DHW control and corrosion

Ross Lindley

Hi there
I have a radiant heating system in my new (1 ½ years old) house. Heat comes from a small Weil-McLean oil-fired boiler w/ tankless heater. I opted for the pre-built Kitec manifold, then had a variable-speed injection system and a tekmar 361 mixing control added. The boiler is in the garage; the rest of the system is downstairs in the utility room.

Unfortunately, the tekmar control I chose does not control the DHW. I knew that when I chose it, but thought I could work around it. The system began operation in November (I live in Yukon, so that was none too soon). The DHW tank was not wired at the time, and the pump circulating water through the tankless heater and DHW tank seemed to work fine.

Then right on Christmas eve we had unusually warm weather and since the boiler was not called on, we didn’t have any more hot water. Of course this had to happen when we had a house full of family from out of town. Anyway, after a couple of days I got the guy who installed the system up here and he made some adjustments or a change to the wiring (can't remember which) so that the pump ran continuously and would kick the boiler on whenever we needed hot water.

Since it seemed to work okay, and I had a thousand other things to do on the new house, I just left it running as it was; not bothering to get the DHW tank wired. I didn’t realize how much more expensive it was to have the boiler heating the DHW in the summer. But around mid August I noticed that our oil tank was getting pretty low, so I decided I’d better do some wiring. Well the breaker tripped right away. I figured I’d done something wrong so I got an electrician to come and check it out. No, I’d done it right. But the elements were both shorted out. This was in an eight-month old tank.

Oh well, winter was coming (comes early here) so I just let it run off the boiler again. Yes, I am a bad procrastinator. Well spring is around the corner so I decided to deal with these elements – finally. They were thoroughly corroded, broken in several places. I’m guessing that the cause is a combination of the pump running continuously and the very hard water we have up here. I didn’t have the right elements anyway; the right wattage, but it looks like the old ones were about twice the length (they fold back).

So after flushing the tank I clipped the corroded elements down to a nub and put them back in as plugs so we could have hot water again. I haven’t checked the anode rod yet, but I’m suspecting that it is probably toast too. What I’m considering doing at this point is leaving the “plugs” in until the system goes into WWSD, and then installing some new elements just for the summer so the tank can run off electricity. Then I’d put the plugs back in for the colder weather and run it off the boiler.

But now I’m wondering what may be happening inside my pump and the rest of the system. Any suggestions out there as far as controlling the tank/pump, or any other ideas would be much appreciated.

Thanks,
Ross

Ross Lindley

Correction

The pump doesn't run continuously - only when there is a demand for hot water.