tankless water heater for backup for wood gasification boiler?

[Galroc]

current post is here but I thought I would ask the question in the boiler forum where storage tanks are used.

I do not want standby losses in a backup system and I am going away from a dual fuel system like the TARM Excel, so have people used a tankless water heater as a backup for their wood boilers?

Instead of using a tankless water heater to heat the house, or DHW, use it to make sure the storage tank doesn't drop below 120 degrees, which in turn heats the DHW, and Radiant system?

This also simplifies having one backup for two systems (radiant, DHW).

 

[Como]

I have seen it advertised, run a UFH system for a tankless heater. I can think of no reason why not.

I will have one spare at the end of our install so I was thinking of going both tankless and electric, electric is currently cheaper than Propane. With a standby generator to power the Propane/pumps etc if the Electric is out.

 

[oilbegone]

I am currently using an on demand oil fired water heater as backup heat. When the tank temp drops below 110 the Toyotomi kicks on and heats the house not the storage tank. I asked the question earlier this winter about dhw in the summer and Nofosil said there would be too much stand by loss to heat the tank to use for hot water during the non heating months. I am trying to figure out a bypass system to use the oil water heater in the summer to heat DHW.

 

[Chris Hoskin]

this will work especially well if you have a low temp heating system. High temps (180F) will tend to drive minerals out of the water as it passes through the tankless' heat exchanger. Think of the crap that builds up on a tankless coil in a boiler, except this happens on the INSIDE of the coil.

Also, running the tankless water heater so that it heats up the storage tank has the benefit of allowing for longer run times for the water heater (better efficiency and no worries about condensation in the heat exchanger or venting). The downside of this may be standby loss from a 500 gallon tank if not well insulated. I believe the Toyotomi has a stainless heat exchanger so is more tolerant of this kind of thing, but all the tankless water heaters I know about have pretty thin copper heat exchangers.

Chris

 

[tom in maine]

I am using a Toyotomi for space heat and DHW. My heat load is quite low and the short cycling that occurs normally occasionally requires a nozzle cleaning with some carb cleaner. I am setting up a relatively small tank, about 350 gallons, that will be heated by an air source heat pump during the summer (giving me AC and dehumidification for free) for DHW in the summer. The oil will be backup for the winter while we use wood to feed the tank for space heat.

I am planning on firing all the heating units directly into the tank, with no heat exchangers other than the DHW one for DHW to the house.
The oil fired Toyotomi, being stainless is fine being fired under minimal pressure. The heat pump water heater is all copper and bronze, so it will also handle unpressurized tank water and the wood boiler will be a site built gasifier with a non-ferrous heat exchanger.

All the heating loads are non-ferrous radiant walls and floors. Again, since the heat load is quite low, the temps do not need to be that high.

It was 86F in Maine today. I better get goin'.

 

[Singed Eyebrows]

I have a Takagi JR that is rated for heating service(natural gas, power vent). When I get my gasifier hooked up I will use this as a backup unit. The nice thing about these is you can keep the chimney sealed up good for the gasser. My Atmos boiler requires a very strong draft & nothing else can be in the chimney. It will run to 70 percent output with fan off(power outage) because of the draft requirement, I'll post again when installed, Randy

 

[Vtgent49]

Word of caution here. Most tankless DHW heaters are not modulating designs, thus are definately not suitable when an application involves using pre-heated water, such as might come from a solar or wood heated tank. In other words, they just have one rate of burn. So, it will make the DHW too hot, and if you then use a mixing valve, the flow through the tankless heater will be reduced, and thus the water will get even hotter.

See Houseneeds.com for an explanation of this.

 

[Galroc]

Great responses..

ok, sounds like I need to get a tankless that has a modulating design. Houseneeds.com shows several tankless hotwater heaters that are also rated for radiant heating, for example http://houseneeds.com/shop/HeatingProducts/WaterHeating/takagi/tm1mainpage.asp

 

[Vtgent49]

Full disclosure, I know the founder/owner of Houseneeds, but I have no affiliation other than living in a small town in a small state. Same with the owner/founder of the Bosch USA distributorship.

I give houseneeds a LOT of credit for being the first that I know to discourage, and even clearly insist, that tankless heaters are not suitable for use with pre-heated water. Better call them, ask for Gary if you don't get a clear answer.

 

[Como]

I have two tankless heaters, they come with examples showing the throughput varying with input water temperature.

Where I am the incoming water is just above freezing for most of the year, if you lived in Florida it would be very different, basically twice the flow. We have one that operates at 140F and one that operates at 125F, the former obviosly has less flow.

 

[ohbie1]

I've had a Bosch Aquastar (no standing pilot) for DHW two yrs. now. The unit is adjustable from adding 50 to 90 degrees to the incoming temp. (you can further control the the temp. by just turning the gas valve closed some until you are happy.) I preheated my water with a DHW coil in the OB. The system worked(works) fine, however there is a maximum temp. that when reached will shut (my) unit down. I think it is 190, but I can look it up in the manual if you need it. Since you are only looking for 120 degrees, I don't see a problem. A tank sensor control hooked up to your circulator should do it.