Back Up Heat Source and Boosting Loop Temps

[wewantutopia]

Hello All,

My long term plans for our home is to swap our nat. gas forced air furnace for a ground source heat pump. These recent cold snaps (it is currently -16*F here and -35*F windchill) has got me thinking about peak load and the backup heat source.

I'd like to eventually no longer use natural gas for anything so it seams electric resistance heat would be the way to go for back up. This seems like a lot of extra electric use. This got me thinking...

Would it be feasible to use a small tankless water heater to raise the temperature of the water from the ground loop before it enters the heat pump? Say raise it only 5-10*F. Since the high SEER/COP of the heat pump it seems like you'd get the most bang for your buck/watt. It would use less electricity then a bunch of resistance heaters and it would be a single device instead of heaters all around the house.

To sum up. Instead of using a back up heat source when it is REALLY cold is it possible to not install a back up but rather boost the ground loop water temps and continue using the heat pump as the only source of heat.

Any thoughts?

[Daox]

If you're setting up a GSHP you should be able to size it to heat in the worst case scenario. Its not like a ASHP where its efficiency drops off as outside temperature drops.

However, if you're wanting to keep the heat pump size down for some reason, you can use electric resistance heat for backup. However, by putting it on the ground loop, you are only increasing your power usage. You are now using energy to put heat into the ground loop, and now you're also using energy to pull that heat out with the heat pump to the home side of the loop. A more efficient option would be to put that heat directly into the home side of the loop. That avoids having to use the heat pump to pull that heat out, and energy will be saved.

Your cheapest option would probably be some electric space heaters that heat up just the room you're in...

[AC_Hacker]

Originally Posted by wewantutopia

...To sum up. Instead of using a back up heat source when it is REALLY cold is it possible to not install a back up but rather boost the ground loop water temps and continue using the heat pump as the only source of heat...

We are a motley lot here with a variety of opinions, so here is mine...

To achieve maximum efficiency, any heat pump system should be sized somewhat smaller that the 98% maximum requirement. To meet the demand on days that exceed the 98%, you would use some axillary heat source like wood, gas or electric.

A heat pump, even a variable speed compressor system (AKA: 'Inverter Technology') will reach its maximum efficiency when running flat out (100%). At utilization levels below that, friction factors will reduce efficiency. (To make this easier to visualize, just imagine that you have your over-sized compressor chained to your foot and you have to drag it around all the time and it NEVER reaches maximum efficiency. It would be far better to have a smaller, lighter compressor chained to your foot and have it reach maximum efficiency more often.)

With a ground source system, by far the biggest part of the cost and/or effort is the earthwork and associated pipe installation. So if you over-size your system, you are paying extra in cost and effort for extra loop field. There are some very good reasons to do this, because during the heating season, there is some inevitable decline in loop temperature. The bigger the loop, the smaller the decline. The smaller the decline, the greater the ultimate efficiency due to the Carnot Efficiency Theorem.

But to get back to your idea of having an electrical demand heater augmenting the loop water temperature just before the point of use, I think that you have a wonderful idea here, and if you are a multi-talented kind of guy, it would make sense to use a Pulse Width Modulation scheme to apply your electrical energy to your water heater, as needed rather than on-at-setpoint, off-at-setpoint.

* * * * *

Some other schemes to consider are that if you use your GSHP for cooling, you will be storing summer heat into your loop field, and your electric heater idea, as clever as it is, would probably not get used.

Also, if you are in an area of summer sun, you could use a hot water solar panel (or several panels) to pump solar heated water into your loop field during the sunny months, to be harvested during the winter.

* * * * *

And as a final word, it is usual that in thinking of ways to stay warm, we tend to think of ways to make heat... I think it's an evolutionary thing.

But it is more modern and often more fruitful to think in terms of how to keep the heat we have. That would lead us to reduce infiltration losses as much as possible and to maximize insulation as much as possible.

Once these steps have taken place, the heating system size and cost will shrink to a more manageable size.

Best,

-AC

[randen]

Wewantutopia

I'm part of the motley crew.

Having first hand experience with Geo-thermal I would compare the price per unit heat BTU Geo vs. High efficiency Natural gas very close dollar per dollar. In reality you would be comparing two highly efficient systems But there are other factors at play. Like what AC-hacker pointed out. Warming a ground loop with summer air-conditioning and using that same stored heat for winter. An central air unit would just pump the summer heat out to the wind and not be able to recapture that heat.

To try to improve the efficiency of a geo-thermal with a resistance or N-gas type boost for the heat-pump could end up to be expensive. After all your really basically heating using an other source and having a heat-pump then move that heat to where you need it. You may be better to use direct source like a gas fireplace or small space heater to get you by the worst condition. This way you don't pay twice. Once for the heat and again to move it.

A heat-pump is just that: It doesn't make heat it just moves it from one area to another. Its less expensive to move it from the warm ground to the home. And the other way as well from a warm home into the cooler ground.

Really the best heat is the free stuff. From the sun. In essence that's what the Geo-thermal is doing is gathering the heat that the sun put into the ground the season before. But keep in mind you still have to pay to move it. As well a Geo-thermal system can be a sizable investment.

The second best kept secrete is OR maybe the first "Keeping the heat you just paid for". (insulation)

Maybe the lack of interest in insulation is the lack of techie stuff. No pumps, temp gauges, heavy power cables and a plethora of plumbing. Insulation is put into cavities and hidden in manners never to be noticed. A handy guy could wrap a house in 6" of foam boards from soffit to footing and then cover with siding or stucco and backfill never seeing his underlying work of art.

For instance using your gas furnace. If your home had an R-100 insulation wrap, that same gas furnace might only operate once every second day. You would keep every BTU. and be comfortable for a long time.

If you check into some of the Net-Zero homes that are heated with solar heat only with possibly a very small back-up heat source the insulation values of the home's envelope are almost extreme compared to a typically constructed home. These homes can almost be heated by the warm bodies that occupy them.

Its a great time to be looking at the possibilities. The technologies are avalible now for a reasonable dollar. Couple that with DIY and sweat equity you can be kept warm and comfortable.

Thinking again in the direction of net-zero home tech.The cost for solar PV panels has dropped significantly. Imagine an extremely well insulated home with geo thermal operated from those same panels heating a concrete floor that remains warm for a couple days. Hot water which is considered 20% of the energy budget heated by the same PV panels. LEDs lighting your home very in-expensively.

We have been collecting solar heated hot water for space heating our home(the free stuff) and Geo-thermal as a back-up or as this winter has been very cold and over-cast more our primary source. Higher performance insulation is our saving grace and a better return on investment.

My humble suggestion is insulation. A deep renovation of insulation and air infiltration curtailment is better bang for the buck. Out-sulation is the tech. of the day. Wrapping a home with layers of foam board clear down to the footings and triple glaze high performance windows.

Randen

[wewantutopia]

Hi everyone, thanks for the responses.

I live in a ~1000 sq. ft. 1910 bungalow that I have insulated to just under passive house standards (the builders grade double pane windows the previous owners installed are the week spot) including R80 attic, 1.5" exterior poly iso, 3" poly iso foamed in between the studs, 1.5" XPS that I hand dug around the basement to below the footings. The walls come to about R30. Done.

I have a prototype solar air heater (really it is version 3.0) and when I rebuild my front porch this spring I will be incorporating 2 of them into the porch build (pictures to follow).

I plan on installing a 10kw solar array in the relatively near future so I'd like to be all electric. I'm sick of paying the gas company for the connection fee (~$30/month) when it is not in use.

I understand the major cost comes from digging the ground loops. I plan on DIY so I'm not concerned.

I've read about using a slightly undersized pump that's why I was thinking back up is needed.

My thinking about the in line tankless just before the heat pump is the heat would be extracted by the pump thus not heating up the ground loop.

Also, I was thinking with the high COP of the heat pump, if you raised the water temp by 1kw you would get the equivalent of 2kw etc. of heat. If you put it after the heat pump you would only get the 1kw worth of heat.

This line of though is purely academic and out of curiosity since the project is many years away.

[AC_Hacker]

Originally Posted by wewantutopia

...that I have insulated to just under passive house standards (the builders grade double pane windows the previous owners installed are the week spot) including R80 attic, 1.5" exterior poly iso, 3" poly iso foamed in between the studs, 1.5" XPS that I hand dug around the basement to below the footings. The walls come to about R30. Done.

I have a prototype solar air heater (really it is version 3.0) and when I rebuild my front porch this spring I will be incorporating 2 of them into the porch build (pictures to follow).

I plan on installing a 10kw solar array in the relatively near future so I'd like to be all electric. I'm sick of paying the gas company for the connection fee (~$30/month) when it is not in use...

Sounds like you really did a lot of work there.

Because you have already lived in it, how much energy is required to maintain your house temperature? I mean this latest Polar Vortex we've had should have been a really good test.

-AC

[MN Renovator]

Originally Posted by wewantutopia

Hi everyone, thanks for the responses.

I live in a ~1000 sq. ft. 1910 bungalow that I have insulated to just under passive house standards (the builders grade double pane windows the previous owners installed are the week spot) including R80 attic, 1.5" exterior poly iso, 3" poly iso foamed in between the studs, 1.5" XPS that I hand dug around the basement to below the footings. The walls come to about R30. Done.

I have a prototype solar air heater (really it is version 3.0) and when I rebuild my front porch this spring I will be incorporating 2 of them into the porch build (pictures to follow).

I plan on installing a 10kw solar array in the relatively near future so I'd like to be all electric. I'm sick of paying the gas company for the connection fee (~$30/month) when it is not in use.

I understand the major cost comes from digging the ground loops. I plan on DIY so I'm not concerned.

I've read about using a slightly undersized pump that's why I was thinking back up is needed.

My thinking about the in line tankless just before the heat pump is the heat would be extracted by the pump thus not heating up the ground loop.

Also, I was thinking with the high COP of the heat pump, if you raised the water temp by 1kw you would get the equivalent of 2kw etc. of heat. If you put it after the heat pump you would only get the 1kw worth of heat.

This line of though is purely academic and out of curiosity since the project is many years away.

I've cut the infiltration on my house down to as little as I can outside of what leaks my fairly decent(but still horizontal sliders) window gaskets and the sliding glass door is infiltrating along with a recently discovered air leak at the baseboards of the two cantilevered upstairs bedrooms. I've got the infiltration low enough to where I have enough pressure through those small gaps get frost at EVERY infiltration point in the house. My house insulation is actually decent for 1985 standards but R13 in the walls and 3/4" rigid on the outside. I'm not even R20, my attic has 7" of cellulose(R25ish) and I'm getting 17762BTUhr days when it is -15f outside, 68 inside and cloudy outside all day.

If you've got an R80 attic, R30 walls and you've got insulation 1.5" XPS for the basement in a 1000 sq ft place, I'm not sure you're going to run into an issue with the GSHP being too small unless you have infiltration issues. What's the CFM on your blower door. Do all the air sealing you can, get a blower door test and if you are under 400 CFM at 50 pascals, you could probably get away with a 1 ton GSHP at any temperature as a rough guess. This depends a bit on your glass too though, if you've got a lot that might add something. Since you are going GSHP you have less to worry about with losing output when it gets brutally cold as long as your loop field is big enough. I'd suggest doing what I did, figure out how to do a Manual J load calc and do one. You'd want to be sure you size for your cooling appropriately.

"I can cool the house with a single well placed 12,000 btu window unit and our current furnace is 60,000 btu. How would one comprimise with the GSHP?"

Sounds like you've got your cooling load figured out. Your current furnace was probably put in before you added all of this insulation and probably did the job just fine without even hitting full duty cycle. Have you paid attention to a period of say 6 hours and timed how long it runs on a day that gets about as cold as it ever gets when there is no sun, you've had the chances so far this winter and if you haven't already you've got a good chance to do it now because if you time how long the furnace is running you can get your heat load that way. It is exactly how I know how much energy my house requires to heat with. It's actually less than my Manual J estimate even if I exaggerate my insulation numbers and skip entering infiltration.

What size GSHP were you aiming for prior to creating this thread anyway?

Remember if you are pulling 5kw of heat out of the ground, its because it had 5kw of heat. You were just pumping the 5kw of heat out using 1kw of energy. If you raise the water temp by adding 1kw to it, you'll only get 1kw out of it. You'd be better off using resistance heating inside the house rather than adding it to the loop.

[AC_Hacker]

Originally Posted by MN Renovator

If you raise the water temp by adding 1kw to it, you'll only get 1kw out of it. You'd be better off using resistance heating inside the house rather than adding it to the loop.

When I first read his idea, I visualized that his GSHP would be extracting heat from the outside loop, and would be yielding the heat up to an inside loop that was part of a hydronic floor heating system (which would be a great idea).

In that case, adding electric heat to the inside hydronic loop would be a viable and very sanitary way to add axillary heat.

It would be the hydronic analog to heating coils in an air handler to supply axillary heat.

-AC

[MN Renovator]

right, adding aux heat to an indoor ground loop or if handled through an air handler heating coils would be appropriate in that case too. It sounded the other way around when the OP described it because adding to the outside loop wouldn't work to boost efficiency.

[Blue Bomber Man]

I just wanted to take the time to address one point mentioned earlier.

The idea was brought up to put say a 1 kw electric heat to prewarm the loop temp and was believed by the original poster to allow the heat pump to perhaps get 2 kw heat back from this extra heat in the loop.

This would violate conservation of energy.

To prove my point imagine a hypothetical ground loop with 0 K temp. Lets say you add 1kwh of energy to it and it raises the temp by some amount. You could not then run this through a heat pump (say a COP of 2) and get back 2 kwh of heat, as the system only has 1kwh of heat in it.

So my conclusion is that it would probably be better to directly heat the space with the electric heat, or add it just after the heat pump in your distribution system