Radiant heat - where does the mixin' come from?

[Telco]

I've been reading a lot of the posts on radiant heating, and I've got to admit you guys are selling me on doing it instead of a forced air hydronic system, mainly due to the lower temps the radiant flooring uses. The hydronic forced air setup will require 160 or better to be effective, but the floor system only uses 100 or so, meaning my water will last a lot longer for heating the house. But, one thing I've seen time and again is the use of a mixing valve to cool the temps down so as to not melt the lines. Where does this mixing water come from, and where does it go? If you have to mix cooler water with hotter water to get, say 100 degrees into the pex loops, where do you get the cooler water from? Once it's pumped through the PEX loop, where does it go? In a standard house a mixing valve just uses cold water from the cold water line to mix water, then it comes out the faucet and does not reenter the potable water system again. But on a hydronic system, these are basically closed loop systems, no water added or removed. Since this is the case there must be a closed source of cool water in the system along with a method of cooling it. Where does this cool water come into play? If it's coming out of the faucet, so to speak, then the pex drains into the ground, I could see how it works but not the attraction to using such a water guzzling system. I hope I've explained question clearly enough. Thanks.

 

[webbie]

Ah, the mixing valve.......

https://www.hearth.com/econtent/index.php/wiki/4_Way_Mixing_Valve/

Consider the figure 8 mentioned in the article.
The circ is on the house side of the 8, so it pumps this relatively cool water to and from the radiation. As you can see, that would make the water very cool - like room temp and falling from there! So what happens is that a small amount of that water is shunted through the other side of the 8 (the boiler) and then returned (mixed) with the relatively cool house side.

So there is no cool source except the house loop, which WOULD be cold if you just pumped it around itself without it hitting the boiler

 

[Willman]

Where does this mixing water come from, and where does it go?

On a 3 way valve the cool water is the return from a zone. If your input temp is 100* then it looses 20* circulating thru your loop the returning water @ 80* will be injected into valve. Return temps will vary due to outside temp and any solar gain on floor. My one floor loop runs a long time without boiler coming on.
Will

 

[BrownianHeatingTech]

On a 3 way valve the cool water is the return from a zone. If your input temp is 100* then it looses 20* circulating thru your loop the returning water @ 80* will be injected into valve. Return temps will vary due to outside temp and any solar gain on floor. My one floor loop runs a long time without boiler coming on.

Just FYI, a radiant loop's flow rate should be designed for a 10-degree drop, not 20.

Joe

 

[Telco]

OK, so let's see if I have this clear then. You have a 3 way valve, which has hot water in, heating loop in, and heating loop out? I'd think there would need to be some way to return water back to the storage tank then, like a 4 way valve. That I can see working. So basically the same water is circulated through the loop over and over, with the mixing valve diverting just enough hot water into the loop to maintain the temp, and sending the difference back to the storage tank then. Wow, this would be even more efficient than I thought it was.

Are all the returns on the loops for multiple zones diverted into individual mixing valves, or is it a manifold type thing? Was thinking that if I'm going to do radiant heat, then it'll be in for a penny in for a pound, would have to do radiant in the garage. Driving to work in the morning would be a LOT more comfortable if the car were already warmed up inside, not to mention that if the car started in a 70 degree room, with the engine warmed up to 70 degrees, then it would be in closed O2 loop that much sooner.

Thanks folks, not near ready to build yet, but every day gets me closer.

 

[BrownianHeatingTech]

OK, so let's see if I have this clear then. You have a 3 way valve, which has hot water in, heating loop in, and heating loop out? I'd think there would need to be some way to return water back to the storage tank then, like a 4 way valve. That I can see working.

The return coming back form the loop goes back to the heat source (storage tank or boiler) - it has a tee installed so that the mixing valve can draw some water out as-needed.

The circulator for the radiant loop is installed so that the mixing valve is before its inlet. It draws water through the valve, and the internal valve element (electric or thermostatic, depending on the valve) adjusts the proportion of hot (from boiler or tank) and cool (returning from radiant loop) water that gets through.

So basically the same water is circulated through the loop over and over, with the mixing valve diverting just enough hot water into the loop to maintain the temp, and sending the difference back to the storage tank then. Wow, this would be even more efficient than I thought it was.

Yup. Radiant just injects a bit of hot water into the loop on occasion, proportional to the heat loss. The cooler your radiant design water temperature, the more efficient your system (and the longer you can run on a storage tank). Sometimes we add more tubing simple to drop the temperature to improve efficiency, for folks who are willing to make the up-front investment.

Are all the returns on the loops for multiple zones diverted into individual mixing valves, or is it a manifold type thing? Was thinking that if I'm going to do radiant heat, then it'll be in for a penny in for a pound, would have to do radiant in the garage. Driving to work in the morning would be a LOT more comfortable if the car were already warmed up inside, not to mention that if the car started in a 70 degree room, with the engine warmed up to 70 degrees, then it would be in closed O2 loop that much sooner.

Every radiant design water temperature needs its own mixing setup. If you can design a whole building's radiant layout so that all the areas use the same design water temperature, then it doesn't matter how many zones you split it into. That can be a big area for cost savings - if you just use a radiant design program and never actually think about what's going on, you may end up with four zones with four water temperatures, that are all within 15 degrees of each other. You may be able to get them within just a couple degrees of each other (thereby eliminating the need for four separate mixing setups) by simply adjusting tube spacing or flow rates very slightly.

Joe

 

[Telco]

Excellent, thanks. I think I'm sold then. But, I'll wait to ask more questions when the time comes. Appreciate the info.

 

[headrc]

I talked with Dave at Cozy yeserday about the mixing valve and application. He actually recommended thast on radiant heat in a slab there is no need for a mixing valve but due to issues of heating and damaging a wood floor you then need a mixing valve ...any thoughts on this? RH

 

[BrownianHeatingTech]

I talked with Dave at Cozy yeserday about the mixing valve and application. He actually recommended thast on radiant heat in a slab there is no need for a mixing valve but due to issues of heating and damaging a wood floor you then need a mixing valve ...any thoughts on this? RH

A mixing device is needed on all radiant systems.

Has it worked without one? Yes. I've serviced a system that was done pumping 180-degree water into copper pipes in the slab. Works like a charm.

Has it completely destroyed a slab by heat stress? Many times. Particularly if you don't give it several years to cure, first.

Occasionally, it can work without a mixing device. But it is more likely to cause cracking than it is to work correctly. For the relatively-small cost of a mixing valve (which will also improve your efficiency), it's not worth taking the risk.

I mean, a mixing valve and a few fittings are easily going to be under $200. For a couple portraits of Benjamin Franklin, it's not worth risking tens of thousands in damage.

Joe

 

[slowzuki]

One thing that Joe didn't mention, in case we're talking non-wood,

If you have a single zone radiant, a mod-con boiler can serve without a mixing valve and works very well when designed correctly. There are lots of mistakes that can be made in one of these setups starting right from the sizing. If you undersize a condensing boiler or you setup the controls using bang-on bang off you will end up pushing it into non-condensing operation and get none of the benefits of the condensing hx. A load calc and pro should be consulted.

 

[BrownianHeatingTech]

One thing that Joe didn't mention, in case we're talking non-wood,

If you have a single zone radiant, a mod-con boiler can serve without a mixing valve and works very well when designed correctly. There are lots of mistakes that can be made in one of these setups starting right from the sizing. If you undersize a condensing boiler or you setup the controls using bang-on bang off you will end up pushing it into non-condensing operation and get none of the benefits of the condensing hx. A load calc and pro should be consulted.

I didn't forget. I don't suggest going without a mixing setup, because you will still need higher water temperatures to produce domestic hot water (either through a plate heat exchanger or an indirect tank). Under those conditions, the mixing setup is required, even if the boiler is ordinarily capable of modulating itself to produce the desired water temperature. I can't recall that I've ever installed (or even seen) a mod-con in a house being used for heat only.

Sizing the mixing device in that case is critical. As you said, there's no point in springing for a condensing boiler if it won't even condense because your piping system will never allow it.

Joe

 

[slowzuki]

Even if you operate a mod-con up in the low domestic hotwater range you're loosing efficiency, 140 F is not condensing, as you say, save your money. All installations I've seen have been large single zone units with separate domestic water heating(ie not in houses). Up here houses pretty much use forced air furnaces, very little hydronic except in the last few years. Most contractors putting electric boilers in for hydronic Don't understand the buyers of spec housing sometimes. At least its easy to change over.

The one thing you'll see them do is install a high efficiency mod con to get the efficiency number for their energy audit rebates, despite the fact it will never operate at that point in its curve. Worth it to get the grants etc for building the "efficient" homes. Drives me nuts seeing a 4000 ft2 "eco" home with energy use double a 1000 ft2 conventional home with the same design number of occupants.

 

[BrownianHeatingTech]

Even if you operate a mod-con up in the low domestic hotwater range you're loosing efficiency, 140 F is not condensing.

I've never used a mod-con that didn't have a separate terminal for setpoint/DHW demands. When the heat is running, it uses outdoor reset. When the DHW or other setpoint demand kicks in, only then does it ramp up to 180.

All installations I've seen have been large single zone units with separate domestic water heating(ie not in houses). Up here houses pretty much use forced air furnaces, very little hydronic except in the last few years. Most contractors putting electric boilers in for hydronic Don't understand the buyers of spec housing sometimes. At least its easy to change over.

Yeah, that would change things a bit. We're mostly hydronic, here. Of course, there are guys installing mod-con boilers on jobs using convective baseboard. Probably actually condenses on two days out of the whole year... For the same money, we can install a conventional (non-condensing) boiler and add electronic heat management, which will actually deliver some fuel savings. But that would require actually thinking about things instead of waving a high AFUE number under the customer's nose, and not bothering to explain that they won't be getting anything like that in terms of actual fuel savings. Drives me nuts, sometimes, how unethical some contractors can be.

Okay, I'm done ranting about that, now

Joe

 

[mpilihp]

Hi im just trying to educate myself on radiant heat and mixing valves, I build a garage and had heat tubing installed in case I ever wanted to heat the garage which Im thinking of doing but want to understand the mixing valve setup. I understand whats been stated about how its plumbed and how it controls the water flow but what I dont get is how it KNOWS when to open and close. Where is it getting the info on the temp of the supply water and know to add return water to it to cool it down? Do you have to put aquastats in line and is there a special controller to operate the mixing valve? Or is the mixing valve SMART enough on its own and have a temp setting on it directly?

THanks

~ Phil

 

[BrownianHeatingTech]

Hi im just trying to educate myself on radiant heat and mixing valves, I build a garage and had heat tubing installed in case I ever wanted to heat the garage which Im thinking of doing but want to understand the mixing valve setup. I understand whats been stated about how its plumbed and how it controls the water flow but what I dont get is how it KNOWS when to open and close. Where is it getting the info on the temp of the supply water and know to add return water to it to cool it down? Do you have to put aquastats in line and is there a special controller to operate the mixing valve? Or is the mixing valve SMART enough on its own and have a temp setting on it directly?

A thermostatic mixing valve has a mechanical element in it (similar to the thermostat in your car's engine) that diverts the right amount of hot and cool water into the loop, to create the set temperature.

An electronic mixing setup (either an electronic valve or an injection pump) uses a small microprocessor control with a temp sensor that you strap onto the side of the pipe. It controls the valve or pump based upon that sensor, and often will also have an outdoor sensor so that it can vary the water supply temp based upon the outdoor temp. Tekmar makes a few of these, as do Erie and a few others. Some manufacturers of valves and pumps incorporate the electronic control right into the valve or pump's control box, so you have an "all in one" solution.

Joe

 

[mpilihp]

HI Joe thanks, I get the three-way mixer but how about the 4-way, appears its still just for MIXING for one zone but is continuously diverting some zone cold return water to the boiler and some back into the zone and the boiler is putting more/less hot water into the zone as it is needed. Appears the only difference is this allows for sending a controlled or somewhat controlled temp water back into the boiler correct? is this so you are not injecting very cold wanter into the boiler to prevent condensation?

Reason Im asking is Eric I think mentioned that with the Marathon addon wood boiler Im working on installing has a failure point in that it condensates at the return port and rusts which caused his to fail.

THanks

~ Phil

 

[BrownianHeatingTech]

HI Joe thanks, I get the three-way mixer but how about the 4-way, appears its still just for MIXING for one zone but is continuously diverting some zone cold return water to the boiler and some back into the zone and the boiler is putting more/less hot water into the zone as it is needed. Appears the only difference is this allows for sending a controlled or somewhat controlled temp water back into the boiler correct? is this so you are not injecting very cold wanter into the boiler to prevent condensation?

Reason Im asking is Eric I think mentioned that with the Marathon addon wood boiler Im working on installing has a failure point in that it condensates at the return port and rusts which caused his to fail.

A 4-way is similar to a three-way. It cycles to blend some of each on the supply, and also handles the returns at the same time.

No matter what mixing setup you use, you need to add some form of return temperature protection to keep the return water above 140 degrees.

Joe