Edit: added second drawing
Good evening guys,
I was wondering if anyone would be willing to give me some guidance on this. This is the job I posted about before, where we lost the bid and they gave the job to the low bidder. They royally messed it up and they're struggling to keep the heat on in the building - an old folks nursing home. I would like to propose a complete solution, not a patch job. Including new boilers, piping, venting, and controls. This is a large customer, 30+ properties and we have their maintenance contract. If they don't approve a complete fix I am going to walk away. But I at least would like to provide them a full solution. I attached a picture of what I had in my head.
I have no idea if this would be the best way to do it. There are two Weil McLain Slimfit 550 boilers. There is a water source loop that stays at 70 degrees during the winter. And there is a 100% outdoor make-up air coil also in the boiler room that needs hotter supply water as it gets colder outside to maintain a 68 degree supply air temp to the building. The pumps, mixing valve, and diverting valve are there already piped to the loop / coil how I drew them. (sorry for the messy drawing, I'm not good with hydrosketch yet).
Am I on the right track? Or is this not even close? I know the boiler piping isn't drawing 100% accurately and if I am not mistaken the two secondary loops with the 3-way valves would need a check valve on them.

Never stop learning.
Comments
NJ Master HVAC Lic.
Mahwah, NJ
Specializing in steam and hydronic heating
I actually drew the condenser loop backwards. You’re right about the tees upstream. That is actually how they are already, I just drew it wrong. The pump and air separator should have been on the left side of the tees.
As for the pump, what is the reasoning for having the pump on the outlet side of the valve (mixing) rather than pumping into the valve (diverting)? Just got my own education.
My thought was, having the pump where it is pumping into the valve, it would allow constant circulation through the header for the boilers. If I were to put the pump on the outlet of that valve it would just sit there and pump in a circle through the close tees and water source loop.
trainer for Caleffi NA
The magic is in hydronics, and hydronics is in me
Their system, which was retrofitted with these boilers a month ago, is a complete disaster. They barely have heat and their make-up air system is completely non-operational at the moment. I just want to give them a good piping option to get things up and running reliably.
NJ Master HVAC Lic.
Mahwah, NJ
Specializing in steam and hydronic heating
NJ Master HVAC Lic.
Mahwah, NJ
Specializing in steam and hydronic heating
For the make up air, I would probably use a variable speed setpoint circulator and run the water in at boiler temp. The circulator will operate off of the supply air temp, speeding up and slowing down as needed.
And yes, you should decouple the boilers either via primary secondary or a low loss header, or Hydraulic seperator. They are high temp boilers supplying low temp loops, no?
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The water source loop is large, the pump for that is correct. It’s 8 inch pipe and a large floor mounted pump. It uses a cooling tower during the summer for cooling. Since these buildings have a lot of core load they don’t need much heat addition from the boilers during the winter. One 500,000 btu boiler was enough to satisfy the heating load.
I went there yesterday and did some experiments. Set the boilers up to run outdoor air reset. That is working much better but the piping and pumping is still wrong. Have to lock the 3-way valve open to the MUA unit to get decent enough flow to run the boilers.
@clammy , the sad thing is, they DID hire a design engineer. They installed it as the engineer drew it. The engineer removed the 3-way for the water source loop and installed a 2-way. Then drew the new boilers piped in series with the main circulator pumps, instead of primary-secondary. The gas lines are also undersized and the venting is completely wrong. I don’t think the engineer knew what kind of system it was. He labeled the boilers and the water source loop as operating at 180 degrees. He labeled the pipes “dual temperature” like a 2 pipe changeover system that uses boilers and chillers. But that’s not what it is at all. The whole job was doomed from the get-go.
NJ Master HVAC Lic.
Mahwah, NJ
Specializing in steam and hydronic heating
Picture attached.