Oil Burner + Tankless Hot Water Issue

I have an oil burner furnace that includes built in on demand hot water. When we moved in back in July the hot water in the house was fantastic! You could take a 20 minute shower and not lose hot water at all. Since we started using heat in October, the hot water in the house sucks to say the least. If I get 2 minutes of hot water, that's a lot. But sometimes the hot water is great. I think I narrowed it down to when the heat is on (and the furnance is running due to the heating demand for hot water), the domestic hot water is also great. If the furnace is NOT on when the domestic hot water demand kicks in the furnace doesn't seem to be turning on to heat the water. So, the only hot water you get is what is in the coil at the time.

We had someone in to look at the furnace because the heating wasn't working properly. Turns out the heating zones were all air locked. They bled the system and solved that problem. But the domestic hot water continues to be a problem. Is it possible that there is a problem with the thermostat (or whatever it is that triggers the furnace to turn on when the domestic hot water is calling for demand)?

Any ideas and suggestions would be great.

I did see the other post that recommended the ERGOMAX heat exchanger system. I understand the principle of that and it looks like a good alternative. I would just like to figure out exactly what is wrong with my current system so I can make an informed decision with how to proceed.

--Stewart McGuire
CT, USA

First, it's a boiler, no a furnace. Furnaces distribute heat as hot air in ducts, that can't be air locked. It sounds like your heating system distributes heat via pumped hot water, so it's a boiler.

To get reasonable hot water performance out of tankless coils typically requires boiler temperatures of 160F or higher. This means setting up the aquastats with a low-limit of 160F. But if you have a lot of radiator/baseboard out there, particularly if they are high-volume high-mass radiators, it's possible that during the zone starts the boiler temps drop well below 160F, or it may be unable to sustain 160F with the combined heat draw of both the hot water coil and the heat emitted by the radiation, even if it has plenty of output to cover each load separately. This is a system design issue that can be calculated. The equipment might be working fine, but under-designed for the combined loads.

An Ergomax is a somewhat oddball variation on an "indirect" hot water heater, and sometimes referred to as a "reverse-indirect" water heater. The more conventional (and usually cheaper) approach is to use an indirect where the volume of water in the tank is the potable water, with the heating system water being pumped through heat exchanger coils inside the tank. With an Ergomax the potable water is in the coils, and heating system water is in the tank. This requires a larger heat exchanger to work, making it a more expensive system.

If the heating system is already cut up into a bunch of tiny zones so small that is short-cycles the boiler, there can be a good reason to use an Ergomax. Plumbed so that the thermal mass of the water in the tank is involved with every burn, whether it's a heating call or hot water, the mass of the tank lengthens the burn times to eliminate short-cycling, resulting in far fewer burns for both higher average efficiency and less wear & tear on the boiler. But since hot water and heating zone calls can't be differentiated, if the boiler is small enough that it can't support both the heating and hot water calls simultaneously, you may or may not be able to get the hot water performance out of it that you want. As the temp in the tank drops the output of the radiators or baseboards fall, so more of the heat goes into the hot water, but whether it balances as a tank temperature that's still delivering suitable hot water temperatures is something that also has to be calculated.

It's both cheaper and simpler to go with a standard indirect, operated as another heating zone, but a "priority" zone, that inhibits calls for heat from other zones whenever the indirect is calling for heat. When you go this route you can also lower the standby temp of the boiler to 140F (or lower if it's a cold-startable boiler) and control the boiler with a heat-purging controller like the Intellicon 3250HW+ or similar, which can maximize the efficiency by lowering standby temperatures (and thereby lowering standby losses), and maximizing the burn times of each zone during extended calls for heat, by utilizing to the extent that that it can the combined thermal mass of the boiler and the zone with maximal high/low temperature differentials as opposed to keeping it in a tight 20F delta.

First things first: If the boiler's low-limit aquastat is set to 140F or something, crank it up to 160F-170F, and the high limit to 190-200F (if you can- some only allow a limited difference) during the heating season, then back it off again during the summer to save on standby losses. If that gets you the hot water performance you want during the heating season, you can take a step back and consider your other options more carefully. A cold-start boiler with heat-purge controls and an indirect can get you efficiency better than 60% during the summer, whereas with most tankless coils even with 140-150F standby temps you're looking at 35-45% efficiency. At recent years' oil prices the indirect will pay for itself well within it's lifespan, as well as give you better hot water performance. More on water-heating efficiency with oil boilers using different approaches can be found here:

http://www.nora-oilheat.org/site20/uploads/FullReportBrookhavenEfficiencyTest.pdf