Air- vs Liquid-Cooled Standby Generator for Winter Outages
We’re debating air-cooled vs liquid-cooled, and we’re debating Generac vs Kohler. Kohler has a strong reputation, but there are proponents of both brands. Reliability and maintenance are important, because we need it to work when we need it most.
We’re in IECC CZ7, and get anywhere from 400-700 inches of snow each winter. 500+ is typical. Elevation 6600 feet. Winter is really the time when the standby genset matters most.
We get avalanches. We wake up to the Gazex every morning for about 30 minutes, and throughout the day when its snowing heavily.
We’re about 5 minutes from the lodge where the 1982 avalanche in Alpine Meadows occurred (documented in the recent Netflix movie “Buried”). I mention this because often it’s an avalanche that’s taken out a power pole or transformer. Fortunately they’ve slowly improved our infrastructure over the years. There’s a lot of granite here, so underground power is prohibitively expensive, as are natural gas lines.
The genset is close to our future family room, and being under the driveway it might echo. I know liquid-cooled gensets can be noticeably quieter than air-cooled by as much as 10 dB.
Hoping to draw on the wisdom and experience of the GBA community!
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Replies
66kW is a mighty high residential load, and would require 400A service, and associated transfer switch (or switchES). I'd be very surprised to see you really use that much.
You can use a load shed kit to lockout the EV charger while running on generator if you want to easily save some load.
The biggest difference between the air cooled and liquid cooled gensets is with the engine. Typical air cooled gensets run at 3,600 RPM, which allows for more horsepower in a smaller physical space. It also means a LOT more wear on ALL the moving surfaces inside the engine (bearings, piston cylinders and rings, etc.). The engines are more prone to overheating, and are typically much lighter built engines. These engines are often similar to those used in riding lawnmowers, as an example. Typical liquid cooled engines are adapted automotive or industrial engines, much more heaily built. Liquid cooled engines normally run at 1,800 RPM, so MUCH less wear on all the internal surfaces, and also a lot lower output HP compared with what the engine could do at higher speeds, so the engine is, in effect, lightly loaded mechanically so there is much more margin between the operating loads and the failure points for internal components. At work, I typically advise customers of around 800-1,000 hours of operational life for an air cooled engine, and 8,000+ hours for a liquid cooled engine. That's assuming both are well maintained. Note that on my normal commercial projects, even small ones like cellphone sites, air cooled gensets are not even a consideration as we NEVER use them for reliability reasons.
I would say a 20-25kW or so genset is a good fit for you. You want to be able to run all your stuff, but you don't want to oversize the unit too much since that just hurts fuel efficiency. Generators are most fuel efficient when running up near full load. Commercially I try to size for around 80% or so of capacity, which is a good mix of fuel efficiency and having some extra capacity for future load growth. For larger plants (megawatt+), I can shrink that margin a bit and run the units at higher percantage of capacity, because the absolute power reserve is higher with the larger units. Note that my typical customers are large facilities with relatively constant loads, so not very similar to residential loads. For residential loads, the peak loads tend to be very much higher than the average loads, so you're likely to see a generator running at around 10-15% or so of capacity on average, with peak loads up to 90%+ or so. This is fine from an operational standpoint, just keep in mind that fuel efficiency won't be as good -- but there isn't really anything you can do about that here. If you look at the fuel consumption tables, you'll often see that the fuel consumption at 25% load is about half that at full load, to give you an idea of how much less fuel efficient the generator is at light loads. This is important for sizing your fuel supply if you will have an onsite fuel supply such as propane or diesel fuel.
My recommendation would be a liquid cooled generator here, because you can expect to need to run on it for extended periods of time, and liquid cooled units are far, far more reliable. I would be SURE there is a block heater on the unit, which is usually going to be one of the Hotstart units or similar in the coolant loop. These work well. Note that you can save some money on electricity by using the 80-100F heater instead of the more commonly seen 100-120F heater (those are the "turn on" and "turn off" temperature setpoints of the internal thermostats in the heaters). The heaters make for easier starting, but also keep the units warm so that they won't see condensation internally or externally, which helps with long term reliability.
In your area, with the VERY heavy snow loads, I would put the generator under a lean-to style shed roof, so that the generator has some protection against being buried in a snow drift. You don't need (or want) walls here, you only want a roof to protect the unit. You need maximum ventillation.
I would pipe things so that the generator is away from living spaces as much as possible, especially bedrooms. At my own home, I reworked the natural gas line so that I could run the generator on the opposite side of the house from the gas meter, which is also the opposite side from where the bedrooms are. This is VERY nice during extended outages (which are frequent here) at night. All generators will make some noise, so it's worth some effort and planning to ensure that noise won't be bothersome while the unit is operating. I have placed residential generators behind detached garages sometimes to get seperation and a sound barrier (the walls of the garage) before too. Anything helps, but be sure the unit is accessible for maintenance.
I recommend you size your fuel supply to run the generator at full load for the full duration of the longest expected outage in your case. This is because in your area, it's reasonable to expect that if you have an extended outage, it's very likely you won't be able to get fuel deliveries due to impassible roads. The telecom industry standard is "48 hours runtime at full load with a full tank". In your case, I'd plan to have 4-5 days of full load runtime as a minimum. Remember that if you run out of fuel during an outage, you no longer have a generator... I would also get your tank topped up BEFORE any predicted large storms, just to be safe. When you size your tank for full load, you'll likely end up with about double the runtime with normal average load, so that becomes a sort of extra buffer for unexpectedly long outages.
Remember that propane does not degrade over time the way gasoline (or diesel, to a lesser extent) does, so if you buy extra propane and end up not needing it, it will stay perfectly "fresh" for years until you do end up using it. There is no worry about propane degrading over time.
Note that you NEED to do one of two things with a propane setup in very cold climates:
1- Ensure the tank is large enough to be able to "boil off" a suffient amount of vapor to supply the BTU load of your generator at full load. This usually ends up being a larger tank than you thought you needed. If the tank gets too cold, and it gets cold as gas is drawn off (physics at work, just like an air conditioner), the gas supply drops off and can starve the generator. This is a Bad Thing. Your propane company should be able to help you size the tank for your BTU load, and keep in mind that a typical generator will be, by far, the largest BTU load of a house, so sizing the tank for the house for heating only (which is how it's normally done) will usually give you a tank unable to reliably deliver the needed gas supply at low temperatures.
2- Have the tank work as a liquid system, which means the first stage regulator will be supplied with liquid propane ("LP" :-), and will need a vaporizer ahead of it. Liquid withdrawl systems do not have the issue with tank size at low temperatures that gas withdrawl systems have, but the vaporizer will need to be heated in very cold conditions.
Bill
Fantastic info, Bill. It could have easily been a blog post.
Are we at a point where there are feasible ways to use the cooling loop on a liquid cooled generator to assist with heating the house? If you are supplying heat pumps from the generator, you could substantially cut the fuel use if you use the waste heat.
Unfortunately, every time I've looked into doing something like that, it hasn't made economic sense. If you were off grid, it would almost certainly work out financially, but for standby applications, there just isn't enough runtime to justify the cost of the hydronic system installation.
For anyone wanting to try this, the easiest way to go is to put a heat exchanger in the coolant loop from the engine on the "hot" side (usually comes off near the top of the engine), then circulate a water/glycol mix through an isolated loop between that heat exchanger and a "water to air" heat exchanger in the house (assuming you're using a forced air furnace for primary heat). A small circulator pump can handle the isolated loop. You need a small expansion tank somewhere on that isolated loop, and an air seperator (which can be omitted if you're very careful when filling).
Personally, I love the idea -- I really like minimizing wasted energy in systems like this -- but it's expensive to implement.
Bill
@Bill - I just noticed that the LP consumption rates are pretty different between the air- and water-cooled, which seems to make sense:
Kohler 24RCLA consumes 598 BTU/hr (24 kWh, 100A, water-cooled, 1800RPM)
Kohler 26RCAL consumes 290 BTU/hr (26 KWh, 109A, air-cooled, 3600 RPM)
If I have this right, then you can get the same amount of power from the air-cooled 26RCAL as you can get from the water-cooled 24RCLA - while using half the amount of propane.
Of course, the 26RCAL, with its 3600 RPM, is 4x noisier than the 24RCLA @ 1800 RPM.
Is this a false economy? It seems that on paper I can get more power out of my propane thanks to higher RPMs and lighter power train components, but at the expense of noise and reliability.
I already know that I probably won't achieve ROI on the water-cooled generator in my lifetime, even if I replace the air-cooled generator three times. And the ROI doesn't include the propane savings and hence extra run-time with the air-cooled generator.
The Faustian deal is that we need to tolerate the noise of the air-cooled generator and give it more maintenance, repair and ultimately replacement to make sure it's always ready to go.
@Akos also makes an interesting observation that you can get alternative heat with a direct vent heater, and you can store electricity for less demanding services in a battery. Switching between utilities / generator / battery is a bit of a pain to install and operate (and batteries are not cheap), but this seems worthy of consideration since we have a direct vent fireplace in the plan.
With these further considerations, assuming I understand them correctly, the water-cooled generator is less of a necessity since you can work around the air-cooled generator (e.g. direct vent propane heating and battery).
We will probably still get the water-cooled because it's quieter, but it's nearly impossible to rationalize water-cooled from an ROI perspective in a residential setting - except the ROI of less stress about the generator starting and fewer service calls to keep it running. Obviously in a commercial setting, the ROI of water-cooled is priceless when it comes to customer retention.
I don't know where you're getting those fuel consumption numbers. I pulled data sheets for both those units and see these numbers on propane:
24RCLA:
100% load (24kw): 113 CFH/hr
25% load (6kw): 63 CFH/hr
26RCAL:
100% load (26kw): 129 CFH/hr
25% load (6.5kw): 64 CFH/hr
You have to be sure to be looking at the numbers for LPG, and for 60Hz operation, and also for 120/240V configured alternators. Lots to keep track of!
From the above numbers, the air cooled unit is very slightly more efficient at 25% load, but less efficient at full load. In actual operation, you probably won't notice much difference in fuel consumption between the two when operating with typical variable residential loads.
The air cooled units are not four times noisier because the run at double the speed. It's more complicated than that. The main reasons why the liquid cooled units are quieter is not only because of the lower operating speed, but also because they tend to have heavier castings and the liquid in the water jacket tends to dampen some of the noise too. You usually find better mufflers on the liquid cooled units as well. With proper enclosures and mufflers, either unit can be made to be quiet, but it takes less effort to quiet down the liquid cooled units.
The main difference between the two will be reliability over time, and during extended periods of runtime (i.e. long duration power outages). If you're expecting long duration, multi-day outages, especially where you're pretty much cut off from the world from the storm that caused the outage, I would very much prefer the liquid cooled unit's reliability because you REALLY NEED IT!
I would not consider batteries. Batteries of the "mount'em on the wall type" like the Powerwall often don't work well with residential standby generators since the generator output is not as stable as the utility, which makes it difficult for the battery unit to track and synchronize with the AC waveform. I think many/most of these battery packs even state that you should not use them with a generator (not entirely sure on that though -- I don't work with them often). I would plan to just use a normal ATS and generator system, and not try to stretch things with a battery. It IS possible to keep a small generator running more efficiently with careful load management and a battery system, but it's much more complex and not something you want to try unless you're really into this stuff. You do get much better fuel efficiency by running a small generator at a higher percentage of average capacity though. Note also that batteries do not store a particularly high amount of power. With your loads, you're looking at a few hours or so of battery runtime. The battery will not be cost effective.
BTW, part of the reason I almost always work with liquid cooled generators is that above about 25-30kw or so, there isn't any other option. Most of my work is with 150+ kw generators, and I frequently work with megawatt size units. Most of my work is with telecom and datacenter facilities, and they use massive amounts of power, 24x7. As an example, there is a planned Amazon datacenter soon to be built in PA that is built alongside a power plant, and they expect to use nearly all (maybe all) of the plant's electrical output when complete. These facilities aren't typical loads :-) On the plus side, it's easy for me to sell efficiency upgrades of even small amounts like 1%, because even 1% is a lot of savings in absolute terms when you have six or seven figure monthly electric bills as many of my customers do.
Bill
@Bill - Oops, wrong specs, not even close (sorry, no excuse for that). And I was thinking power and not perceived loudness. Thanks for your patience, and all the wisdom (i.e. battery, smaller generator).
TL;DR
Perceived loudness at full-speed / normal operation:
Kohler air-cooled is ~1.6x louder than Kohler water-cooled (no surprise)
Generac water-cooled is ~1.9x louder than Kohler water-cooled (surprise)
Generac air-cooled is about the same as Kohler air-cooled (no surprise)
I was surprised on the water-cooled Generac vs Kohler. Perhaps the Generac has a lighter weight housing (cost) or more free-flowing muffler (squeezing out every little bit of power?).
You've got me back on water-cooled (noise and reliability), no battery back-up (complexity), load-shedding (smart), and Kohler for more reliability and less noise. Will also do the block-heater and battery heater.
I'll talk to the installer about liquid withdrawal. Not sure if we can do that with the LP tank already buried and paved over by the driveway? The buried tank is at a higher elevation than the generator, if that makes a difference.
We get up to a week's advance notice on big storms, so we can top-off the tank ahead of the storm. And we have a direct vent fireplace if we have to go into survival mode when the propane is getting low and we can't get off the mountain (and the propane supplier can't get up). We've got a wood-based fireplace as well, but it mostly blows heat up the chimney - but it looks and sounds nice.
Details (hopefully I got it right this time)
When sound is measured in dB:
3 dB is doubling the power
6 dB is doubling the pressure level
10 dB is doubling the perceived loudness
From the spec sheets:
Kohler 24RCLA (water-cooled) = 53 dB(A) exercise, 61 dB(A) full-speed
Kohler 26RCAL (air-cooled) = 56 dB(A) exercise, 67 dB(A) full-speed
Generac Protector QS series (water-cooled) = 61 dB(a) exercise, 70 dB(a) full-speed
Generac Guardian series (air-cooled) = 57 dB(A) exercise, 67 dB(A) full-speed
We usually use 6dB as "perceived as twice as loud" at work. Honestly though, that "how loud is it" stuff is very subjective -- some people hear different frequencies in different ways, so sometimes the character of the sound is more important that the actual level. It depends in part on the person. If you want quiet though, keeping the sound energy as low as possible is a plus regardless.
Generac might not be using as much acoustic insulation in their inclosures. I don't really work with their product much, so can't really compare things there. That would be my guess as to why one is louder than the other though, aside from differences in engine and alternator design (especially the cooling fans).
Liquid withdrawal needs to be drawn off of the bottom of the tank, but I think they can use drop pipes in the larger tanks to allow for liquid withdrawal with a top-mounted fitting. You'd need to check with your propane supplier about that. Gaseous withdrawl is much more common, and is what I usually see in the field.
BTW, power is based on a lot more than RPM too. There is also cylinder size, compression level, probably other stuff (I'm an EE, and this is ME stuff :-). For a given engine though, you generaly get more horsepower out at higher speeds, but you also stress all the engine internals a lot more. Engines have a "torque curve", so they have an RPM for peak power, another for peak torque, and I wouldn't be surprised if there is a third point for peak efficiency in terms of energy conversion. I don't really work with that stuff though, for me, the rating on the generator set tells me what to design with electrically, and the generator manufacturer has already worked out all the mechanical stuff with the engine.
Bill
With Bill having covered most bases very well already, I'd say it's worth seeing what local dealers your neighbors have been happy with. While I would prefer a Kohler, if Generac had a better service network in the area that would sway me more towards them.
I have a much more modest setup of an 8kw portable inverter generator that runs everything I need including 3 ton heat pump, induction range, lights, fridge, and freezer. A liquid cooled standby generator would be nice along with an automatic transfer switch and someone else to maintain it for me.
The power just came back here after a three day outage due to a storm. During the past decade many houses in our rural community have switched from small portable generators to Kohler or Generac ones of about 17kw wired to run virtually everything.
To add to Bill's comments: The most common problem I saw in the past few days was owners not anticipating the fuel consumption of these units even when not drawing much load. Many resorted to turning them on and off, leaving the houses without any power at all for long periods. If you size your generator so that you can continue a lifestyle completely unimpeded by outages, make sure you are aware of how much propane that takes, and size your tank accordingly.
That's an excellent point, Malcolm. Many of my clients have either Kohler or Generac standby generators and they indeed use far more fuel than people usually expect.
I see that issue fairly often with people adding generators to existing propane systems, because of the tank's inability to "boil off" enough gas to supply everything.
A typical permanently installed generator is likely to be the largest (often by far) BTU load in a house. My 18kw genset here, as an example, uses 280,000 BTUH worth of natural gas at full load. That's almost double the load when BOTH of my home's two heating furnaces are running! At 1/4 load, it's still using 140,000 BTUH, and it's doing that continuously for the entire time it's running. I'm converting from the cubic feet per hour numbers in the datasheet here, using 1,000 BTU per cubic foot for NG.
Adding a unit to an existing natural gas service without upgrading the meter and regulator can be a problem too, since insufficient gas supply will result in low gas delivery pressure, and that can cause erratic operation of the genset. When installing a generator on an NG system, it's a good idea to upgrade to 11 inch WC pressure (standard for residential is 7 inwc) too, to help ensure a bit of extra wiggle room when the generator is operating.
I would stay away from the surplus gov't generators. Those are the MEP-xxx series. They are way overly complex for what they do, and they are a huge pain to work on. They're typically well built, but aside from that, they aren't a good option for most people. If you want to go the surplus route, look for a used Kohler or Commins/Onan in the 10-25kw or so range. Those are much easier to work on.
BTW, Malcolm: I hope you made it through that storm without taking any serious damage. I heard it was pretty brutal, winds off Vancouver I had heard peaked up over 100MPH in some places offshore.
Bill
Bill,
We did fine. The winds went right over our place. Not even enough debris to warrant getting out a rake. The areas that saw damage were mostly predictable. Any south facing slope that had been recently clear-cut near a road. They leave an insufficiently deep visual buffer of trees, which then blows down every time there is a storm.
Have a look at the government surplus gen sets.
Yes, you will not be sere that the one you buy will work out of the box but you are getting a much better build quality and generally they have very low hours on them.
Walta
People tend to overestimate average power use in a house. Even with electric heat, it will be a small fraction of a 15 to 20kw genny, that means the fuel efficiency will be pretty bad. Generators also have a nasty habit of not starting, so not sure I would want to rely on it for keeping your house from freezing.
Even with a heat pump especially in cold climate, the efficiency of propane->electricity->heat will be around 40% to 50% best case.
I think a much simpler setup is to use something like a larger through the wall propane heater in the core of the house to keep the place from freezing and combination of fire places and running the heat pump in circulation mode to even out the temps. This would also reduce your peak power use and also the size of the propane tank you need. It would also be much more reliable and you'll always have heat even with the genny off.
Charging an EV from propane makes even less sense. If you really must charge, I would get a smaller plug in cable that would get you enough juice for daily trips.
I think a smaller and cheaper propane inverter generator with a reasonable sized battery pack (say 5kW, 10kWh) would be better use of your dollars. This would still get you reasonably high peak power plus you can add some PV to offset electricity costs the rest of the year. It is more complicated install though.
I agree that a direct vent propane heater would make more sense for providing heat, and batteries can be great for providing extended periods of not needing to run the generator. Even if planning on using the heat pump, the propane heater would be a good backup in case there was an issue with the generator or heat pump. If you just need to run the generator for an hour or two in the morning and evening to power higher draw items and recharge batteries then a 3 day propane supply could last closer to 3 weeks.
I have found the emporia vue energy monitor very useful for getting a good picture of energy use. Over the past month even with my 3 ton heat pump running with a soft start my peak use hasn't exceeded 6kw. Prior to changing the heating elements in my old electric water heater from 4500 to 2500 watts I would occasionally reach almost 8kw, but since the switch I've only hit that when charging a friend's EV, and if for some reason I needed to do that while on generator power the charge rate could be adjusted to stay within the limits of the generator. This is for a 2100 square foot house with 4 people, 2 dehumidifiers, an ERV, fridge, separate freezer, induction range, 3 ton heat pump, microwave, electric water heater, heat pump dryer, a 2nd oven, instant pot, and air fryer. My base load is around 300 watts.
Yup and that is peak power.
An all electric house might use 1000kWh in a month so the average power would 1.4kW. When it comes to fuel use, what matters most is average power not so much the peak, so using a 20kW genny to supply 1.4kW average power is a recipe for bad fuel economy.
Peak power for things like motor starting doesn't really affect generator sizing much, if at all. For motor starting, you look at a generators "peak motor starting KVA" rating, not it's KW rating. If you've ever looked at a transformer in a commercial building, you'll see an "impedance" number, given as a percentage. That impedance number is used for calculating motor start ability and deliverable fault current. These are different things from the long term energy capacity of the device.
Residential generators can usually start an A/C compressor, and that's typically the biggest motor load in a house. Everything else is small enough to not really matter. I do recommend the hard-start kits (that's what they're called, not "soft start") for the A/C compressors though, which does make things easier for the generator.
Bill
@Akos - I appreciate this alternative solution (smaller generator, direct vent heating, and a battery). Some heat and some electricity is certainly better than none.
You suffer more noise from the smaller air-cooled generator, but you use less propane at the same time. No free lunch, but definitely some wins.
Roughing it, but not putting yourself in harms way, is acceptable to me during an emergency, but for some family members, perhaps not so much.
Or maybe I completely misunderstood how air-cooled vs water-cooled generators work.
EDIT
See post #18. I made an egregious error on propane consumption. Air-cooled and water-cooled gensets are about equal for the same electrical power output. However, your comments on the direct vent heater has me looking at our direct vent fireplace in a different way - the fireplace is a very viable source of emergency heat (so long as you have propane) that doesn't require the genset. We also have a traditional wood-burning fireplace, but it's way too inefficient - mostly just good for mood and a nice crackling sound.
>"Or maybe I completely misunderstood how air-cooled vs water-cooled generators work."
They both work basically the same: the engine converts chemical energy from the fuel into mechanical energy (what I like to jokingly call "twirly force"), the generator end (actually an "alternator") converts the mechanical energy into electrical energy. It's a rotary machine, basically an engine and a generator on the same shaft.
The difference is in how the ENGINE is cooled (the generator is ALWAYS air cooled with these types of generators). An "air cooled" engine will have fins cast into the cylinder walls, and excess heat from the cylinders is removed by airflow over those cast fins. Some generators also have an oil cooler, which is a small radiator with oil circulating through it from the engine's lubrication system. A fan then blows air over the oil cooler. Air cooled engines tend to operate at 3,600 RPM in generator applications, which allows for smaller and lighter (read as "cheaper" ) engines to be used for the same horsepower output. The downside is air cooling is less efficient, there are more chances for hot spots to occur in the engine, and the temperature is less stable, which means less consistent fuel economy.
In a "liquid cooled" engine, there are water passages cast into the engine block, and coolant (a mixture of water and glycol) is circulated through those coolant passages by a pump, and ultimately goes through a radiator. A fan blows air through the fins on the radiator to cool the system. Water cooled engines also tend to operated at 1,800 RPM in generator applications, which is where the big jump is reliability comes from, but they also tend to operate at more consistent temperatures (due a thermostat in the coolant loop), which helps with both efficiency and reliability. The downside is these engines are physically larger and much heavier than an air cooled engine of equivalent HP output.
Note that RPM is directly related to AC output frequency with these types of generators (that's not the case with "inverter" generators). 1,800 RPM engines are mated with 4 pole generator ends to get 60Hz (North American standard line frequency). If you run them at 1,500 RPM, you get 50Hz (European standard, and a lot of the rest of the world too). You get less HP out at the lower speed, which is the reason for the reduction in output power you see in the datasheets when running the generators at 50Hz (the generator end contributes a bit to this too). 3,600 RPM engines use a 2 pole generator for 60Hz. Some big diesel generators run at 900 or 1,200 RPM, which use 8 or 6 pole generators, respectively. Fun stuff.
Bill