Electric heat source for radiant

chezking

I'm a retired CA general contractor that built my own home in NorCal, completed in 2005. The heating system was a professionally designed hydronic system. It uses a Munchkin T-80 boiler anchoring a 3 zone closed loop system; indirect DHW, Hydronic forced air for 2000sf upstairs, and infloor hydronic in a 2000 sf slab. I also insulated well above Title 24 and added an ERV.

Unfortunately, the energy costs were horrific for an area that rarely sees temps below freezing. in spite of only intermittent loads from electric appliances like oven, dryer, fridge, PG&E hit me for more about $300/mo for power. In addition, the propane use exceeded over 500 gals year so yearly energy costs were $5-6000. We mainly used a MasterCool evaporative for cooling which worked fine and wood heat to ease the heating costs with little effect and more hassle. I also became somewhat competent at keeping the Munchkin operational, including getting a warranty replacement after only 3 years of use. even that didn't end the reliability issues.

Last November, I finally couldn't keep the unit running. We survived the winter on wood heat and a propane on demand for hot water. We had installed PV solar so we are changing over to electric based comforts. Though we installed a York air to air heat pump and I thought it would handle the basic needs for the whole house but downstairs really needs the hydronic back on. I foolishly let the old air mover unit go but all the other pumps, controllers, indirect storage are still (somewhat) intact.

My question: I've prepaid my electric for the rest of my days with a massive PV array. Ideally, I SHOULD have installed an Air to Water unit and built from that. Now, after a very expensive winter and also concerns on if this is my "forever" house, what suggestions are there to spec an electric heat source for the infloor system. BTW, rooftop water heating would be a stretch due to short solar exposure during the heating season. I also set the t-80 aside knowing there is a source for replacement parts but it would be a hard sell after the problems I've had. Hell, an inline hot tub heater would do as well for $150.

Thanks in advance for your thoughts, Brad King.

mattmia2

What is the heat loss that the radiant has to cover? An electric water heater is an option if the heat loss is less than the wattage of its elements. There are also numerous electric boilers out there. Probably would want a modulating boiler for radiant.

pecmsg

Have the existing system revaluated. Something is drastically wrong. A well-insulated house in your mild environment shouldn't use that much LP!

Hot_water_fan

Air to water is the solution to keep the setup similar to what you have now, while another solution would be: air-to-air heat pump for upstairs (who cares if that's hydronic) and electric boiler for the lower floor if you want to keep the in-floor.

Jamie Hall

Hot_water_fan said:

Air to water is the solution to keep the setup similar to what you have now, while another solution would be: air-to-air heat pump for upstairs (who cares if that's hydronic) and electric boiler for the lower floor if you want to keep the in-floor.

Agreed. There is no reason that I can see why an air to water heat pump for your downstairs radiant shouldn't be quite adequate, and that's the way I would go for it. You should be well within the output temperature range of the heat pump for that floor.

There are a number of manufacturers.

Since they exist, don't go to electric resistance -- the heat pump should cost less than half as much to run. Now would it be hot enough to restore the hydronic air upstairs? Maybe. But you could certainly get run it as a chiller for the upstairs for air conditioning.

chezking

Thanks for the quick replies all.
First, a typo for downstairs slab 1200sf not 2000.

I agree the downstairs system could be handled by a small A/W heat pump, especially with the possibility of incorporating the indirect storage tank. It's probably the way I'm most inclined though I can't find anything close to the $800 a resistance type heater (spa or on-demand) prices out at. Plus I have to set up another condenser outside. Some now apparently use CO2 which is cool! The A/W unit might also heat some radiators, towel warmers or ? with the old zones and thermostat I already have. Some article also mention reversing the pump to cool the slab. This is attractive because summer temps here rarely go below 70º for months so the slab never cools below that. All those reasons push me towards the Air/Water option.

JeffM

I also like the air-water heat pump solution, but don't forget about the "massive PV array". If it has enough capacity to offset the load of an electric resistance boiler, then that's a cheaper option (though without the cooling option). Electric boilers take more electricity to run than heat pumps, but if you're not paying for that power then it doesn't matter - and electric boilers can be simple, reliable, and not very expensive to purchase.

Zman

What are your design temps for the infloor and fan coil?
Maybe air to water for the infloor and a ducted mini-split for the fan coil?

Jamie Hall

I didn't mention the "massive" (presumably meaning large, not heavy?) solar array for a very simple reason: it doesn't work very well at night, when it's cooler. Also, double check the power consumption -- even of a heat pump, never mind electric resistance. Remember that even in full sun, a PV array will produce -- at best -- 20 watts per square foot.

chezking

Regarding the PV. 32 panels produced 11.1 KWh in Feb, my first month of operation and about 4 hours of production due to trees and topography. Now I'm over 60KWh / day so with Net Metering and the meter has always shown a daily net loss. I'll be fine on anything I can throw at the system. It wasn't cheap but taking charge of one's own power-particularly as we start using EVs as backup-is a blessing. @Zman I didn't design the old system but the boiler ran a 180º primary and was supposed to modulate back for the floor through a mixing valve set at about half that (I could look it up). Floor has 6 zones on and eight zone manifold.

Jamie Hall

Yes. So long as you remain connected to the grid, and the grid has constant back up power (nuclear or fossil fuel are your choices), and so long as the folks who can't install PV will pay for your system, you'll be fine with it.

May I suggest, however, that you mind your units. KW is power. KWh is energy. Your figures show a power output of around 100 KW which is indeed a large PV array. More than enough for your heating needs, so long as the sun is shining.

Do you have any battery storage? Or do you depend on the grid to supply your power at night or in cloudy weather? If you truly want to take charge of your own power -- and I wouldn't blame you, where you are located -- you need battery storage. Otherwise you are still dependent on the power company...

Dave Carpentier

I wonder how much of that hot water propane is going for heating vs for domestic hot water ?

Hot_water_fan

I didn't design the old system but the boiler ran a 180º primary and was supposed to modulate back for the floor through a mixing valve set at about half that (I could look it up). Floor has 6 zones on and eight zone manifold.

Low temperatures for the floor will be well within the range of an air-to-water heat pump, but hydronic air handlers operate at the higher end. You could probably use 120 for that, but it'll be less efficient than using 90 or whatever for the floor. Your heat loss seems pretty low thankfully, so you'll have little problem finding a suitably sized heat pump. At those outdoor temps, you'll have wonderful efficiencies.

Storage wise, water offers some interesting opportunities if you're so inclined.

pecmsg

Hot_water_fan said:

I didn't design the old system but the boiler ran a 180º primary and was supposed to modulate back for the floor through a mixing valve set at about half that (I could look it up). Floor has 6 zones on and eight zone manifold.

Low temperatures for the floor will be well within the range of an air-to-water heat pump, but hydronic air handlers operate at the higher end. You could probably use 120 for that, but it'll be less efficient than using 90 or whatever for the floor. Your heat loss seems pretty low thankfully, so you'll have little problem finding a suitably sized heat pump. At those outdoor temps, you'll have wonderful efficiencies.

Storage wise, water offers some interesting opportunities if you're so inclined.

That can be overcome with oversized coils.

Larry Weingarten

Hi @chezking , When you built, did you have opportunity to use a blower door to help with air sealing/shell tightening? I'm wondering if combining using a blower door and IR camera now might show you places that could be improved in the shell.

Yours, Larry

JK_Brown

I recently ran across a British youtube channel 'Heat Geek' that has videos for their switching over to using heat pumps as the heat source for hydronic systems. It's being mandated in the UK and EU areas. That site covers doing calcs, set up and such. They are not just drop in replacements as the HP systems will likely need to run at lower supply temps using modulating thermostats.

That channel may have some good info you can use.

mattmia2

pecmsg said:

That can be overcome with oversized coils.

And slowing down the airflow one way or another to match the output. It is possible they already have oversized coils and they just need to deal with the airflow so it doesn't feel drafty.

jumper

If you have enough PV then use resistance to make very hot water for energy storage. Goal is to eventually be off grid because utility costs will increase no matter how much surplus electricity you send back to Pac. G&E.