Condensing appliance exhaust heat recovery?
[I tried to add a comment to https://www.greenbuildingadvisor.com/community/forum/energy-efficiency-and-durability/28393/hrv-flue-combination but it flagged this as spam and I could not.]
I’m looking for products or methods used to preform exhaust air to intake air heat recovery for a condensing furnace or boiler.
Heat recovery on the exhaust of a condensing appliance (boiler or furnace) would seem to make sense. There is a large delta T between inlet & exhaust gas (maybe 100 F here in NY today), and there wouldn’t seem to be a downside to reducing the exhaust gas temperature like there would be in a non-condensing application (AFAIK).
The acidic contents of the condensate might be a practical concern. If you used an aluminum core like a in a residential HRV it might corrode?
Searches turn up wood stove flue-to-indoor air exchangers in the US (like “Miracle Heat Reclaimer — 6in., Model# MH6”), but nothing for condensing appliances. Unless you knew the effect on the flue temperature these really seem like a bad idea for a wood stove!
From the UK there do seem to be some products that are air-to-water heat exchangers. (Such as: “Flue Gas Heat Recovery Systems (FGHRS)” mentioned on TheGreenAge or GasSaver a “Passive Flue Gas Heat Recovery Device”) but I can’t find anything air-to-air.
Anything exhaust-air to inlet-air for condensing boilers in the US?
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1. The available energy that might theoretically be extracted from the flue gases of condensing appliances is quite small. Most of the heat has been squeezed out already.
2. As you guessed, corrosive condensate is the main obstacle to the use of metal flue pipes. If this weren't an issue, pipe-within-a-pipe systems (common on wall-mounted non-condensing space heaters) would be an obvious solution.
3. If you want more info, see this paper: Condensing Furnace Venting.