Radiant loop off existing baseboard boiler

[GSSFC]

I have a Weil Mclain hot water oil boiler. It is set-up right now with a 4 zone baseboard system. The upper limit is set at 200*F and the lower limit set at 180*F.

I recently added a radiant system to the addition of the house. I have 4 loops (3) 225' and (1) 245' (slight miscalculation on my part!)

The issue I am having is getting the supply temp down to 120-125*F. I have utilized a tempering valve. The problem is the heat loss through the loops is so minimal, I cannot get the return water "cold" enough to mix with the supply water to get the desired supply temp.

I installed the pex 8" on center staple up under the plywood subfloor. Installed a reflective radiant barrier, and then closed cell spray foam insulation.

I have heard of utilizing a "cold loop" to the supply, but I don't see how that will work. I have a closed system, and would prefer to keep it that way!

Any help would be greatly appreciated.

Thanks, Tim

 

[trythis]

Are your underfloor tubes in contact with anything other than the foil shield? I woul dthink they would need to be in the snap in aluminum reflectors.
It sounds like you might have used bubble wrap or something like that with foil on one side. The radiant in pex pipes is contact radiant and if it is surrounded by a foil/bubble barrier with foam behind that, there isn't really anywhere for the heat to go except the small surface that touches the wood, which isn't really going to transfer much heat.
IF you used this kind of heat spreader, it should be transferring heat enough to affect the temperature.

Without something like that you are trying to heat the foam, which is mostly air.

I hate to say it, but if you arent losing heat in your loops, you have to fix that problem.

Could the pump be too fast? If you can slow the water down that might help.

 

[GSSFC]

The pex is stapled up to the underside of the subfloor with about a 1/4" gap between the pex and the subfloor (by design of the clips). The radiant barrier (foil faced bubble wrap) is installed with a 1.5" space between the pex and the bubble wrap leaving a total air space of about 2". The closed cell foam on top.

The pex is properly installed, and the insulation is done correctly (I did it!).

The problem I am having is getting the 180* boiler supply down to 125*F. Properly installed the temp differential between the supply and return manifolds in radiant system should only be about 10*F.

Thanks, Tim

 

[Friartuck]

Tim,
Do you have any form of transfer of heat from the Pex to the subfloor, as shown in the aluminum plate by RHT? I have seen both solid plates like the RHT and suspended air versions from RadianTech.

As for getting the temp down, don't they make a temp valve to mix or divert water that's too hot just for this problem??

Where did the 10 degree differential figure come from?? With such a low differential, that suggests to me that heat is NOT being delivered and extracted in the loops, hence a heat transfer issue.

 

[redsky49]

A couple of questions:

From your post, am I correct that you are firing the boiler when the temp drops below 180 degrees, and continuing firing until the temps reach 200 degrees? Is the burner control temperature being monitored from somewhere in the circulating loop or distribution manifold?

Where did you get the 180/200 degree design parameters? Was this from an actual heat loss calculation?

To get a handle on system capacity, and therefore flow rates, what is the PEX diameter and boiler volume?

Typical deltas for a heating system is generally 20 degrees. A 10 degree delta merely adds to the cost of the system.

Did you also design and install the hydronic system?

If you are using a 3-way tempering valve, is the valve marked for inlet and outlet? Can you describe how it is piped? Where is the sensor/control for the tempering valve? Have you confirmed that the valve actually cycles?

Probably have more questions but this will get us started.

 

[GSSFC]

Tim,
Do you have any form of transfer of heat from the Pex to the subfloor, as shown in the aluminum plate by RHT? I have seen both solid plates like the RHT and suspended air versions from RadianTech.

As for getting the temp down, don't they make a temp valve to mix or divert water that's too hot just for this problem??

Where did the 10 degree differential figure come from?? With such a low differential, that suggests to me that heat is NOT being delivered and extracted in the loops, hence a heat transfer issue.

No transfer plates. The dead air space created by the radiant barrier and spray foam are the "transfer". I've done this before hundreds of times and it works great, but I am not a plumber!

My issue is not whether or not the radiant system works, it is how to get 180* water down to 125*

Thanks, Tim

 

[GSSFC]

A couple of questions:

From your post, am I correct that you are firing the boiler when the temp drops below 180 degrees, and continuing firing until the temps reach 200 degrees? Is the burner control temperature being monitored from somewhere in the circulating loop or distribution manifold?

***The burner control temperature is being monitored at the boiler via a thermostat. The control is done through the aquastat. ***

Where did you get the 180/200 degree design parameters? Was this from an actual heat loss calculation?

***I am in the building trade, so a friend who is an HVAC contractor for 20+ years, recommended the 180/200. It works great with the baseboard setup in the rest of the house. Not sure how he arrived at that setting.***

To get a handle on system capacity, and therefore flow rates, what is the PEX diameter and boiler volume?

***The pex diameter is 1/2" inside diameter. How would I determine the boiler volume?***

Typical deltas for a heating system is generally 20 degrees. A 10 degree delta merely adds to the cost of the system.

***I was quoting a design spec I had read while I was researching the system. I cannot get a grasp on the actual heat loss from supply to return, because I cannot get the desired supply temperature.***

Did you also design and install the hydronic system?

***I did install and design, hence my "problem".***

If you are using a 3-way tempering valve, is the valve marked for inlet and outlet? Can you describe how it is piped? Where is the sensor/control for the tempering valve? Have you confirmed that the valve actually cycles?

***I am using a Taco 3/4" thermostatic tempering valve designed for heating. The supply from the boiler is piped into the "Hot" side, the return line going back to the boiler is tee'd with the tee going into the "Cold" side, and the "Mix" side is piped into the supply manifold. The problem is the 180* water is too hot, and there isn't a supply of water cold enough to temper the water to 125* as desired.***


Probably have more questions but this will get us started.

Thanks, Tim

 

[trythis]

Perhaps you could use a water chiller coil. I have one on my system and use it to heat the air if the floor is a little behind. It also is my AC coil with my water to water ground source system.
Put a fan (air handler style) behind it and use it to throw heat into the garage or someplace with COLD air for the intake. That should cool the water down.
The problem would be piping to the coil, it would have to be metal. Then drop that water into a storage tank that would keep the cold water for use when the pex loops have demand.

 

[HoosierBuddy]

Seeing as how you've done this hundreds of times, and it's worked there's probably nothing I can tell you.

But...what you are running into is exactly what I ran in to with my 1 installation that was installed exactly as you are describing. I couldn't get enough heat transfer from the PEX to the floor to warm the room. My "fix" was to use the aluminum plates similar to the ones shown above, and use 4" fiberglass insulation in combination with FBF to futher insulate under the PEX.

I'm running my water at 160 degrees and it performs adequately (barely). It actually performs pretty well down to about 15 degrees outside temp (55-degrees differential) then it can't keep up. That is an improvement of 25 degrees differential from the same setup with clips and no plates.

Phil

 

[trythis]

another thought in terms of your mixing:
Use a separate storage tank for the pex loops. Give it a separate aquastat that pulls water from the boiler tank as needed. Keep its temperature at a 120 to 140 diff there and supply the pex from that water only, not directly from the boiler.

Air is a terrible heat sink and just cant transfer heat very well. Put your hand 1 inch from a stove eye: hot, but not painful. touch it and the heat transfers quite well. Pex needs surface contact to transfer heat, otherwise people would suspend it in foam when pouring concrete over it.

 

[72Tunaboat]

If convenient, would you post a picture of you boiler and the piping?I would like to see how you configured all of it.

If you piped it correctly, it should work, unless the tempering valve is faulty.

Thanks
Roger

 

[GSSFC]

I understand the concern about heat transfer, and the performance of the radiant etc, but that is not the issue.

My question however, is how to get 180* water down to 125* without a cold water supply to temper the water.

In a domestic heating situation, the hot water off the boiler is tempered with the incoming ground water from the well to get 120* hot tap water. I don't have or desire to use cold water from the well to get the desired temperature.

I'll have to look into a cold water coil as someone mentioned.

Thanks, Tim

 

[z28snksknr]

To get the return water cooler, you would either have to increase the heat transfer (more area, more conduction through srufaces rather than convection through air), or lower the flow rate through the system so that the hot water has more time to transfer the heat.

A gate valve on the supply or return side to throttle the flow to the radiant section would control this adequately (ball valves aren't good for flow control unless it's a V-ball). Lower flow = less energy in the system with the same heat transfer rate out = lower outlet temps. Then just use the cool return water to temper the hot supply water. You'll have to play around with setpoints of of the gate valve and wait for the system to stabilize to see how it works (lowering the flow lowers the return temp which will lower the supply, which lower the return, etc. until it reaches steady state)

I'm not all that informed about radiant heat installations, but I know A LOT about heat transfer (I design heat exchangers),

 

[trythis]

It sounds like you want a second tank. That way you can run the pex off the same water, but it would be regulated separate loop. I am not suggesting using new water here, just recycling the boiler water in and out of tank2 to adjust the temperature. Use the boiler's water as the heater for the second tank.
Have an inlet and outlet from tank2 that have a solenoid on one pipe and one way valves on both that connect to the boilers tank in a circular flow (might need a small pump here.) Open the solenoid when the water in tank2 is too cold, close it when it is hot enough via separate aquastat form the main boiler's. The floor pex is now running at whatever temperature you set it's aquastat to.

The water in the pex loop will not need a mixing valve this way, tank2 with its own aquastat would do that job for you. Even if the pex loop never loses any heat at all to the floor, (impossible of course) it will never go above 125 if you set tank2's aquastat to 125.

 

[nate379]

Try this, that is what I just posted as well and then realized to posted the same exact thing I said pretty much.

Maybe an indirect fired water heater using the DHW ports to run to the pex. T Stat calls for heat and pump on that line runs the water through. It starts getting cold and the aquastat calls for boiler heat and would kick on pump #2 to heat the water in the water heater.

Maybe there are better ways to do that, just my thoughts on the little HVAC and plumbing knowledge I have.

 

[sneezer41]

A true primary secondary would have a pump on the secondary loop.
Do you have a pump?

A three way mixing valve would then open and shut to give the 'exact' temp you need. The pump then runs and circulates the water within the loop to heat the floor. I imagine in a steady state it draws a little hot water constantly from the main loop.

If you are not pumping the secondary loop then it is never going to work right. I suppose you could use diverter T's with a simple thermo valve on the output and it would shut down the loop at high temps, but I dunno if it would be effective

Other things.
No way you need 200 degree water. Lower the temp on the boiler until you start seeing the room not maintain temp on a very cold night. My guess is that you could run it as low as 160 and still get adequate performance.
Your new radiant floor is helping out now.

Consider outdoor reset. Then you can run between say 150 and 200 and it will always be right.

The system of hanging tube is the least effective way to do radiant. Sure it works, but not is not very efficient. Staple up with plates is better, direct under floor the best. I did sleepers under hardwood in my last house, and 125 degree water heated great in any weather[actually too high in warmer weather] I am only slightly south of you.

 

[mikefromme]

http://www.taco-hvac.com/en/product...0%28XPB%29/products.html?current_category=345

This is a nice off the shelf solution for your situation. It can obviously be done with individual components.

If you download the instruction sheet, page 2 has many sample diagrams -- find the one that most closely matches your system and adapt it to work with what you have

There is also a couple videos that help explain different systems and how they work pretty well.

 

[trythis]

I don't see how using any sort of mixing valve on its own will drop the pex loop temps down below the boilers minimum when it is 40 degrees above the temp he wants in the pex loops. It either has to be isolated, or have fresh cold water that he doesn't want to introduce.

 

[72Tunaboat]

I don't see how using any sort of mixing valve on its own will drop the pex loop temps down below the boilers minimum when it is 40 degrees above the temp he wants in the pex loops. It either has to be isolated, or have fresh cold water that he doesn't want to introduce.

If the mixing valve determines the water is too hot, it will just recycle the return water ONLY, until the water is to the set temp. Pretty simple.....

 

[72Tunaboat]

http://www.taco-hvac.com/en/product...0%28XPB%29/products.html?current_category=345

This is a nice off the shelf solution for your situation. It can obviously be done with individual components.

If you download the instruction sheet, page 2 has many sample diagrams -- find the one that most closely matches your system and adapt it to work with what you have

There is also a couple videos that help explain different systems and how they work pretty well.

This is good idea. I have installed a few of these on retrofits. They work well.

 

[trythis]

If the mixing valve determines the water is too hot, it will just recycle the return water ONLY until the water is to the set temp. Pretty simple.....

Gotcha, same thing I am talking about, but in a small package. I just assumed that if you open the valve, the water at the valve would instantly get too hot and end up shuttering back and forth.

 

[GSSFC]

If the mixing valve determines the water is too hot, it will just recycle the return water ONLY, until the water is to the set temp. Pretty simple.....

Now that makes sense!

I do need to install a pump between the mixing valve and the supply manifold, that is one of the errors I realized I had made, but wanted to address the temperature situation before I did anything else.

Thanks, Tim

 

[72Tunaboat]

Now that makes sense!

I do need to install a pump between the mixing valve and the supply manifold, that is one of the errors I realized I had made, but wanted to address the temperature situation before I did anything else.

Thanks, Tim

There is a pump between the mixing valve and the supply manifold on ALL hydronic applications that require a mixing/tempering valve. It won't work (properly)without it.

I hope you get it sorted out.

 

[GSSFC]

There is a pump between the mixing valve and the supply manifold on ALL hydronic applications that require a mixing/tempering valve. It won't work (properly)without it.

I hope you get it sorted out.

Yes I know, I haven't even filled the new radiant system yet, I have been picking away at the job for the last two months.

Thanks for all the help and advice guys!

Tim