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Your contractor purchased the wrong tankless heater for your steam boiler. You got the correct boiler but the supplier screwed up. The end section has both openings. The tankless coil is the wrong one.

The Left section with the coil openings is part number 311-800-014. That comes preassembled from the factory when you specify the tankless coil on either a steamer or water boiler.    With a steamer you should get the Operating Control and Tankless Heater 35-S-29.  This is the part available from supplyhouse.com WM Tankless heater Your contractor should be able to get that part from the their supplier. Your contractor is responsible for verifying the supplier's error before installing the boiler. With the incorrect coil you will never get hot water in the summer. You will get hot water in the winter whenever the boiler is making steam.

You can look at the openings in the exploded parts view. The round opening for the water boiler tankless coil is located above the water line of the steam boiler. the water in the steam boiler will not get that high so the coil will never get hot enough for a shower. If the water does not get up that high then there is no hot water until the boiler makes steam.  The rectangle opening is at the water line so you get most of the coil in the water below the water line. When you make that water below the water line 150° or 160° there will be no steam, but the tankless coil is in the hot water and will get hot water all summer.  

If the contractor made the mistake of installing the wrong equipment, and you can not get satisfaction from the contractor, you need to call Weil McLain Tech support and have the factory representative show up at the job with the contractor and get this taken care of. 

CAUTION

Once the contractor gets the proper coil installed, they MUST install a proper automatic mixing valve to prevent scalding.  In the summer the boiler water temperature may only be 160° to make 120° water to the shower.  That same water will be 213° or higher when the boiler makes steam.  That difference in water temperature that the tankless coil is submerged in will cause scalding water temperatures to leave the tankless coil.  A proper mixing valve with an ASSE 1017 certified rating is necessary to prevent scalding water from leaving the boiler room and ending up in your shower or sink.

You have to get the coil thing fixed first and then:

1.Make sure they follow the piping instructions for the tankless coil. Automatic mixing valve, flow control valve and unions. Mixing valve should be dropped 12" below coil as drawing shows.

2. Install a skim tapping according to the manual and skim the boiler (again in the manual)

3. Pipe the Hartford loop correctly (again in the boiler manual)

4. Start the boiler and combustion test it, check gas pressure and adjust pressure control to cut off the burner at 1 1/2-2psi max. Test low water cut off. Set aqustat at 160 burner off wit 10 degree differential. Can be adjusted later if need be.

They should have included all this its all in the manual. Don't let them off the hook.

If it was me the tech (questionable) that came out and flooded the boiler to cover the tankless coil I would not let him back in the house.

Maybe I don't know what I am talking about, but I looked at the boiler manual this morning. They show a round tankless coil for a HW boiler and rectangular coil for steam. I think if you buy the hW boiler you do not get the opening for a steam tankless.

To me the picture shows after blowing it up the round HW tankless coil. It is too high above the water line to work for steam. Look at the attached brochure and call me stupid. To get the section for a steam tankless is special order optional equipment.

Wrong boiler. Fix the Hartford and run it for heat disconnect the tankless and put in a gas fired HW heater or change the boiler or section.

Boiler is not piped right. Hartford Loop is not right. Look in the manual it calls for a 2" riser and 2" header (which you have).

Did the installer skim the boiler and clean it?

Hartford loop needs to be repiped ….see man

Tankless coil does not have a mixing valve

NOT YELLING HERE JUST DIDN'T WANT YOU TO MISS THIS:

I think your installer installed the wrong boiler. He used the tankless coil opening for a hot water boiler. The steam boiler has a square opening not round.

Tankless coil will not work with a normal steam water level and heat will not work with a high water level.

Sorry to say your installer is an idiot.

Post your location but another contractor will probably not touch this if it is the wrong boiler.

You may have to ditch the tankless and go with a gas HW tank if he won't change the boiler.

Boiler is piped wrong

What is the water heater temperature setting on the aquastat?

Difficult to see the hot and cold water pipes that enter the DHW tankless coil.

There is no mixing valve to control the DHW temperature leaving the boiler.

Need a repipe of the boiler and a repipe of the potable water lines. The diagrams are in the instruction manual.