2 pipe dry return with a mechanical condensate tank

mpitt

Good morning. I have a two pipe dry return system with an oil fired boiler in a 3000 square foot house, eastern Canada. The system does not have any main steam vents but rather a condensate tank with mechanical pump and and open vent on it. I have an older slant fin boiler that I think is going to need replacing in the next year or two, possibly sooner. It's got a lot of rust and pitting around some of the connections. When I bought the house the pressure troll was turned up to 5 PSI with a three differential. There was a lot of banging and hissing. The radiators in the house on the top two floors are a original convection style built into the walls where it pulls cool air from the bottom gap and exits out of the top in a grate. There's only one rad that I had to change a steam trap on the rest function well with reasonably even heating. I did lower the pressure troll down to 0.5 with a 1 differential. The boiler now short cycles 2 minutes on 1 minute off when things are up to temperature. Being that it's oil it's quite loud on and off all the time and I'm guessing it's not great for longevity. Good news is there's no banging in the pipes now it sounds like things flow well. My guess is the system was designed for coal originally 1930's with a slow steady steam throughout the day. I guess I would need to do an EDR to get an understanding of the required BTUs but I feel this boiler is oversized. My question is would it be wise to switch to a propane based steam boiler when it comes time? We do not have natural gas but I do like the fact that the gas systems in general run much quieter. If so what are some good brands these days that could operate with the condensate tank return system. I heard that some of the smaller boilers now won't do so well with the call for refill coming from the float inside the condensate tank rather than the boiler asking for return condensate perhaps I'm wrong. Thank you for all your wisdom

Jamie Hall

A lot to unravel here.

First, the low pressure settings are good — keep them that way.

Now a question: why is there a condensate return tank? I'll grant it may be necessary, but… that's rather rare. But I'll get back to that.

Your existing boiler is clearly rather badly oversized. In fact, it's probably very nearly twice as big — certainly half again — as you need. When you start looking for new boilers, start off with figuring the EDR rating of all your radiation, and sizing the new boiler to be no bigger than that (a bit smaller is OK — but no bigger).

The question of fuel is a matter of dollars and cents. Which fuel in your area is generally cheaper? Do not go by cost per gallon — what counts is cost per BTU. This is sometimes a bit confusing, but remember that a gallon of LP only has about half the heating content of a gallon of fuel oil. Modern power gas burners are a little quieter than modern oil burners, but not much, so that should not be a consideration.

Now back to piping and that tank. You say, and I don't doubt, that you have a dry return system — but just to be certain, where are these dry return pipes located? Up near the overhead in the basement, very likely near the steam mains? And how, if they are, are they connected to the steam mains at the distant ends? Directly, or is the connection through steam traps? If it is through steam traps, how might condensate get from the steam main to the associated dry return? A picture might help…

Are there any wet returns? That is condensate return pipes located below the boiler water line — often right at the floor? I presume if there are such, that they simply drain into the condensate tank at floor level? If this is true, are there any vertical pipes which drain condensate from the steam mains into these wet returns? If so, they must have float and thermostatic traps on them. Do they?

Now that tank. Basically there are two types of tanks which might be used: boiler feed tanks and condensate receiver and return tanks. They differ in one critical way: a boiler feed tank holds condensate — and often additional water — which is pumped to the boiler as needed, controlled by a float or other level sensor in the boiler. A condensate receiver also holds condensate and often additional water, which is pumped to the boiler — but not as needed by the boiler, but by a float in the tank which turns on the pump when the water in the tank rises to a certain level.

You are absolutely correct in thinking that newer smaller boilers don't play well with condensate receivers controlled by a float in the receiver tank. In fact, even older larger boilers don't. That will have to be changed to a boiler feed tank arrangement on any new install — if the tank is even necessary at all. As I noted above, it's very likely that it isn't, unless your system is rather unusual.

mpitt

Thanks so much for the reply. The first picture above is the end of main line with the steam trap going to a return line. The second picture is the actual return coming off of the radiators on that side of the house. The third picture is where the end of the other side of the house finishes and as you can see the return lies tie into. The fourth picture is the condensate tank which does sit below the water line. In the last picture is just the header.

Steam rating for the boiler is 592 square feet. 189000 gross btu, 142000 net.

Curious if perhaps there was vent at the end of the main on on that 3rd picture?

mpitt

This is the feed back into the boiler

mpitt

Jamie Hall

Thank you for the pictures! You are entirely correct in your thinking about the way the system operates! Sorry to sound doubtful, but sometimes…

There is no need for a vent at the end of the steam mains. The crossover traps you show between the steam mains and the dry returns take care of that very happily, and the entire system is vented at the condensate tank. That particular crossover trap arrangement will also take care of condensate in the steam mains.

I still doubt that you need that tank, but if it is rewired so that it operates off the water level in the boiler rather than a float in the tank it will do no harm. You would also need to have a float in the tank still, which will control any makeup water if the tank should get low. The existing tank vent will do very well as an overflow should that get stuck!

Now back to the boiler choice. Do add up the EDR in your various radiators. That will determine the new boiler size — and there is no need to convert to BTUh, as steam boilers conveniently are rated in EDR, as well as BTUh, and that rating is conservative (there is a lively debate about just how conservative it is, but leave that be — suffice it to say that you don't want to go over your total radiation with the boiler, and under will be OK at least up to a point).

As I said in my first post, in my view the choice of fuel is open, but compare costs between LP and fuel oil very carefully. In some areas one may be less expensive than the other; in other areas the other will be.

ethicalpaul

One question for Jamie— you compared the noise of an oil burner with the noise of a modern power gas burner:

Modern power gas burners are a little quieter than modern oil burners, but not much, so that should not be a consideration.

But wouldn't a better choice be a propane atmospheric boiler? Fewer parts to fail, almost no electrical use, and much much quieter. I actually don't know if propane atmospheric boilers exist, but I would think it would be just a question of the correct fuel valve/regulator and burner tray. No?

EBEBRATT-Ed

Yes propane atmospheric exist. It's just a regulator spring change and oarfice change.

I also agree that a boiler feed tank is a better choice than a condensate pump.

If the new boiler does not play well with the existing condensate pum (if you choose to keep it) you can increase the water volume so it will work by using a surge tank. Weil McLain shows ho to pipe them in some of their install manuals.

As far as gas atmospheric I would use Weil McLain or Peerless.

mpitt

Awesome, this is such great help. I'm wondering if because we have these wall cabinet 3"*5.5" convection style rads how much of a difference it might make. I'm assuming these would be much quicker and heating up and then cooling down versus a traditional cast. Perhaps that would also attribute to short cycles in general. I believe it's about a 360sqf EDR based on my measurements.

EDR of 1sf per inch of measured length of rads +/-

7 - 30" rads on the main floor

5 - 30" rads on the upper.