how do TMVs function?

millhouse

Do TMVs (manual dial) operate by allowing a maximum temperature to pass through? I have a 3-way TMV for my radiant floors (~115 deg). The floor return tempers the water. I was told, that ODR would not work on my system as I would constantly need to adjust the TMV whenever the ODR changed boiler temps. This doesn't make sense to me. I thought, if the boiler was running at 180 degs, the TMV would mix more return water to drop the supply to 115 degs. Similarly, if the boiler was running at 140 degs due to ODR, the TMV would mix that much less return water to drop the supply to 115. The same temp would go to the floors, just in the latter case, the boiler would burn less oil.

Am I far off base?

Unknown

Your logic,

sounds fine to me. The floor MV is manually set, the boiler water temp could be set from the ODR,,isn`t that what its supposed to do? I can`t really see a prob in adding this to your set-up if you wanted.

Dave

Unknown

if you only need 115, why would the boiler ever need to run at 180?

Mike T., Swampeast MO

If yours is a conventional (non-condensing) boiler, the generally preferred (and safest to the boiler) way of adding reset to your system would be by using a motorized thermostatic mixing valve connected to the reset controller. This allows the boiler to operating in the proper temperature range while ensuring that the temp of the water circulating through the floors is best suited to the weather.

If yours is a condensing boiler, there is usually no need for a mixing valve unless you also emitters other than floors which in general require not only a higher temperature, but a steeper reset curve. Four-way mixing valves are highly preferred with condensing boilers as three-way mixers will raise the return temperature to the boiler and thus reduce efficiency needlessly.

Unknown

how does a four way change that, exactly?

Mike T., Swampeast MO

YIKES!!!

What was I thinking?

Three-way, NOT four-way, valves are preferred for use with mod-cons as four-way valves can result in higher return temps to the boiler.

As to why, I must confess that I'm relying on what manufacturers say and have never made drawings and compared flow path to see just why. My assumption is that when a mixing valve is used with a mod-con it's in conjunction with primary/secondary and a dual-temperature system.

Will try to add some sketches later, but have a big job looming that will take nearly all my time for the next few days.

millhouse_2

thanks for the replies.

We're upgrading to a 3 pass oil boiler. It seems, if you peruse this board, ODR is the way to go. In fact, it almost seems common place unless you're looking at EKs. However, it’s been difficult to get a buy-in on ODR…for all sorts of different reasons, such as, it won’t save money, it won’t work with oil boilers, it needs expensive floor sensors to work, it needs a mechanical contractor to design it…and just recently, I would need to manually adjust the TMV every time the ODR changes the boiler temp. I’m starting to second guess myself in my quest to use ODR.

NRT.Rob—That’s what I asked and was told that the water heater needs 180 degrees…yet another reason why I don’t need ODR

Mike T – Is the point of a motorized TMV is to balance the floor load to the outside temp (ie, the motorized TMV works on the floors, the same way the ODR works the boiler)? So that on a cool spring day the floors might be getting 95 degrees instead of 115? But with the manual TMV, the floors will get 115 regardless of outside temps?

Dumb question, but is it acceptable to hire a professional to design my system and then ask installers to bid on the design…like those fancy schematics that John Siegenthaler has all over the web? IMO it seems overkill for a boiler replacement, but I’ve burned 6+ hours talking to these guys, they burned 6+ hours talking to me (plus travel time) and all of our time is valuable. It seems I could send out a single design to potential contractors and if they’re interested we could take things from there. Is this acceptable practice? Right now I feel like I’ve been on a bunch of blind dates when I should be using an internet dating service. sorry for the vent!

Vinnie Smedick_2

Millhouse,
You just need an informed heating contractor! I have no idea why someone would say ODR isn't justified. That is just way out of base. There are products and controls out there for every application. An informed contrator ( one who invest his time and money into learning all there is about his field, like most of us on this website) are the contractors you should be looking for. Give the find one in my area search a try I'm quite sure you'll be pleasantly informed by one of us.

To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"

Mike T., Swampeast MO

Yes, the purpose of using a motorized mixing valve is so you can reset only the supply temperature to the floor without lowering the temperature in the boiler to the point that condensation damage could occur.

In general, non-condensing boilers need to see about 140F return to ensure that no condensation damage can occur. By resetting only the mixing valve a conventional boiler will come up to temp rapidly so it can operate near the conditions intended by the designers which--ideally--will also be the most efficient.

Only one conventional oil boiler that I know of has no low temperature limit. There is at least one "near condensing" oil boiler that can operate safely at lower than normal temps, but it cannot recover energy from the condensate--instead it is routed to a highly corrosion resistant area where it is safely evaporated. Condensing oil boilers are becoming a bit more common, but low sulfur oil is required. While low sulfur diesel fuel is now mandated, I'm not sure if low sulfur fuel oil (identical other than color and taxes) is widely available. The only fuel oil that seems to be used in my area is waste oil.

Mike T., Swampeast MO

I agree Vinnie. None of this is in any way esoteric and any contractor with reasonable hydronic knowledge--not just residential plumbing--will have no problem offering you a number of options of both boiler and appropriate control systems.

Unknown

it sounds like his plan is to use the indirect water heater as a buffer tank.

thus, all heat demands are "domestic" demands.

this is pretty typical for an EK setup and means that the EK itself has no reset and really, not that much need for it. COMPARED TO OTHER OIL BOILERS, with its purge cycles and low mass, you dont waste that much energy running the higher temp, as you can be sure it will pretty much all hit the buffer tank and be utilized unless lost by standby lost. Not like a conventional boiler where you are heating up a quarter ton of mass just about, then wasting all that heat in the metal, etc.

If you went gas, a mod/con would do better most likely.

I would agree in this setup no boiler reset is needed and is not that helpful. However, we typically use a shell in tank indirect, and use "injection mixing" out of it to the floor, not a dumb (fixed temp) tempering valve. That's a radiant issue, not a boiler firing issue.

If you REALLY want to turn down the boiler temp, you would need a separate buffer tank, and you could just run the boiler at 140 for heating demands and 180 for domestic demands with domestic priority, provided domestic demands did not heat up the buffer tank. That might be marginally more efficient, but I'm not sure it would be worth the additional tank and electronics.

millhouse_2

hi nrt.rob

No, that wasn't his plan. He was going to put the indirect on a separate zone, as it is setup now. He argued that since domestic hot water was needed to be heated to 180 (at any given point in the day) setting the boiler to lower temps w/ ODR would be of no value.

what's "a shell in tank indirect"?