Gas heater short cycling on cold mornings, heat pump runs fine

Started by btu_frank · · 4 replies
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#1
Got the dual fuel setup, AquaCal for shoulder season and the Raypak for the real cold snaps we get down here a few times a winter. Past two mornings when water's in the low 60s the gas heater fires, runs maybe 4 minutes, shuts off, fires again 5 minutes later. Heat pump never does this on the same mornings. Flow looks fine, pressure gauge reads normal, filter was backwashed last week. In the trade we'd call this a limit trip but I can't tell if it's a bad thermistor, airflow around the vent getting blocked by wind, or something simpler. Anybody run into short cycling specifically on the gas side while the heat pump behaves. Not ready to call a tech yet till I rule out the dumb stuff. FN

4 Replies

#2
Frank, before anything else, what's your actual flow through the gas heater versus what it's rated for. Gas heaters have a much narrower flow window than a heat pump, usually something like 20-30 gpm minimum depending on the model, and if you're running your VS pump at a lower rpm setting for the heat pump stage then switching over without bumping speed back up, that alone can trip the high limit on the Raypak even though your gauge pressure looks normal. Pressure reading fine doesn't rule it out since a restriction downstream of the gauge or just running too slow won't always show up there. What rpm or gpm are you actually running when the gas heater kicks on.
#3
No gas heater here so take this for what it's worth, but is the short cycling only happening when the wind's up, or has it done it on a dead calm morning too? Vent clearance issues usually show a pattern with the weather, constant cycling regardless points more toward Marcus's flow theory.
#4
Marcus is on the right track but I'd check the bypass valve on that heater too if it's plumbed with one, a lot of these dual fuel setups have one that's supposed to stay mostly closed and if someone bumped it open even a little it'll starve the gas heater of flow while the heat pump downstream never notices.

Also worth pulling the union right before the Raypak and looking for scale buildup in the elbow, that heater exchanger's a lot less forgiving of restriction than the AquaCal. Dave
#5
I don't run gas so can't speak to the Raypak specifics, but the VS pump speed thing Marcus brought up rang a bell for a different reason, I had my pump set low for overnight filtering and forgot to bump it before I started messing with equipment one morning, and anything downstream that actually needed real flow just acted up for no obvious reason. Took me way too long to realize the gauge looked normal because the restriction wasn't where I was looking. If your controller lets you log actual gpm at the exact rpm during the gas call instead of just eyeballing the dial, I'd do that before pulling anything apart. Dave's bypass valve idea sounds like the cheaper thing to rule out first though, that's a five minute check versus pulling a union apart.