TRV with radiant heat

Midnightsnacker41 · September 19

Can TRV be used with in floor heating? Where do you install it? I'm imagining the TRV for each zone would be in the boiler room, but they would need to know the temp for the zone they are controlling.

Are there TRVs that have a temp sensor on a wire? Or some other remote sensor? Or do you need to run a tube in the wall so that the TRV can attach to it?

I'm working on installing radiant heat in my house. I would like to have micro zones so that the temp in each one is fairly tightly controlled.

DCContrarian · September 19

Put a thermostat in each room and connect it to a zone valve. It will cost less and work better. TRV's are for retrofits.

Jamie Hall · September 19

This ^^^.

But, while you are thinking, remember that radiant floor heat responds only very slowly to changes in setpoint…

DCContrarian · September 19

That's why you want your heat emitters to have as little heat capacity as possible. The lower the heat capacity the more quickly they can respond to changes in either setpoint or heating load. A major selling point of systems like WarmBoard is that they're "highly responsive" — they have low heat capacity.

Midnightsnacker41 · September 19

https://forum.heatinghelp.com/discussion/comment/1811030#Comment_1811030

The thing I find attractive about TRV's is they are not just on/off. They modulate the flow, which seems like it would make temps more stable

Midnightsnacker41 · September 19

https://forum.heatinghelp.com/discussion/comment/1811031#Comment_1811031

I'm not planning to change the setpoint very often. I mostly want to avoid a large variation in room temp. I know zone valves and balance valves should able to achieve this, but I would prefer a system designed to be closer to constant flow.

DCContrarian · September 19

If you do the math, you'll find that it takes large changes in flow to create small changes in heat output. If you have a room where the heating load changes significantly — say, a room with solar gain during the day but not at night — you're not going to be able to keep up just by varying the flow.

The key to responsiveness is a low heat capacity in your emitter.

Midnightsnacker41 · September 19

Not sure I understand your point. Are you saying that because large changes in flow are required to make a small change in room temp, nobody will notice the on/off of a zone valve as opposed to a more steady modulating flow from a TRV?

DCContrarian · September 19

Large changes in flow are necessary to make a large change in floor temperature. The amount of heat the floor puts out is determined entirely by the difference between the floor temperature and the room temperature.

No matter how responsive you make the floor, it's always going to have some heat capacity, even if it's just the water in the tubing. When you shut off the flow the floor temperature isn't going to change immediately.

But we're getting afield. The best way to modulate the floor temperature is to modulate the water temperature and keep the flow constant. Then have a thermostat and zone valve to adjust the flow.

Jamie Hall · September 19

Let's step back one square here. What material is this radiant floor to be? How much mass (weight) per square foot? It makes a huge difference.

If the floor is very lightweight — say radiant panels such as Warmboard under minimal flooring — you can reasonably expect to see the room feel respond to changes in flow and temperature within an hour or two. If the floor is somewhat heavier — say radiant tubing under a subfloor under wood flooring or tile — several hours. If the floor is heavier — such as might be a slab — the response can take from the better part of a day to days.

Radiant flooring (or, for that matter ceilings) works best if the flow is constant through the tubing and if the input temperature of the water is varied in response to the heating load — not the air temperature in the space. The ideal arrangement is what is called outdoor reset, where there is an outdoor temperature sensor which varies, though a mixing valve, the temperature of the water being supplied to the floor.

Now let us consider using a TRV. There are two problems. First, it is sensing space temperature — if the space below the setpoint, it will open some (some are nicely proportional to the amount the temperature is off). This will supply heat to the floor as desired (but see the second point…). The problem is that by the time the setpoint is reached and the TRV closes, the floor will be nice and warm — and stay that way, and the space temperature will overshoot. The second problem is a little more subtle: ideally the flow through the floor tubing will be enough so that the temperature drop from inlet to outlet is no more than 10 degrees — and thus the floor temperature will be within that range. However, if the flow is less and the floor is cool, the outlet temperature will be less — and the floor near the end of the loops will be cool (or even cold!) which the floor near the beginning will be warm or even hot.

So what might be a solution? Basically you need to do one of two things: vary the temperature of the water and keep the flow constant (the mixing valve approach) or turn the flow on or off completely with a constant water temperature (the zone valve approach).

If you need tighter control of space temperature, or to occasionally raise the space temperature, a much more satisfactory approach is to use the radiant floor maintaining a constant temperature — set it and forget it (use a thermostat in the floor as the control) and use auxiliary radiation — often wall panel radiators — at a much higher temperature — which are controlled by the space thermostat.

GroundUp · September 19

Constant circ on ODR with proper balancing is almost unbeatable for radiant floors.

Midnightsnacker41 · September 20

https://forum.heatinghelp.com/discussion/comment/1811069#Comment_1811069

This is what I am trying to do. I haven't picked a heat source yet (likely a gas combi boiler), but Out Door reset is a must.

My question is about balancing. If I am understanding correctly, systems with a high thermal mass (slabs) need something like a sensor in the slab for in door feedback. However, (if my understanding is correct) systems with lower thermal mass can be balanced using air temp. At least that is what the suggestion to put a thermostat in every room and use zone valves seems to be saying.

I have 2 parts of the system:

Basement - 2 inchs of rigid foam, with 2 layers of OSB on top sandwich style. The upper layer is in strips and has the tubing in it. The tubing is inside hear plates with LVP on top.
Main floor. This will be staple up from the bottom. OSB subfloor with mostly LVP, some tile.

Seems like both are fairly low mass. If I have outdoor reset, is air temp enough for balancing? Or do I need in floor sensors?

DCContrarian · September 20

Let's spitball some numbers. Let's say you've got a room with a heating load of 20 BTU/hr/sf, you've plumbed it with loops of 1/2" PEX on 8" centers, 300' maximum loop lengths. So a 300' loop covers 200 square feet and delivers 4000 BTU/hr. With a 10F delta that's a 0.8 gpm flow rate. To get 20 BTU/hr/sf your floor surface needs to be 10F above room temperature, so if your room is at 70F you need the floor to have an average temperature of 80F. Let's say further that there's R1 of insulation total between the water in the tubing and the surface of the floor. So to get 20 BTu/hr/sf through R1 you need a temperature difference of 20F between the water and the surface of the floor. With the floor at 80F that means the water is at an average temperature of 100F; with a 10F delta that means the water is being sent at 105F and returning at 95F. Does that sound good so far?

OK, so what happens if you want to cut the heat output in half, without changing the delivered water temperature, just the flow. Well, the surface temperature has to be 75F. The difference between the surface and the water has to be cut in half, so now it's 10F. So your average water temperature is 85F. You're still delivering at 105F, a straight average would be returning at 65F but you can't get colder than the room, so you're returning at 70F, a delta of 35F. Half the output is 2000 BTU/hr, which gives a flow of 0.11 GPM. So to cut the output in half you have to cut the flow down to one eight. And one half of peak isn't really that low, in mild weather you might want to cut to 10% or 5%. That's going to require quite small flows.

You get better control by reducing the water temperature rather than trying to modulate the flow.

Jamie Hall · September 20

If you have outdoor reset, and your house is reasonably tight and not too much glass in the sun, chances are the outdoor reset alone will be adequate. Floor sensors in such a system act as a trim rather than the main control. You can also use in-room space thermostats as a trim, but keep in mind that the lag is long (that is, it will take a while for a call for heat to produce results, even with your relatively low mass floor). Probably the best strategy will be to use the space thermostats to close the zone valve for that zone, relying on the ODR to provide the main control.

You may also — indeed, would be wise to — place flow control valves on each zone as well as the zone valve. That will help in the initial stages of getting the system dialed in, as you will be able to use them to get a little less heat to one zone, should that zone — for whatever reason — tend to overheat while the other one is OK.

It's going to take some time and some fiddling to get the zone balance and the ODR curve to play nice together, but it will be worth it to minimize the amount of trim needed from the zone valves.

DCContrarian · September 20

With the same hypothetical floor, let's look at the effect of different materials. First, imagine you have a slab of concrete, 4" thick. Concrete is roughly 150 pounds per cubic foot, depending on the formulation, so that's about 50 pounds per square foot. Concrete has a heat capacity of about 0.5F BTU per pound per degreee F. If the surface of the floor is at 80F when that floor is at full output, and the water is at 100F, let's say the average temperature of the slab is 90F. If the slab starts at room temperature, 70F, to get it up to full output requires increasing it's temperature by 20F; at 50 pounds per square foot and 0.5 BTU/lb/F that's 500 BTU per square foot that needs to go into that slab.

The entire heating capacity of that slab is 20 BTU/sf/hr. So that's 25 hours of full output to get the slab up to temperature — not counting any heating of the room that happens as the slab starts to warm.

Now let's look at a wood floor with aluminum plates. Let's assume 3/4" of subfloor and 3/4" of finish floor needs to be heated. I'm going to ignore the heat capacity of the aluminum because it's low. Wood weighs roughly 40 pounds per cubic foot, although it varies a lot by species. So an inch and a half of wood is one eighth of a foot, or about 5 pounds per square foot. Wood has roughly the same heat capacity per pound as concrete, 0.5 BTU/lb/F. To get the same 20F increase in the wood takes 50 BTU per square foot, or about 2.5 hours of output.

When you're heating the slab initially, you can speed the process by sending hotter water into the floor. What you can't speed is the cooling process. If the sun starts shining and all of a sudden your heating load vanishes, there's nowhere for the heat in the floor to go but into the room.

Midnightsnacker41 · September 20

So it sounds like out door reset needs to be the primary control.

For trim, you need either floor sensors, or a space thermostat that would operate zone valves. Does a thermostat designed for a hydronic system do a decent job of avoiding the overshoot issue? Maybe a learning one?

I guess in also not understanding why a trv can't be used for trim. It would basically act like a zone valve and a control valve in one.

hot_rod · September 20

TRV controls are the most commonly used control in most countries. Panel radiators work well with TRVs

Several companies make TRVs that mount in a wall control.

Oventrop UniBox is one, but they recently pulled from the US market

@Paul Pollets and a few others on this site have used the UniBoxes

TRVs are modulating, proportioning, wireless controls, handy for many types of installation

Jamie Hall · September 20

A TRV can be used as a zone valve, provided it has the flow capacity. Never said it couldn't. In my simplicity oriented opinion, it would be overkill…

TRV with radiant heat

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