How much oil am I using for my DHW?

depends a little if your furnace is one that runs all the time, or a cold start type. Also, if your hot water is "tankless coil" in your furnace or if yo have an indirect tank.

I have cold start furnace, pretty efficient new furnace, 50 gal in-direct tank. We use 800 gallons of oil, each of the last 3 years. energy auditor has figured the DHW accounts for about 200 gallons year round. In the winter, my system is VERY economical in producing the hot water... In the Summer, it's not efficient at all - even with my indirect set-up. I'm in the process of a DIY project adding solar preheat to the DHW - my goal is to knock off at least 100 gallons of oil for the hot water. 100 gallons of oil = what $450?

hope that helps a little.
dave in Maine

I have a cold-start oil boiler with indirect DHW. Over a 5 month period (May-August) I used almost exactly 20 gallons per month. Family of 5 with teenagers. This was before solar and wood and preheat and storage - back when oil was $0,85 per gallon.

I would opt for larger than 35 gal. I assume the indirect tank your looking at is one of the super insulated tanks. Some claim a heat loss of less than a degree an hour - which is great. What I have learned with our indirect system (cold start, oil burner - 50gal indirect, only 2 of us using) that the boiler run on average of 15-20 minutes when the water tank calls for heat. At least in our case - there is about 12 gallons of water in the furnace that need to come up to temp (and the mass of the boiler itself) - therefore, in my mind, a larger tank would take better advantage of getting the heat out of each cycle. In other words, if the boiler runs for 15 minutes to bring 35 gal tank back to 130 degree - it might only run for 18 minutes to bring 50 gal up to 140 degree.

Something to consider is to put your hot water tank on a timer so it only calls for heat from the boiler during the times you are going to need a lot of hot water, like when you're taking a shower. Instead of a timer you could also use a simple light switch and turn the tank on when you need it and off when you're done.

I do this at home and my boiler only fires in the morning when we're getting ready for work and the timer shuts it off after we leave the house. It's on long enough to fully heat the tank up and the tank is insulated well enough so we have plenty of hot water in the evening to do the dishes and bath our baby girl.

So our boiler really only runs for about 1/2 hour in the morning.

thats is exactly the ticket.

It was driving me crazy hearing the furnace come on, whenever we used 4-5 6 gallons on hot water. And again, the entire boiler had to come up to temp before it really did anything to heat the indirect. (a summer problem only). I started off just using the emergency switch - with the two of us, we could go every other day before turning the boiler on.

After lots of research, last weekend I installed a simple 24/7 Programable timer - the perfect unit - intermatic self-adjusting wall switch timer - model ST101c. From Home Depot - $34.00. Battery operated - allows for mulitlple on/offs each of the 7 days. It's wired in to "intercept" the signal from the aquasts on the indirect tank to the furnace. 24V. So, the furnance swich is left on, IF the timer is in the OFF state - even if the hot water tank wants heat, the signal doesn't get to the furnace. If the timer is in the ON state and the tank needs heat, the signal gets to the furnace and the boiler turns on.

This is working out great for us.... I have it set up for every other day... and there is an easy manual override switch, so we can turn on the circuit manually when need.

I think during the summer, that I have easily cut oil consumption in half - at least a 50-75 gallons of oil saving ($200-$300 savings) for a $34 timer.

Dave from Maine

Dave,

I got the same style switch from Home Despot but I got the model down from that and it cost me $19. Works great!