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Thread: Cabin Heat, Water Cooled Engines - Which Way is Best and Why?

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    Registered User Dan Thomas's Avatar
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    Re: Cabin Heat, Water Cooled Engines - Which Way is Best and Why?

    Quote Originally Posted by lr27 View Post
    Wondering how much all this stuff would weigh compared to a more powerful alternator and heated seats or something.
    You need a big alternator to get any usable heat out of electrical heating. Heated seats do nothing for your face or hands or feet anyway, so you'd need heating elements to heat the cabin air and for defrosting. The overall weight would add up real fast. With coolant, you need maybe another quart in the system, a heater core, maybe a light fan, and an aluminum box with a couple of flapper valves. Not much weight at all, really. And the engine's waste heat is there and you don't need to rob any more power from the engine to make it.

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    Re: Cabin Heat, Water Cooled Engines - Which Way is Best and Why?

    Not so sure about that. Hi output alternators are getting pretty compact these days and with some strategic positioning of heating elements (to include heated clothing which would remain in the closet much of the year) might get you a net savings in weight. No, you're not going to flip a switch and get the cabin all toasty warm so you can wear shorts and T's in February, but you could get it safe and functional. A look at all the attributes of the airplane (composite, glass area) as well as the existing heat sources right near the windscreen (avionics), might be the basis for a simple, but comprehensive heating system that checks off all the boxes.

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    Re: Cabin Heat, Water Cooled Engines - Which Way is Best and Why?

    Quote Originally Posted by lr27 View Post
    Wondering how much all this stuff would weigh compared to a more powerful alternator and heated seats or something.
    Let's remember that even if you heat the seats and clothing, to fly in the upper teens in winter, you still have to heat the windows or they frost up from your breath. Once you have a way to heat the windows, cabin heat is a cinch...

    Billski
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    Registered User Toobuilder's Avatar
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    Re: Cabin Heat, Water Cooled Engines - Which Way is Best and Why?

    There is still going to be considerable heat bleeding through the firewall. Agreed, a cold start after sitting out on the ramp all night is going to take some significant BTU's to get up to temp, but for most of the flight it's going to only require a fraction of that. Kind of like A/C... Its real value is the first 10% of the flight. After that, the excess capacity is just dead weight.

    I'm not suggesting its a slam dunk, but certainly worth a critical review of requirements and a trade study.
    Last edited by Toobuilder; February 21st, 2019 at 10:52 PM.

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    Re: Cabin Heat, Water Cooled Engines - Which Way is Best and Why?

    Quote Originally Posted by Sockmonkey View Post
    In an emergency there's always the old spit in a rag and wipe the inside of the windshield with it to keep it from fogging up.
    That works just fine in the hanger. Bad idea at 40 below at altitude.

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    Re: Cabin Heat, Water Cooled Engines - Which Way is Best and Why?

    Modern cars also use the always hot heater core with the air conditioner on. Dehumidifies the air for defogging.

    Electric heat takes power from propulsion. Taking heat from coolant is free energy. At the cost of weight and complexity, but that's a given.

    Standard heater valve, steel, through the firewall, and a simple plenum holding the heater core fwd of the firewall should satisfy most safety concerns. A fresh air source out of any possible exhaust leak air path should take care of the remaining safety issues. I'd think a hose to the front, top of the engine, taking in some of the cooling air good enough, but ymmv.

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    Re: Cabin Heat, Water Cooled Engines - Which Way is Best and Why?

    Quote Originally Posted by Aesquire View Post
    Modern cars also use the always hot heater core with the air conditioner on. Dehumidifies the air for defogging.


    Standard heater valve, steel, through the firewall, and a simple plenum holding the heater core fwd of the firewall should satisfy most safety concerns. A fresh air source out of any possible exhaust leak air path should take care of the remaining safety issues. I'd think a hose to the front, top of the engine, taking in some of the cooling air good enough, but ymmv.
    I'm not aware of any car using the heater core airflow with a/c to aid in reducing humidity. That would pretty much negate the a/c. A/c already de-humidifies by the fact that the evaporator is cold and moisture in the air condenses on it and drips off to a vent line under the car- the same way house de-humidifiers work.

    As I said before, cars haven't used coolant heater valves for decades now, they mix cold and warm air for temperature control. Some cars require the heater loop to activate the thermostat so you can't throttle coolant flow here unless you have another, separate bypass loop to perform that function.
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    Re: Cabin Heat, Water Cooled Engines - Which Way is Best and Why?

    Quote Originally Posted by rv6ejguy View Post
    I'm not aware of any car using the heater core airflow with a/c to aid in reducing humidity. That would pretty much negate the a/c. A/c already de-humidifies by the fact that the evaporator is cold and moisture in the air condenses on it and drips off to a vent line under the car- the same way house de-humidifiers work.
    I think he's saying that defrost mode uses the A/C to dehumidify, then the heater core to heat the air for defrost, with the idea being not to blow humid air onto the cold windshield. At least, this is what I've noticed my cars doing.
    I reserve the right to be smarter tomorrow than I was yesterday.

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    Re: Cabin Heat, Water Cooled Engines - Which Way is Best and Why?

    [QUOTE=rv6ejguy;463150]I'm not aware of any car using the heater core airflow with a/c to aid in reducing humidity. That would pretty much negate the a/c. A/c already de-humidifies by the fact that the evaporator is cold and moisture in the air condenses on it and drips off to a vent line under the car- the same way house de-humidifiers work.

    When you turn on car windshield defogger anytime, even in the winter, A/C compressor will "kick on" and A/C system will help to reduce amount of moisture in the car to help defog windshield faster.
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    Re: Cabin Heat, Water Cooled Engines - Which Way is Best and Why?

    Quote Originally Posted by Toobuilder View Post
    There is still going to be considerable heat bleeding through the firewall.
    Not much once you know what is in a composite airplane firewall. 1/4" plywood with glass on both sides, then on the engine side, 1/8" Fiberfrax insulating/fireproof layer, then a thin sheet of stainless to protect the Fiberfrax from being destroyed by air flow and brushing it while doing mechanics' tasks on the engine. This amounts to about 7/16" of layers of varying insulating qualities. There will be penetrations, but they all must be designed to keep fire on the other side, and so are inherently rather low heat transmission as well as being relatively small. This will be orders of magnitude less heat transmission than a typical sheet metal airplane firewall.

    Some folks have been talking about heat from the electronics. I summed up the nominal operation on the panel mount stuff - from Dynon and Avidyne sites, it is 12 amps or about 169 watts. I seriously doubt two light bulbs will do much for keeping 10 ft^2 of windshield frost free when we are breathing on this side and have -25 F air at 300 ft/s on the other side...

    Agreed, a cold start after sitting out on the ramp all night is going to take some significant BTU's to get up to temp, but for most of the flight it's going to only require a fraction of that. Kind of like A/C... Its real value is the first 10% of the flight. After that, the excess capacity is just dead weight.

    I'm not suggesting its a slam dunk, but certainly worth a critical review of requirements and a trade study.
    Understood and agreed. Building these parts is a ways off. I have to go back to my heat transfer and HVAC books, estimate the min temp and flow rate of air (from my flight profile) blowing on the inside of the windshield to prevent condensation/frosting while we run really -25 to -40 F air at 300 ft/s over the outside of 1/4" Plexiglas (windshield) and 1/8" Plexiglas (other windows). While I am at it, I can also estimate the heat transfer through my composite and Fiberfrax firewall and compare all of this to the 169 Watts of waste heat the avionics will make.

    Billski
    Billski's opinions expressed here are available free and may be worth the money you paid for them. Understand that they are based upon a successful combination of education and a lifetime of experience using that education, but I can not know everything about your circumstances. Your choices are yours alone, and you must be the final judge on what you do. No whining...

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    Re: Cabin Heat, Water Cooled Engines - Which Way is Best and Why?

    Quote Originally Posted by Aesquire View Post
    Modern cars also use the always hot heater core with the air conditioner on. Dehumidifies the air for defogging.
    Actually, they run it the other way around. When you turn on defog and the evaporator is above freezing, the AC is run to dry the air being blown on the windshield. Raising air temp does not take out moisture, it does reduce relative humidity, which helps with evaporating water from the windows too. Lowering air temp to the dew point condenses moisture onto the coils and is dumped out the scupper, which really helps on above freezing dewy days. Once air temp at the evaporator reaches freezing, the compressor is turned off to prevent icing the coil...

    Electric heat takes power from propulsion. Taking heat from coolant is free energy. At the cost of weight and complexity, but that's a given.
    Well, sort of. A kilowatt of electricity (1.5 hp or so) would be hard to notice at cruise or even in climb.

    Standard heater valve, steel, through the firewall, and a simple plenum holding the heater core fwd of the firewall should satisfy most safety concerns. A fresh air source out of any possible exhaust leak air path should take care of the remaining safety issues. I'd think a hose to the front, top of the engine, taking in some of the cooling air good enough, but ymmv.
    That has become part of the plan, if the room exist FWF for it, and I think I do have that space...

    Bill
    Billski's opinions expressed here are available free and may be worth the money you paid for them. Understand that they are based upon a successful combination of education and a lifetime of experience using that education, but I can not know everything about your circumstances. Your choices are yours alone, and you must be the final judge on what you do. No whining...

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    Re: Cabin Heat, Water Cooled Engines - Which Way is Best and Why?

    Quote Originally Posted by rv6ejguy View Post
    I'm not aware of any car using the heater core airflow with a/c to aid in reducing humidity. That would pretty much negate the a/c. A/c already de-humidifies by the fact that the evaporator is cold and moisture in the air condenses on it and drips off to a vent line under the car- the same way house de-humidifiers work.

    As I said before, cars haven't used coolant heater valves for decades now, they mix cold and warm air for temperature control. Some cars require the heater loop to activate the thermostat so you can't throttle coolant flow here unless you have another, separate bypass loop to perform that function.
    There are full bypass valves for heater cores on the market. Four hose connections return flow looks like it is maintained. Less bulky option than a mixer door and chamber, but a hit on FMEA for five added possible leak paths.

    Agreed on the AC for defog. AC compressor is cut off when evaporator temps approach freezing to prevent coil icing.

    Billski
    Last edited by wsimpso1; February 22nd, 2019 at 01:15 PM.
    Billski's opinions expressed here are available free and may be worth the money you paid for them. Understand that they are based upon a successful combination of education and a lifetime of experience using that education, but I can not know everything about your circumstances. Your choices are yours alone, and you must be the final judge on what you do. No whining...

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    Re: Cabin Heat, Water Cooled Engines - Which Way is Best and Why?

    Quote Originally Posted by gtae07 View Post
    I think he's saying that defrost mode uses the A/C to dehumidify, then the heater core to heat the air for defrost, with the idea being not to blow humid air onto the cold windshield. At least, this is what I've noticed my cars doing.
    That might be what Aesquire meant, but it was not what he said. Both Ross and I felt it necessary to not convey that to other folks who might read our threads.

    Billski
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    Re: Cabin Heat, Water Cooled Engines - Which Way is Best and Why?

    [QUOTE=Malish;463153]
    Quote Originally Posted by rv6ejguy View Post
    I'm not aware of any car using the heater core airflow with a/c to aid in reducing humidity. That would pretty much negate the a/c. A/c already de-humidifies by the fact that the evaporator is cold and moisture in the air condenses on it and drips off to a vent line under the car- the same way house de-humidifiers work.

    When you turn on car windshield defogger anytime, even in the winter, A/C compressor will "kick on" and A/C system will help to reduce amount of moisture in the car to help defog windshield faster.
    Not on my BMWs, Hyundai, Nissan, Toyotas... How would the a/c help de-humidify -30C outside air? Reduce it below that temperature and cause the evaporator to freeze over and block flow? You can't condense moisture out of the air for long on a freezing surface as you get ice which is why refrigerant flow is throttled to keep evaporator temperature just above freezing. You can't de-humidify without condensing. Condensation freezes below 0C.

    A/c systems on some cars will run in the winter months on certain settings to exercise the compressor and seals.
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    Registered User Toobuilder's Avatar
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    Re: Cabin Heat, Water Cooled Engines - Which Way is Best and Why?

    Not going to argue the science behind it, but all the GM cars I have owned from 60's vintage through today turn on the AC compressor when "Defrost" is selected. Move it off defrost and the compressor disengages.

    At any rate, I think the true requirements need to be explored. I have flown in hard IMC which included rain and water intrusion followed by a climb to altitude and airframe icing. This was in an aluminum airplane with no cabin insulation and no heater of any kind. It was not safe or comfortable by any means, but the canopy fogging was easily removed with a towel. The towel didn't do anything for the 1/8 inch of solid ice on the outside of course, but we are not discussing anti ice requirements here. The salient point is that in my experience it does not take much more than airflow on the windscreen to keep it clear, and for those "corner cases" where it isn't, a towel is an effective backup. Heck, even the U-2 cockpit has a rag on a wooden stick just in case the anti fog can't keep up.

    Once again, the minimalist system I'd go with is not going to provide the firebreathing output of a typical modern car, but is that the requirement?

    One other thing to consider. I agree that the risk of failure is pretty minimal for either a wet system or a dry system, but that is only one side of the risk analysis process. There is also the severety of failure. And to me, this bears strong consideration. Even a small leak in your "wet" defrost ductwork is going to steam up your windshield in a split second. Unlike with a dry system, windscreen obfuscation will happen in summer or winter and is not going to be cleared with a towel. So with the possibility of failure low, and the severity of failure high, you are looking at an overall risk score of "moderate" every time you fly.

    Not trying to steer the decision, Just food for thought.

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