Radiant Floor Heating Navien plus Heat Box, heat box, navien

Radiant Floor Heating Navien plus Heat Box, heat box, navien

Radiant Floor Heating /Navien plus Heat Box



QUESTION: In a previous answer you said:

«This is particularly true when using Mod/Con boilers and Geo-thermal heat pumps, as they will operate in a very efficient mode when presented with low return water temperatures.»

Why is efficiency higher for these appliances when return water temperatures are lower?

ANSWER: Both systems operate at higher efficiency when return water temperatures are kept as low as possible. I design my radiant systems to operate below 140°F design water temperatures. This always requires a heat load analysis which leads to window and insulation specifications. Comfort, efficiency and reliability will all be effected by good design practice involving low water temperatures and out door reset.

QUESTION: Is it the delta T that you are speaking of when saying ‘low return water temperatures’ (i.e. a large temperature drop) or is it keeping temp of the whole system lower?

I am trying to understand our system better. We have 2 zones of our house with staple up and 2 that are in concrete (Navien 240a and Heating Box). The system runs at 125F. The staple up areas are for keeping the floor warm underfoot, not heating for which we have a furnace. In the staple up areas, the return temps rise quickly when those zones run alone. The water heater turns off until the return water temps drop to 7F below output temp. Is this a situation where we would experience lower efficiency because of the low delta T?

Thanks for you help.


A tankless water heater is not a boiler.

Delta T and return water temperature are not the same. You may have a large Delta T (temperature going out minus the temperature returning to the boiler) but the return water temperature could be anywhere. Condensing boilers and geothermal units do not operate at their peak efficiencies when presented high temperatures (over 130F). Geothermal units can’t produce them.

Radiant Floor Heating Navien plus Heat Box, heat box, navien

You may be suffering from a product designed to make a water heater serve as a boiler AND a DHW heater in a package made easy to understand and geared to the DIY crowd. When using the heat exchanger model (not the silly ‘open’ system) you have a safe arrangement, but it lacks outdoor reset (one of the critical components when efficiency and comfort are considered).

Outdoor reset keeps a modulating boiler running practically non-stop in the heating season, delivering water temperatures designed to match the heat lost in a given space. This is the most efficient and comfortable way to heat any space unless you have geothermal with the same control arrangement.

If you have the Navien and it is short-cycling (on and off in less than 5 minutes) you may suffer loss of efficiency because the appliance is over-sized and never sees a steady state. It it is designed to modulate on a fixed water temperature rather than a reset temperature dictated buy outdoor temperature and design settings.

If return temperatures reach 130F and below, some level of condensation should occur if the unit runs long enough.

These systems are heavily marketed to the DIY crowd, but I fear (while promoted as cheap and easy) they are not as easy as one is lead to believe and should probably be reviewed by an experienced designer before installation.

I do use dedicated conventional water heaters as boilers for small systems occasionally (especially when designing for mild climates) and I like the Combi1 for combination systems. For systems over 1000sq. feet in colder climates, as you know from my previous posts, the ModCon, with indirect water heater is the «best buy» in hydronic radiant floor heating available today.

Leave a Reply