Tankless DHW heater with Wood boiler. Wintertime DHW Options?

I have an ordinary 80 gallon electric water heater.

It heats from wood using a 20 plate flat plate HX, and a B&G Ecocirc on the domestic side. Controlled by a Johnson A419 controller. The supply side of the HX is just another heating zone - and all my heating zones are pumped by a Grundfoss Alpha. If I could have found an indirect tank around here that also had a heating element in it, I might have gone that route instead. So, I can either heat DHW with wood, or if storage isn't hot enough the elements will just kick in. I can get 6-7 days on a burn with storage. Even if using just electricity, it only costs $25 or so per month.

Is the propane unit in use now, or is it something you plan to get? Do you use propane for other things?

You could likely do that - so when the wood boiler is hot, the FPHX would function more or less as an on-demand water heater. I think mine is almost to that point now - when hot water is being used & the Johnson senses cooler water entering the bottom of my DHW tank & kicks the FPHX circuit on, I think it is doing most of the DHW heating when the pumps are going. My FPHX is a 20 plate B&G (5x12?) that I got the local supply place to order for me, for no more than what I could find anywhere online. It works pretty good.

I have absolutely no experience with propane or on-demand hot water heaters or that one in particular, so likely can't offer much more. But - does it get called by a thermostat/aquastat, or a flow switch? Or maybe even both? Either way I would think you should be able to make it work - if it gets called by a flow switch you might have to add a controller to get some temperature related controlling.

Hopefully you will get more feedback from others.

Just curious, what's the resistance to using a conventional tank/water heater? Many people (myself included) have had good success using some type of a heat pump water heater (whether integral or add-on) for summer DHW and then a small side-arm HX to heat the DHW during the winter. The HX I'm using is a very small refrigerant HX that I purchased on ebay for literally $1+ a few bucks S&H.

That's a pretty hefty HX, but may very well be the size you need in order to meet the huge single pass delta T. I understand your logic and can understand the desire to move away from the oil fired DHW heater, but really urge you not to dismiss conventional tank heaters. For as much criticism as tank style heaters get for their standby losses, they really don't get the credit due for their significant benefit - the ability to provide time for recovery, allowing much slower, low grade heat than is necessary for instantaneous water heaters.

Although indirect water heaters are really nice, they're not the only way to do indirect heating. You can do this with some type of external HX (sidearm, flat plate) and a small circulator plumbed into any available ports on the tank.

xcgreene said:

Those indirect water heating tanks seem expensive? and using a tankless water heater with an indirect water heating tank to accommodate for the option of heating my DHW with the wood boiler during the winter, seems nonsensical during the summer. Perhaps my issue is wanting a tankless water heater? I guess my old, oil fired 80 gallon water heater has left a Horrible taste in my mouth for wanting hot water stored all the time.

Here is the plate type heat exchanger I'm talking about:
http://www.supplyhouse.com/Bell-Gos...-Heat-Exchanger-With-Mounting-Tabs-12700000-p

looking at flow rates and temps of this heat exchanger, it may be able to keep up with a shower running. The idea is that, should this heat exchanger not be able to keep up, the tankless heater will kick in, as it will be plumbed in series with this plate type heat exchanger. The heat exchanger will be acting like an economizer, and save some propane, even if the Tankless water heater has to fire up.

I'm open to all thoughts, and willing to disassemble ideas in my head that have been instilled by bad experiences.... maybe.

Click to expand...

Mine is a BP400-20MT. Not sure what the difference is between a 400 & 410, but I paid $190 at my local supplier for it (plus tax - and they had to order it for me).

How much will your on demand propane unit cost?

Just to give more to think about - going with a tank heater vs. a tankless on-demand will give the ability to store DHW (so you can have wood-heated hot water even if the fire dies down or goes out for a few hours or even a day or two), and also the ability to use a smaller & cheaper HX. The only extra thing you would have to buy over what you are planning is another pump to pump the DHW side of the HX (I use a B&G E3Ecocirc, model 4V/BTXYZ, was $178). You might also then be able to use the tank in your solar setup.

Have you figured out how you will control things? You will need to start your system cirulator when there is a demand for DHW - I think.

Xgreene - your tankless heater is likely designed to only operate on a pressurized water system of around 15 psi (minimum), so if you don't have a closed system with a heat exchanger, pump and pressure tank then you can't use a tankless heater. A downside of tankless units 'in my experience' is that you need good, clean water, with very low mineral content - think scale build-up if you have hard water. If you have electrical power, save your money an buy yourself a standard electric water heater.

So assuming you have a LP-fired tankless heater or an electric water heater/tank for summer use (when the wood boiler is down), install a small brazed plate heat exchanger and tempering (mixing) valve on the downstream side of the electric tank and open the breaker serving the electric tank. Now you have a wood-fired instantaneous water heater that see's 180F entering source water and will give hot water as long as you keep stoking that boiler. Be sure to install the tempering valve to avoid scalding the folks in the shower.

Some would prefer the side-arm tube style water heaters, where I prefer the smaller (more efficient) brazed-plate units but you should install an inline filter/strainer - brazed plates don't handle fouling and can't be cleaned easily or effectively.

My 2bits.

I'd suggest a bit larger HX, especially if you want some reasonable DHW at low "A" side temperatures. My experience with two different HX systems at my place is a 5X12 30 plate is about ideal. I also run an 8X3 40 plate at the house. These allows a pretty good DHW flow, around 4 gpm, 110F with 60F incoming temperature. With a supply side down as low as 120F, 4 gpm flow rate.

There are a few free online sizing programs that let you play "what if" examples, if you really want to nail down a tight design. No harm in oversizing if you watch for a good $$ on a larger one.

When you undersize you end up needing much hotter fluid temperature on the "A" side or slowing the DHW down to a trickle to get hot enough to even take a shower. it's n ice to be able to generate DHW at boiler temperatures down to 120.

This example is from the link posted at the very top of this site, the NYSERDA trainer.

I bought a 40 plate on ebay for 80 bucks with shipping I set it up as on demand on my pellet boiler there are 4 people in my house and I have plenty of hot water. David PS they still sell the same unit for the same price

it is possible and i am doing it. the way i have it set up for the DHW, is cold water from the well runs threw a 20 plate heat exchanger. water then travels into the tankless. my tankless must senses the incoming water temperature. since the temperature is above the preset temperature the tankless dose not fire up. the only down fall to this is the draft fan dose come on and off depending on water flow. i do turn my tankless DHW temperature down during the time my gasser is running.