In-slab hydronic augmented w/ baseboard
| Posted in Energy, Heating & Insulation on
What are the elements of system design when doing in-slab radiant heat, and augmenting it with hydronic baseboard units to assist in deep winter times of low sun and below-zero temps? My design has a lot of windows, and I think I may need this. Electric rates are ridiculously low, and we will heat using an electric boiler. Are additional thermostats used to control the baseboard units to kick in when needed? Are loop lines run from the manifold dedicated only to the baseboard units? I want a little basic education here, before talking to my plumbing and heating sub.
Replies
Gene...............it seems that no one wants to bite at this one!?
It is a HUGE & COMPLEX question that you have asked............plus all sorts of permutations with exceptions and contradictions.
Let me give it a shotgun blast of assorted info.
Have you calculated your heat load requirements?.......Step one of the answer to your question. Also required are solar heat gains/loses and ratios of glazing area versus passive solar floor absorption area. Your sub should be able to do these calcs for you.
It is unusual that electric boilers are more economical than gas fired...but you may have an exception. Collect the cost of natural or propane, also. Once again your subs will be able to inject their experiences and opinions.
Heating loops in slab are run at about 100 degrees as opposed to a baseboard loop which is about 160-180. Temperatures in floor over 100 degrees tend to leave blisters on bare feet! So loops are separate and the floor loop requires a mixing system to combine returning cooler water from the slab with the hot boiler water to be sent back to the slab manifold. Baseboard loops are straight from the boiler.
Your heating sub can set you up with separate thermostats or sequenced thermostats or manual set-ups or inslab heat sensors or sensors attuned to exterior temps. Prefences and $$ may make your decision for you.
The design of the slab system is usually done by the supplier of the tubing, and upon purchase of the system the drawings are included. Once again your sub should be able to help you at this point.
Maybe this will start you on your way.....................Iron Helix
I've not yet come across using the baseboard as a supplement to the RFH, nor the RFH ever being insufficient, so I'm interested in learning if this is common practice anywhere. Typically a tubing plan will have lines spaced closer nearer exterior walls and windows to provide more heat in that area.
I know that our 500 sf window wall is estimated to lose 11K btu/h on a cold winter night. Instead of paying to provide more heat there to compensate, I'm wondering if providing window insulation, such as a window quilt, would be money better spent. What _are_ you electric rates, btw?
Most recent bill, $68.03 for 2381 kwh consumed. Works out to $0.02857 per kwh.
I have done this several times, and personally, I think it makes sense, but most people don't seem to do it. I am doing it on the job I am now because the radiant slab only takes up a portion of the addition, not enough to provide all of the heat. But I have also done it when I wanted to provide some heat to a room that had a more rapid response time than just using radiant. I size the units based on a heat loss survey, and zone them using a separate thermostat. Obviously you can have either a zone valve or a pump to operate the baseboard zone. Usually I would leave the baseboard thermostat set a degree or two behind the floor so that they only come on when the floor isn't keeping up.
Thanks for the input, Nick. Are dedicated loops run to the baseboards, with mix valves delivering hotter water to them, or do you come right out of the adjacent in-floor loop, and have the local baseboard thermostat control valving, to bring on the baseboard radiator only when needed? I know I can get into this with my plumbing and heating contractor, but I want to be as informed as possible before soliciting bids. It can hang below zero here for weeks, with lows sometimes plunging to 45 below. My floorplan has three areas of concern, a 10x12 breakfast room bumpout with windows on most of the three walls, a corner of the LR with large windows on adjacent walls coming tight into the corner, and an 8x10 bumpout off the LR with windows on most of the three walls.
I would normally run a separate loop to the baseboards. If you look at the output of cast iron based on input water temperature( http://www.burnham.com/radiator/58100.cfm ), you will see that you lose a lot of heat output running them at lower water temps. However, since you are running off of an electric boiler you have to make a choice. You can either run the boiler at a higher water temp, feed directly to the baseboards, and use a mixing valve on the floor, or, if you were planning on running the floors directly off the boiler water temp, then you could just use more cast iron to make up the heat output that you need.
Additionally, since you are talking about three separate areas that may need supplemental heat, you will have to decide whether or not each needs to be zoned separately, or if they can all be hooked together. Logically, although each area is different, it is likely that the same conditions will exist at all three areas when you need more heat. Plus, you can use the balancing valves on the baseboards to get each area fine-tuned. Zoning each separately will be expensive.
Frankly, since you are using electricity to begin with, you might want to look at the option of putting in an electric radiator at each area instead of baseboard. Look at Runtal and others that are nice. They have the added bonus of having built in thermostatic control. By the time you pay for the cast iron, plumb to it, and the controls, you could easily buy an electric radiator.
Should you choose to use the cast iron, look into plumbing them with 5/8" or 3/4" HePEX. I use this frequently instead of copper, and the installation is a LOT cheaper and easier, especially if you already have an investment in the tools and fittings (which assumedly someone does if you are using PEX for the radiant floors).