How do you heat you Domestic Hot Water?

Tankless Coil hot water heater - off boiler

How do you heat your homes Domestic Hot Water (DHW)? If you’re from the east coast and have an oil boiler, the chance you heat with a Tankless Coil is quite high. Tankless coil systems are very effective at producing extremely hot water; however, it’s one of the most inefficient ways to heat DHW. A simple upgrade to an alternative Hot Water Heater or indirect storage tank could save you 5-15% on your annual fuel bills. 

How to tell if you have a Tankless Coil system. Track down your boiler and take a look to see if you have a hot water heater anywhere. It could be a standalone 40-100 gallon fuel fired tank for residential use (oil, gas, or electric) or can be an indirect hot water heater (which stores preheated DHW from your boiler, 30-50 gallon). If you don’t have a DHW heating tank, then you are producing your DHW from a Tankless Coil system, and you are wasting allot of money to do so.

(Some people mistaken there expansion tank for a hot water heater – the expansion tank usually hangs above your boiler and for residential purposes looks like a smaller version of a propane tank)

Why is a tankless coil system so inefficient? When it is 90° outside and you look up to your chimney and see heat waves or smoke coming out – that is because your boiler is running full blast to heat hot water for: your clothes washer, dish washer, to clean dishes, showers, etc. Since the boiler (which is usually oversized) is firing full blast to heat small increments of DHW, it is wasting most of the heat produced as it has nothing else to heat. (During the heating season this efficiency is higher as your boiler is firing for longer periods of time to send hot water to your baseboard/radiator/ radiant heat, but during the non heating season your boiler is firing full blast to heat only small amounts of DHW.

Another way to look at the issue. When you have your oil boiler cleaned annually you are given a percentage of efficiency from your technician (should be listed on a tag or piece of paper near your boiler) and should be around 80-84%. This efficiency reading is taken at what is called Steady State, which is reached after about 5-10 minutes of continuous use, and is basically the most efficient your system will ever function after that cleaning. This means that up until your boiler reaches Steady State it is not operating at its maximum efficiency.

AGAIN, WHAT DOES THIS MEAN? Most oil boilers are around 76-83% efficient at Steady State. If your boiler efficiency reading says 80%, then in reality your System Efficiency (distribution) is measuring around 70-75% efficient - accounting for heat loss and lower efficiency readings when the boiler initially fires and then shuts off (that residual heat has to go somewhere when it shuts down).

This means that in the prime of the heating season maybe you’re supposed 80% system is actually 72-75% efficient. How about during the non heating season? This is hard to track due to living standards (amount of inhabitants & DHW consumption), but most unbiased professionals will tell you that the average 4-5 person home using a Tankless Coil DHW system sees a Heating System efficiency of around 15-20% during the non heating season! Ever wonder why you go through 100+ gallons of oil from May to October? So much for your oil boiler being 80% efficient. In reality, energy advisors and heating professionals look at annual system efficiency for a Tankless Coil DHW system as being roughly 50% efficient. So if annually you need 200 gallons of oil strictly for DHW, then you’re only really getting the use of about 100 gallons and you’re basically burning money in your boiler.

Standalone hot water heating system

So what are the solutions? GET SOME SORT OF HOT WATER HEATING SYSTEM: indirect storage tank (still heated by your boiler, but in larger quantities and stored in an insulated tank = more efficient production of hot water), gas fired storage tank, on demand instantaneous gas heater (~90% efficient), electric hot water heat pump or Solar Hot Water.