Cheap diverter valve solution

Kewlwhip

Ok, here is what I am trying to accomplish....I'm going to have a tankless water heater radiant floor system....and I want to have a fairly large Delta-T on the system, and I want to limit the amount of flow to the HWH.

I only want the return water to be sent to the HWH once it goes below a certain temperature. I know there are pumps that accomplish something like that by having sensors on the supply and return line, so water is only pumping quickly once it goes below the delta-t....but I already have a gifted groundfos pump that senses the water temp at the pump...

So I know a diverter valve is generally the answer here...but I am trying to pench pennies...so my idea is a thermostatic mixing valve on the return side of the system...with a T-fitting on the cold inlet and the hot inlet capped off....and the valve setting set at the bottom of the delta-T so it only sends water through the "mix" port once the cold sides falls below its setting.

Will this work? Will the valve stay closed as long as the water is above the valves setting? And Instead be diverted through the other port of the T fitting on its inlet?

Ironman

You're already gonna have problems by using a water heater where a boiler belongs. The high resistance to flow from the heat exchanger is gonna cause poor flow and the issues that accompany it.
By limiting flow, you limit the water heater's firing rate and you also limit the output of the floor.
There's no cheap shortcut to doing a radiant floor correctly. If it's not done correctly, it's not gonna perform correctly.
The laws of physics will outweigh the laws of economics every time. You need a properly sized boiler.

Kewlwhip

It's a low temp system in a 1100sqft house in a temperate climate....why in the world would I want a boiler? I'm very rarely, if ever, going to need water temp higher than 120 let alone 140.

And if I HAD a boiler, the LAST thing I would want to do is let the return line go STRAIGHT back to it...so a diverter valve would be EXACTLY what I would need.

As far as the SYSTEM is concerned this isn't even necessary, the pump will limit the flow based on temp, and the tankless won't use anymore energy than is necessary to bring it up to its(low) temp setting.

It's just a way of limiting the wear and tear and the HWH and extending its service life by limiting scale formation, galavanic corrosion, velocity wear, running time, etc.

In fact it might make the system slightly less economic as far as energy expenditure, but not be enough to matter, not for what I am doing. Although it might free up a little heat production for domestic hot water during peak usage periods if I get my settings right, by allowing the system to drop to the bottom of its delta-T during times I know hot water usage will be high....or by letting me lower the delta-T during times when I WANT the heat output to drop, such as certain times of day or when the house will be empty. Not that its even a concern, I could have very fixture pouring hot water and the floor would still stay warm honestly.

So can you answer my actual question....

Will a TS valve operate as I intend through this method? My other idea was instead of capping off the hot, just running the return line to both ports....here's a couple drawings.

https://drive.google.com/file/d/1bDOhByiseRZ0xjGwlkjTQuMTBYDvkuAK/view?usp=sharing

Considering the difference in price between TS valves and diverter valves I think this a solution worth exploring for a LOT of people though....and I am having trouble seeing why you can't see the value and utility of it, unless you can tell me the TS valve won't function correctly in this capacity....and if so please explain why....

hot_rod

Not sure I follow. A tankless heater as a combined system. DHW and radiant all the same water?