TG, I think you're right about the mornings. 2 hours shouldn't be too big a deal.

Amount of system idle time during the 5 hour peak in the afternoons (even with a ramp up and then a setback) will depend largely on outdoor temperature, sun, wind, etc. I'm sure there are savings to be had, just don't know how much.

> I believe that approximately doubles your required tonnage.

While an off peak storage system should involve some upsizing of the compressor capacity, one doesn't design for 100% capacity during off peak hours - the system usually has excess capacity. Or in other words, design load doesn't happen every day and you can use the system during peak hours on occasion - it just costs slightly more than the normal rates. A system with no upsizing at all would still provide a significant shift to off-peak usage (most days) and keep the house warm (always). All use of aux heat can take place during off peak hours.

Posted By jonr on 11 May 2010 01:55 PM
> I believe that approximately doubles your required tonnage.

While an off peak storage system should involve some upsizing of the compressor capacity, one doesn't design for 100% capacity during off peak hours - the system usually has excess capacity. Or in other words, design load doesn't happen every day and you can use the system during peak hours on occasion...

You'll have to do a cost / benefit analyst I highly doubt it's going to be cost effect to double the size of your system or install some kind of storage tank to try and buffer the load.  I would guess it's going to be more cost effective to ride out the morning spike and turn up the thermostat a few degrees (or down in summer) before the afternoon spike, and if the system has to kick on during the spike to maintain your comfortable temperature settings, your just going to have to live with the extra operating costs.
  

In geome's case, probably true. In some other cases, a larger buffer tank looks very attractive financially. Hopefully the part about doubling the compressor size (or even the more accurate 25-60% increase) being unnecessary was understood and we can stop bringing that up.

jonr, I get at what you're saying. For example, if a system is designed for let's say a 95% load, then whenever the load is at less then 95% during non-peak times, there may be extra capacity to store heat during these times. I don't want to do this, but I understand. If we had this rate structure before the system was installed, I may have factored this into the design. Not saying that it can't be done now, just don't know how to easily and cheaply integrate it into our system while keeping things simple.

TG, I agree that the system may turn on in order to maintain a bearable temperature during peak times. I can experiment further with adjusting the thermostat to see what works well, if anything, on these days.

I'm not suggesting this is practical or overall a good idea, but with an air based system, one could do what some solar guys do - a basement room full of rocks.

> there may be extra capacity to store heat during these times. I don't want to do this

I think this _is_ what you are proposing with normal house mass as the storage. If you don't have extra capacity, you won't be able to heat above the normal temp or catch up quickly when the peak period ends. Ie, your proposals will not help on a design day (but will work on a typical day).

Wife wants the basement cleaned out. Substituting rocks for junk wouldn't go over too well... Having enough trouble on the hairdryer front. :-) You're right - our system is water to air.

Not surprising, I think it would take some unused basement space, a low cost supply of rocks and last but not least, an understanding spouse.

Posted By jonr on 11 May 2010 04:50 PM
Hopefully the part about doubling the compressor size (or even the
more accurate 25-60% increase) being unnecessary was understood
and we can stop bringing that up.

Hmm... you weren't planning to recharge that very financially attractive
storage tank (or rock pile) at anything like the heat pump's normal COP
or BTU/hr output capacity were ya? If so, I can't help but ask:

a) What 10:00 AM temperature do you expect to need inside the
tank/rocks in order to maintain reasonable output temperatures
until after the evening news?

b) What kinda BTU/hr and COP do you expect to see from the heat
pump at that temperature, ...uh, assuming that it's even possible?

c) How many gallons of water/rocks will be sufficient to store 9 hours
worth of BTUs, at the heat pump's maximum affordable output temp?

d) How do you pump luke warm rocks through a fan coil -- without
violating local noise ordinances?

...best o' luck, Bunky,

Looby

Looby, you loony, you have it all wrong!

The rocks are only temporarily stored in the basement. During peak times you carry them up into the rooms where the load dictates.

Little room - little piles of rocks under each window so that the natural convection off them counters cold drafts.

Dining room - pile them under dining table, use as foot rests while eating

Great room - Great big pile of rocks in center of room - great conversation piece

Tuck a few beneath bed covers as foot warmers

Must consider also extra latent and sensible heat given off by family members engaged in rock moving - excellent exercise.

Having piles of rocks convenient to hand enhances home security as well.

Curt...Thanks for a good laugh.

Since we're on the subject, heating rocks with a phaser always seemed to work well on Star Trek.

Posted By geome on 11 May 2010 05:18 PM
Wife wants the basement cleaned out. Substituting rocks for junk wouldn't go over too well...

I think I can make use of this, It's not junk honey, it's thermal mass!

Posted By geome on 11 May 2010 05:18 PM

It's not junk honey, it's thermal mass!

stuff: Junk we keep.

junk: Stuff we throw away

Posted By Looby on 17 May 2010 02:12 PM

junk: Stuff we throw away

stuff: Junk we keep.

So, by substitution:

junk: "Junk we keep" that we throw away?

stuff: "Stuff we throw away" that we keep?

Boy, this forum is slow today - at least we can amuse ourselves.  LoL 

The forum is still slow, so here is an update.

As it turns out, the rate structure has changed from what I was previously told. We will have a choice between the new "normal" rate of $0.12/kWh
OR
Time of use rates as follows:
Peak = $0.178/kWh
Off Peak = $0.084/kWh

Based on our kWh usage for the preceding 12 months (and compared to the "normal" rate):
If we can shift 80% usage to off peak (with 20% peak) would roughly save $230/year.
If we can shift 90% usage to off peak (with 10% peak) would roughly save $350/year.

Unfortunately, this is not as much savings as I envisioned due to the rates changing. I still do not know how weekends will be handled (i.e. peak/off peak or just off peak.) If off peak, the savings would be slightly better since there would not be any peak time on those days ($300/yr for 80% off peak usage and $388/yr for 90% off peak usage.)

Does anyone have productive thoughts that don't involve capital investment (i.e solar, etc.)? Those of you that have experience with time of use rates, can you change your selection monthly, or is it annually? I'd hate to get stuck paying peak rates if we decide we don't like this setup as we get into winter.  Edit: I'm just not sure it's worth taking the risk for $20-$30 savings per month.