Low flow through heat exchanger

Krash

My system is not providing enough heat to the heat exchanger.

This is a split system with the heating side, and fresh water side separated by a heat exchanger. That heat exchanger is connected to a 199,000 A.O. Smith propane heater by about 3 feet of tubing. The heater requires about 15 degrees or more difference between the output and input temperatures or it shuts the heater off until the inlet temperature cools.

According to A.O. Smith, the heater should operate at between 2 and 4 gpm flow. I am only getting 1 gpm. This is not enough to keep the heat exchanger hot enough to feed the coolant side to heat the whole house. I need more flow.

The pump that I use for this short loop is a Taco 0015. This is a three speed pump, and when set on high, only produces 1 gpm. In my view, the pump is just too small.

I contacted Taco, and they recommended a 0011-F4, which I ordered. This is a much bigger pump being about 3-4 times the size of the 0015 I had installed.

I changed out the pump, and fired everything up again. No change!! Still only 1 gpm going through the heater.

Just to be sure the flow meter on the heater was accurate, I used an external pump I had to circulate thru the heater, it produced 2.3 gpm.

I am not sure where the problem is, and would appreciate any help.

EBEBRATT-Ed

pictures

hot_rod

A new installation, or new problem on an old install?
A flat plate HX?

Any y strainers in the piping loop?
If it is a system that has operated for some time, the plate heat exchangers can foul after time. You may need to run a cleaner through it.

Krash

Only one season on system, but that was isolated to just two rooms.

Hx has find of that helps. It rated at 125k BTUs.

Small strainer in heater, but flushed system out two months ago, and clean as a whistle. No debris of any kind came out.

Krash

Hx has fins!

Jamie Hall

I'd need to see how this thing is piped...

hot_rod

What size pump between the tank and HX. How do you know it is flowing 1 gpm?

LRCCBJ

Krash said:

Only one season on system, but that was isolated to just two rooms.

Hx has find of that helps. It rated at 125k BTUs.

Small strainer in heater, but flushed system out two months ago, and clean as a whistle. No debris of any kind came out.

You are attempting to send 199000 BTU into a HX that is rated at 125K BTU. The resistance in this HX is massive. Even the larger pump cannot make much of a difference.

If you have the details of the HX including headloss at various flow rates that would be helpful.

hot_rod

Here is an example of one of the free online HX sizers. You need a few pieces of information to properly, accurately size a heat exchanges

That 125,000 capacity needs to be defined a little better. A lot of the online sales have no idea how sizing works

Krash

OK, lots of questions to answer, so I will do my best.

First, here is a picture of the heat exchanger side of the system. In other words, the fresh water side.


Link to all the data on the heat exchanger.
https://www.supplyhouse.com/Bell-Gossett-BP400-20MT-125000-BTU-Hr-Hydronic-BPX-Brazed-Plate-Heat-Exchanger-With-Mounting-Tabs-12696000-p?utm_source=google_ad&utm_medium=shopping_neutral&utm_campaign=Shopping_Neutral_New_users&gad_source=1&gclid=Cj0KCQiAnrOtBhDIARIsAFsSe51ZlWuCMuWkwPtWD7hb9Y5pFcDpA0NrGTPKiIGX-WCc7BDU0VSpshgaAkCnEALw_wcB

Yes, there is a y strainer, but it is on the radiant side of the system, and should have no effect on the low flow problem.

Here is a link to the pump I have been using for the past 1.5 years.
https://www.supplyhouse.com/Taco-0015-MSF3-IFC-0015-3-Speed-Cast-Iron-Circulator-Integral-Flow-Check-1-20-HP

Here is a link to the much larger pump that Taco recommended.
https://www.supplyhouse.com/Taco-0011-F4-0011-Cast-Iron-Circulator-1-8-HP-1987000-p

I know the flow rate thru the heater because it has a built in flow meter. I have verified it is working properly by flowing water into a 5 gallon bucket.

I am not trying to send 199,000 BTU's into the system. I admit the system is oversize, but I have never required the heater to put out much at all. The house heat loss is approximately 55k BTU's, so that's about what is required by the hydronic side of the system. Then, there is also the hot water required by the house for showers', etc.

I am fairly certain the although oversized, that is not my problem. The heater will provide from 16k BTU's to 199k BTU's.

As a note, the HX specs says it will handle 6.5 GPM flow rate, and the heater requires from 2 to 4 GPM for radiant heating application. I just can not figure out why I am not achieving anywhere near those flows. Probably pilot error, but I can't see where the problem is.

I hope I answered everyone's questions.

Krash

Just to simplify my set-up, here is a diagram of the part of my system that I am having problems with.




1) What pump should I use to achieve a flow rate of 2 to 4 GPM?
2) IS there a problem with the flow being blocked/restricted by the HX?

Thanks to all.

Jamie Hall

Any idea what the total head loss int the heat exchanger and the water heater is? Water heaters of that sort usually have a very high head loss (relatively speaking, compared to heating boilers) and my thinking here is that you are trying to use a heating system circulator, which isn't built or intended for high head loss use.

Krash

I have no way to measure head loss. But I know it is easy to circulate at least 2.5 gpm through the heater because I have done that.

And the HX says it will flow 6gpm. There is only 3 feet of 3/4" between the two devices, so I am confused as to why the flow is so low.

Can anyone advise me on what these pumps will provide in gpm with no loss?

hot_rod

The installation manual will probably show the pressure drop of that tankless. It should be a stainless circ on the water heater side 

which model do you have

Krash

Zman

You have some unaccounted for head loss.My guess is that the boiler resistance curve is very steep. It is also possible that scaling has occurred in the boiler or HX.