One thing to check on is the minimum temperature that your current boiler is allowed to operate at. For a lot of boilers, this can be rather high -- 140F+. This is bad for trying to work solar in because the solar collectors don't work as efficiently at these high temps.
We like the radiant floor solar heat more than our "backup" forced air heat, and we are in the process of trying to work out a good way to add a boiler style heat source to the existing solar system (kind of what you want to do, but from the opposite direction).
Right now, as Zap mentioned, my system circulates water directly from the storage tank to the collector loop and the floor loops -- no heat exchangers. I think that I may have to add a heat exchanger on the floor side to keep the boiler happy -- still not sure yet. I'd rather not, since the heat exchangers reduce efficiency and cost dollars, but may have to.
On the controls side, I talked to a local HVAC guy who is doing a solar/boiler system of his own, and the system he is thinking about works like this: at any given moment it would use EITHER the solar heat tank water OR the boiler heat source. Which heat source is used is controlled by an electrically actuated valve that basically connects the floor loops to the appropriate heat source. When there is enough heat in the solar storage tank it uses that source, if not, it uses the boiler. I like his idea in that it allows you to use the solar heated water at lower temps and keeps the solar collectors in an area where they operate more efficiently. I think that under cold conditions where the solar heated water is not hot enough to keep up with the house heat loss, it could even switch between the two heat sources every once in a while to allow you to keep up with the house heat loss by using the boiler once in a while to "catch up". Anyone see any downside to this approach?
GaryGary gary@BuildItSolar.com www.BuildItSolar.com
I'll check about the minimum temperature for the boiler. I would not think it would be a problem since it is made to heat th efloors at just enough above the set point to maintain the temperature in the house. My boiler has a sensor under the soffit that tells the boiler to fire up one two or three elements depending on the outside temperature.
I don't see the point in adding controls to have the system chose between the storage tank or the boiler. Why not simply run the piping in series, if there is not enough heat in the storage tank, the boiler turns on?
MartinEau, soleil, le vent[ Parent ]
lets say that you have a pipe coil heat exchanger in your solar storage tank with the output side running to the boiler inlet, then the boiler outlet goes to the floor loop, then the water goes through the floor loop and returns to the other side of the heat exchanger coil in the solar tank -- i.e. a series connection.
Suppose the solar tank is at 100F, and suppose that its cold enough outside to require 130F water running through the floor to heat the house. The water would come out of the solar storage tank at 100F, the boiler would boost it to 130F, the water will probably only lose about 10F going through the floor loop, so it will be returned to the solar tank at 120F. It seems to me that you will end adding heat to the solar tank rather than getting heat out of it. Probably not what you want to do.
This is fundamentally different than a solar water heating system, where all the water comes in at 50 or 60F and needs to get up to 120F -- then its fine to let the solar provide what it can, and boost up the rest of the way with a boiler or tankless heater. But, with a floor heating system, if you need 120F water in the floor to meet the heat load, its going to come back from the floor at around 110F -- this return water from the floor may already be hotter than the water in the solar storage tank.
There may also be limits imposed by the boiler manufacturer. Are they OK with circulating water through the boiler without the boiler adding any heat? It would be good to ask your boiler builder what they think if this. Tankless water heaters will handle this OK, but not sure about boilers.
If you do the other system where the floor loop feed is from either the boiler or the solar tank, then you can heat from solar whenever there is enough heat there to do it, and, if not, heat from the boiler without having the two interfere with each other? I think you could even alternate between the two to make use of solar when its not hot enough to take the full heat load.
Or, I may be totally missing the boat??
GaryGary gary@BuildItSolar.com www.BuildItSolar.com[ Parent ]
Very interesting point. Everything seems right with your reasoning. The thoughts that arise to me are the following:
I put the sketch that our local HVAC guy Eric drew up for me here:
http://www.builditsolar.com/Projects/SpaceHeating/SolarShed/AddBoiler.htm
It seems like a pretty simple way to go, and he does a lot of radiant floor systems (but no solar ones so far)-- so I think he knows the area well. This is what he is thinking about for his home when he adds solar.
Have not thought a lot about the control, but maybe a two stage thermostat could select solar for the first stage, and then switch to the boiler when the 2nd stage is needed. Heat pumps often have two stage thermostats, so its not uncommon. You would want to have some way to go right to stage two when the solar tank is too cool to provide any heat.
Right on the pump, you should use stainless or bronze. I use a Taco bronze pump -- they offer most of their pumps with a choice of case metal.
Number (1) is just the regular pump in the boiler that circulates water, not an electrically controlled valve as the label says.
So, when you want the boiler to supply heat: energize the boiler pump(1), deenergize the solar tank pump(3). Boiler heated water flows from the boiler, through the floor loops (on left of diagram), and back to the boiler.
When you want heat from the solar tank: energize the solar pump (3), deengergize the boiler pump (1). Water then flows from the solar tank, through pump 3, out through the floor loops, and back to the solar tank via the top line.
Check valve (2) prevents the solar pump from pushing water through the boiler in the wrong direction when in boiler mode. The check built into pump (3) prevents the boiler from pushing water back through pump (3) and into the solar tank in the wrong direction when in solar mode.
Maybe its confusing that the floor loops are not completely shown? The four floor loop lines on the bottom left of the diagram go around and connect to the 4 lines coming into the top left of the diagram.
This paragraph: Check valve (2) prevents the solar pump from pushing water through the boiler in the wrong direction when in boiler mode. The check built into pump (3) prevents the boiler from pushing water back through pump (3) and into the solar tank in the wrong direction when in solar mode.
Should read like this: Check valve (2) prevents the solar pump from pushing water through the boiler in the wrong direction when in SOLAR mode. The check built into pump (3) prevents the boiler from pushing water back through pump (3) and into the solar tank in the wrong direction when in BOILER mode.
Thanks MartinEau, soleil, le vent[ Parent ]
