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I've got some computer equipment that burns really hot (90 celsius exhaust), and I thought it'd be a great idea to recycle that heat to keep the house warm in the winter.

The equipment is in the basement, and I routed the exhaust through 6" insulated duct into a heat register that blows in the home. The problem is it's too damned hot! As a test, I've put the thermostat at 19 degrees - and the house is at 24-25. Furnace hasn't run in a few days.

I'm considering venting outside instead, tapping into the exhaust for my HRV. I worry though, that sucking air in the basement and pushing it outdoors like this could have adverse effects such as negative pressure.

Should I worry about this? Sucking air in from inside the home, and pushing it straight outside?

Thanks for your guidance.

  • Since its cold outside, would a heat exchanger work to cool off the air, or do you really just want to dump it outside? Also, could you suck in cold air from outside to cool the equipment? – JPhi1618 Dec 6 '17 at 17:00
  • Hey @JPhi1618. I have a heat exchanger, it's just not enough. With it running at full blast, the basement's ambient air goes to approx 27-28 deg which is too high for the equipment. Dumping it, or at least most of it outside seems like a need-to-have. – Saeven Dec 6 '17 at 17:01
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    Rather than dumping the heat outside I'd look for another use for it, (it costs energy/money to make that heat), something like getting another heat exchanger and setting up a hot water heater might make sense. – Josh King Dec 6 '17 at 17:08
  • What do you mean HVAC pipe? A duct? Also, are you trying to solve the problem of 24-25 being uncomfortably warm or are you trying to solve the problem that your computer is overheating despite pushing its air exhaust away? – Dean MacGregor Dec 6 '17 at 19:29
  • Hi @DeanMacGregor - the conundrum is: recycling the heat is warning the house far too much. The furnace isn't running at all, and the house is at 25 deg. Opening windows during the Canadian winter is not a good idea. Duct - yes. Right now, the miners at a comfortable temp, but there's just too much heat being recovered. Wanted to send it outside, but worry about neg pressure in sucking from ambient air in the basement, forcing it directly outdoors in a sealed type of exhaust. – Saeven Dec 6 '17 at 19:34
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Negative presure is actually a serious problem. Modern homes are tight enough that a negative pressure device like a dryer venting has the potential to pull negative pressure on the house and interfere with gravity (convection) exhaust stacks on fuel furnace, water heater, etc. This is why heat exchangers are a thing, and many machines are now direct vented, with double vents, inner for exhaust, outer for combustion air. (Bonus, the exhaust heat warms and dries the combustion air).

Also, it is a shame to throw away perfectly good heat.

The cheap way is to block doors open and set blowers/fans in doorways to pull the heat around the house. In summer, open basement windows. However this is not a reliable solution and requires a lot of awkward fans-in-pathways and manual arrangement.

Other venting plans are up against a basic fact: air is not a very good thermal transfer fluid. Water is ideal, and adding antifreeze doesn't hurt much.

from here on out, when I say water, I mean antifreeze, ideally propylene glycol which is basically edible. A spill will bring vermin rather than kill cats.

Surprisingly little water flow will do a fine job of cooling a PC, even several chips plumbed in series. (Another way of saying this is, heating water is hard).

Water cooling - push the water around

If practicable, use water-cooling on the equipment. This detailed "whole room water cooling" video series goes deep into the nuts and bolts of it. They do a few things I don't agree with, like dingdonging together a bunch of PC radiators instead of just getting an old automobile radiator (with attached thermostats and electric fans) from a junkyard.

But the basics are the same: cooling plates on the hot running chips, pumps in the PCs, a hot/disposal water trunk, and a cool/makeup trunk.

Now in the simple version, you plumb all the heat outside, to an automobile radiator. Keep its built-in fan thermostat and fans. Wire it off a 12V supply, so the thermostat operates a 12v relay which supplies power to the fans. Plumb it so water reaches the thermostat last. That way the fans only run if they're needed, just like in the car.

In the more complex version, you make stop(s) and get to the automotive radiator last, but first, you make a tour around the house and use the water-heat "any which way you can". Now this is very similar to hydronic heating systems used in wealthier homes in the US, and multipurpose boiler" systems in the UK. I'm not an expert in those systems, so I'll explain the general gist. And they may not be in this sequence.

First stop: household heat

You take hot water either to a heat exchanger in the furnace, or baseboard heating around your house, whichever is more practical.

In the normal state, water flow bypasses your furnace or baseboards and goes onto the next thing - that way it doesn't overheat your house. When your house thermostat calls for heat, it stops bypassing and flows througH the furnace/baseboards. You'll need a 2-stage thermostat (or 3-stage if you already have 2-stage). You would want to program this as the first stage. When the house is not warm enough, the thermostat will call first to use this free waste heat. If that works, the thermostat will cycle it on and off (bypassing or not). If that is not enough, it will then call for your normal heat to fire up.

You need a diverter valve. During a call for heat, the valve operates and forces water through the circuit. If the little pumps inside the PC's can't push water around the heating circuit, you may want to have a booster pump turn on with the diverter valve. If the booster pump is a type that tends to block flow when it's off, you may not even need the diverter valve!

Other heat dumps, er, I mean uses

Next stop is the water heater. The bypass valves will be a bit more complex, because if your heater is already 60C, you don't want to run 50C water through it. Or you may have an on-demand heater and let your system "help it out", but in that case don't heat standby water 24x7, as this will breed legionella. (A recently discovered problem that reared its ugly head in the Flint, Michigan water crisis). There's a real science to getting hot water right, without scalding or breeding legionella.

Then you go on to any hydronic floor heating you may have. Now, this is a bit of a problem, because if your servers aren't running for some reason, or if the furnace/baseboards have already taken all the heat out of the water, then no floor heat unless you have an alternative heating method. The other option is to do the floor-heat circuit first before the furnace/baseboards, since the furnace has a plan B, and the floor heat does not.

Finally the outside radiator

Now if everything bypasses, or if the servers are making more heat than the non-bypasssing things can use... Then the heat winds up outside at the aforementioned automobile radiator.

The only other serious concern is condensation. You don't want fresh cooling water arriving inside the PC colder than about 10C, as it risks causing condensation inside the PC. This problem could be reduced by careful ducting around the outside radiator, so it doesn't convection cool very well, and needs the radiator's fans to run to be effective. When the fans cycle off, it shouldn't do much cooling.

If all else fails, have the PCs gravity draw from a tank of water above them, and have the radiator discharge into that tank. That way the tank temperature changes slowly. Then a bypass valve on the radiator when tank temp is below 15C. The tank is to keep the bypass valve from rapidly cycling.

  • Thanks for such a thorough response. These are Antminer s9's, bitcoin miners -- a water peltier type approach wouldn't work very well because of how they are built. You'd have to rig 63 peltiers x 3 per unit, and the ASICs are very very small. The home is currently heated with a gas furnace. Mineral oil might just work... I hadn't thought of oil cooling... – Saeven Dec 6 '17 at 19:30
  • @Saeven LOL I had a hunch Bitcoin had something to do with this! My favorite kind of electric heat. Note I didn't say Peltier. Peltier is something else entirely, is too weak for this, and only makes even more heat in your house. Mineral oil would be a fantastic way to do this, and would lick the condensation problem too. However, I would do an oil-to-antifreeze interchange, so you are pumping antifreeze all over the place instead of oil. – Harper Dec 6 '17 at 19:39
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No, you can't just exhaust that air without adding a makeup air intake. As long as you balance the intake and exhaust air flow, then you're good.

The commercial solution is to add a ductless mini split AC unit.

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    Hey Jeff. What is a ductless mini split AC unit? – Saeven Dec 7 '17 at 3:58
  • @Saeven A mini split is an air conditioning unit that has a separate condenser and evaporator coil that are connected with two insulated refrigerant pipes called a line set that can be 50+ feet long. These units are called heat pumps if they feature both heating and cooling, but heat pumps are more expensive and you don't need the heating feature. energy.gov/energysaver/ductless-mini-split-heat-pumps – Dotes Dec 7 '17 at 16:49
  • @Saeven Here is another better link:energy.gov/energysaver/ductless-mini-split-air-conditioners – Dotes Dec 7 '17 at 17:07
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Unless your home is a retired space station , it has many air leaks. When a home is tested , they measure airflow volume not if it leaks. So no worries about negative pressure. However a tight house may not have insufficient leaks for natural convection of gas water heaters and space heaters /furnace, so modern units have a fan/venturi boost for the flue. Why not use circulation fan to blow the heat where you want it?

  • I don't think the concern was about popping ears, but about the creation of cold drafts and poor efficiency. Maybe I'm wrong. – isherwood Dec 6 '17 at 18:06
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    That, and for example, creating a suction that'd cause off-gassing cars in the garage to start seething into the home. – Saeven Dec 6 '17 at 19:35
  • Or the suction of pulljng furnace or water heater exhaust gaes back down their gravity/convection stacks. – Harper Dec 7 '17 at 0:53
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The temperature is not the key value here. The value that really matters is the heat power output of your equipment.

You can suck fresh air from the outside through a filter to cool your device down. The temperature on the output is influneced by the intake temperature, the device's power output, heat exchanger efficiency and the flow rate. The last can be regulated effectively to get desired air temperature.

If the air heat sink is not enough, you can use water as a main heatsink and air as a backup. Install water cooling to your device and instead of a radiator use the input of water/water heat exchanger.

If you can invest even more money, you can buy earth/water heat exchanger that uses huge heatsink below your house. Use one heat exchager to cool your device down by "pumping" the heat down in the heatsink. In summer you can use same heatsink to cool your AC down aswell. In winter you can use this heatsink as a heat source for your house. With this solution you will waste she lowest ammount of energy possible. What can warm your heart as well is the fact that you wont contribute to the hell-like temperatures in the summer as your neighbours using air/air AC units.

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