upgrade to mod con question

pat_3

existing heating system is aprox 1/3  recessed cast iron rads [not oversized], 1/3 low temp high mass radiant and 1/3 higher temp low mass radiant.system also has two 56gallon dhw storage tanks.the system has two injection pumps for the radiant sides of the system controlled by two tekmar 356's.my question is do i basically leave the controls and injection pumps intact and just set the new boiler to modulate and not condense or do i take out the injection pumps and try and set the boiler up so it will condense at least some of the time?or is there another way i should be setting this up? i have no experience using a mod / con in this type of multiple temperture application ,any input is greatly appreciated.

Brad White

If each

emitter type now has its own circulator and controls, leave it.  I do not know how it is now distributed, that may be another topic, but let's assume it works fine right now, correct?



Each type/zone needs the ability to "call" independently of the others and  will heat and cool at different rates compared to others.



As you mentioned, not all of your radiation is generous and each may need individual temperatures. Find out which one needs the highest. That will be your efficiency limit on the coldest day. Figure out if you can what the ratio of radiation to heat loss is. For example, if your design day temperature is five degrees and your coldest zone, most demanding zone needs 180F water on that very day, that is your target and what your boiler must produce. The others may be less but you have to satisfy the worst.



Think of the ModCon as the heat source and an efficient one which can modulate and adapt, becoming the boiler you need when you need it.



There is more to it than that, but just to get you started.

pat_3

thank you

thank you for your response,but just to make sure i am on the same page...utilize existing piping and controls and just set boiler to maintain the design day temp and don't try to use the boiler ODR option.correct?

Steve Whitbeck

Mod con

Brad is right on the money. Let the boiler outdoor reset but don't let it get too cold.

Maybe 120* at 68 ODT.

Brad White

No, not that~

I was not at all clear, that is my fault.



I do not have the entire picture of your existing piping, but assuming what you have works right now, yes, use the ModCon boiler as your heat source AND use the outdoor reset feature inherent (or as an accessory) to it.



My point  was that the highest temperature circuit will govern what your highest design boiler temperature will need to be on the coldest day.  Basically the parameters to set it up. Allow the outdoor reset feature to take it downward from there as the outdoor temperature warms up. Set the bottom temperature on a mild day.



As Steve noted, it may be a bit higher than you think it should be. I would start at a 100F minimum when the outside temperature indicates a day when you *just* need heat. If that does not cut it and you need more heat (or it takes too long to warm up), kick it up in ten degree increments until you are satisfied. That will set your bottom limit.

pat_3

thanks again

thanks again for your time.,but now if i use the boiler ODR won't my injection pumps and piping  size  for the injection piping  be undersized since they were designed to draw off a primary loop that was never intended  to go lower than around 160.?

Brad White

Never intended to go lower than 160?

But would that be on the coldest day? Every other day is warmer and the water temperature could be less. The entire concept of ODR is built upon that.



That is what I see, anyway. Most systems benefit from ODR. Only those systems using such low temperatures such as radiant floors, where there is little room to drop, can work reasonably well at a fixed temperature. Or rather, you can drop the temperature but from 80F when your room is at 70 is not much room.

pat_3

ODR

totally agree with everything you are saying, i think i wasn't very clear on my description of the system i am working with. i have two injection pumps serving two seperate secondary sides of the system,both of those pumps are already using ODR via seperate tekmar 356 controllers.if my primary loop temp gets lowered using ODR,  will my small injection pump and lines that were designed thinking it would always have a high temp primary to draw off of be able to keep up when injecting a much lower temp?

Brad White

Getting there

You are finding one of the tenets of Primary/Secondary heating, that the boiler source (I call that the primary) has to be kept about ten degrees warmer than the secondary temperature.



Otherwise, (with the boiler and secondary loops at the same temperature),  the secondary control valves will tend to "yawn wide open", taking whatever the boiler can give. With the secondary control valves wide open, they are no longer "in control".



So yes, this is why the zone with the hottest requirements sets the tone for the entire system.



Your boiler would have a steeper curve (or higher parallel offset), than your radiation-side curve. Both would work together.



I was going to mention that in my first post but did not want to confuse the issue, to keep it simple to start. We got there anyway!