2nd Boiler for Addition?

Judson Broome

We have a tankless hot water heater. I've never had one before. Are they efficient? It seems like the boiler runs more than it should. Also, depending on where the oil-fired boiler (single pipe steam) is in its heating cycle, sometimes the hot water heater won't keep up with our hot water requirements....just running the shower, for example.

With several kids, this can be a pain. I was thinking of installing a hot water heater. Would that be more efficient? What are the venting requirements for a gas hot water heater? Any recommendations on brands to look at?

Constantin

Oh Boy...

... from what I understand so far, you seem to have a tankless coil in your boiler to heat your water. Typically, they aren't very efficient, particularly during the summer months, when the boiler has to be kept "hot" in order to heat water quickly as needed.

The reason your boiler cannot feed steam and plentiful hot water is that it simply doesn't have the BTUs to do both at the same time. Modern hot water boilers sidestep this issue by putting an indirect water heater on a primary loop, which is a fancy way of saying that there is no heat to the house while the boiler heats the indirect. Only once the indirect returns to the setpoint does the heating system get heat again. Usually, that's OK because of the thermal flywheel effect in houses (i.e. no one notices that the heat is off for an ½ hour).

With a steam heating system, it is a bit more difficult to put a indirect on a primary loop. Very few indirects can use steam for heating the water (the only one that comes to my mind is the Viessmann Vitocell 300, horizontal). But it can be done, if need be.

I think your ultimate choice depends a bit on the loads. For example, you might find that a large indirect water heater has enough buffer in it to allow you to keep everyone in the house happy with hot water. The tankless coil can heat the water in the indirect, so there would be no additional venting requirements, etc. If the loads aren't too egregious, your boiler/IDWH combo should perform just as well as just about any regular gas water heater you can buy.

Alternatively, you can go tankless with gas and only heat water as it's needed. That gives your steamer the summer off and is likely to be more efficient than your current setup. If properly sized, a tankless gas water heater will give you unlimited hot water. The effectiveness of such a setup depends on the heaters being able to handle the maximum hot water flow and temp rise they might encounter. However, tankless water heaters are not exactly cheap.

The cheapest option is probably a gas water heater. Not my first choice, but inexpensive it is. Even a dedicated gas water heater will run into trouble if it is undersized relative to the loads it is supposed to supply. Thus, no matter what, take a look at the flows in your home as a function of time and figure out what appliances can either heat that much water quickly enough or sustain the prologned draw that teenagers and other house inhabitants entail.