Radiant heat headaches

cmc

As part of a gut/rebuild project recently I had a new bathroom built with radiant floor heating. The plumbing for this was added on to our existing forced hot water furnace that otherwise supplies a three story Victorian house with about a dozen big old cast iron radiators.

When the new radiant system was first installed it seemed to work fine but as it started getting colder and heating more we became aware of a rattling sound in the old radiators. Long story short, the plumber who installed the new system couldn’t figure it out (he bled the system then replaced the circulating pumps in the radiant system) so a second plumber was called in by our contractor.

This second plumber made two changes; first he re-piped the mixing configuration for the radiant heat, now shown in "radiant heat " photo. Note that before his re-do it was simpler, looking more like what’s "towel warmer" photo which is for a radiant towel warmer in the same bathroom. This re-configuration didn’t solve the rattling noise so he then thought it might be the back flow preventer in the downstream end of the radiant heat return pump (blue arrow, "furnace" photo). This stopped the rattling instantly and we all did virtual high fives.

However we now have a new problem. Whenever the radiant heat comes on, hot water is getting into the return pipes to the old radiators. For example, the return pipe marked with the blue X in the “furnace” photo gets hot as soon as the radiant return pump kicks in. The result is that our main house radiators get hot and the house temperature goes up even though the main house thermostat isn’t calling for heat. Yesterday the house was set at 66 F and went up to 71. I should point out that we have the bathroom rad heat set to 70F – 4F warmer than the main house. If I disconnect the power to the radiant return pump (blue arrow in ”furnace” pic) but leave the radiant supply pump (shown in the “rad heat” photo” ) running this doesn’t happen and the system seems to work OK.

As I look at the set up of the return pipes in the “furnace” photo it looks to me like this is an inevitable consequence of the way the return pumps are placed, upstream of the returns to the old radiators. If those return pipes offer less resistance than the furnace, could water being pumped by the rad heat return pump go there rather or at least in addition to the furnace?

So that’s my story. My question is, what should we do?

Zman

The way your pumps are oriented, you are not "pumping away" from your expansion tanks. The ghosting issue is usually caused by a lack of functioning check valves.
It would help if you could sketch the piping, circs, boiler, exp tank, radiators and check valves.

hot_rod

A couple ways to pipe a mixed zone off your boiler. The left drawing just needs one circulator pulling from the mixed port of the 3 way.
The second drawing establishes a primary loop, it needs a circ in that loops, then the mix valve tees into that loop and needs a circ to "pull" through the mix valve.

You could probably get to the right drawing easiest, some repiping at that mixing station, both circs need to run to make it work.
And the boiler of course

cmc

Thanks Bob Rohr. My system looks like your second drawing on the right. the problem is that the return pump is adjacent to the returns of the old radiators. this is apparently leading to ghosting up those return pipes. so when i disconnect the return radiant pump no ghosting and the radiant still seems to work OK with just the supply pump on the panel in the picture. I'm not sure this is kosher but it seems to work. the only other solution I can see is to re pipe the radiant return pump so its downsteam of the old radiator returns.