Fat Pipe

insulated water tank

Recall that I am not a professional.



I was thinking about your question of where to place the tank that is there just to increase the thermal mass of the system so as to reduce short cycling. And I was surprised at my conclusion. Which is to put in on the return. I wonder what the professionals will think of this.



Consider: the thermal mass of the boiler must be too low for your application or you would not be doing this. (If the real problem is an oversized boiler, different thinking is required.) Therefore, if you place the tank in the supply, the boiler will heat up quickly, but the heat will not be delivered to the house until the tank is pretty well warmed up, where if it is in the return, the heat will be delivered to the house quickly. Now on the return, the water is not cold, but colder than the supply temperature. This water will heat up the auxiliary tank before returning to the boiler. And since it is cooler than the supply, whatever heat loss there is from the tank will be less because of its lower temperature.



My guess is that if money is no object, I would put a modulating boiler with outdoor reset in there and skip the tank. But money is usually an object, and by Murphy's Law, the existing boiler is too new to replace.



My only experience is with my own boiler in my own house. The boiler is the smallest the manufacturer makes in the mod|con line. If the entire house is calling for heat, the outdoor reset works fine and it does not short-cycle. But the heat load in the upstairs is very low even when it is at design day temperature outside (14F), so on normal days, the load is way under what the boiler can produce even when modulated all the way down. So I might actually benefit from such a tank, if I had a place to put it and the money to pay for it and its installation. But maybe a better solution if I wanted to spend more money would be to double the amount of baseboard in the upstairs instead. This would increase the thermal mass some, get rid of more heat in a given amount of time, and perhaps even permit me to run that zone at a lower temperature and get more condensing.