ODR Mixing Valve for Fixed Temp Boiler

mostlybirch

Hello, I am trying to add temperature control to our radiant slab system.. The small oil-fired boiler (Toyotomi 128-HH) is set at a fixed ~120F to provide DHW, and currently feeds a glycol loop in the slab through a HX, Would a 3-way mixing valve with ODR like the Taco i-series be a good fit to modulate the temperature into the HX? Attached a diagram of the current plumbing.

Thanks

Big Ed_4

Add the ODR mixing valve on the radiant side of the heat exchanger . Maintain max boiler supply to the heat exchanger .

mostlybirch

Would this be correct?

hot_rod

Screenshot 2025-01-26 at 1.31.36 PM.png

Kaos

Except for improperly insulated slab, adding ODR on the floor heat won't help much with fuel use. What you want is ODR on the boiler supply temp which is not really an option if you are using it for DHW.

It also doesn't look like a condensing boiler so you don't really want to run it that cold.

mostlybirch

Thank you for the replies and diagram.

What is the effect of mixing after the HX versus after the circulator? Is it related to staying downstream of the point of no pressure change?

And as far as the options for mixing valves: as currently set up I have no temperature modulation outside of changing pump speeds. Which mixing valve would best integrate into a temperature control system? I was leaning towards the ODR valve to help handle the big swings in temperature we get in interior Alaska. And would I tie the circulators and ODR valve into one relay? Thanks

mostlybirch

https://forum.heatinghelp.com/discussion/comment/1838006#Comment_1838006

Slab is well insulated, below and sides. Building is well insulated as well. It is a non-condensing boiler. Small oil-fired with 5 gallon heat exchanger. I was trying with my original diagram with mixing valve at boiler circuit before the HX to lower the heat coming into the radiant circuit without lowering the temp for DHW

hot_rod

The motorized mixers often have boiler return protection. tekmar does, and I thing the I-valve has a tekmar brain.

You need to pump away from a mix valve to have them work properly.

ODR helps from over-heating and flywheeling the slab. Saves some fuel and gets you close to constant circulation. Which is the best controol option for slabs.

Screenshot 2025-01-26 at 1.48.45 PM.png

mostlybirch

Wouldnt a mixing valve downstream of the DHW tee as in the original photo keep the boiler temperature ( and DHW) temp constant but lower input temp into the radiant system?

mostlybirch

Which would also reduce sending cooler water into the boiler supply?

Kaos

With anything combi heat using DHW the goal is to minimize water flow through the heat source. Since you don't have zoning and your DHW temperature is fixed, I would not bother any mixing valve. Adjust the potable side pump flow rate to get the supply water temperature you want on the floor heat side.

mostlybirch

And for controlling long term, just set the circulator(s) onto a thermostatic relay?

Kaos

I'm assuming there is a thermostat the controls the floor heat. That should turn both circulators on.

If you want to get fancy, you can add an adjustable strap on aquastat on the floor heat return water. You can use this to turn off the potable side pump even if the thermostat is calling for heat once the return water is warm enough. This would further reduce the potable side runtime on warmer days when the slab heats up quick.

mostlybirch

That sounds like what I am looking for. I currently don't have a thermostat wired in, just relying on manual circulator control.

What type of relay or thermostat would I need to utilize the aquastat and shut off circulator?

Kaos

Simplest is a line voltage mechanical thermostat (ie Honeywell baseboard one). You can wire it to provide power to both pump.

For the aquastat you can use something like a Honeywell L6006C. Strap it to the return manifold and feed the power for the potable side pump through it.

So it would be:

thermstat→floor heat pump

thermostat→aquastat→potable pump

If you want better controls, you can use a programmable thermostat driving a Taco relay module instead of the line voltage thermostat.

mostlybirch

Thanks Kaos this is very helpful.

Looks like a simply Taco SR501 single zone relay, with a thermostat that can accept a slab sensor (I ran an empty pex line in the slab for a sensor) would get me going. I have an ecobee 3 laying around but looks like it isn't compatible with a slab sensor unfortunately.

Kaos

Slab sensor is not really needed. About the only use I got out of it is to allow me to set a minimum and max floor temp which is helpful with cold floors in an overhang and to keep hardwood flooring from overheating.

The Ecobee to an SR would work.

hot_rod

that is basically an oil fired tankless, a plumber friend in Alaska, on the Kenai, works on many of them. A fairly simplistic control, and aquastat and jumpers for 3 different deltas. Circulator runs whenever the boiler is powered, even with no call, intended to be a recirc pump.
Toyo recommends a buffer tank when used for heating, but he rarely sees them piped with tanks. Typically sold direct to diyers and homeowners, so the systems he sees are often creative handyman installs. Although they have a plumber in his area set up as a parts distributor now, making service easier.