Poorly performing tankless coil

Brad White_27

Just a gut-check:

1. Is there a nameplate rating on the coil?

2. Do you know the actual flow rate? Thinking here is that with two showers at the same time (6 GPM?) you might be double the flow rate of the heater so half the temperature rise.

3. You did say the water leaving the heater was so-so warm but not hot. Just wanted to verify if it was after a mixing valve (which you should probably have to prevent scalding).

Potential solution: Add storage. Get a Super-Stor Ultra or other insulated NSF rated tank and run a circulator round-trip between the tank and heater to charge the tank. Set the flow rate for an appropriate temperature rise (less is better in your case is my hunch).

Draw your showers and baths off of the tank which buys you time while the unit re-charges.

The above is a stop-gap measure to get you going with what investment you already have made.

Here is a better idea:
You can also get a Super-Stor or other tank with a heat exchanger coil inside of it. That is the one you want, even if you just do storage off of the tankless. In the future (after this heating season) you can pipe up the tank heat exchanger directly to the boiler as the first zone. A Tekmar or Taco control setup to give you domestic hot water priority and you and the Mrs. can take simultaneous showers again. Or save water and shower together :)


Best,

Brad

Jim Franklin_2

tankless coil has low GPM after replacement

We had the tankless coil in an old Peerless oil fired boiler replaced. The service tech said it was bad and since there was no hot water we believed him. Since then, we can't have two showers running simultaneously (both get ice cold immediately) or draw a bath in less than 15 minutes, both of which were possible beforehand. The boiler is firing while the bath is filling, and the water temp is 170, but I can leave my hand on the hot water pipe exiting the tankless coil so it's maybe 105? 110?.

Is there anything that could be wrong besides having the wrong coil installed, or the tech cutting out and patching a bad part of the existing coil? He did a crappy job sealing the coil and he had to come back, so I'm not too confident in his work...

thanks,
jim

ChasMan

Could it be...

On my tankless I dont have a mixing valve but I do have a loop with a valve on it that when opened allows cold water to mix with the hot. If left open the hot water comes out cold. I have to keep that valve closed. On mine I can have one shower but can not have a bath. FUBAR idea these tankless coils if you ask me.

Long Beach Ed

They Work for Me...

I have lots of luck with these in two family rental units. Yes, the boilers are a bit oversized and that makes the whole thing a little less than efficent, but they work well in 30 year old steam boilers.

Here's a few ideas.

If it a steam boiler, or hot water? If steam, make sure the water level covers the coil. There is a separate aquastat for the coil. This would be a low limit. Turn it up. I find I need at least 155 degree boiler water to get anything good out of the coil.

If there is a mixing valve to interject cold water into the coil output (and there should be), make sure it is set hot enough and that it wasn't fried when your Master Plumber soldered it.

Lastly, get those water-saving showerheads that transform a shower to something less than a mild fog. I drill and pin them so the tenants don't take 'em off.

Think of the water you'll save!

Long Beach Ed

Jim Franklin_2

The plate says Therma-Flow, 5 gpm, and the hand-test was before the mixing valve. Girlfriend's system so I have no history with it.

I don't know what you mean by the the actual flow rate...do you mean what we're using out of the tap? If so, I have no idea. The upstairs shower is no water saver, but the thing is, it never used to be a problem. 2 showers going for over an hour at a time (lots of guests needing to be up & out at the same time).

jim

ChasMan

Could be a lot of air in the system? Thats not an OEM heater but a universal replacement Perhaps Universal doesnt fit all.

Brad White_27

Flow rate

Yes, the flow rate at the shower. Take a bucket with one-gallon graduations.

(Use a Sharpie or Magic Marker and some bleach jugs in a pinch to "calibrate" a mop bucket; make a large measuring cup essentially).

Take a stop watch and time the flow in one minute. Do it for one then the other when both are running to compare.

If that exceeds the flow rate on the name plate (and it well may) you are on to something and may want to add storage.

Yes, it may not have been a problem before but it also was not the same exchanger for whatever reason.

Remember too, if the primary heater is 5 GPM and you draw more than that, you also might be diluting it at the mixing valve near the boiler and again at your shower valve (but I am sure you have that one pegged all the way to "hot").

A.J.

Flow rate

The flow rate is the amount of water you can put through your tankless coil in G.P.M. Find out your flow rate. If your trying to use two 2.5 G.P.M. showers off of one coil rated for 3 G.P.M. you know your in for a cold shower.
Also check the temp. going into the mixing valve, it should roughly be about the same temp. as the boiler.
If it isn't you have a bad valve.
Also you should have a flow restrictor that matches your coil capacity, this allows the water ot stay in the coil long enough to have the rated temp. rise.

Hope this helps

Guy_6

change

If the old coil ran 2 showers and the new does not, I would also suspect the change. I'm not saying that the tech did anything wrong, but perhaps they changed the limit setting on the boiler, or something happened to the mixing valve (debris from the work?).
That's where I would start.

Ken D.

Tankless

I would think a mixing valve, or even high water pressure. The faster the water goes through the coil, the less time it has to pick up heat. Perhaps the old coil had a flow restricter, such as a Dole valve or Watts P3.The addition of one might be the key. Perhaps you need to increase the temperature to compensate for heat losses of long pipe runs or piping through cold areas. We assume he piped right. If possible look for any piping that could allow the hot to mix with the cold water. Maybe there is a bypass (tempering) valve that is open too much or has a bad seat, allowing cold to bypass all the time. HTH