associated piping? The diagram doesn't make much sense.

You said the system is 12 years old. Did it ever work right?

dpenny

I've been here 6 yrs, and it seemed OK. We must have had colder seasons than this one.

Photo attached. Hope it + my drawing make sense now.

Rinnai1.JPG

jesmed1

@dpenny Here's my attempt to understand your photo.

We need to know where every supply branch goes and every return branch comes from, because without a full piping diagram it's impossible to know what all the possible flow paths are and which way the water "wants" to go.

1. Just before the pump, there's a tee where a return is coming from above, and another return is coming from under the slab. Where is the upper return coming from?
2. Just before the Rinnai, there's a tee where a return is coming from above, and the other return is coming from under the slab. Where us the upper return coming from?
3. The supply main goes off to the left out of frame, beyond the expansion tank. Where does it go?
4. The supply seems to branch off somewhere out of frame, then it reappears coming down the wall to the left of the Rinnai. Where is it coming from?
5. The supply seems to branch off into a tee at lower left, going horizontally out of frame, before it goes down into the slab. Where is that supply branch going?
6. If there is only one supply going down into the slab, why are there two returns coming out of the slab? You have to show all the piping and radiators "below the slab," otherwise there's no way to know what the likely pressure differentials are between those two returns.
7. Is there a circulator built into the Rinnai?
8. What controls the Grundfos circulator? Does it always run when the Rinnai is running, or are there some times when the Rinnai is running but the Grundfos is not?
9. Are there any other circulators not shown?
10. I don't see any air eliminators. Are there any?

dpenny

1st… THX for your detailed thoughts and annotations to my photo. My eariler "drawing" showed the key point/parts. The key point and Q is #6. Regardless of deltaPs below slab, the highest pressure is right after circulator, so therefore I think the flow is down into slab, with H2O at recirc temp, not Rinnai hot output temp. I am thinking the circulator is mis-place and has been for 12 yrs.

7. there is no circulator in Rinnai

8. Control: single theromostat (on/off) controls circulator thru AC relay. Flow turns on the Rinnai cuz it is a tankless domestic water heater.

10. there is one air eliminator to left of pressure vessel, and one way above your label "upper return" which is not a return, but air eliminator up in attic.

psb75

You are not providing adequate representation of this installation. The photo and your drawings are woefully inadequate for a proper diagnostic by those of us on the forum. Provide some more anecdotal operational history of this installation as well. It should be stated right off the bat that this Rinnai on-demand water heater is the wrong machine for this purpose. Has it ever been flushed? There don't seem to be any temperature or pressure gauges. Provide an electrical (control) schematic of the installation. How big is the slab (sq.ft)?

jesmed1

@dpenny I am not a heating pro, just homeowner/mechanical engineer. The pros here know a lot more than I do, so I would defer to their expertise, but here is what I see:

1. As @psb75 said, a tankless water heater is wrong for this application. But since it's there, you're stuck with it until it gets replaced. It's possible the heater has "scale" in the heat exchanger which is reducing its output. Google "tankless water heater descaling" to learn how to descale it.
2. Air elimination is wrong/inadequate. Optimum location for an air eliminator is near the heater output, where water is hottest. Typically the air eliminator is on a fitting at the expansion tank. With poor air elimination, it's possible the system is air-bound, causing poor circulation.
3. Pump is in the wrong place. Ideally it should be on the supply, just downstream of the expansion tank and air eliminator. Without seeing the entire piping/radiator diagram, there's no way of estimating the balance of flow in the two return lines you're asking about.
4. It looks like there's a swing check valve just to the right of the circulator. It's possible the swing check is stuck in a partially open position, obstructing flow.

Any or all of those could be reducing heat output and/or flow rate. There may be other problems that we can't see.

dpenny

THX to all for your comments. Given its 12 yrs old, I am going to replace it. The two who have come out (for svc) say they will ONLY replace with a true "boiler." This is BS in my opinion (and my checkbook agrees).

1. a tankless DHW worked OK for 10+ yrs
2. Rinnai makes a "solo home heating unit I-series." It is much less than a "boiler."
3. we have a "lite" winter here, 2-3 weeks of 20-26F.

Am probably going to design it myself (with help from a PE if I can find one), and then hire a $50/hr plumber to install it.

Tune in next heating season.

jesmed1

https://forum.heatinghelp.com/discussion/comment/1846454#Comment_1846454

OK, good luck. You can get some free advice from the pros here on a simple design if you want to start a new thread.

psb75

Rinnai DOES make good boilers. Get the right one of the right size. Have it installed correctly. And have all of the necessary devices for a proper hydronic radiant floor installation. What's not to love?

mattmia2

There isn't a lot of difference in price between a simple mod con boiler and a tankless water heater. There is a huge difference in performance.

EdTheHeaterMan

https://forum.heatinghelp.com/discussion/comment/1846454#Comment_1846454

"This is BS in my opinion (and my checkbook agrees)."

I have heard it said that "When spending too little on a job you will get what you pay for." If you need to replace a heater every 12 years with another lower cost heater then in 25 years you have purchased three heaters.

When the proper heater designed for space heating is used for this job, You may need only one heater that will last over 30 years. But that does not help you today (as did the previous heater installed by others). So I guess it all depends on how long you plan on owning this home. If you are just going to flop this house, then I understand. I just don't want to buy a home from you.

For example, I searched "Tankless water for space heat" and found on this site alone over 500 discussions like yours. And each one is having a problem related to a water heater designed for an open system being used on a closed system for space heating, and the problems associated with that setup.

I sampled discussions from 2002 to 2025 and found the one of the big problems is getting sufficient water flow to activate the burner, using the proper size circulator pump for that application. It seem that the foot per minute of the water flow, usually does not open the gas valve enough to get the highest fire capacity. Think of it like opening just one faucet just a little bit to wash your hands, but the water is not flowing fast enough to turn on the gas, so you open the hot water tap full in order to turn on the gas to heat the water.

Another problem is that these water heaters are designed to make the water flowing thru the system 60° to 80° hotter than the entering 45° cold water compared to the usual 20° temperature rise of a space heating system. and lastly the relief valve and expansion tank that would normally be used on a DHW open system is the wrong one for a closed system used in space heating radiator type systems. Just to name 3 problems that are a recurring theme here.

I have actually visited one of these home designed Tankless water heater used for space heating jobs a long time ago. I spent hours trying to solve the problem of no heat. Eventually I discovered that the system only worked for about a month when it was first installed, and there were problems getting that to work in the first place. Assuming that this was at one time a problem free operating system, i spent too much time diagnosing the system, only to discover that the water heater was no actually a boiler designed for space heating. S oafter two hours of wasted time I was able to collect my minimum $39.00 only to recommend that they need to purchase a boiler designed for space heating.

One successful water heater system I personally installed used a 40 gallon tank where I could set the water heater temperature to 125° and used a circulator pump to feed radiant floor heat tubing in a concrete basement floor. But that was not a tankless, there was a storage tank involved and the water heater was designed to maintain 125° water in the tank. That was an inexpensive boiler substitute that I knew would work. The operating efficiency was poor but the amount of space they were heating was insignificant, so it worked.

Just thoughts from an old guy that has already learned from the mistake you are about to make because I already did it years ago.

GGross

Hiring a $50/hr plumber will get you exactly what you pay for (an unlicensed handyman) good luck!

EdTheHeaterMan

I just heard on the news that D Penny cost 2 cents to mint and that there is a movement to stop making D penny. I hope that @dpenny is still around next year so we can hear the success story.