Steam Boiler with Indirect Hot Water Problem

Larry1

I recently had a new steam boiler installed with indirect hot water tank attached. The thermostat on the hot water tank is wired in series with an aquastat set to a high limit of 160. I understand that the purpose of this aquastat is to prevent the boiler from making steam when there is only a call for hot water and not for heat. This makes sense for the summer time when i don't use heat. However, what seems to be happening now is that when the boiler was recently on to heat the house, and the temperature of the water is still very high, the aquastat is not allowing the circulator and boiler to come on to produce hot water. Only when it cools back down to less than 160 does it begin to make hot water again.



Is something wired incorrectly here? Is that aquastat supposed to be controlling the circulator and the boiler? It seems to me that if the boiler water is already 200 degrees because it just finished heating the house, the circulator should just come on when there is a call for hot water.

Mark N

deleted

AlexR

two controls

What you're asking for certainly makes sense- I had a similar issue with the way that some knucklehead wired my baseboard hot water loop.



What model is the indirect tank?  Do you know whether it has an aquastat/thermostat output that you can use to control the circulator?

EBEBRATT-Ed

Indirect

Usually the thermostat on the indirect operates the indirect circulator and the boiler when the indirect needs dhw. If you want to limit the boiler water temperature you would need to use a relay to only put the aqustat  in the boiler circuit when calling for dhw.

Larry1

Larry1

The hot water tank is HTP Superstore. The limiting aquastat is positioned on the return line right before the water re-enters the boiler after going through the hot water tank. It is a very basic Honeywell aquastat that opens the circuit when the temperature gets above 160. It's is wired in series with the tank thermostat and then the single zone relay that simultaneously turns on the pump and fires up the boiler. So, if the aquastat is open and the house heating thermostat is off, the pump can not start, nor can the boiler fire. So, my questions are:



1. Is this a common setup? Is it common to not be able to make hot water while the boiler is making steam? Or even after before the water cools significantly?



2. When making hot water without heat, it's it normal for the boiler to always fire up with the pump, even if the boiler water is already more than hot enough to make dhw? Or, are the pump and boiler supposed to be on separate controls?



3. Is there any reason to limit the temperature of the water going into the water tank during the heating season? The tank manual says it can handle a supply up to 250 degrees? The thermostat on the tank would stop the dhw from getting too hot.



4. If I'm right, what is the easiest way to separate the pump from the boiler control so that the boiler will be inhibited by the aquastat but not the pump?



5. Or, should I simply bypass the aquastat during the heating season?



Thank you!

AlexR

I think you're correct

As a non-pro, I think you're correct in your thinking.  If I were you, I'd



1) move the aquastat to the supply line to the tank.  Measuring temperature on the return seems a strange way to get hot water without making steam since 160F return could come from boiling water if the tank is cold enough.



2) control the zone relay for the circulator with the tank thermostat.  There will be an input wire from the tank thermostat into the relay's control and then another wire from the other side of the relay's control back into the common side of the boiler's transformer (I'm assuming the relay wants 24VAC control signal).  If the tank's below temp, it should always get water from the boiler.



3) run a second output wire from the tank thermostat through the aquastat back to the boiler (there will only be one input wire that goes from the 24VAC hot on the boiler to the tank).  This goes in parallel with your house's thermostat so that either the tank+aquastat or house can turn the burner on.  So this wire runs from the tank thermostat into one of the aquastat wire.  Then you run a wire from the other aquastat wire back to the boiler and tie it into the same point where the wire runs back from your thermostat.



If that doesn't make sense, yell and I'll make a diagram.  Also, I'm assuming that your tank doesn't have it's own power supply.



Alex

Steamhead

You have to use the right relay

one with double-pole switching. Here's a link to a thread where we installed a SuperStor with a Smith G-8 steamer:



http://www.heatinghelp.com/forum-thread/132567/One-of-the-worst-maintained-oil-fired-boilers-weve-seen



On this one, we used a Honeywell R845, but an R832 or a double-pole normally-open or double-throw fan center relay will work too. Both types have their own 24-volt transformers to power the "thermostat" circuit, which in this case is the aquastat on the SuperStor.



One pole (relay contact) in the R845 is for the 120-volt circuit to the circulator. It is fed from the same source ("hot" wire) as the R845, thru a jumper wire. The neutrals from the circ and the R845 are wired together.



The other pole switches 24 volts from the burner control. It is wired in series with the aquastat on the boiler. This circuit connects to the burner control in parallel with the main steam zone thermostat, so that either one can start the burner.



This way, whenever the tank is calling for heat, the circulator will run. But unless the main steam zone is calling for heat, the aquastat will shut off the burner (but not the circulator) before the boiler water gets hot enough to make steam.

Larry1

Thank you

Thanks for your replies. I'm gonna go ahead and rewire the way you suggested. I was thinking in much more complicated terms, but this sounds straightforward. I'll post again when it's done.



One other question... I looked up the circulator pump and found it can handle water up to 230 degrees. I know water usually boils at 212. Is this enough of a cushion? Do I need to worry about damaging the pump if I allow it to run when the boiler is going full steam? It is a Grundfos UPS 15-58.



Thanks!

Larry

Steamhead

Look at my other thread again

the pics show a bypass that routes some return water into the pump suction. This keeps the temperature down enough so the pump won't cavitate.



The Grundfos 15-58 is a wet-rotor circulator, which uses the system water for lubrication. This is generally not recommended- for a hot-water loop off a steam boiler a bronze, 3-piece circ such as the B&G 100AB or Taco 110B is the best choice even though it costs more. The bronze, oil-lubed unit handles the harsh water in a steam boiler much better.

Larry1

A new wrinkle

So, I rewired things a bit, and now the aquastat is only controlling the boiler, allowing the pump to run independently, any time there is a call for hot water.



But... I think Alex hit the nail on the head with the placement of the aquastat. Now, when there is a call for hot water, the boiler is cycling on and off every couple of minutes, even when I know the water in the boiler is more than hot enough (just finished heating the house with steam). I think that because the aquastat is on the return line, the water has already been cooled down by running through the water tank, and is tricking the boiler into coming on. Since it's not really cold, it shuts off again in a minute or two.



So here are my questions:



1. If I allow this to go on, will all of that short cycling damage my boiler or cause another problem?



2. If moving the aquastat to the supply line for the tank is above my ability level, are there any other long term or short term workarounds?



Thank you. I really appreciate the help!



Larry