hot water radiant heat with hot water heater, trouble with return

[Bob Keingstein]

I have a problem getting my system to work. I have a detached 2 story garage with radiant pex tubing in the concrete floor and base board heaters on the 2nd floor. I have cold water entering the garage on the first floor and the hot water heater (40 gallon electric unit, for safety and ease of installation, in spite of added cost to operate) is located on the second floor. The design is a series loop. Hot water is intended to leave the heater unit on the 2nd floor, go downstairs through the floor in the concrete and then back up to through the baseboards on the 2nd floor and back into the heater for reheating.

Here is the setup. Cold water supply line passes a shutoff valve, then a back flow preventer and then to the pressure reducer to 12psi and then into the cold supply inlet on the top of the heater. Just before the cold supply enters the unit, there is a monoflow T there with my return line/loop coming back in for reheating.

Now, from the hot water out side of the top of the heater, my pipe passes over the expansion tank as well as an air float vent, then to a shuttoff valve and then into the taco 1/25th HP crculator pump switched to a line air thermostat. Hot water is supposed to flow through the circuit and then it passes evenutally all the way back to another air expansion tank, air vent and into the monoflow T.

When I turn the circ pump on, hot water will get the inlet manifold to the concrete loop hot, the return manifold for the concrete floor gets warm and the baseboards get only slightly warm. Eventually, after running the pump for a half hour, the pipes all get cold and it won’t reheat.

I am in the process of adding another circ pump on the return just before the monoflow T and just after the return expansion tank and air vent. Before I do this last fix, I am curious if my design is fundamentally flawed. This is a closed system. No showers will be taken and no warm water drunk.

I have been thinking that the water in the loop has air in them and is thus creating a block for the water to return adequately. I figured the return pump would help to bleed the air out and to push the warm water back into the heater through the supply inlet. Any help would be appreciated. I know this might be an underpowered heat source, but if 40 gallons of 150 degree water comes back warm on the first pass it ought to be able to keep the flow warm on the reheating phase, but it doesn’t seem to provide a continuous flow of water for reheating.

[Harold KestenholzParticipant]

You have a lot of ‘magic’ in your description. There is nothing in your description that gives an idea of the load you are to heat, the size of the slab you are heating, whether the slab is insulated, the length of the baseboard elements upstairs, etc. In other words, you do not give a description of the heat sink. A garage floor would be 4 or more inches of mass that is usually uninsulated from the ground. A two-inch slab in a heated house on an insulated raised floor takes many hours to get to 70F.

People raised on warm air heat have no concept of what it takes to heat several tons of concrete. Your 30,000 btuh of electric heat would not heat the slab any faster if you attached a 350HP Chevy engine to a fire hose pumper.

[mielefParticipant]

I need to clarify the load. My concrete floor is a 22’x20′ slab 4″ think, which is insulated with 1.5″ blueboard. There are 3 baseboard elements, 8′ slant fin baseboards. I think the second pump should make a difference because the problem I think I have is that the water meets some resistance from air in the line. My logic is that if the water comes back warm on the first pass, it should maybe stabilize in temperature (if it’s recirculating) rather than go to very cold after a 90 minutes with the pump on. I like the analogy about the 350 chevy, I have a 427 center oiler under the hood in the garage, maybe that can be incorporated into this albatross. Does it seem plausible that air is blocking flow? I have a hosebib on the return pipe before the monoflow T on supply line back into the heater. If I run a hose out the window from the hose bib and put my thumb over the hose end, you can hear little popping sounds of air, but there has got to be a whole lot more in there! When I first filled the hot water heater tank with cold current, there was a long and continous hiss from the air float vent. I have not yet been able to purge a similar volume of air out of the loop circuits. I originally thought that the air would get forced back into the tank when I had the return reentering the heater through the bottom of the tank at the drain. Since I wasn’t convinced that the return water was getting back into the tank, I converted to the monoflow T with another expansion tank and vent on the return before the monoflow T. Still not purging the air and I believe that the single pump is not pushing all the water through. Any thoughts on whether a second pump will help maintain a fully bled supply of return water for reheating and redistributing?

[Harold KestenholzParticipant]

Your 22×20 slab is 440 square feet. If you spaced the tubing at 6-inch centers in one loop then your loop is over 800 feet long. If the tubing is 3/4-inch then you can’t expect to pump enough water through a loop 1/3 that long. Better get that 427 hooked up to a pumper or add maybe 4 pumps in series.

[Robert Stephen MortonParticipant]

I once sawn a 3/4 pipe laid along a footpath aprox 1 mile to a stop cock on a distant property & not a drop came out the end, yet there was 100psi at the meter. This bloke is talking about 12psi though 800ft.
Also when we used low pressure “Tank Pressure” we sometimes used to get air locks these were in in straight copper pipe. PEX pipe bends like crazy! maybe a jetter to purge would work?
Bob

[mielefParticipant]

so it sounds like the fundamentals of this system should work, it’s just going to take more pressure. As I mentioned, the water does flow back to the heater with one pump. I cannot ensure that it has enough pressure to overcome the resistance of the 12psi pressure on the other side of the monoflow T but when I have shut off the incoming line at the ball valve just before the Back flow preventer, to make the loop all the more closed, the pump should be able to recycle the water into the “cold in” riser. If I try to purge some water from the hose bib at the terminating end of the loop, there is flow, I think I have air trapped in the lines and until I can adequately purge it out, there will always be that resistance. If I have dedicated pumps at either end of the series loop, one pushing and one pulling, maybe between the 2 expansion tanks and 2 air bleeders, it may just purge the air after running the pumps for a few hours with cold water. Appreciate the help, not sure what a “jetter” is but if it provides an alternate way to purge air. Furthermore, if you go to http://cowan.bendnet.com/volcalc/CylinderH.htm you can calculate the gallons that would be in 622 ft of 5/8 pex tubing to be about 10 gallons, add the rest of my system as approximately half of that and round up and you still have about half of the 40 gallon capacity of the hot water heater, in the piping. The unit should be able to keep up. The water return should drop the total temp inside the heater unit by half or ((150+75)/2)= 112.5 degrees after one full rotation of the water. It should build from there on upwards at maybe 5 degrees per hour? but again my concern is on flow right now, and this is probably hindering any positive outcome.

[Harold KestenholzParticipant]

You are still trying to apply magic to the system – not sound physics. You are not just heating 60 or so gallons of water, which would take an hour to heat up with the electric element. The pex tubing is connected to the concrete slab (or it wouldn’t be considered a heating system, correct?), so the mass of the slab is equivalent in mass to another 600 gallons of water. This would take another three hours to heat up from 50 to 70F – IF the slab wassn’t losing heat from its surface at the same time.

The other defiance is that the PEX company wasn’t consulted as they would tell you not to make the length of a run more than 300 feet. The 800 feet should have been divided into three shorter circuits so an ordinary heating circulator can handle the flow.

Two expansion tanks in different locations with two circulators will confuse the pumping rates of the circulators and probably result in repeated pump failures.

According to the Bell&Gossett System Syzer, the friction loss per 100 feet of your tubing is 7 feet per 100 feet which is normal work for one circulator. You have 8 times that – or more than 56 feet of head; no circulator is made to overcome that pressure. It is a GOOD thing to know physics.

Go to the end of the slab where you know the circuit loops are and break out two loop ends approximately where the loops would be equal to making three loops and repipe.

Go to http://www.hydronicnetwork.net and read the free lesons on hydronic design so you can understand the process of developing a system without these errors.

[mielefParticipant]

I am planning on completing the installation of the second (sucker) pump. I am still unclear as to what physcially happens inside the loop with the one circulator pump pumping away from the heat source. Is the water returning back through the monoflow T or is the flow just running out of steam. It seams that if it just a flow being reduced to 0, then the water inside the heater still at the full rate of volume maintained from the cold supply from the town, should begin reheating. I have not found that to be the case. The return water comes back through all the piping lukewarm while the hot water just before the pump is still hot and after about 30 minutes, everything is ice cold, almost as though the water stops circulating. However that cannot be, if the pump is no longer capable of circulating the water, then the water in the tank should get hot after an hour. I understand there are some flaws to the setup but before I can start adding more modifications, I need to assure that there isn’t something preventing the water from running through the piping and being brought back in through the monflow. There must be a huge volume of air that is creating some kind of blockage in the line after the pump is run for a while. I could always hook up 2 more pumps to the manifolds before and after the loop on the pex tubing in the floor. Harold I emailed a few pictures to you to see the layout and the manifolds on the slab.

[Harold KestenholzParticipant]

The pump should be pumping away from the heater where the expansion tank is located. Circulating water will draw the temperature of the system down to the slab temperature and will rise as the slab is heated. As your flow is slowed down to .5 gpm due to the tubing resistance, there is plenty of time to cool on the way through the slab.

[mielefParticipant]

I buy everthing you’re saying but if I have a tremendous volume of air in the loop, water won’t circulate. When I initially filled the empty hot water heater, the air vented at the top of the expansion tank throught the air float vent. To date, I have not experienced a similar release of air from the lines when warm water was introduced to the loop back through the return. My initial thought was that the return loop would reintroduce air to the tank and purge it on the way back out but that never happened. Results improved substantially with the monoflow T instead of using the drain as a return. I believe a second pump could pull the water and trapped air through the system and out through the second vent, but I think you said that 2 expansion devices will confuse the pump rates. It might make more sense to move the second pump down to the floor of the garage to help it pull from the return manifold, as the trek straight uphill is likely the most difficult leg of the trip.

[Harold KestenholzParticipant]

Consider how a gravity hot water system heats. Where was the pump installed in a gravity hot water system and why? You really need to get by this magic.

[mielefParticipant]

so if i am going to install this second pump tomorrow, will I get a bigger bang for the buck by placing in on the first floor on the return from the manifold for the radiant heat loop to send the water back upstairs, rather than install the 2nd pump as a sucker which is looking like a wasted attempt. Clearly there are single story homes that have no basement that are circulating the heat through hot water and a circulator pump and likely with no elevation for gravity to aid in the traditional sense.

[Harold KestenholzParticipant]

It is becoming obvious to we dense people that you are not going to read the instructions for designing a system, so I will copy it to here from the free education site at http://64.226.150.132/hydronic/des/start.htm#Circulator
:

“Select a Circulator
Some pumps do not move enough water to carry the required btu through the heat distributing units. A pump must develop enough pressure to overcome the resistance (friction) created in the boiler, piping, fittings, heat distributing units and other parts of the system. Each pump manufacturer publishes charts that show how much pressure his pump will overcome. The charts are usually stated in terms of feet of head pressure, because it is possible to see how strong a pump is by observing how high the pump (even though it is used as a cirulator) will be able to raise water in open pipes.

The height of the heating system above the boiler does not change the head pressure rating required by the circulator. A sealed, filled system has all the interior parts under water. It is already filled to the top. A heating circulator does not have to raise water from one height to another. This is why a pump used in a sealed hydronic system is called a circulator. When making your selection, do not add the static head pressure (due to the height of the system) to the head pressure rating of the circulator.

Rating pump head Pressure
It is possible to test and rate a circulator by the flow it produces in a standardized circuit that includes typical pipe sizes. Most residential circuits incorporate 3/4 inch tubing and baseboard and are less than 250 feet in length. Ordinarily, such circuits have no more than 22 elbows and 2 valves, altogether producing a head pressure rating of less than 12 feet.

Some confusion exists because the circulators are rated for the purpose of raising water above the pump, even though they will be used for circulating water in a system that is already filled. Although circulators can be rated by using a laboratory circuit, there is not an economic benefit in doing so; hot water systems have been in use for a century without a demand for changing the current rating system.

Engineers pump water up lengths of open pipe to observe the gpm pouring over the top. Then they mark their observations on a graph.

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