Here's the situation. We are currently running a system of fin baseboards in most rooms and at the end of one of the zones,the water goes into in-floor heating in two rooms in our house. The heat on the boiler is currently set at 140F but it's not warm enough to heat the rooms with the baseboards. We want to turn it up to around 180F but we're worried that it might be too hot for the in-floor part of the system and cause damage to the concrete subfloor or the PVC tubes.

How hot can we safely go?

180 F may be appropriate for the baseboard heaters, however, even 140 is likely too hot for the floor circuits. You will likely need to use a mixing valve to provide a lower supply temp to the floor circuits. Typically, for a well insulated building, the floor surface temp is only a couple degrees higher than the desired room temp (perhaps in the 70-85 F range) and the floor supply temp is only several degrees higher than the floor surface temp (perhaps in the 75-110 F range). The required supply temps really depend on many factors like the heat loss of the room/building and the circuit length/spacing, etc.

How hot you can safely go really depends on what you are using for your circuits and your operating pressure. For example, PEX might be rated at 100 PSI @ 180 F, but only rated 80 PSI @ 200 F.

140 is a max temp for 'crete radiant. and even that isn't really ideal. You would only need it that hot if this is a thin pour (2" or less) and you have a high load with resistant floor coverings like carpet.

There are a couple of other concerns here:

1. How big are your radiant rooms?

2. Do you know what make/model of boiler you have?

those questions are to ensure you are not going to rot out this boiler with cold return temps from your concrete zones. If they are small, or the boiler is big, that's probably not a concern, and you can add a 3-way valve to limit the radiant temps as sailawayrb mentions. but if these are big rooms that represent a significant portion of the boiler output you might want a more intelligent version that can protect your boiler as well. That's unlikely, but "2 rooms" can mean a lot of things.

" floor surface temp (perhaps in the 75-110 F range"

You might want to read that chapter again sailor.

You will need to add a sub-system circulator and mixing valve to lower the radiant floor panels supply temperature to a more appropriate maximum. The design water temperature is determined by the heat load for the room, the type of radiant panel you have and the floor coverings between panel and room itself e.g. wood flooring, carpet, etc.

An experienced radiant floor designer would be the answer.

bad cut there badger. his statement was correct. 75-110 was the supply temp, not the surface temp.

I stand corrected and apologize. My speed reading is not what it used to be.

Why is the maximum temperature 140 degees? I will check my pressures and get back to you.

Here's some more info - It is a thin pour - 2" or less poured over a wooden subfloor, suspended over an unheated cellar below. The two rooms with in-floor heating are two small bathrooms - the total square footage of both rooms is about 100 square feet. The return temps at 140 are about 7 degrees lower than output temps so that's not a big difference.

The boiler is a Laars mascot II Combi Boiler.

apparently above 140 you can damage 'crete products. now, I happen to know of a lot of old 180 degree concrete systems, so this is hardly a prediction of doom. but then again, 140 is more than enough to cook feet in a tile floor situation in concrete. you must have some darn hot floors. or maybe you didn't insulate below and you're dumping a lot of heat downward?

I don't believe it either but cooler is better, all "extra" heat, is wasted heat as it is with flow. You can quote me on that Sailor, to make up for my earlier foible :-).

Badger, no harm and no foul…apology accepted.

Sierra, have you contacted the clown who did your design/installation to see about making it right? If you are in state that requires HVAC licensing, you should be able to contact your state’s contractor board to seek/enforce a remedy. Or was this a DIY project that was not properly researched and accomplished?