You can disable the aux heaters during defrost fairly simply.
That way it will defrost ok, just it will blast cold air inside during the defrost cycle.
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My Electricity Use for my Heat Pump
- Thread starter dreancestere
- Start date
That way it will defrost ok, just it will blast cold air inside during the defrost cycle.
dreancestere
New Member
I agree that with that generator, having him turn off the aux heaters might be the only way to keep things running. But, in rare cases, it might lead to issues. Here's our worst day during last year's January cold snap (no sun, really cold, a grid voltage so low that the compressor wouldn't run and the system switched to non-heat pump mode (i.e., we had to rely on those auxiliary strips)):
EDIT 1: In our case, if the grid had failed entirely, our 26kW generator (though hooked through the GEN terminal and thus limited to 19.2kW) would have theoretically (and hopefully) picked things up. But, it's a scary thing to think of.
EDIT 2: This was when we had all 3 aux strips connected. So, you can see the 16kW defrost surges when the compressor was still running. That puts us right at the edge of what the generator could supply.
EDIT 1: In our case, if the grid had failed entirely, our 26kW generator (though hooked through the GEN terminal and thus limited to 19.2kW) would have theoretically (and hopefully) picked things up. But, it's a scary thing to think of.
EDIT 2: This was when we had all 3 aux strips connected. So, you can see the 16kW defrost surges when the compressor was still running. That puts us right at the edge of what the generator could supply.
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Quattrohead
Emperor Of Solar
I don't think this is the case, how could this be so because a big enough heat pump to cool a Florida or Texas summer has got to be plenty enough to provide heat during the winter. I'm not sure I understand what you're getting at.
Quattrohead
Emperor Of Solar
Ah I understand your situation now, very similar to mine where I could probably have got away with one 18K PV but having two definitely make sense.
Sounds like you need to bulk up your inverters, batteries and panels to make yourself totally comfortable with your system.
What I am getting at is the house will always have more heating load than cooling load in the middle south.
And the system must be sized for the cooling load to keep moisture in check.
In Florida, the cooling load FAR outweighs the heating load, so, of course it would be fine. In MANY parts of Texas, they are finding, the heat pumps properly sized for cooling, are inadequate for heating in these cold storms…
Some 5 stage inverter systems are either 3 ton, or 5 ton, so they are capable of locking in cooling loads below the heating capabilities. My house has a 3 ton Bosch unit, capable of the home’s 2ish ton cooling load, but ramping up to 3 ish tons for heating. So far, it has proven reliable heating into the 20’s, without aux strips (I didn’t install strips in the unit.)
But single or two stage equipment doesn’t have the range to size them to handle all weather.
But single or two stage equipment doesn’t have the range to size them to handle all weather.
HarryN
Solar Wizard
One of the challenges of modern heat pumps is that part of the efficiency comes from running the coils very close to the air temperature setting goal.
Older heat pumps were less efficient, but they included a coil that always sub cooled the air to dehumidify the air. They used more power, but were very good at controlling humidity.
If my home were heated by electricity only, especially given that the grid is becoming less and less stable, I would definitely have a back up plan. Generator or fuel heating.
Older heat pumps were less efficient, but they included a coil that always sub cooled the air to dehumidify the air. They used more power, but were very good at controlling humidity.
If my home were heated by electricity only, especially given that the grid is becoming less and less stable, I would definitely have a back up plan. Generator or fuel heating.
dreancestere
New Member
Another minor cold-snappish minor update. We got past the previous cold-snap, but now have arrived at another one. This morning we dropped to 16 degrees F. At that point, in addition to the Stage 5 heat pump compressor going (pulling roughly 5kW), one of the auxiliary heat strips kicked on. I'd have hoped with a heat strip on, either the compressor would ramp down a bit or the heat strip would pull it's full power. But, it looks like that was a vain hope. The heat pump is currently pulling a full, constant 10kW. Eeeek. Something to be aware of.
Zwy
Emperor Of Solar
I have a Senville 18K btu Aura rated to -20°F in the kitchen. Running wide open this morning at -9°F it draws about 3400W when first turned on but ramps down within a half hour as the room heats up.
10Kw is just mind boggling. Of course, the propane furnace was running this morning too when the programable thermostat kicked in. I'm running an electric radiant in the living room as that heat pump is not rated to -20°F. I learned not to buy another one like it if I want to use it for heating.
dreancestere
New Member
Ours is a whole house, 3 ton unit (in an 1,800 sq. ft. house). It looks like that works out to roughly 36k BTU. I'm guessing yours is a mini-split, so it's probably more efficient than my whole house unit. Plus, it doesn't have to worry about heat loss from the air handler and air ducts in the unconditioned attic (and isolated from the conditioned part of the house by spray foam on the attic floor, i.e., it's cold up there).
I'm thinking our heat pump doesn't ramp down like yours because the house stays at a constant temperature all the time. So, once the external temperature gets cold enough (apparently 16 degrees F) to cause the auxiliary strips to turn on, there's no other variable to cause it to ramp down. That does seem weird to me. I'd have thought each auxiliary strip would be variable draw depending on the inside/outside temperature. But, it looks like they come on one at a time, full bore, as the outside temperature drops. The individual strips are what make up the variability in radiant heat power draw.
It looks like the single strip stayed on for a half an hour until the outside temperature rose back to about 18 degrees F. At that point, we dropped back down to pure Stage 5 compressor mode drawing a "mere" 5kW.
EDIT: And -9 degrees F. Ouch. Thankfully, down here in Wichita Falls, TX, our average temperature at this time of the year ranges from a low of 30 degrees F to a high of 55 degrees F. These way-out-of-bounds swings down into the mid-teens aren't supposed to happen that often or for that long.
I'm thinking our heat pump doesn't ramp down like yours because the house stays at a constant temperature all the time. So, once the external temperature gets cold enough (apparently 16 degrees F) to cause the auxiliary strips to turn on, there's no other variable to cause it to ramp down. That does seem weird to me. I'd have thought each auxiliary strip would be variable draw depending on the inside/outside temperature. But, it looks like they come on one at a time, full bore, as the outside temperature drops. The individual strips are what make up the variability in radiant heat power draw.
It looks like the single strip stayed on for a half an hour until the outside temperature rose back to about 18 degrees F. At that point, we dropped back down to pure Stage 5 compressor mode drawing a "mere" 5kW.
EDIT: And -9 degrees F. Ouch. Thankfully, down here in Wichita Falls, TX, our average temperature at this time of the year ranges from a low of 30 degrees F to a high of 55 degrees F. These way-out-of-bounds swings down into the mid-teens aren't supposed to happen that often or for that long.
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5kW seems like a large draw for a 3 ton unit, my 4 ton unit draws about 3.8-4kW including furnace fan. Is your unit running 100% duty cycle prior to heat strips kicking in? Are you able to tweak the point where the heat strips kick in?
Our 4,500 sq/ft equivalent home(loft and vaulted ceiling main floor), including basement, runs a 4 ton Tempstar TSH6 central system with a 100kBTU propane furnace for backup, no heat strips. Nothing fancy as far as I'm concerned, but does output down to -3F with a COP of about 2. Currently set to switch to propane at 5F(around 25kBTU at that temperature) as it won't keep up below that. Also have a heat pump water heater sucking about 4kBTUs. We have 2x needed cooling, fortunately our humid snaps are short when the corn gets tall. Heat pump water heater helps.
Our 4,500 sq/ft equivalent home(loft and vaulted ceiling main floor), including basement, runs a 4 ton Tempstar TSH6 central system with a 100kBTU propane furnace for backup, no heat strips. Nothing fancy as far as I'm concerned, but does output down to -3F with a COP of about 2. Currently set to switch to propane at 5F(around 25kBTU at that temperature) as it won't keep up below that. Also have a heat pump water heater sucking about 4kBTUs. We have 2x needed cooling, fortunately our humid snaps are short when the corn gets tall. Heat pump water heater helps.
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dreancestere
New Member
The heat pump is a 5 stage one. Very roughly, each stage of the compressor adds about 1kW to the power draw. "Normal" use sees it usually in stage 2 (roughly 2kW). Even in 100℉+ summer temperatures (for instance, this past August, we spent much of the month around 110℉), it doesn't go into stage 3 very often. Ditto for "normal" winter temperatures. But, once we drop below 30℉ and down towards 20℉, well, those compressor stages ramp up pretty quickly. This morning, we were running at stage 5 from about 0300hrs on (and then there's the hourly defrost cycle spikes). The first auxiliary strip ran from 0630hrs to 0700hrs.
I don't think there's any way to program different set points. I was surprised that the only way to restrict how many auxiliary strips will turn on was to physically disconnect them. I really would have thought that all those things could have been set in the super-secret installer menus. But, I guess not.
EDIT: Given what I've seen around here, I probably should have considered individual mini-split heat pumps for the various rooms of the house instead of a single, central, whole-house heat pump. But that would mean somewhere between 3 and 5 systems (3 bedrooms, possible the central portion of the house, and given an infinite pile of Scrooge McDuck gold, the garage where the inverter and batteries are). But, I'd think the wife-acceptance factor on that would be pretty low and the maintenance requirements pretty high.
I don't think there's any way to program different set points. I was surprised that the only way to restrict how many auxiliary strips will turn on was to physically disconnect them. I really would have thought that all those things could have been set in the super-secret installer menus. But, I guess not.
EDIT: Given what I've seen around here, I probably should have considered individual mini-split heat pumps for the various rooms of the house instead of a single, central, whole-house heat pump. But that would mean somewhere between 3 and 5 systems (3 bedrooms, possible the central portion of the house, and given an infinite pile of Scrooge McDuck gold, the garage where the inverter and batteries are). But, I'd think the wife-acceptance factor on that would be pretty low and the maintenance requirements pretty high.
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Quattrohead
Emperor Of Solar
You need a central system to keep the air moving around the whole house, especially when the kids leave and you have empty rooms haha.
But we like our bedroom cold so I added a mini split to it when we built the house.
There have been a few occasions in the morning recently where I've actually put it in heating mode to get me out of bed LOL
k490
Solar Enthusiast
Can lose lot of efficiency in ducted HVAC system compared to a ductless evaporator is right in the room, and run it in the rooms your instead of relying on vents. Ductless has been used for years outside the US probably because electricity was lot more expensive we got spoiled on cheap electric power until 10-12 years ago. I remember my summer bill was $250 for 3-4 months winter bill was $120 a month same AC brand and size unit. Now I'm paying $580 summer months, and $210 winter months not using the heater at all.
HarryN
Solar Wizard
Sure, but air flow / oxygen / fresh air are just as important as the temperature and humidity.
ERV or HRV solves that.
k490
Solar Enthusiast
Not sure how fresh air changes in a ducted system? Just moving air from one part of the home to another. The biggest problem here in Phoenix we now have 118F days for 30-40 days at a time. My master bedroom is the farthest away from the return air we have to run the main AC unit down more to keep that room cool to sleep even with fans going. Duct work is probably leaky also I'm really considering a mini-split in our bedroom for sleeping at night.
Quattrohead
Emperor Of Solar
Best idea since the round wheel !!!!
In properly designed homes, hrv system offsets exhaust fans with make up air conditioned for exchange.
Also, fresh air return opening should be installed.
k490
Solar Enthusiast
Flat roof on either side of a pitched roof, so installing more duct work would mean tearing out ceilings. If I was going to do that would just ditch entire system and go ductless. I do have access from the pitched attic of part of my master bedroom it would be easy to add a mini-split head unit there.
HarryN
Solar Wizard
Makes sense. We do some fresh air intake at night when the temps have dropped below 80 F.
Agree completely on the idea of a heat pump for those difficult rooms. Wish I had added a dedicated mini split for one end of the house and garage long ago.
ksmithaz1
Solar / EV Junkie
Now I'm paying next to nothing in the summer months, and next to nothing in the winter months. Power is cheap here even if you factor in TOU rates, but my solar is now providing 99.99% of my power needs, and I could care less what my HVAC draws.
ksmithaz1
Solar / EV Junkie
Not 118F x 30+ days, don't get carried away
. 110+ or something for 30 days straight, this past year, and we didn't get over 120. Further when it hits 118 and up it's generally very dry, so it falls off quickly with the sun. It's the moderately humid 105-110+'s where it stays over 90 at night during the monsoon season when it get's tough. . . To the sleeping point.
What's cheaper? More Solar or a new/Additional HVAC? Umm, Which is more fun? Seriously, if we get the ridiculous temps, we get the ridiculous sun to go with it. I probably should insulate a bit more, my house is slump-block, it was engineered for a swamp cooler, and retro-fitted with a package unit on the roof. I'm pretty light on attic insulation, but I added a ridge vent and soffit venting with the new roof, it helped tremendously with attic temps, but when it hit's 120, I can't pull it down below 85. Then again 85 feels like a refrigerator when you walk in from 120, and I can get it down to 76 once the sun goes down, but it runs pretty much wide open all night. I let it float up to 78 around 0500.
The yellow line is the HVAC This is august 8th this past year, It flatlined from 1700-1800 at about 5KW, and again at 2200 as I pull it down to 76.
YMMV.
Ot is crazy how efficient heat pumps have become.
The old stuff in the 80s topped out at a op of 1.5to2, new stuff can hit 6! And down to 15F, it is simply amazing.
I really believe a set of minisplits can make a huge power load difference over a central setup.
Also, minisplits are available with ductwork blowers...
I am installing more and more of them, it is great.
During my HP installation all ducting was replaced and separate zones for each area to heat or cool only what is needed.
Works wonderfully.
Otherwise a larger battery is probably in order. Not sure I would disconnect all the resistance heating but maybe cut it down 50% to 80%.
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