Heat Pump How Water Heater in series with Boiler Indirect Tank?

jmbarndt

Hello Heating Help World!

Let me begin by stating I am not a tradesman and have just been learning about HVAC and plumbing over the last few months. My wife and I are in the process of renovating an 1856 farmhouse in upper Maryland. We inherited a Burnham V8 v84 oil boiler that was heating coils up in our attic and then blowing out through forced air ducts. The duct work was almost all flex duct and to no surprise almost complete junk with leaks everywhere and crazy bends to fit through opening between attic spaces. Due to really high fuel oil prices we have decided to go with 2 mini splits downstairs and a 2 ton heat pump with new duct work upstairs. I will also be bringing the air handler into the building envelope by putting it in a guest bedroom closet and will be burying the duct work with cellulose when done. At this point I feel good with our plan for the lower level and upstairs heating and cooling but am struggling with what to do with hot water and heating the finished part of our basement. Currently there is a small old radiator in the finished part of the basement that is the sole source of heat for that area. Given that there is asbestos tile on the floor that needs covered anyway my thinking was to keep the boiler (and converting from oil to gas) and pulling out the radiator to put in radiant floor heating in the basement instead. I would of course insulate under the poured slab and then use pex to the boiler. However using this approach I would still be getting our hot water through an indirect tank attached to the boiler. Even if I convert it to gas I still feel like I will be wasting a lot of money for gas throughout the year. I am also unsure if boiler would be so far oversized for the basement that it wouldn't meet the minimum fire rate often aside from hot water.

My proposed solution to this is to move some of my water treatment (softener, and acid neutralizer) over to where a current sink is in the basement and add an electric tank heater in series with the indirect tank. I would favor a heat pump water heater if it can fit since I can get rebates and they appear to be so efficient (not to mention the basement is not insulated and humid in summer)but if it wouldn't make sense I would be fine with normal electric instead. Under this plan I would pipe the output of the indirect tank to the input of the electric/hphw and turn the boiler off during warmer months when the basement would be plenty warm enough already. Then when I need to heat the basement floor in colder months it will be able help heat water before reaching the electric unit. Is this a bad idea or are there other ways I could arrange that would be more efficient? We have a 500 underground propane tank that feeds a furnace for our attached in law suite so conversion isn't a big deal and I could look at other options to heat floor/water besides converting boiler.

Thanks!

hot_rod

Is there a recirculation on the hot water system?

I would valve them separately. If you run the indirect to the HP when it is not in service, the water will cool in the tank.

If the HP is always plugged in, and piped in series, make sure it could handle the 140F or whatever the indirect sends to it.

JUGHNE

Have the minisplits proved themselves thru your real winter temps yet? There is some debate over their output at design low temps.....zero degrees or so.
Any back up resistance in the 2 ton heat pump air handler?
Do you still have any old radiators in place?

jmbarndt

They just got put in so can't say on those. There is backup resistance for the 2 ton. I also put in an efficient wood burning insert and make a fire basically every night so think we should be able to handle things especially now that I am air sealing and getting attic insulation up to R49. Honestly most MD winter nights only get down to freezing so heat pumps shouldn't lose all efficiency. No radiators except for the one in the basement. They had electric baseboards which is why I am getting a huge rebate from utility company for the mini splits/heat pump. Any thoughts on water heater/basement heat dilemma?

ethicalpaul

I like your idea of the heat pump water heater. I just installed one myself and it’s great so far.

I wouldn’t tie it into your boiler. Too much complexity and failure points for the possible gain. Just keep it simple

jmbarndt

So are you saying to only use a heat pump hot water heater and remove the indirect tank? I would still keep the boiler only for basement radiant floor under that plan right? Seems like I am wasting the indirect tank that was already installed before we bought the house is my main concern.

hot_rod

pipe and valve them for options.

If you want the indirect to pre-heat the HP. Or let the indirect just go room temperature in summer, then to HP. HP need to run also to keep itself hot

Or indirect only if the HP quits

Or HP only in summer, no flow thru indirect.

Yo could valve all the connections so a tank could be removed fro replacement, this show a simple method to show flow directions.

SuperTech

> @hot_rod said:
> pipe and valve them for options.
>
> If you want the indirect to pre-heat the HP. Or let the indirect just go room temperature in summer, then to HP. HP need to run also to keep itself hot
>
> Or indirect only if the HP quits
>
> Or HP only in summer, no flow thru indirect.
>
> Yo could valve all the connections so a tank could be removed fro replacement, this show a simple method to show flow directions.

I completely agree with this, my setup allows for warm weather shutdown and electric hot water in the summer. If you have an indirect tank in good condition, you might as well use it during heating season and put less hours on the components of the heat pump water heater. It's never a bad idea to have two ways to heat your home or hot water.

Erin Holohan Haskell

Since this was posted multiple times, we've merged the threads here. Thanks!

jmbarndt

Thanks for your thoughts and the diagram! That is exactly what I had in mind. Just wanted to make sure it wouldn't be a problem for some reason. Only thing is I was thinking of adding valve to hot out as well (at least to HP since is at the end kd the series but possibly both) so I don't have to have tanks filled unnecessarily. Would that be an issue?

hot_rod

sure you can add additional valves

If the heat pump will be the primary source, I would run them in series, always flow thru the indirect first, so you do not have stagnant water trapped in a tank and the potential for legionella growth. Whatever temperature the water picks up in the unfired indirect, lowers the load on the HP.

I think the only concern with HP water heaters if they can keep up with your demand. They are slow to recover from what I hear. You don't want any resistance element kicking in or you end up with expensive DHW.

ethicalpaul

Even in the mode where the resistance kicks in when needed, the energy star annual running cost of these units is extremely low.

Plumb the indirect as a backup if you wish but the complexity isn’t free.