Water to Air Heat Exchanger in Furnace problems

[dirtyedge]

We currently set up our CB eclassic 2300 and the unit is working great. I have one set of 1 1/4" Thermopex and another set of 1" thermopex running 150' to the house, with Taco 014 pumps in the house. Currently the pump on the 1 1/4" pex is running to two furnaces 140K btu and 100K btu heat exchangers.

The water coming in is extremely hot and the heat exchangers take about 40-60 deg out of the water, but the air in the plenum and the air coming out of the registers is only mid 80 deg F.

What are others seeing for temps out of their registers or plenum?

The OWB has no problem keeping up and only has to be filled once a day but the house will only heat to mid 60's right now and the current outside air temp is 0 deg F.

Any ideas what the problem could be?

Thanks for the help,
Aaron

 

[Slick]

Maybe air trapped in the heat exhanger? Are the exhangers the highest point in the system? My shop exchanger is the highest point in my system, have a boiler drain on the high side (return line), just crack it open to bleed out some air.

 

[Laird]

No expert here having just finished my install over the weekend, but I damn near forgot to take off one of the protective pieces of cardboard on my heat exchanger when I installed it. Any air would have had to go around it rather than through. Not a likely source of your problem but since I almost did it.........:monkey:

 

[dirtyedge]

I know that the air is passing throught the heat exchanger because the line coming out is a lot colder than the one going in, and the way the exchangers were installed the air has no choice but to go straight through.

I will have to try and purge the system tonight and see if that helps. It still seems like with the size line and pump that I am running it should not cool the water down that much, and should heat the air more efficiently.

Thanks again,

 

[zeliespecwriter]

This is just a guess, but......I am assuming your heat exchanger is in the plenum of a forced air oil or gas fired furnace. If so, check your flue damper leading to the chimney. It may be allowing cold air to rush down your chimney into the furnace cooling your exchanger. Just a thought.

 

[oneoldbanjo]

A big temperature drop between the OWB inlet and outlet of the heat exchanger would be an indication of low flow. (If only a small amount of water is passing through your heat exchanger there is very little energy/heat to be exchanged). The low flow could be a result of air trapped in the exchanger - you want the flow into the heat exchanger to be in the lowest port and the flow out in the highest port so that the air can be purged out of the exchanger by the incoming water - if you have the flow coming in the high port it cannot push the air out.

 

[Hawken]

I just finished my install of a CB 5036 with a 149 btu water to air HX. It took a good 3 days to purge all of the air out of my exchanger and when it did it nearly roasted me out of the house. The heat exchanger is on my furnace in the basement of the house.

Make sure your hot water going in is on the bottom fitting of the HX and the discharge is on the top.

Good luck and let us know how it turns out.

 

[dirtyedge]

My water inlet is on the bottom and outlet is on the top. My current setup has a T from the supply to the inlet on each HX then the outlets go out to ball valves then to a T and return to the boiler. The ball valves were just in case we had to add more or less back pressure to one or the other HX depeding on size, but so far they both seem to flow the same amount.

Tonight I am going to add some valves and boiler drains, at the high point in the basement so I can try and purge the air out. I have boiler drains after the pumps so I should be able to flush water with a hose through the bottom and out the top one until there is no more water. (Hopefully)

Thanks for all the pointers so far, any other ideas or opinions are greatly appreciated.

 

[Slick]

Actually you shouldn't need the garden hose to purge it, if you can get a boiler drain at the high point you can just keep the pump running and crack the valve, in my shop that valve is on a T at the output of my exchanger. When I crack that valve I hear it leak air then gurgle until water comes out slowly.

 

[Hawken]

You should have the HX running in series not parallel. In one HX and out then over to the next in and out. The water is going to travel the path of least resistance. If you have the lines t'd so that the HX are parallel then the water is just going to flow by the HX and around the loop. After the air gets purged out of the HX you should not have a 40 to 60 degree drop in water temp. The water is moving fast enough that you should only get maybe a 10 degree drop from the entire loop in and out.

One guy out where I live has his loop hit a side pipe water to water HX, then a air HX then another water HX for infloor radiant in basement and finally to a air HX in a pole barn and he only looses about 10 to 15 degrees.

 

[Slick]

Even if he's parallel and T'd...how would the water bypass anything? It would still have to go through the heat exchangers.....it could possibly prefer to go through one more than the other (which is why he installed ball valves on the outputs) but how can the water get back to the boiler without going through and exhanger? Sounds like he has his house setup just like my shop...two exchangers, T'd with balls on the outputs...I sized one way to large so I throttle it down with the valve....with both wide open I can loose 20 degrees, less with one throttled down.

I've seen it in series like you drew, I just didn't like the thought of the second exchanger getting colder water.

 

[iCreek]

What are others seeing for temps out of their registers or plenum?

Aaron

My buddy had a similar problem with his new OWB install, the water returning to stove was really cooled down. He ended up having a 'bad' new pump.

My temps are around 120 - 125 depending on the location of the register. OWB set at 170.

 

[Hawken]

The lines in the HX are smaller like 3/8 or 1/4 inch. the water has to be forced to flow through them. A ball valve would work but it has to be closed off enough so that the water takes the easier path through the HX. If the water is not forced through his HX the pressure from the air in the lines might be enough to keep the hot water from flowing freely through his HX.

If the valves are open then the water is just going to flow past through the 1" or 1 1/4" lines.

With a series set up the water when forced through a smaller opening speeds up. The water moves through so fast that it only spends a little time in the HX. The water just looses a little fraction of heat but because of the volume and speed of the water passing through it, it adds up to a great deal of heat transfered.

Its kinda like if you have a small leak in a garden hose. If the nozzle is closed even a small leak will spray out, but if you open the nozzle up you might not see the leak at all.

 

[Laird]

Does the amount of heat increase from one heat exchanger if you close the valve to the other? How do you know if you have the same flow rate through both? I run three HX's in series and get plenty of heat out of the 3rd one, but I'm not sure that is your problem.

 

[Geez]

If they are parallel then in theory each would only have half the pump's volume going through each, right? I would think that would give the same result as when I tried to run my Grundfos on low instead of medium to my shop. Only took a few minutes and the air coming through the heat exchanger was ice cold. Moved it back up to medium and it was just fine. So I assume the flow was giving up it's heat faster than the pump could replenish it.
Now that I think about it, would it be advantageous in my case to run the pump on high? Maybe that would increase the temp at the ducts? Would that be more efficient or no?
Not trying to hijack this thread. Just thinking it may have some bearing on the OP's problem as well and help us understand the process at work here.
Just what are some of the pros and cons of increasing/decreasing the volume of flow?:monkey:

 

[ericjeeper]

slow your blower speed down

Sounds to me like the furnace blower is running on the cool side of the blower. for heat you need the slowest setting.Might have to move a wire on the control box.

 

[mtfallsmikey]

Again...

1. Bleed!...Isolation/blowdown valves should be installed...that is is only way to thoroughly purge air out of anything.
2. Slow down the fan
3. If you pump thru HX's, or anything else in series, when the water gets to the last device, it WILL be a lot cooler.

I use/used primary/secondary piping for my OWB, separate circ. for HX loops in house and shop. I get 130 deg. air with 180 deg. water. High Delta T (differential) temp drops in the primary loop are eliminated. Get Dan Holohan's book "Pumping Away"....www.heatinghelp.com
Also...look at the FAQ's/Resources section of the website.

 

[oneoldbanjo]

I got thinking about this last night.....and although I believe the exchangers run in parallel should work.....the flow being split between them may be dropping the flow velocity through the exchangers enough that the trapped air cannot be forced out.

I would suggest that you close one of the exchangers off completely and see how it operates alone. After a day or so of it operating with the other one turned off - open it up and close the other one off.

This would hopefully incease the volume and flow enough that any trapped air could be purged.

 

[dirtyedge]

I replumbed the HX's to run in series again, last night. I think a main part of my heat problem is air trapped in the lines.

Which leads to my next question, I have 150' of 1 1/4" thermopex feeding a taco 014 pump to 1" pex to both HX's in the furnaces in my basement, is there any chance that this pump is not getting the amount water that it needs from the boiler?

I also have a 150' of 1" thermopex feeding a second taco 014 that goes into 1in pex that feeds a 30plate water to water HX for a indoor pool. When trying purge the air out of this line I put a hose on the boiler drain which is at the high point in the system and closed the return valve off and the water comes out of the hose very slowly, maybe a gallon every 1 min. These pumps are supposed to flow 32gpm.

The furnace side flows faster than this but still is not as fast as what I would think it should.

When purging the air can I just leave the boiler drain open? I did that last night and sometimes the system would run for a couple of minutes before water trickles out. Before I switched the HX's back to running in series I shut one valve so all the water was running through one HX and I was seeing 125deg air out of the plenum but now I with both in series I only have 95-105deg air out of both of them.

I guess for now I will try the techniques mentioned in previous posts and see what I can do.

 

[Hawken]

You should have plenty of water from a 1 1/4 line if the pump is below the boiler tank. If the tank is level or above you should have the pump on the boiler.

Someone wrote about slowing your fan speed. That is probably a good idea. I installed a 8 inch booster fan in duct in the farthest room from the HX. Even when the furnace fan is not on the little fan keeps pulling warm air through the duct. It is just enough to keep the duct air warm so when the furnace fan kicks on it is instant heat.

What do you have your boiler temp set at? I started out at 180 degrees but was going through tons of wood. I have set it back to 172 and am going through much less wood and still getting plenty of heat.

Where is your cold air return air coming from? Once the air hears up in the house the furnace will be recycling warmer air. 50 or 60 degree air is going to take a lot more energy from your HX than 60 to 70 air. Once you get everything purged and running for a while you should get warmer and warmer air at your registers.