Mixing radiators with in-floor hydronic

[75gmck25]

I have a standard efficiency Buderus gas boiler that runs water at about 135-140 degrees with my cast iron hot water radiators. I am considering using in-floor hydronic in a new addition, but my understanding is that it will have to run at a much lower temperature.

I have seen a few articles about using mixing valves to get zones with different temperatures, but none of them have a clear explanation of how it works. It seems like they may mix the return water from the high temp zone with the water going to the lower temp zone, but I'm not even sure that is how it works. I think my return water is probably about 100 degrees (IIRC), so even that might be a little too warm for in floor heat.

My boiler also has a minimum temp of 100 degrees when not circulating, so that the response is faster when the heat comes back on, but I know that minimum can be turned off.

Anyone have pointers to articles that explain the concept and physical setup when you use a mixing valve to get a lower temp in one zone?

Thanks,

Bruce

 

[finn]

Is your addition in floor staple up or is it pex buried in tha concrete slab?

Buried pex is normally run at 100-110 degrees to protect the concrete, but staple up requires more heat. My hvac guy suggested 125-130, but has seen up to 150 degrees. Mine runs ~140 in sub zero ambient temperatures, but I run it lower in milder conditions.

A mixing valve is in order if you are putting pex in a concrete slab.

There are stand alone mixing valves mounted on boards, complete with circulation pumps available. Not inexpensive, but easy to install.

You’ll probably want to replumb to a primary/secondary loop configuration.

 

[Radix2]

A simple way to think of a thermostatic mixing valve is that it only mixes in enough hot water to stay at the set temperature as the water circulates - so you have water going around and around, as it passes the mixing valve, some water from the boiler is added as needed, the rest just goes back around.

Assuming you are set up with primary secondary then there should be no big mixing of the return water between zones prior to the mixing valve. You could might be able achieve the same thing in other configurations, it might take some thought to optimize the layout.

 

[75gmck25]

My addition is not built yet, but my current plan is to use a perimeter foundation with a short insulated crawl space. Both 1st and 2nd floors would be wood underlayment with either hardwood or tile on top.

My current radiator setup is fairly simple. Water comes out of the boiler into a 1" pipe, which has a T that connects it two PEX manifolds. From the manifolds there is 1/2" PEX running to each radiator. The return lines run back to a 2nd set of manifolds, 1" pipe, and T that connects back to the boiler. My manifolds are similar to these. https://www.pexsuperstore.com/pex-m...x-manifold-with-adapters-new-flow-meters.html

I still have 4 available 1/2" PEX ports between the two manifolds, so I could run lines to 4 radiators or to 4 runs of in-floor PEX. The manifolds have a flow meter and I can manually adjust the flow for each connection. There was an option to add thermostatic valves, but it has worked fine running everything with the valves wide open.

I wonder if I could just adjust the flow to the in-floor runs and use that to reduce the amount of heat provided for the in-floor zones.

Bruce

 

[finn]

I wouldn’t recommend underfloor heat with a hardwood floor unless the room is really small.

I have hardwood in my living room, and it isn’t dimensional stable.

The rest of the house is mostly tile, with one bedroom a Pergo type composite.
Both are dimensionally more stable than the hardwood, although there are a few cracked tiles beginning to show up. The tiles are probably twenty years old, though.

As I said before, you may not need the mixing valve as it sounds like you are going with a staple up system. A mortar based system probably needs the valve.

 

[75gmck25]

That may be good justification to go with tile for the entire flooring.

About 1/2 of the new floor space on each floor will be bathroom (accessible 1/2 bath on 1st floor; full bath on 2nd floor), so using tile for the entire area won't look out of place. It will also be very comfortable if the heat in the room is coming from a warm floor.

Thanks,

Bruce

 

[chinboys]

I too have mixed hydronic heating in my split level home on a slab.
I tried to make my own low temp mixer and decided to go with a Viega mixing block Model 2887.3.
It isn't cheap but it works like a charm as PEX in floor heating only needs 110 F water and my radiators are set for 170 F during cold days. I have a modulating condensing boiler.

 

[75gmck25]

My boiler is a medium efficiency and the max temp is set to 140 degrees. Based on my Northern VA climate (outside DC), there are not too many days where water temp even reaches 140 degrees, and its more typically about 135 when the system is running.

It looks like the Viega mixing block goes for about $1290, so its not cheap to get that 110 degree water. However, in the grand scheme of renovation costs it may be worth it. I want to heat the new rooms, but not end up with floors that are so warm that feel too hot and throws off the temperature in the rooms.

The entire addition is only about 200 square feet total for both floors, so it doesn't really add that much to the existing 1340 square feet that uses cast iron radiators. I have the opportunity to make sure I get the addition well insulated and sealed, so it should be easier to heat than the existing 1940 construction that has no insulation in the exterior walls.

Bruce

 

[LS6 Tommy]

Running any fired, non-condensing boiler much below 140° F is not good for it. You're making acidic condensate in the chamber in the off cycle...

Tommy

 

[Chris705]

OP - take a look at the Caleffi web site. In their menu is link to education. (Idonics)(browse the series) They have a series of books that do a great job of explaining all the in and outs of hydronics. I am sure they describe how to mix radiators and in floor hydronic heat using a mixing valve. And there is a good explanation of how mixing valves work.