Adding a Heat Pump Water Heater in series with an Oil-fired Coil
mistered1957
| Posted in Energy, Heating & Insulation on
I’m looking into installing a Heat Pump Water Heater (HPWH) in series with an oil-fired boiler with a domestic hot water heating coil.
My present thinking is to use the oil-fired boiler coil to pre-heat the water and have the HPWH finish the job, particularly in the winter months. I’m also thinking that I would plumb in a by-pass capability so the HPWH would do the entire job of heating the water in the warmer months.
Does this make sense or should I just disable the coil in the boiler? Does anyone have any experience with doing this, or have any suggestions?
Replies
Thats really resourceful to save a few dollars. Have you calculated how many years you'd need to do it that way to make the money back?
I haven't done any calculations, as this installation is for my son-in-law. Due to incentives in New York State, the Heat Pump Water Heater only costs about $200 out-of-pocket, and the coil in the boiler has significant limitations. So, my primary goal is to improve their home's hot water capability using some of the latest technology, and moving away, in the long term, from using oil...
I would expect that the return on investment would be less than 1 year, due to the government and utility subsidies for the purchase of the water heater and installation costs... Without the subsidies, well that's a different story...
I have been running a HPWH for over 11 years. (currently the heat pump part quit and I have been running in resistance heat mode but have a replacement waiting for me to hook it up)
The great thing about a heat pump water heater is that it reduces humidity and cools the air in addition to heating your domestic hot water.
The drawback in the winter is the same thing.
(so I like the idea to complement the HPWH with a fuel-source method.)
I previously had a house with a boiler and indirect water heater.
Do you already have the indirect fired heater?
Hi UncleMike42,
I've enjoyed reading your many inputs on this forum on a wide variety of topics!!!
The house where I'll be installing the heat pump has a "tankless" coil in the boiler, so it doesn't have an indirect tank.
I recently installed a HPWH for a friend where I replaced an oil-fired hot water heater (separate unit from the oil-fired boiler), and he loves it! I should add that he also has a PV array on his roof that generates the bulk of his electricity needs, including installation of mini-splits for heating and cooling needs...
As you mentioned, the HPWH will cool and dehumidify the basement, in this case, for the summer months, but will also cool the basement during the heating seasons. On this point, I'm also thinking about configuring a venting system that ducts the cold air to outdoors, but only during the heating season.
So, I'm trying to determine the benefits and pitfalls of using the oil-fired boiler's heating coil to preheat the water upstream from the HPWH...
Thanks...Ed H.
Thanks for the nice words, Ed.
You could just hook this in series, but it would work out better to integrate the controls.
If you install the HPWH and hook the cold water input to the output of your heater coil, the boiler will heat water on the way in when you are drawing hot water from the HPWH tank. The HPWH would have to provide heat to make up for thermal storage losses, between when the hot water enters, and when you need it. HPWHs still have electric resistance elements, and have electronic controls that let you select all resistance heat, or all refrigeration driven heat removed from air (and by condensing water in the air), or a hybrid of both.
It might be better if you can use the HPWH as an insulated storage tank during heating season, heating with your boiler heater coil, and use a circulation pump, to keep the HPWH full of hot water heated from the boiler.
but a HPWH is not really designed as a storage tank. It is designed as a water heater, with temperature sensors and heating elements located to take advantage of thermal stratification. So that you can pull hot water from the top, while cold water enters from closer to the bottom, and you get a good supply of hot water while the incoming water gets heated and mixed with what is in the tank. Cold water enters near the bottom, and hot water is drawn near the top.
You might be able to repurpose the HPWH tank drain, and integrate it into a loop to let the boiler heat water in the HPWH when there is no demand. ( I am thinking out loud here)
You would have to be good about draining the bottom of the tank periodically, (not like me who waited too long, and now the bottom of the tank is full of rocks.)
it might work to have a circulation pump push water from the HPWH drain to a tee which joins cold water input to the boiler coil, and then to the HPWH cold input.
Part of the puzzle would be the temperature settings of your boiler, and where you like to keep the hot water temperature.
UncleMike42,
Thanks for the additional input. You've given me some things to think about... As you pointed out, a key consideration is the temperature settings of the boiler... I also liked the creative thinking regarding the use of the HPWH drain!!!
Best wishes...Ed H.
“[Deleted]”
Did you end up finishing this? How did you like it. I. Thinking of doing the same thing. The only difference being plumbing the it up stream of the coil so that the hot water runs thought boiler reducing the need for the boiler to run.
I ended up disabling the coil in the oil-fired boiler, and not having the capability of preheating the water with the coil. The decision was made by the homeowner who was looking to shut down the oil-fired boiler as part of a larger electrification project. So, it made no sense to add in capability that would not be utilized...