big modcon, DHW priority, secondary waiting between DHW cycling

lolo21

is it normal that a modcon cannot be programmed to run the secondary when there is a call for heat while the boiler is cycling in DHW priority because of bigger btu output?



The modcon doubles the btu requirements of the indirect; basically the boiler will go up to 194F water and swicth off while the DHW pump keeps on running till the return water on the primary goes down to 154F-160F aprox.



I find very limiting that I cannot program the secondary to pump away and keep the boiler firing when there is a call for both heat and DHW, and this will affect the total output of the boiler if really push to the limit=lowest design temp.

ALP285+superstore45 and I use two relays (DPST) to control the pumps.

DHW and Secondary pumps run each from their own relays, the primary is conected two both relays second poles (so it runs with either of them)

I know they recomend the taco503 but I see no use in this set up as I have only one zone, and a big boy pump (1/3HP) which probably will end burning the tiny relay in the 503. In general I like to keep it simple instead of adding unnecesary 3rd party control boards.

This is an ALP285 with MCBA, could the sage2 do this?

Jean-David Beyer

is it normal

Since I am not a professional, I do not know what "normal" is.



I do have a mod-con boiler. It has three thermostat inputs. One is priority 1, one is priority 2, and one is priority 3. Everything in it is programmable from the front panel. My system is set up so that the indirect hot water heater is Priority 1 (most likely to be serviced). It is set so that only the circulator to the indirect runs. However, after 1/2 hour of continuous running (it never runs that long), it looks at the other priorities, and gives them a chance. In this case the boiler supplies 170F water to the indirect (default value for this is 190F).



Priority 2 is connected to my downstairs radiant heating setup. The controller is set to run both the boiler circulator and the circulator to the slab. This is also set to run 1/2 hour continuously and then look to see if Priority 3 wants service. But it also lets go after 15 minutes to let Priority 1 run again if it still wants to. I would probably increase the 15 minutes if I had to, but since the indirect is rapidly satisfied, I have not had to change the default value. Water to the slab is in the range of 75F to 120F depending on outside temperature.



Priority 3 is connected to my upstairs baseboard zone, and it runs both the boiler circulator and the circulator to the upstairs zone. This will run as long as it likes, but if a higher priority zone wishes to run, it will let go after 20 minutes. Water to the baseboard is in the range of 110F to 135F depending on outside temperature.



All these times are programmable; they are just what I happen to be running. The reason for these timeouts is so that all zones get some heat even if all thermostats are calling for it; the lower priorities are not shut off indefinately.



Actually, the upstairs zone circulator will also run if the radiant slab circulator is also running. Each priority setup has its own outdoor reset curve, so different temperatures can be used for each. The Indirect is not affected by the outdoor reset.

Gordan

Doesn't sound like your limitation is in the control.

Rather, it sounds like your limitation is in the indirect's ability to accept the boiler's output. What's the point of having all that boiler power on hand when your DHW call is still not getting satisfied, and you're bouncing off of boiler high limit instead? The cure would be to get an indirect that can actually transfer the full boiler output through its coil to the tank water, which would allow you to be done with the DHW call quickly and move on to space heating.



I'm assuming that you're still getting a DHW call, rather than running off of post-purge. If you don't want DHW priority, and your other zone is high-temp as well (or uses a mixing valve) then you didn't have to use the DHW priority features on the boiler at all - you could have just piped the indirect in as another zone.

lolo21

boiler needs to be bigger

The boiler is sized for the size of the building meanwhile the indirect is sized to heat water for only two apt.

The Burnham Alpine control does not seem to have any other option which I find anoying for the price of this toys, and the litherature for the Taco 503 is not clear either as to how would it work.

thanks everyone

lolo21

what control is that?

thanks for your detail explanation.

What system or control you have ?

thanks