ideas for hydronic heating with a solar powered water heater

[11thhourfabrication]

Hi
New to the forum but have question about water heaters. My house in Marin County CA. currently has hydronic baseboard heaters with a 115K BTU/hr gas fired boiler in the basement which runs $250 to $450 during the winter months. No issues with the operation of current system but I am generating so much extra solar power that I would like to heat the house instead of giving PG&E free power. So far I have generated over $1200 in extra power with 2 months to go before the year is up. I would not have a issue if I could apply towards gas or another property or cash back, but that is not the PGE way. I was thinking of a hybrid hot water heater running during the winter to heat the home which would eliminate the bulk of my gas usage. I talked to a tech a Rheem and he said that the heater could easily get to 140 degrees and it would be possible to reach 160. My boiler runs at around 180 degrees and the only thing I see is it taking more time to heat the house. I also have 20K in battery backup so I could in theory keep the whole system running all night since the hybrid hot water heater only consumes 5.5 amps when the heating elements are not used. Anybody have any ideas or alternative ways of setting up the type of system I am proposing. Thanks in advance.

 

[OzSolar]

Welcome to the forum!

You also have to look at the BTU's you can generate. EG: if your 160F tank cools off to 100F in 15 minutes and takes 2 hours to recover your house will also cool off.

I don't know how many BTU's the the HP water heater would make in HP mode but I suspect it's less than it makes in straight resistance mode.

4.5kW x 3,413 BTU/KW = ~15.4 KBTU's best case.

Comparing that to 115K BTU and I think you can see where I'm going.

I'm in a similar situation with excess credits and hydronic heating. I purchased a new electric water heater with 2 x 5,500 watt elements. I rewired it and added a 2nd 30A circuit so I could run them both at the same time. I can get 38.5 KBTU and that keeps up with my house most of time.

 

[upnorthandpersonal]

I don't think this will apply to your situation, but just in case. In my set-up (off grid) I use a heat-pump in Spring and Autumn. I use a 3000L insulated buffer tank to do this. I can easily store a day's worth (more really) of heating in that tank with the amount of solar power I generate. This means that I have essentially a large battery storage when my LFP batteries are full. I can't use that principle here in winter since there is no sun (63 degrees north), but others could. Together with the LFP battery, you can cover several days, only running the heat pump where there is solar available or you are able to drain the battery if the forecast predicts sunny weather.

 

[efficientPV]

I love complicated control systems. Are you talking a standard water heater? I can see you spending a lot of time and money and not getting any results. If you have to ask questions, you are not up for this. I suggest going the mini split route.

 

[11thhourfabrication]

I love complicated control systems. Are you talking a standard water heater? I can see you spending a lot of time and money and not getting any results. If you have to ask questions, you are not up for this. I suggest going the mini split route.

Hi
Thanks for responding. I was thinking about a hybrid hot water heater in place of the boiler. With battery backup and excess solar power it really is not a issue of running it constantly during the winter. Just wondering if there is a better way to achieve the goal of heating my house with my excess power. I am asking questions to see if anyone has tried this and to see what their results were before I consider moving forward.

 

[dustyPanels]

What if you added an electric water heater to supply warmed water to your boiler? I don’t know how you'd control it though. You'd only want your excess power to be used.

People used to use solar hot water systems to pre-heat water for their water heater.

 

[Crowz]

This is seems to be the modern version of what we used for heating in the old days.

Solar Storage Tanks - SunMaxx Solar

Experience the durability, customization, and engineering excellence of StorMaxx™ Solar hot water storage tanks from SunMaxx, the industry's leading solar

www.sunmaxxsolar.com

Basically solar collectors which are copper pipes running thru a black backed panel with glass over it but could be open faced.

The water circulated thru the panels and into the tanks. Most of us used large open topped tanks and put insulated covers on top of the tanks. Tank water is heated during the day and the heated the house during the night. Works perfect as long as you have enough water storage to handle the temperature drop off during the night.

I'm not sure where all of the research stuff I had from back then went. I had all of the calculations for how much water you need to heat x sq ft vs outside temp vs insulation etc.

Should work fine with based board heating since that's what most were using.

Far more efficient than electrically heating the water. Of course that depends on clouds but if you have to heavy a cloud cover to store heat you won't be making any electricity either.

 

[upnorthandpersonal]

I had all of the calculations for how much water you need to heat x sq ft vs outside temp vs insulation etc.

I have mine written down here https://medium.com/@upnorthandoffgrid/6-heating-systems-5c5727c1607e

Metric units though...

 

[OzSolar]

Hi
Thanks for responding. I was thinking about a hybrid hot water heater in place of the boiler. With battery backup and excess solar power it really is not a issue of running it constantly during the winter. Just wondering if there is a better way to achieve the goal of heating my house with my excess power. I am asking questions to see if anyone has tried this and to see what their results were before I consider moving forward.

You need to compare the energy of the gas you are purchasing during the winter to how much excess energy you have. Until you do that calculation you're just guessing.

 

[S Davis]

If standard fine tube baseboards you will get about half the output at 140 degrees instead of 180 degrees.

140 degrees is 340btu

160 degrees is 480btu

180 degrees is 610btu

This is per foot of baseboard.

 

[circus]

KISS I'd probably just buy a cheap electric space heater and plug it into a digital line voltage thermostat and call it a day. Keeping it separate. Cheap, fast, easy, redundancy if something breaks down. Less risky to existing boiler. Maybe a heat pump?

 

[11thhourfabrication]

You need to compare the energy of the gas you are purchasing during the winter to how much excess energy you have. Until you do that calculation you're just guessing.

Hi
Thanks for the reply. I realize that I need to do that but I was looking to see if anybody else has done something like this and what kind of results they had. I used 486therms last year Dec to April which translates to 14.5KWh and over the course of the year I exported 9MWh to PG&E. I think I have the generation side covered, I just need to find the best way to use my excess power to heat the home and domestic water

 

[OzSolar]

KISS I'd probably just buy a cheap electric space heater and plug it into a digital line voltage thermostat and call it a day. Keeping it separate. Cheap, fast, easy, redundancy if something breaks down. Less risky to existing boiler. Maybe a heat pump?

Right, and if you get three of them, each with a high/low setting you'll have 6-stages. They'd all have to be on separate breakers of course but the point is that 30 minutes after getting back from the "space heater store" you've got a system set up. A clumsy system that takes lots of manual control but still.

 

[11thhourfabrication]

If standard fine tube baseboards you will get about half the output at 140 degrees instead of 180 degrees.

140 degrees is 340btu

160 degrees is 480btu

180 degrees is 610btu

This is per foot of baseboard.

Hi
Thanks for the reply. That is the type of information I was looking for. If I read your figures correctly I will have to run the system twice as long to bring the house up to the same temp as the boiler.

 

[OzSolar]

Hi
Thanks for the reply. I realize that I need to do that but I was looking to see if anybody else has done something like this and what kind of results they had. I used 486therms last year Dec to April which translates to 14.5KWh and over the course of the year I exported 9MWh to PG&E. I think I have the generation side covered, I just need to find the best way to use my excess power to heat the home and domestic water

Have you done that math?

1 therm = 100k BTU's.

Can you show your math on how 486 therms = 14.5 kWh.

 

[11thhourfabrication]

Right, and if you get three of them, each with a high/low setting you'll have 6-stages. They'd all have to be on separate breakers of course but the point is that 30 minutes after getting back from the "space heater store" you've got a system set up. A clumsy system that takes lots of manual control but still.

Hi
Any type of space heater you would recommend. I realize that is a simple solution but I was looking for something a little more permanent with my existing infrastructure.

 

[OzSolar]

Hi
Any type of space heater you would recommend. I realize that is a simple solution but I was looking for something a little more permanent with my existing infrastructure.

Hi

I'd be glad to look later. I'm trying to get my wife's parking spot back her at the moment. Building a new ground mount in the shop and I'm trying beat sunset. I've had to steal her indoor parking while I build it.

 

[S Davis]

Hi
Thanks for the reply. That is the type of information I was looking for. If I read your figures correctly I will have to run the system twice as long to bring the house up to the same temp as the boiler.

Depends on how the current system is sized, it might not have the capacity at the lower water temperature to overcome the heat loss of the structure.

 

[circus]

just buy a cheap electric space heater and plug it into a digital line voltage thermostat

A clumsy system that takes lots of manual control but still.

No monitoring needed. If for some reason you just want to use the boiler, have it's thermostat a degree or two higher than the space heater thermostat. If you have excess solar energy to burn off set the boiler thermostat a degree or two lower.

 

[OzSolar]

No monitoring needed. If for some reason you just want to use the boiler, have it's thermostat a degree or two higher than the space heater thermostat. If you have excess solar energy to burn off set the boiler thermostat a degree or two lower.

What sort of space heater are you talking about? The common plug in the wall style is limited to 1,500 watts. One of them is not going to replace a 115k BTU heater.

 

[circus]

What sort of space heater are you talking about? The common plug in the wall style is limited to 1,500 watts. One of them is not going to replace a 115k BTU heater.

Of course not. Doesn't replace central heat, reduces fuel used. Sixteen years ago I built some solar air heaters, cut my propane use in half. Wisconsin

 

[JLV03]

How are you heating your domestic water (not used for your heating) now? If it is a tank off of your boiler, you could get an electric water heater and put it before/after/in-place of that tank.

Another method would be to get an electric boiler and switch between gas and electric based on time/fuel cost/net metering excess bank.

I use a 6kW electric boiler to heat my garage - haven't had an excess banked since I turned it on!

Thermolec

Dual-energy standard boilers for fossil fuel backup or standalone installations. Standard models include great features like a modulating electronic controller with outdoor sensor as well as integrated load management control.

www.thermolec.com

 

[11thhourfabrication]

How are you heating your domestic water (not used for your heating) now? If it is a tank off of your boiler, you could get an electric water heater and put it before/after/in-place of that tank.

Another method would be to get an electric boiler and switch between gas and electric based on time/fuel cost/net metering excess bank.

I use a 6kW electric boiler to heat my garage - haven't had an excess banked since I turned it on!

Thermolec

Dual-energy standard boilers for fossil fuel backup or standalone installations. Standard models include great features like a modulating electronic controller with outdoor sensor as well as integrated load management control.

www.thermolec.com

I currently have a gas tankless heater for domestic water heat. It is not a large part of my energy usage that is why I was trying to tackle the radiant heat boiler first. If The hybrid hot water heater did not work for heating the house I would just switch it over to my domestic hot water and the get a high temp heat pump for the boiler. I was trying to go the cheap route first to see if it will work.

 

[11thhourfabrication]

Depends on how the current system is sized, it might not have the capacity at the lower water temperature to overcome the heat loss of the structure.

I have not measured the heat loss of the structure, would not know where to begin. After putting a second story on with r30 insulation and radiant heat foil (walls and roof), insulating space on the original roof (1 inch air gap), double pane windows in the whole structure I can feel the difference in the summer and winter. The solar panels in the summer shade the roof so well that I have no need for AC, when it is 100 outside it is 78 inside.

 

[Madcodger]

If you wish to use a heat pump water heater for space heating, and locate it within the same space (structure) you are trying to heat, you aren't accomplishing anything. Heat pumps of any type don't generate heat - they move and concentrate it. So, you'd be removing heat from the air, putting it into the water, and then releasing it back into the air. That doesn't get you anywhere.

Instead, look at air-to-water heat pumps, which capture heat from the outside air (yes, even when it's cold outside, there's heat energy in the air). The problem is, they are relatively uncommon in the U.S. (much more common in Europe) so they are expensive and not many techs can work on them. But they would likely do what you describe wanting to do.