Mixing valves in oil fired radiant floor heating system

skippy_ · July 9

I have an 1100 sqft house that I am moving onto a new ICF basement and will be installing radiant floor heat in the basement. The main floor will be heated with forced air. I will also be adding an 800-1000 sqft garage in the future which will be radiant floor heat. The boiler will also provide DHW with an indirect tank.

I have been looking at an NTI Caprice oil boiler or a Viessmann Vitorond oil boiler. I've been told by the plumbers that due to the higher temp of oil boiler vs propane, I will need to have mixing valves installed in the loop to reduce the temperature enough for the floor heat.

However, the marketing materials for both boilers talk about using a hydrostat and outdoor ambient temp sensor to lower the boiler water temp and account for varying heating loads. I'm having trouble understanding how this would be any different than a propane boiler. I understand that oil has more heat per volume than propane, but shouldn't this just cause the oil boiler to come up to temp quicker and run less than a propane boiler?

I would imagine the system would have 3 zones - basement, garage, DHW. I'm not quite clear on how the mixing valves would be integrated into the system and if they are even necessary.

If anyone could clear this up for me and explain if I am misunderstanding something, I would appreciate it.

Jamie Hall · July 9

Let's get one thing out of the way first: the temperature of an oil fired boiler will be no different from that of a propane fired boiler under the same load and BTUh input conditions.

Where a propane boiler may run at a lower temperature is with lower load conditions and modulating the burner flame to match — what is called a mod/con boiler.

Now your sales people do have a point: residential oil boiler power output can only be modulated by turning them on and off, not by lowering the flame.

Both oil and propane systems will only work well for radiant if they use a mixing valve and outdoor control of that valve. They both will also need a control to modulate the boiler to match the load — which may take the form of a modulating control for a propane mod/con, or a simple aquastat for the oil boiler or a conventional propane fired boiler.

skippy_ · July 9

Your first 3 paragraphs all make sense to me and pretty much align with what I had in my mind. I suppose the on/off control of an oil boiler could become an issue if it there is a minimal heating load causing short cycling? Or is this not usually an issue? If it is, how is this usually overcome?

Your last paragraph is likely where my confusion comes from. I have attached below a diagram of the propane boiler system in my parents house. One zone for basement, one for garage, and one for DHW tank. There is no mixing valve that I can see. It does have outdoor control, which I assume is just varying the temp of the boiler water. Is there an implicit mixing valve or something I am missing here?

mattmia2 · July 9

A propane or natural gas condensing boiler can output low enough temp water to feed radiant directly. a cast iron boiler(weather it is gas or oil) can not. It would condense at low supply water temps which will destroy the cast iron. Condensing boilers have special heat exchanges that can handle the condensation. I thought Viessman used to have a condensing oil boiler but i'm not sure. Energy Kinetics also makes some higher efficiency conventional boilers for gas or oil.

hot_rod · July 9

Use a 4-1 type hydrosep for air, dirt, magnetic and hydraulic separation. Eliminates the y strainer

P 1 is the boiler pump, it runs on any heat call. Needs a check valve

P2 is the indirect circ, it runs for DHW, P1 is off. Needs a check valve

P3 is the low temperature, always pump away from the 3 way mix valve.

The boiler runs on outdoor reset, 135 minimum for boiler protection.

The lower drawing is without a separator, if the manufacturer allows direct piping.

P1 indirect

P2 radiant mixed down via 3 way

Expansion and fill connect at the separator into either of the bottom ports.

Depending on the pressure drop through the boilers, you may not need a hydraulic sep. Unless the manufacturer requests it? The smallest zone needs to assure enough gpm through the boiler.

Bottom drawing is a two temperature system with indirect

EdTheHeaterMan · July 9

@skippy_ said: Your last paragraph is likely where my confusion comes from. I have attached below a diagram of the propane boiler system in my parents house.

I don't see in that diagram how the radiant floor water temperature can be different from the DHW tank high temperature without a mixing valve of some type. The only way this might work is if the DHW has priority and can send the boiler to 160° or above to make DHW. When there is a call from the lower temperature radiant zones during the DHW high temperature, the higher temperature boiler water must drop quickly so the floor does not get the 160° (or higher) water so not to overheat the floor. That can end up working with the proper control system. There may be some short bursts of higher temperature water to the radiant loop(s) but that will dissipate in the radiant floor concrete quickly and the lower temperature water from the boiler will take over heating. There can never be a simultaneous call for radiant and DHW.

I don't see that same design working on an oil fired boiler system, as of this date there are no ModCon oil fired boilers on the market. Oil boilers are either on or off. There are 2 stage flame burners for oil, but those burners are only offered in commercial sizes. Also, the nature of Cast Iron boilers indicates the minimum return water temperature of 135° to prevent flue gas condensation and ultimately premature boiler failure, means that the supply temperature must operate above the maximum water temperature of the radiant zones. A mixing valve is a must for the oil fired systems.

Robert O'Brien · July 9

https://forum.heatinghelp.com/discussion/comment/1859763#Comment_1859763

Viessmann does make an oil-fired condensing boiler, unfortunately it has never been imported here

Vitorondens 200-T Cast Iron Oil Condensing Boiler | Viessmann UK

EdTheHeaterMan · July 9

https://forum.heatinghelp.com/discussion/comment/1859792#Comment_1859792

It is not modulating though… so not a ModCon

skippy_ · July 10

I appreciate the responses, there is a lot of good info here

@mattmia2 said: A propane or natural gas condensing boiler can output low enough temp water to feed radiant directly. a cast iron boiler(weather it is gas or oil) can not.

This is the piece of info I was missing. It is not an oil vs propane issue, it is a condensing vs non-condensing boiler issue. That clears up a lot of confusion.

@hot_rod said:
P1 is the boiler pump, it runs on any heat call. Needs a check valve
P2 is the indirect circ, it runs for DHW, P1 is off. Needs a check valve
P3 is the low temperature, always pump away from the 3 way mix valve.

I do not see how P1 can be off if P2 is on. I assume this is a typo unless I am missing something.
For 2 low temp zones, would I tee in before the mixing valve and add a second mixing valve for the other zone, or would one mixing valve serve two zones each with their own pump?

@EdTheHeaterMan said: I don't see in that diagram how the radiant floor water temperature can be different from the DHW tank high temperature without a mixing valve of some type. The only way this might work is if the DHW has priority and can send the boiler to 160° or above to make DHW.

This must be what it is doing. I double checked the boiler piping and the diagram is correct - there are no mixing valves. This is what was confusing me as I never realized it must be doing some kind of control scheme in the background to allow different temp water to the floor and DHW tank.

So as I understand it, for 2 floor zones (basement, garage), and DHW, I'll need:

2 (?) mixing valves
4 pumps (boiler, basement, garage, DHW)
hydraulic separator
oil boiler with hydrostat
Tekmar or equivalent controller

I assume manual mixing valve(s) would be used since the floors are large thermal masses. They would be set such that the max boiler temp would result in the max acceptable floor temp and then as the ODR adjusts the boiler temp, the floor temp would track as well.

Essentially I am looking for the simplest, most reliable system that still gives good performance. I don't want to spend $$$ and add extra complexity just to get a marginal performance/efficiency bump, but I also don't want to cheap out and have a system that does not operate properly.

Let me know what you think. Thanks

SuperTech · July 10

I think a buffer tank with a mixing valve would be ideal for hydraulic separation. This would eliminate any potential for issues with short cycling and flue gas condensation. A reverse indirect is another option.

EdTheHeaterMan · July 10

A fixed temperature mixing valve will not track with the boiler's ODR. A fixed temp mixing valve will take the changing boiler temperature and add more or less return (cold in) temperature water to create the set temperature of that mix valve. In order to get true ODR for the floor heating slabs you might consider using something like this: Taco I-Series ODR valve This will do the mixing from the higher temperature Boiler/Buffer Tank/Separator water. I don't know if @hot_rod's company has a similar product but if they do, then He would know the best Hydraulic separation/air elimination/pumping away/mixing valve design for your situation. I might start here:

Screenshot 2025-07-10 at 12.43.51 PM.png

2 Taco iSeries mixing valves
5 pumps (boiler, system, basement, garage, DHW)
hydraulic separator if needed to maintain boiler temperature and/or reduce short cycling
oil boiler with hydrostat
Taco iSeries valve motors or equivalent controller

You can place the mixing valves near each floor system.

EDIT: To save on pumps and on iSeries valve you could eliminate the Separator and use one mix valve for both floors. The mixed water can be made in the boiler room and lower temperature water piped from the boiler room to the two different floor locations.

Screenshot 2025-07-10 at 12.29.31 PM.png

1 Taco iSeries mixing valve
3 pumps (boiler, low temp, DHW)
2 Zone Valves
oil boiler with hydrostat
Taco iSeries valve motors or equivalent controller

hot_rod · July 10

If you use a hydraulic separator you could pipe the indirect on the primary side of the sep and you only need to run 1 circulator when the tank calls as a priority zone. Also you get full boiler output to the tank. The indirect on the secondary side requires you run the boiler pump and the indirect. You get some temperature blending in the separator, so the indirect may not get the full boiler temperature.

Proportional reset uses a boiler with ODR, then a manual “dumb” mix valve supplies low temperature zones. So that manual mix valve tracks along with the ODR reset. I had this mixing option my last home.

Here is a better explanation from Idronics #7

This example has a mod con modulating on ODR feeding a panel radiator system, the radiant is via a manual valve.

Thr HydroLink is not required to make it work. A hydro sep or close tees works IF the boiler needs a dedicated circ

EdTheHeaterMan · July 12

I made an edit to the Taco Mix I-series Mix valve. I did not select the outdoor reset offering. this is it:Taco I-Series ODR valve here and it is corrected above. I also agree with @hot_rod Bob on piping the indirect before the separator. Good call on that one.

skippy_ · July 17

The system using "dumb" mixing valves described by @hot_rod 's post are what I had in mind. I very much like the simplicity and reliability achieved with this setup.

Are there any significant benefits to going with an active mixing setup such as the Taco mix valves?

EdTheHeaterMan · 5:31PM

The supply water temperature to the floor will not change with the weather for the most part.

Lets say the fixed temperature mix for the floor needs to be 125° for the design days. (the coldest days) and the boiler will create 160° for that using the ODR when there is no call for DHW and the minimum temperature for the boiler return of 135°. The fixed temp mixing valve will take 160° supply water and add the proper amount of return water from the radiant in order to make 125° to the floor

When the boiler temperature drops lower temperatures based on the boiler or Tekmar ODR control sensing a warmer outside temperature (say 145°), the fixed temp mix valve will take the 145° reset temperature and mix less return water from the radiant return to make the supply 125° to the floor. So when the floor needs the lower temp on the warmer day, it will still be getting 125 °. That will over heat the room enough to satisfy (turn off) the thermostat until the room temperature drops below the thermostat set point and it calls for heat again. Room temperature thermostats and radiant floor heat are not the best combination to avoid large temperature swings in the rooms because of the slower reaction time of radiant floor systems.

Eventually the Tekmer or boiler ODR control will drop the supply temperature to 120° when the outside temperature is above 50°. That is when the 125 mix valve will not allow any return water from the radiant loop to mix with the 120° supply water. the 120° supply water will go straight through the mix valve. You will then get the lower floor temperature. Now this 120° boiler supply temperature will only happen when the boiler's burner is off. If the boiler's burner is firing then the minimum return temperature of 135° will need to happen by some function of the control system to protect the boiler from flue gas condensation.

I'm not saying that it won't work, I'm just pointing out the reason you might want to consider the Taco iSeries ODR valve. They were not available when I installed my radiant floor system so I installed fixed temp mix valves and used room thermostats. It worked fine for me. I just wanted you to know that without the ODR valve, you may be wasting your $$$ on using a Tekmer or other boiler ODR system.

What you think you are achieving with a Tekmar or other boiler temperature ODR may not happen after you spend all that $$$ on the control system. Save on the system ODR and use the ODR mix valve where it will do the most good. Or forget about ODR completely and save $$$ in installation costs.

hot_rod · 6:24PM

The forced air runs off the boiler as a high temperature loop? Or is all the hydronics low temperature. If so a mod con lp boiler would be my choice, running ODR.

the manual mix valve will in fact track with the boilers changing temperature. As the boiler drops so will the mix from the manual valve. The manual valve does not have a sensor, it is a fixed blend.

On the mix down manual valve you can also adjust the offset, dotted red lines, how it tracks with the higher temperature.

This shows the relationship with a 130 panel rad system, but it could be a 160, 180 or higher zone also.

There is no reason to have ODR on the boiler and also on the low temperature zone.

Of course with a cast boiler you will have a minimum return to be aware of.

EdTheHeaterMan · 10:22PM

I agree @hot_rod. There is one DHW high temperature loop. I like the mod-con for the ODR idea, because the mod-con will do the low temp and DHW temp on priority. The ideal situation with the logic already built into the boiler controls. But the original post indicated that they were using oil fired boilers. Probably cast iron. So the non condensing Cat I venting system would require boiler return protection. That is where I am looking at the Taco iSeries ODR valve for floor temperature control as a better choice for ODR. Spending $$$ on a Tekmar or other ODR to control the boiler temperature will be wasted for over 50% of the heating season, if at all.

Ideas Bob?

Mixing valves in oil fired radiant floor heating system

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