Hydronic radiant floor heat loads when combined with a forced-air system
Pretty good home, Climate Zone 6a ~ 3900sq.ft. This is not technically a green building question but more of a systems integration question.
We had always planned Hydronic Radiant floor heating in our home based on our general liking of the type of heat and warm toes in the winter. That was supposed to be my only system, no cooling. However my general contractor has been pushing me to add a forced air heating system to prevent pipes freezing should the boiler go out. He tells me he typically installs forced air with a radiant floor system for that reason, and to accommodate quick changes in temperature of the house such as unexpected cold spells.
I had been resisting until this summer. It has been so stinking hot in our area and we are just miserable with the heat even although I’m managing the house temp with open windows and fans at night and closing the windows during the day it’s not enough. So I’ve decided we are definitely going to put in a forced air system, primarily for cooling but with the quick heating benefits of a furnace system too.
My contractor and subs seem to be treating each system as a separate entity. The radiant floor guy doesn’t talk to the HVAC guy and visa versa. My gut tells me I should be designing conditioning of my home as a single system with two subsystems. How do I go about doing that or finding someone who can help me?
My thought process is this:
1) I’ll only do forced air on the upstairs level, but have radiant floor on both levels. We have large enough stair well and open area to connect both levels that there will be “some” air exchange between levels.
2) I’ll size the radiant floor system to accommodate the full heat load.
3) I’ll plan the forced air heating system to accommodate only a partial heat load. Let’s say I plan the radiant heat load to the 99% Dry Bulb of 2.6F I’ll plan the forced air heat load using a 20F or warmer cold condition.
This way I’m not over sizing the furnace for what really is a supplemental role.
Other thought’s I’ve had about whole system design is how would I set up the controls (if it can be done) so that the radiant floor heats the house to say 60F and then the forced air takes over the load to get me up to the 68F? The forced air would also deal with any quick temperature changes required too.
My thinking is that the thermal mass of radiant floor doesn’t like to change its temperature and when it does, it is very slow. Having the furnace provide the heat from 60F to 68F means that at night we can drop the temp to 60F to keep it cooler for sleeping. I understand I need to ensure I don’t short cycle the furnace. Can this type of system be designed? Does anybody of any info on how to do it?
p.s. In general I like the new site
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