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Daox 10-09-12 01:07 PM

Low head loss hydronic floor heating
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We have an excellent thread on DIY hydronic floor heating here. Admittedly I haven't read all of it, but there is tons of good information in there.

One topic I haven't seen come up in the discussion much is minimizing head loss in a hydronic system. I'm curious why low head loss hydronic heating isn't a larger topic. I realize that hydronics requires much less energy to move its heat around vs a conventional gas furnace, but it seems like it can be further optimized. I see many images of people heating a room with a single loop of 1/2" pex as illustrated below:

However, if one were to route the plumbing in many parallel lines, the head loss would be significantly reduced.

In a properly designed system, the two hydronic floors would both recieve the same flow rate, same thermal transfer and heating abililty. However, the second floor would require a substancially smaller pump to achieve the same flow rate. This means less electricity consumed and more efficient operation.

I think that the only downside here would be all the tees and connections you'd need to make within the floor.

Lets take a look at an example to see what kind of gains could be made. In this example, we'll look at putting hydronic heating into a 10'x10' room.

With a single run of 1/2" pex with 6" spacing, this would require about 170 feet of pex. At a flow rate of 1 gpm (total guess, sorry, I haven't done any calculations for hydronic floor flow rates yet, it might be too high or too low), we'll have a pressure head of 6.6 inches of water.

With a parallel setup of 1/2" pex with 6" spacing, this would also require 170 feet of pex. However, now we have a 1/2" headers on each end of the room and 18 parallel run lines of 1/2" pex. We would like to keep that 1 gpm flow rate. This means we have 1 gpm through the headers, and .055 gpm through each parallel line. The pressure head of this system turns out to be .39 inches of water for the 9 or so feet of header tubing. Pressure head for the parallel lines is a bit more tricky because the flow rate is so low (1gpm / 18 lines = .055 gpm). I haven't been able to find any head loss charts that go anywhere near this low. However, I took the 1 gpm rating and divided it by 18 to get a rough idea (actual head loss will be lower) which gives us .23 for the parallel lines. You then add these together for a whopping .62 inches of water.

.62 inches of water vs 6.6 inches of water to do the exact same thing to the exact same room. Add in a bunch of tees and crimp/cinch clamps.

Besides having trouble finding a pump small enough to give us the required flow rate at this rediculously low pressure head, does anyone else see any major problems? Ideas and thoughts are quite welcome.

mephistopheles 10-09-12 03:39 PM

All those tee fittings will increase the required head significantly through friction losses. I don't have a Crane or Cameron book that includes PEX but I would use 1.38 or so for the K value of each tee fitting. You also run the risk of short-circuiting your heat transfer if for some reason there is a flow obstruction or something to absorb more heat on one side of the room than the other (such as a rug covering the middle).

Daox 10-09-12 03:51 PM

Welcome to the site mephistopheles!

Excellent point. The tees will definitely cause a restriction. I had not thought of that. I would imagine we'd still be well under the 6.62 inches of water of the initial setup. I'll try to find a crane catalog and see what all the tees would introduce. However, we could fairly easily fix this by enlarging the header pipes if it were an issue. You would just have to use reducing tees.

Flow obstruction IMO is a non issue. If you have something clogging your pipe in the standard version you simply have no heat/flow at all. At least with the parallel setup you would have some heat. But, in either case you would still want to flush the system and try to get it out.

Another alternative would be to reduce the number of parallel lines. Instead of 18 parallel lines perhaps you reduce that down to 9 or 6 and just make some small loops of those to make up for it.

Daox 10-09-12 04:10 PM

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A quick google search came up with this chart:

from this PDF

Lets use the above parallel example of 18 parallel lines and using 1/2" pex for everything. It is going to have to flow through the tee as a run for 7 fittings, and as a branch for 2 fittings. That gives us an effective additional length of:

16 * 2.2 = 35.2ft
2 * 10.4 = 20.8ft

Or a total of 56 extra feet.

56 extra feet at 1 gpm gives us an additional pressure head of 2.3 inches of water. That would bring the total pressure head up to just about 2.9 inches of water even. Still quite a bit less than the 6.6 of the standard setup.

mephistopheles 10-09-12 04:54 PM

The way I would calculate that would be 34 tee fittings and 2 elbows to get 18 parallel pipes between two headers of equal size. That's 34*2.2+34*10.4+9.4*2. That's 447.2 additional feet. If we assume laminar flow (since we're at speculating a low flow rate) we could half that and assume that layout will cost us about the same as pumping 223 feet of straight pipe. Fittings add up in a hurry!

Daox 10-09-12 05:18 PM

I was definitely wrong with my initial calculations! I have since revised them. I've come up with an additional 2.3 inches of water for a total of about 2.9 inches of water.

I must say I do not agree with your method of calculation. You're counting the fittings as if they're all in series, and you're even counting them twice. A single molecule of water passing through the system is not going to flow through every single fitting. It should only flow through 18 fittings, 16 straight and 2 elbow. That is why you see a benefit from running parallel pumbing lines.

ecomodded 10-10-12 01:24 AM

All connector restrictions in your diagram will be elevated by the many new paths the water has to flow threw, there is also more pipe on the floor in this design so it would increase the head, the water weight would be more of a concern then the flow restriction as the flow rate and resistance will be lowered threw each pipe. I think.

brian 10-10-12 02:40 AM

this is the same system as ice rinks. there is usually a shut-off valve at each return location, usually every year or so at start-up most returns are shut off to keep all pipes pressurized and a main header drain is opened or the air eliminator in the system will remove air which becomes lodged in one of the pipes. Usually most hydronic system will have some air migrate into the pipes through the non-heating season. At the initial start-up the (sounds like a river in my pipes) air will be pushed into the expansion tank. The more parallel runs, the more chance of cold spots.

mephistopheles 10-10-12 09:20 AM

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If you had an indirect return I would agree with your calculations. With a direct return flow, however, you will have short-circuiting of the system and all the flow will go through the shortest branch; to keep that from happening you would need to assume the system will operate as a series flow system.

You will develop cold spots overtime in parallel flow systems--of that there is no (cheap/easy) cure!

Daox 10-10-12 09:40 AM

Why are assuming that the branches aren't all the same length? I would think that it wouldn't be very difficult at all to design the system to have equal length parallel lines.

Why do you say that you'll develop cold spots over time?

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