How do I calculate the radiator flow of this monoflow loop system?

Jells · November 2022

I've built this system and I'm concerned I don't have enough BTU throughput using the monoflo loop system the mfr of the K120 Twin Flow kickspace heaters recommended in the datasheet. I have a 20 deg temp drop through the heaters, which seems like a lot and I think tells me I have slow moving water through the monoflow loops. Right now temps are in the mid to low 30s and it's calling about 13-14 hrs a day. I'm worried that it'll be maxed out when it gets down to the teens in a NJ cold snap.

So I'm wondering how to calculate the flow on this setup.

It's all 3/4 PEX from halfway down the WH, and this kind of fitting that I don't know the Cv for, it's not listed on Supplyhouse:

I used 3/4 monoflows on the return side of the loops with 3/4 sweat to 3/4 pex adapters. The feed tees are regular brass 3/4 pex. The pump is a Taco 006, there's 2 spring check valves, in addition to the kickspace heaters and like 13 ells in the main loop and 6 on each of the monoflo loops. The K120 datasheet says a friction loss of .43' @ 1gal and 3' @3gal.

If this were a simple copper baseboard loop I think I could calc the Cv and flow. But this has me baffled. I'm seeing 2 possibilities to add more heat throughput. Try a direct loop without monoflows, and/or add a 3rd heater in the kickspace of the cabs of the kitchen that's between the 2 rooms with already installed heaters.

FWIW my calculations show that theoretically the heaters should be able to throughput enough BTUs with 140 deg water to heat just fine. Identical 450 sq ft units with a WH and 22 ft of standard baseboard stay warm just fine.

EdTheHeaterMan · November 2022

20° is spot on!

Are you using that same water heater for potable water?
How are you going to keep the microorganisms from growing in those radiators when there is no flow for several months in the summer?

MonoFlow seems like a waste of time, I might have used a pair of home runs from the radiator to the boiler room and just do a parallel design. Only 2 rads only 1/2" copper or PEX with a Tee fitting to 3/4" would be just fine. This is not Rocket Surgery!
Do you have another boiler for space heating in the building?
IF NOT. I might recommend a brazed plate heat exchanger to keep the potable water from the heating system water. That would require 2 circulator pumps (one SS or Bronze on the DHW side and and one cast iron for the heat side) and a separate relief valve and expansion tank.

EdTheHeaterMan · November 2022

Just using rule of thumb, your design is requiring about 4.5 ft Head Pressure according the the diagram. If that pump os a Taco 006-B4 bronze pump for open system water, then you are moving about 5 GPM or circulating 50,000 BTU at a 20° temperature drop. But I believe that those radiators can only distribute 10,000 BTU each at 3 GPM. So to be sure of the numbers,
What circulator pump are you using?
What is the actual inside diameter of the pipe/tubing?

I believe you are using a pump with less GPM at 4.5 ft of head pressure.

Jells · November 2022

EdTheHeaterMan said:

20° is spot on! Are you using that same water heater for potable water? How are you going to keep the microorganisms from growing in those radiators when there is no flow for several months in the summer? MonoFlow seems like a waste of time, I might have used a pair of home runs from the radiator to the boiler room and just do a parallel design. Only 2 rads only 1/2" copper or PEX with a Tee fitting to 3/4" would be just fine. This is not Rocket Surgery! Do you have another boiler for space heating in the building? IF NOT. I might recommend a brazed plate heat exchanger to keep the potable water from the heating system water. That would require 2 circulator pumps (one SS or Bronze on the DHW side and and one cast iron for the heat side) and a separate relief valve and expansion tank.

Thanks. At this point changing to home runs would be a little problematic, I'd have to be pulling up some of the laminate flooring. You think 20° is just right? That might be fine if I was feeding it 180° water. Just worries me that the last legs of the fins are only getting water close to 120 and that's not going to throughput as much heat.

One idea I have kicking around is convert the far unit's monoflo loop to a straight through and measure the temp differential there. Less temperature drop would indicate more throughput. It happens that I don't have the sub floor covered up where that connection is.

The plan to avoid the legionella issue is a timer on the system that circulates water periodically in the off season. That's far easier and cheaper than the heat exchanger system. It could be daily but my research shows that probably 5 minutes of circulation weekly would be fine! Just got to replace the water, and no one contracts legionella after simply being away from their house for a week. But it could be daily with no issue, it's not like I'm going to heat up the apartment in 5 minutes.

Jells · November 2022

@EdTheHeaterMan Sorry, saw your 2nd after I posted. The pump is a 006-BC7-IFC. My caliper says 3/4 pex is 0.66" id, but all those those wide elbows are 0.575 id. Don't all the constrictive fittings enter into the calculation?

Yes, those K120s can put out 10k btu@140, the 24' of baseboards in similar units at 140 are radiating like 340/ft, or 8160 btu. This was why my confidence was high that I'd get enough heat from 2 K120s. I had >100% overkill!

EdTheHeaterMan · November 2022

So the rule of thumb pipe sizes are not to be used. your system is using a pipe that is in between 3/4" copper and 1/2" copper. my guestimate is more like 3.(?) something gallons per minute is flowing thru that 3/4" PEX. That is still 30000, BTUh @ 20° difference.

As I said in my first post. Try removing the Mono-flo Tees and make those heaters parallel supply and return to 3/4" copper tee fittings near the pump and water heater. The pressure drop across the Mono-flo system needs to be properly calculated. You may be getting 3 or 4 GPM though the loop but only .5 GPM or less actually going into the radiators.

What is the temperature drop from the circulator to the outlet to the Hot water feed to the loop? That will tell you how many BTUh is being used by that mini heating system.

And Again... Is that tank also being used for potable hot water (Aka: DHW) ?

Jamie Hall · November 2022

Is there any way that you can at least block the two bypass lines and convert that setup into a series loop? You'll be much happier and get a lot more heat out of it. Monoflow Ts are wonderful -- but they allow a substantial fraction of the water (usually more than half) to bypass. Though you may be getting decent flow in the loop as a whole, it's not getting to the heaters.

I won't comment on using a domestic water heater as a space heating source, and certainly not on using the same one without separation for domestic water -- except to say that if I were still inspecting buildings for occupancy, you'd never get a permit.

EdTheHeaterMan · November 2022

With the pump specification I can find the pump curve. Using the rule of thumb to calculate the pump head, I can determine the approximate GPM flow rate thru the monoflo loop. With the temperature drop at the supply and the return near the source (Top Left) I can calculate the flow rate. With that estimate and the pipe size inside diameter I can come up with a reasonable close BTUh estimate for the system. Then we can determine the approximate flow rate thru the mono-flo branches based on the ∆T of each radiator. That will give the BTUh of each radiator.

I believe your hunch is correct, you are not getting anywhere near 10,000 BTUh from those K120s

EdTheHeaterMan · November 2022

One idea I have kicking around is convert the far unit's monoflo loop to a straight through and measure the temp differential there. Less temperature drop would indicate more throughput. It happens that I don't have the sub floor covered up where that connection is.

Do That!

Jells · November 2022

Jamie Hall said:
Is there any way that you can at least block the two bypass lines and convert that setup into a series loop? You'll be much happier and get a lot more heat out of it. Monoflow Ts are wonderful -- but they allow a substantial fraction of the water (usually more than half) to bypass. Though you may be getting decent flow in the loop as a whole, it's not getting to the heaters.

I won't comment on using a domestic water heater as a space heating source, and certainly not on using the same one without separation for domestic water -- except to say that if I were still inspecting buildings for occupancy, you'd never get a permit.

Converting to a series loop was one of my ideas in my OP to get more heat. It sure seems like just not enough flow is being diverted. The mfr recommended the monoflos, and I was concerned that in a series the 2nd rad would be cooler. But it's not a huge project to convert the far rad to straight through and check the temp drop, that will tell me what the temp drop would be if I did series for both.

If I do it, and don't pull up the flooring already down near the WH, I'll end up with a dead end of pex a couple of feet long. Not good as far as the health thing, but some of this stuff is over the top. People have baths or slop sinks in big houses that don't get used for months! My regular licensed plumber who works all over this town scoffs at the legionella concerns. He's seen a lot of these units, recently replaced the WH on one of mine when I was busy with other stuff. I do not scoff at it, all my WH are set to 140 with tempering valves, and I'm happy to put these on a timer to not let the water stagnate, but the trouble and expense of the plate exchanger system seemed unnecessary.

EdTheHeaterMan · November 2022

This illustration shows a typical mono-flo design.

Each branch is allowing about 1/2 GPM flow off of the main perimeter loop. (as designed) My guess is that you are only getting 1/2 gallon per minute or less into each radiator (as designed) with leftover heat returning to the water heater.
Maybe you are getting 4000 to 5000 BTUh from each radiator if you are lucky.
You got to find a way to get rid of those mono-flos

Jells · November 2022

@EdTheHeaterMan I guess I'll get rid of the monoflos. I may try a hack to get rid of the unsanitary dead end by making an EPDM plug on a stick that I can shove the 2' to the tee that's now inaccessible, before plugging the end of the pex.

EBEBRATT-Ed · November 2022

Just know if you convert the far rad to straight through and leave the monoflows on the first rad the first rad won't heat at all. All the water will go to the far unit which is the path of low resistance

Jells · November 2022

EBEBRATT-Ed said:
Just know if you convert the far rad to straight through and leave the monoflows on the first rad the first rad won't heat at all. All the water will go to the far unit which is the path of low resistance

Sorry, but that doesn't actually sound right. The whole point of the monoflo is it doesn't interact with the other units on their own monoflow loops. Just one, or 12 like in your drawing, should not make much difference.

Actually, I have found the hard way that if one monoflo loop is way longer than the others it doesn't get as much heat. One of my rental properties had been owned by a family of plumbers and the ground unit had a 1" copper loop in the basement feeding a bunch of monoflow rad loops, and a monoflo loop for a 13'x25' extension that was always too cold. Partly it was because it had NO insulation, but partly the loop just wasn't enough flow. I took it off the loop and made it it's own zone by adding 2 zone valves.

EdTheHeaterMan · November 2022

You now, Primary secondary piping was invented (or discovered) by Gil Carlson in 1953 because someone did a poor mono-flo design. Gil was called to find out why all those B&G parts and pumps did not heat the building. In order to solve the problem with the bad design, he added circulator pumps to the branch circuits that were not getting the heat. Those "Secondary" pumps would pull (or push) the heat from the 200° loop that was going around the building from the boiler supply to the boiler return. @DanHolohan has a story about that. (Go Figure). That story is in this podcast https://heatinghelp.com/dead-men-tales/the-legacy-of-gil-carlson-hydronics-pioneer/

How do I calculate the radiator flow of this monoflow loop system?

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