Hydronic Radiant Floor Help (Codeword I'm Clueless)

cayenne

I'll be the first to admit - I'm a complete newbie to hydronic radiant floors. I got relocated to Minnesota this year and am starting to see astronomical energy consumption. Temps have recently dropped into the 20's and are now heading towards 0F later this week.

I do not have a slab sensor, and have installed a Honeywell thermostat measuring ambient temp. No schedule, no weird smart thermostat stuff. I have it set to 69F... Our heat pumps are still running in the house but will continue to lose efficiency as we approach 0F

What I'm seeing is the thermostat will make a heat call, turn on and then run for 8-10 hours and then it will shut off and the floor will cool back off and the next day we do this all over again. I'm consuming 8-10 kWH per hour during the hours that this is running so each time it cycles like this we go through 60-80 kWH

I'm new to all this but it seems like we heat the water, the house is nice and then we let the slab cool off because the house doesn't need it and the thermostat isn't making a heat call. Seems like a viscous cycle to me.

Sorry for all the rookie questions, but I'm trying to learn and understand how I should manage the situation.

Jamie Hall

I presume that something -- electric resistance? -- is heating the water going to the slab? Ideally, a slab should have "just right" water running through it all the time and be maintaining an even temperature. This is usually done by having a mixing valve in the slab circuit, so that some return water is mixed with the water from the boiler or whatever, to obtain the desired temperature. That valve should be controlled either by a floor sensor or perhaps better through and outdoor reset.

The boiler or whatever then runs just long enough from time to time to ensure that there is, in fact, enough hot water getting to the slab supply line. If it a modulating boiler (or an electric boiler) it should be controlled to get the just right temperature, and a mixing valve may not be needed depending on what else is being heated by the same source.

cayenne

Jamie Hall said:

I presume that something -- electric resistance? -- is heating the water going to the slab? Ideally, a slab should have "just right" water running through it all the time and be maintaining an even temperature. This is usually done by having a mixing valve in the slab circuit, so that some return water is mixed with the water from the boiler or whatever, to obtain the desired temperature. That valve should be controlled either by a floor sensor or perhaps better through and outdoor reset.

The boiler or whatever then runs just long enough from time to time to ensure that there is, in fact, enough hot water getting to the slab supply line. If it a modulating boiler (or an electric boiler) it should be controlled to get the just right temperature, and a mixing valve may not be needed depending on what else is being heated by the same source.

Correct - utilizing an electric boiler. I looked in the junction box and it has wires on the outdoor reset terminals. Just moved into the house in August and didn't build it so I'm left trying to learn what the heck is going on.

mattmia2

8 kw is about 27,000 btu, that isn't an unreasonably amount of heat loss especially if it is only being replaced about half the time. Changing the outdoor reset curve(or enabling it if it isn't set up) could make that evenly consumed without the slab heat up and cool down. Does it overshoot after the thermostat turns it off?

Unless electricity is really cheap, heating with electric resistance is expansive.

Ironman

The Out Door Reset works from a mathematical curve to vary the water temperature based upon outdoor temperature. The colder it gets outside, the warmer the water temperature and vice versa.

Your slab is high mass, a huge rock. It takes a long time to get it warm and it will take it a long time to give off its heat and cool down (the flywheel effect).

When the ODR is set correctly, the slab should gently and unnoticeably idle along giving  off just enough heat to match the load. The thermostat becomes almost unnecessary, just a high limit control. I’ve got a place next door working like this. It hasn’t had a thermostat installed in 12 years since I installed it. It stays a constant 68* all through the heating season.

You either have a problem with the ODR not being setup properly or the thermostat - or both. This assumes that everything else in the system is correct.

Ironman

What’s your KWh rate? 

A straight electric resistance boiler may be the most expensive of any fuel to operate.

cayenne

Ironman said:

What’s your KWh rate? 

A straight electric resistance boiler may be the most expensive of any fuel to operate.

kWh is .0969 unfortunately I don't have an option. Our heat pumps will lock themselves out approaching 0F and we will be forced to utilize the electric boilers. We have two propane fireplaces as supplement, but propane isn't going to heat the entire house nor is it cheap these days.

Ironman

A propane boiler should be substantially cheaper to operate, depending on the cost of LP in your are.

There are also air to water heat pumps which work well with radiant floor. You could keep the electric boiler to supplement in cold weather.

If you’re waiting until it’s near 0 to turn the electric boiler on to heat the slab, that may be your main issue. A radiant slab is not designed to be operated that way. It will several hours, maybe days, for it to reach equilibrium doing that.

Hot_water_fan

A propane boiler should be substantially cheaper to operate, depending on the cost of LP in your are.

I'm not sure: $.0969*293 = $28.39/MMBtu. Propane needs to be under $2.46 to beat that with a modcon. Probably not worth replacing for sub-zero temps.

hot_rod

This may help determine which fuel source will be least $$.
https://coalpail.com/fuel-comparison-calculator-home-heating

cayenne

Ironman said:

A propane boiler should be substantially cheaper to operate, depending on the cost of LP in your are.

There are also air to water heat pumps which work well with radiant floor. You could keep the electric boiler to supplement in cold weather.

If you’re waiting until it’s near 0 to turn the electric boiler on to heat the slab, that may be your main issue. A radiant slab is not designed to be operated that way. It will several hours, maybe days, for it to reach equilibrium doing that.

Thanks for all the info... Much appreciated! I'm debating about swapping out boilers but it is a large investment and I need to look at the ROI. We aren't waiting until we reach the shutdown point of the heat pumps as the boilers were turned on when we dropped below freezing. The cycling is difficult to understand though - as an example

Saturday 151 kWh
Sunday 35 kWh
Monday 144 kWh
Tuesday 42 kWh

All the same temperature swings and high/lows each day