Hydronic Floor pump frequency vs water supply temp?
 

I am in the beginning stages of designing an hydronic in floor setup for our main level. I always hear that the lower the supply temp the better the efficiency, but what I take from this the pump has to run longer times than correct? What I am thinking is that instead of having 90 degree water and the pump running half the time wouldn't it be more efficient to have the pump run for shorter periods with water at a higher temp?

For this discussion I would like to take away the energy loss from heating the water 120 degree vs 90, reason is my hot water source is already producing water at 140 about so it really is no difference.

So summarized I am wondering why it would be so bad to put 140 degree water into floor at a flow rate to get a delta t of 15 degrees with less frequency needed vs a lower water temp.

Thanks everyone for your input!

bump* I am sure somebody has an idea about this? Any ideas to throw around are appreciated. Am I stupid in thinking this way?

where are you getting your info from that you want a lower temp for more efficiency?
You want a lower temp for comfort, a floor that is to hot makes your feet sweat, 140F sounds about right for heating a concrete floor because it will take a while for that floor to soak up the heat from the piping, the thiner the floor the cooler you want the water going in to it to be.
With solar hot water systems I've seen 160 to 170F water going in to a sand bed under the slab and you would turn that system on to start heating up that floor in september or so, but with solar you are going to see those higher temps on almost any sunny day.

Ryland thanks for the reply! I have a above floor sandwhich system so it should heat up pretty quick, is the reasoning behind lower temps for thinner floor is that the floor is more even of temp then? I have about a 2" thick mass to heat up.

Thanks!

The purpose of lower temperature water is to allow heat pumps and solar hot water systems to be more efficient. If you don't have either of those then there isn't a huge benefit to sizing a system for lower water temperatures. There are smaller benefits to be had though. Closer tube spacing will allow for more uniform and faster heat up times, this should reduce the amount of time the pump must run.

Perfect that is exactly what I wanted to hear! So is there really a need to mix down my 145 degree water to 120 to run into the system or does it make more sense to just run it what it currently is. It is an open system so I already have the water at 145 for household hot water. Thanks

I have to disagree here. The MAX floor surface temp should be about 85F. By using a high input temp you are using a big V8 engine, gunning it, then putting on the brakes till it comes down then repeating. This is not the way to heat. If you are using a gas boiler or tank, the lower your temp the higher your efficiency, as much as 20% better. Plus every start and stop of a pump or boiler is done at a very low efficiency. The electrical cost for a small circulator is about $20/year. There is no point in not running it flat out.

Higher input temps also mean higher heat loss near the tubing and we don't want this. So, the rule of thumb is as tight a tube spacing as possible, and as low a water temp as possible. If you every want to use a HP or solar for heating, you will be able to use much more of your heating source with a lower temp.

Also 145 is way too high for DHW, most showers run at 105F or less when measured and 145F is a waste of energy. Anti-scald valves are set for 120F max so why heat higher?

From experience we have in our home a heated slab of concrete 6 inch th. finished with ceramic and porcelain tiles. We began heating it with a oil-fired hot water tank. It worked very well. The input temp was 180 degF and the circulating pump ran and stopped keeping the floor at a constant temp. and it was very comfortable. During the winter months and as the price of oil began to shoot up I noticed the oil burner seemed to run a long time. Makes sence. The flue gasses flowing up the stove pipe in the furnace room made the room quite warm. It made me think! there is a lot of heat going up the chimney. If I only need the floor at 85 degF and for showers only 105 degF why am I heating the water to 180 degF. I lowered the temp on the water heater to 110 degF the house stayed as warm and we didn`t notice the difference with the showers but the oil consumption dropped way down and the furnace room was no longer like a sauna. The oil burner ran a lot less.

Long story short. Lower temperatures mean lower losses. The little circ. pumps are designed for continuous operation and are very inexpensive to operate.

We are now heating with solar hot water and geo-thermal which never exceeds 106 degF
The tubes in the concrete are 12 inch centre to centre and .62 inch dia.

However this is an older technique, better is as Mikesolar states smaller tubes placed closer together 4-6inch and closer to the surface to be heated. I had also learned by experience heating between the floor joist below the plywood dosen`t work. The tube needs to be placed on top possibly in a routed groove and covered with a flooring that will conduct heat like ceramic tile or a thin hardwood, but never carpet. Check (Uponor)

Randen

Hey compman723,

You know what my system looks like ;)
I'm just turning on and off the pump based on various data gathered by sensors over time. Running about 110 degree supply, and I'm getting round about 100 degree return temperatures. It's working well so far - due to the relatively long latencies of radiant floors, I haven't seen a need yet to adjust supply temperature, and turning on and off the pump works well enough. However, I'm still keeping the option of adding a mixing valve and controlling it based on the difference between supply and return temperatures as well. That should be relatively easy to add, so I didn't worry too much about it for the initial setup.

I don't know what your heat source is but if it is a non condensing boiler, a mixing valve is necessary. They don't like low water temps. Your best bet is to get a Tekmar 256 outdoor reset boiler control (or pump control) which will vary the boiler output temp based on outdoor temp. You need to pick one temp to monitor, supply or return (supply is better, IMHO) and the control will do the rest. This would be for a traditional boiler as most condensing boilers have this feature built in.

If you are working off a water heater, which you run at 130F, the control can run a mixing valve or injection system to give you the desired water temp.

I am a fan of thermal mass (I have installed over 1,000,000 ft of tubing over the years) while others in the biz like the aluminum plates. To me, a 6-8" slab is the best way to hold the heat and is so resilient as far as water temps (within reason) that the comfort is always great. You can experiment with water temps better than with an aluminum plate floor. Others may disagree but this has been my experience.