check my math

[junkhound]

new poco (PSE) time of use option, selected for 2000 customer trial.

7 cents kwhr, except 7-10 AM and 5-8 pm. when it goes to 32 cents kwhr (I assume using cent/kwhr does NOT violate any pricing rule)
Own house i 5300s sq ft, 2 story plus full basement, Seattle area weather

Heat is a 20 SEER GSHP

So, turn off heat at 7 AM and 5 PM for 3 hours. Last January used 65 kW hrs on a typical day probably 90% of that was for heat
65*3412*5.6= say 1.2 million BTUS or 600,000 BTU per each high cost period.

If I reroute my condensor lines thru an old free 500 gal water tank (old oil tank, I do have space for it) and preheat to 120F with another HE to recover heat to house down to 80F during the 2 ea 3 hour power off period, I can save 500#*8.3#/gal*40FdeltaT* 1 BTUdF/# = 166,000 BTU

I would need pay (1/5.6)*(166000/3412)*32 cents or $2.78 vs. $.52 off peak power.

So, maybe 80 days per yar would need the 166,000 BTU thermal storage so save $$. only 20 or 30 days more thant that (e.g the 600K BTU during cold spells.)

Thus final query ( IF my calcs are correct) = should I convert one or 2 old oil tanks to thermal water storage? I give up 100 sq ft of basement space for about $160 power bill savings. A 1000 sq ft apt here is about $20k per year, so I 'paying' #200 a year rent for tank space (to myself <G>) to save $320???

Of course, not even counting the $$$ to add another water to air HE in the ducting or the extra condensor loop.

PS: above ignores thermal mass of house, which for 40 F outside drops from 70F to 67F in 3 hours.

 

[wwhitney]

Seems simpler to use the thermal mass of the house: if 70F is your desired set point, set the thermostat for:

12 AM to 5 AM 70F
5 AM to 7 AM 72F
7 AM to 10 AM off
10 AM to 3 PM 70F
3 PM to 5 PM 72F
5 PM to 8 PM off
8PM to 12 AM 70F

I'm assuming the GSHP can raise the temp 2 degrees in 2 hours; obviously for some outdoor temps it won't be able to do that. But if it can maintain 70F when it's 10F out, then it can raise the house 1F / hr when it's 40F out.

Cheers, Wayne

 

[junkhound]

We already do that, but the tstat goes from 65F at 3AM to 72F, since we wake up and start our day at 4 AM, Works out for us also as we east supper at 3 PM and generealy go to bed at 8 PM, so heat basically goes off at 5 PM if not colder than 40F outside (which is seldom)

Clothes dryers only on Saturdays and Sunday, and I try to not weld or use the 50A pressure washer in peak times, etc.

 

[__dan]

Three words, sweat wicking baselayer. If you feel chilly and want to ride through the day, part of the day, while the heat is off during peak demand charge time, sweat wicking baselayer. They go on sale for $20 each all the time.

If you're wearing a fabric with any percentage of cotton content, the cotton traps perspiration against the skin and gets very uncomfortable very quickly outside if a very narrow temp range. In the old days with cotton thermals you would work and sweat, then freeze as soon as you stopped working for break. That went away with polypropelene baselayer. Now there are similar and bettter fabrics, Tecasafe and Drifire fabrics among them which are excellent for sweatwicking, which are hard to find but do go on sale and clearance if you know what to look for.

https://www.sportsmansguide.com/productlist?k=drifire

It's outside my expertise to check btu math but the scenario you're trying to propose is something I am pretty deep in.

The GSHP distribution system, from the equipment to the loads, is probably forced air. I would never do forced air (just me). If it was a hot water coil it would be looking for 180 F water to make heat. To run from storage you would want a distribution system that works with a low temp water, like hydronic radiant, which will work with water from 90 to 130 F. Trying to make hot forced air from storage is Sisyphean. Getting 100 F from storage for radiant would be doable, probably the only way to do it is hydronic radiant, imo.

The oil tanks are not pressure rated so has thin steel to start with and rust through would be the problem. Then water on the floor.

With a closed loop and water treatment, would be prescribed to get some minimal life from it. But to take all the air out the water storage system, would have an expansion tank, and it's a big bladder tank just for that, in that size range of storage. I cannot see it working over the expected life of many years. I don't see it.

If you did water thermal storage, there's significant delta T just getting across the HX to get the heat out of the tank to the next equipment type. People do unpressurized storage with HX's but pressurized storage with no HX is probably more commonly advised and done.

That's a very expensive install and people do still have trouble with it. It's not a money saver. It's jungle rules, how do you battle the heat loss of a large older house by putting more heat in.

Which brings the next problem, service and maintenance. If the next guy calls a contractor to fix some problem down the road and he sees the oil tanks used as storage, with all the piping and circs, as soon as he sees something out of the ordinary and non standard, not what he's used to, he might be inclined to fix it by ripping it all out. That's what I see. Save the ripping out step by not building it that way.

Save the high rate kwh by scheduled off time on the existing system, swap over to sweat wicking baselayer for the chilly ride through times. If that's not enough, swap over to another fuel during those times, pellet stove would do it, or gas log fireplace would be much more common and expected for the next guy who has to call a contractor to service it.

Forced air distribution is just the hard way to do it, and water thermal storage is one of the much harder ways to fix that. With the large house you probably have a significant heat loss and hard to heat spaces like tall ceilings and lots of glass. Combat that with another fuel to burn. Baselayer and fire up the wood stove.

 

[__dan]

Actually the oil tanks used as HW storage would never take any pressure and have to be open to the air. That means water treatment would be essential and still, failure from rust through would be foreseeable at some time in the future.

One of the ways people do that, for wood boiler burners who batch burn, is a large box lined with EDPM rubber. Then piping loops are immersed in that as the HX to get to the next system. I would not do that myself.

Other than that I have heard people love their GSHPs.

 

[GoldDigger]

We already do that, but the tstat goes from 65F at 3AM to 72F, since we wake up and start our day at 4 AM, Works out for us also as we east supper at 3 PM and generealy go to bed at 8 PM, so heat basically goes off at 5 PM if not colder than 40F outside (which is seldom)

Clothes dryers only on Saturdays and Sunday, and I try to not weld or use the 50A pressure washer in peak times, etc.

If. like most, your heat pump has secondary resistance heating which kicks in both when the outside temperature is below an operating threshold and when the difference between the internal temperature and the set point is greater than a threshold value, you need to advance the thermostat setting programmatically in the morning to make sure that the temperature delta never exceeds the threshold to trigger the aux heating. In severe cases the energy cost of the resistance heating will outweigh the cost savings of a nighttime setback.
If the thermal mass of the building is large enough and the outside temperature high enough this may not be a problem.

 

[junkhound]

We do not have any secondary resistance heating. The WSHP works off 60 ft deep aquifer, constant 56F source temperature, has kept house at 65 F when 16F at night. Have never set it at 70F for night, but do have Tstat set for 70F or 72F at from 3 AM to 8:50 AM. We are up at 4 AM most days.
Have HPWH also, timer for turn OFF during high rate times. Can get over 100k btu hr from our WSHP 5T scroll compressor running 9.6 GPM water loop, custom concentric tube evaporator, 'oversized' 7.5 T air duct condenser charged by subcool.

Am going to wait till February to see if we do need any added thermal mass, am suspecting the thermal mass of the house is enough, there is over 100 cu yards of concrete in the construction and a few tons of basalt in interior stone work. good old days, concrete was only $14 a yard here when we built the house, and you could hand load as much basalt boulder as your regular size PU truck would hold for $7.

 

[__dan]

If there was any way to embed HW radiant tubing, PEX, into the interior stonework, that is one of the top line methods I am considering for a future build.

That interior stonework would become the radiant emitter as well as the thermal mass at that point, and I am sure it would give you enough (heat from storage, from itself) during a 24 hr ride through. Simple, passive, using itself and not much more. When you are directly in contact with a huge emitter like that, it does not need to be much above 70 F to keep the house feeling like it's 70 F.

One of the scenarios I like but have not done, would be water to water heat pump, no water to air. Then the delta T from the ground source of 56 F to the distribution radiant water temp (of 100 to 110 F would be fine) is much less work for the compressor, meaning a much smaller equipment and equipment load.

Something I want to try.

 

[junkhound]

Probably would have done the pex thing if pex had been around 50 years ago when we built out house.
Actually considering trying a direct exchange pex GSHP for granddaughter's house.

 

[__dan]

I know Emerson makes a suitcase sized water to water heat pump. How it applies is not in my expertise. But it's a multibillion dollar industry. If anyone does it right it would be Emerson.

You can see what's going on. The heat pump heats the air then the air heats the stone. The condenser gas / fluid has to be *very* hot to get medium air, to affect the cold stone. Plus the time to call for heat to get the stone warm is a long wait. Delta T from the condenser fluid to the stone is over 100 F (guessing). The living space air is one of the the heat exchangers, just a poor one.

Heat pump heats the water, let's say / guess 10F delta T there from the condenser HX to the water, water in PEX to stone another 10F delta T (guess). The living space air never needs to get warm and I am guessing it could be 65F air temp, with the stone at 90 F from the water, you might be in a position to turn the heat off or open a window (that's what I do).

Everyone does it the hard way. Then the news reports how bad the results are (energy crisis), then they go back to copying the guy doing it the hard way. Why idk.

I have not done it myself or seen it done but it's on my list to try. Water to water heat pump. Make a low temp water sufficient for in slab in floor radiant (90 to 130 F max). And done. Everyone loves in floor radiant.

With the right build, thermal mass with the insulation and building envelope exterior to *all* of the concrete, Passivhaus type, heat loss could be under 10 btus / sf (my guess). So in that range solar powered heat pump is doable. Conventional stick built at 30 btus / sf, probably not.

One thing the Passivhaus (imitators) get wrong is building tight with non opening windows. Heat the mass, not the air, and run year round with a window cracked open (I do). I'm at 4 cord for the year with a window cracked open.