So we have been building our own house for a couple years now (living in it for a year) and with winter coming are trying to work out the last kinks in the radiant system. The puzzler is our relays. The heating system is small - two zones each with its own pump. The pumps work fine when plugged directly into a standard outlet. However, the dont run when hooked up to our relays (controlled by our thermostats). Each pump is controlled by a separate SSR, through a dedicated outlet. Because the SSRs control an outlet, we can test the outlet to see if it works. We first had cheap china made SSRs and the outlet would not light a CFL lamp, but WOULD intermittently run a jigsaw. So we switched out one SSR for a US made one. That outlet now runs the lamp, but still wont run the pump. The pumps are small, 25 watt. Any ideas at all? I dont want to replace both relays and have it still not work.

Need more information. What is the brand and model number of the relays.

No problem, wasnt sure where to start. The china one is "lllager" or "111ager" brand, serial MGR-1 D4825. It has 3-32VDC control; 24-480VAC, 25amp output. The US one is Continental Industries, serial S505-OSJ440-005. It has 4-28VDC control; 24-280VAC, 40amp output.

Oh and both have heat sinks with compound in case overheating is a thought.

What type of thermostats are you using.The problem you are having is compatibility of thermostats and ssrs.You need a solid state output controller which the relay can respond to. Most ss devices use a pulse train from a gating controller. Inductive loads are the worse type of devices to control from these types of relays.I do not know what type of setup you have cause I can not see everything there, but it sounds as if you just have the wrong type of relays for what you are attempting to do.You may want to consider electromagnetic relays which will work just fine.

We have Aprilaire 8448s. The signal for the SSR control side comes from a transformer in our ERV. The LED indicator on the SSR was steadily lit in all of the test scenarios when the thermostats called for heat. Despite that can your analysis still be accurate? If its just "switch to electromagnetic relays", I will be thrilled.

Your thermostats are what is controlling the pump relays.The problem you are having is the thermostats are sending out voltage to either energize or break the low voltage at the relay coils,(W1 and common to the coil) The SSR relays are needing a on - off voltage which would be derived from a SCR controller which you do not have.When 24 volts is applied to your relay it is as if it is being told to stay off.It can get complicated in a hurry.Switch out the relays to electromagnetics and all of your problems will disappear.

If I am switching to electromagnetic relays, wont it need diode protection for magnetic field collapse?

I think you are way over thinking the problem. I want to make sure that you are not trying to pulse the pumps as if they are inverter driven.The relays will work fine if these pumps are your basic circulators.

I checked with Aprilaire and our thermostat (8448) just acts as a switch, when no heat is needed no current is sent. The current that would flow through is from a transformer wired to run the thermostats. If this is the case, why wouldnt it provide the needed "on-off voltage" needed by and SSR and why would 24 volts being applied to the SSR be telling it to stay off? I really appreciate how much you have already helped, I am just hoping to understand this before rewiring.