Tankless DHW heater with Wood boiler. Wintertime DHW Options?

[xcgreene]

Set up will consist of a Biomass 40 Gassifier and a Noritz 120,000 BTU tankless water heater. I cannot afford storage currently, but will be planned for down the road.

During the summer I will be heating my DHW with the tankless propane water heater and, of course, the wood gassifier will be offline. During the winter months I'd like to heat my home and DHW with the wood gassifier and save some propane. I'd like not to have an indirect DHW tank. I have seen some other heat exchangers that may do the trick online, but I’m not confident enough in my understanding of what I need to make a purchase.


What are your thoughts on this?

 

[maple1]

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?

 

[xcgreene]

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?

I currently have a propane cooking stove and generator. I do not yet have the tankless water heater, but really like the idea. I've seen a number of plate type heat exchangers for around $500 that seem to match up to the small GPM I require for my small DHW operation. around 4.5GPM that is. I suppose if anything, I could get the plate type heat exchanger, and plumb it in series with the tankless heater, so if the water were not quite hot enough after leaving the heat exchanger, The propane could bring it the rest of the way up. Assuming the unit can accept a hot water feed. I'll have to see about this.

 

[maple1]

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.

 

[xcgreene]

I'm not getting much help from the manufacturer. Does anyone have any insight on this? I’d like to plumb my already heated DHW through a Noritz NRC661-DV-LP tankless water heater, so the unit will only fire if the water drops below set-point.

 

[maple1]

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.

 

[brant2000]

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.

 

[xcgreene]

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.

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.

 

[brant2000]

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.

 

[maple1]

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.

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?

 

[xcgreene]

Thats a great price for the BP400-20MT. It's 8.4"x3.1" and 20 plates thick 125,000BTU/hr ideal. The BP410-20MT I'm looking at is 12.2" x 4.4" and 20 plates thick 350,000BTU/hr ideal. I will have to investigate prices at home for this. the interwebs has it at $450. I feel I need this larger heat exchanger in the wintertime if I am to utilize the full benefits of my wood boiler. greatly diminishing the probability of needing the tankless water heater during those months, so long as the boiler is on.

I have heard back from the manufacturers at noritz about the tankless water heating unit, and they are, indeed, designed to take in 120F water and kick on when needed. They even sent me a stock piping diagram showing a solar water heated tank plumbed into the noritz inlet. I feel this may work out well.

The propane tankless water heater will cost $850 out the door. I could probably squeek by with the next size down, but screw it. So I'll have $1300 in my DHW. After that cost I shouldn't be spending much on making DHW, and I won't have any tiny tanks to deal with.

This should all keep my mind at peace while I work on the solar project.

 

[maple1]

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.

 

[xcgreene]

As far as DHW is concerned, the cold water supply will run through the HX, then through a mixing valve to bring temp back down should it be above setpoint, then into the propane tankless water heater should the water be too cold, then out to fixtures. The tankless water heater will only fire if the temp is below set-point and flow is above 0.5gpm. No circulation pump required in this side of the loop, other than the well pump, which is run off a pressure switch.

I will have water storage soon, just not for the moment. That's why I like this streamline, should it work out anyway.

 

[Kelster]

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.

 

[Bob Rohr]

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.

 

[David7636]

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

 

[treefrog359]

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.