At our FloPro classes held at Taco in Cranston (http://flopro.taco-hvac.com/2015flopro_training.html) we ask what is the ONE thing you want to know about Taco, Hydronics, Life…before class starts. One recent question, which I have seen before, was “How is boiler sizing affected when you have a low temperature radiant floor heating system and a high temperature baseboard system with a modulating-condensing boiler?”
Well, the short answer to this question is; it doesn’t!
The long answer to this is; it depends. It depends on the best way to control and maximize the comfort in the home with the most economy of operation but the heat loss of the home IS the heat loss of the home. It won’t change based upon the type of heating system installed. That’s the beauty of Hydronic heating. We get to mix and match all types of heat emitters in one project and have a central boiler plant. Finned-tube baseboard, cast iron radiators, fan coils, radiant floors….
The first thing to look at is the controller and programming of the boiler along with how much baseboard is installed. Baseboard manufacturers will state how much BTU output you can get from a linear foot of the element based upon an AWT (average water temperature). Most will stop their charts at 140 degrees or at least “gray” them out. This gray area really says, if you are going to operate in this range, you better know what you are doing! I was working on a project with a friend of mine a while ago (https://dhblogtaco.wordpress.com/2012/12/18/ask-dave-fuelmizer/) where I was asked to size a new boiler for a replacement. We not only sized a smaller, more efficient heating appliance, we also lowered the supply water temperature.
Next, you need to take a look at how much baseboard is installed and compare it to the actual heat loss of the boiler, for example;
The heat loss of the house is 48,600 BTU’s/hr and you have a total of 110’ of baseboard on two zones; the math is easy to figure out what we need. 48,600/110 = 442. That number is the BTU output needed per linear foot to heat heat the house on a design day. So how do we get baseboard to do that? Simple, change the delivery water temperature to the baseboard. Take a look at the chart and we see the BTU output of 400 and 480 per linear foot. At 150 degree water, we are going to be a little cold in the house, at 160, we are a little higher than needed, but it is a lot lower than the 580 per foot.
So setting up the equipment, the maximum boiler water temperature needed when there is a demand for heat from the baseboard would be a maximum of 160F. This water temperature is a maximum temperature for the coldest design temp of the winter. With some of the modulating-condensing boilers, this can be programmed into the controls of the boiler. Use your design day temp, say 10 degrees outside and maximum water temp of 160. If your controller doesn’t have this capability, then using the FuelMizer or the PC-700 can accomplish this with relative ease. Why bother with this? Extra components, electronics, design time…….Well, by adding controls such as this, you can add quite a bit of comfort to the system as well as economy of operation. Longer run time of the equipment which eliminates some short cycling as well as fuel economy!
The radiant floor system will depend upon the installation method, floor coverings and heat loss. If your design water temperature is kinda close to the 160F mark (plus or minus 15F), then you are good to go. No other components or thoughts of the system are needed. Just consider the zone of radiant floor just like a zone of baseboard. However if the water temperature is a lot lower than 160 (and it typically is) then a mixing device will be needed to lower the temp like the i-Valve.
Enjoy and I’ll see ya out there!