Given my water pump timing needs, is this the right device for me?

September 28, 2016 4:37 PM

After three water pump failures in 5 years, I need a way to control the behavior of my radiant heat system's water pump during summer and winter months. I think I know what kind of timer device I need, but before I buy something, I'd welcome some advice from plumber MeFites. (Lengthy, possibly unnecessary details inside.)

We have a radiant floor heat system for our master bath that runs off a household water heater (not its own boiler). It's set up with two circulating water systems: an open system that pulls hot water from the heater, through a heat exchanger, and back into the water heater; and a closed system controlled by a thermostat that kicks in to run water through the heat exchanger and circulate it through the floor when there is demand. Each of these systems has its own water pump.

Here is the problem: The open system pump has failed three times, and I'm about to have it replaced yet again. I think I know why it fails, but I'm unable to prevent the failure with the current setup (that came with the house when we bought it). I think I need an Intermatic ET110, but I'm not 100% sure it's the right thing.

Here is the situation:

The water pump is wired to an on/off light switch, helpfully labeled "Winter" [on] and "Summer" [off]. The idea appears to be that we should shut the pump off during summer months when heat is not needed. There is no household-type plug for the pump, but instead a thick electrical-looking wire from the pump directly to the lightswitch.

During our first spring in the house, when we no longer needed the heated floor, I shut the water pump off at the switch. When fall came, we powered up the system and it didn't work. As new homeowners, we assumed the pump had failed from old age, so we replaced it.

The following spring we turned the pump off and once again in the fall it failed. A smart plumber told us that when we turned the pump off for the summer, it was rusting and locking up (and he showed me the rusted guts of the less-than-a-year-old pump). The answer: Just let the pump run all the time, summer and winter, "because it's built to do that."

So we let the pump run continuously. 18 months later, it failed a third time (it makes a very faint whirring noise). This leads me to believe that having the pump run hot water continuously shortens its lifespan.

I think I need a way to run the pump in two modes: ALL THE TIME, but also in a second mode where it runs, say, once a day, every day, for 10 minutes or so to prevent rust. (The closed system's thermostat runs its pump for a few minutes each day regardless of demand, which I suspect is intended to prevent rusting.) I don't need the switchover to be done automatically -- I'm happy to switch modes when the seasons change. But I do want some kind of timer to manage the pump during the off season (so to speak).

So -- given the above -- will the Intermatic ET110 work? I would set it with a timed mode (10 minutes a day) and "override" for run-all-the-time mode. Would a different flavor of the same timer be better? (Please note that I'm looking for an off-the-shelf product here, as I'm not really qualified to build something on my own.)

Thank you!

posted by woot to Home & Garden (11 answers total)

Most fluid-based radient system use their own closed-loop system. Your problem is that you are using your regular hot water system, but the disabled pump is probably collecting crud and just generally seizing up due to lack of use. The new pump thar runs occasionally all year might work better.

It still needs to be on it's own closed loop, and not water-based.
posted by Thorzdad at 5:00 PM on September 28, 2016 [1 favorite]

I haven't done the math or read the timer specs to know whether it will work electrically, but your plumber's explanation of the problem doesn't ring true. All else being equal, I don't see how running the pump constantly or periodically could slow the rate of corrosion.
posted by jon1270 at 5:04 PM on September 28, 2016

Best answer: A pump in an open radiant system needs to be Stainless because the continously replenished dissolved oxygen will chew up the pumps. Warning they are (at least when I bought mine 10 years ago) three times the cost of a iron pump.

I really doubt running an iron pump occasionally will significantly lengthen it's life.
posted by Mitheral at 5:50 PM on September 28, 2016 [3 favorites]

You could try a marine pump, like this bilge pump with a plastic impeller, for the low low price of $15.76. Don't know if it would be happy handing hot water, though.
posted by monotreme at 6:06 PM on September 28, 2016

The pump needs to be line pressure rated.
posted by Mitheral at 6:14 PM on September 28, 2016

You have to drain the lime deposits out of the bottom of your hot water heater at least once a year, some say once every six months. But if you have a heat exchanger, maybe you can run Bray Oil or something different in the floor loop. A good plumber can help you figure this out. The bray oil or whatever other heat exchange fluid is recommended, doesn't gunk up the pump. The systems have to separate. It sounds too difficult, and like there is some misunderstanding on the plumber or original installer's part. I have see whole house systems that run off a single water heater, that heats the house, and the water they drink.
posted by Oyéah at 7:06 PM on September 28, 2016

The pumps in my radiant system spend the off season not running simply because the thermostat doesn't' kick on. They are 7 or 8 years old, never had an issue. I would suggest a different pump, even if it requires a bit of pipe fitting.
posted by humboldt32 at 9:14 PM on September 28, 2016

Response by poster: Huh. It sounds like there's more going on here than I thought. We have the plumber coming on Friday (and I asked specifically for the one I like, who is one of those old dudes who is really good and likes it when I hang out with him the entire time and ask a lot of questions) and I'm going to have an in-depth talk about what's going on.

jon1270, the idea that turning the pump off hastened corrosion was driven by the reality that the open system can't be bled and has air in the lines. The plumber's explanation for the rapidly rusting pump was that when the pump is stilled, the air and water settles into it and facilitates corrosion. If this is true, would not running the pump for a few minutes effectively flush it and slow the corrosion? I'm no expert here, so please enlighten me!

Mitherai, thanks for your comments -- I have two followups:

First, I didn't know that there was such a thing as a stainless pump, so thanks for that information. The two we have look to be iron to me.

Second, what's the significance of pressure rating for the pump? I know conceptually what pressure rating is, but what do I need to look for in this application?

Thank you for your answers so far!
posted by woot at 3:29 AM on September 29, 2016

A bilge pump only has to handle a few pounds of pressure so generally has seals and housings designed for that use. Your current pump will be rated for the higher pressure that exists in your water supply.
posted by Mitheral at 7:31 AM on September 29, 2016

I may be misunderstanding the setup, but if you have two different radiant heating systems for one bathroom, one of which (the open system_ is experiencing all these failures, why not ditch that system and expand the other one (the closed system) to cover the whole room?
posted by beagle at 12:51 PM on September 29, 2016

If it were me I would bag the heat exchanger and just make the floor an open loop system using a bronze circulator pump. It might be that the added complexity was code driven but it is not hard to plumb an open loop such that the cold water supply intake to the tank is drawn through the tubing of the radiant portion when domestic hot water is needed. This is so that you never have stagnant water, the terror of lysteria being one reason for having a heat exchanger and why open loop systems are not approved in some jurisdictions. I don't understand the use of an iron circulator in this application, nor do I find the use of a constantly running pump persuasive, winter or summer, (why waste electricity? One thing that will destroy pumps fast is cavitation, when the self destructing circulator runs do you hear a quiet crackling sound? This can come from incorrect pump selection which, given the use of a cast iron pump, doesn't sound beyond the range of possible .
posted by Pembquist at 11:55 PM on September 29, 2016