Comments on: Hydronic Furnace & Tankless Water Heater — A Great Combo!

By: Todd

Todd — Wed, 18 Jan 2012 18:17:19 +0000

A question for the HVAC
A question for the HVAC professionals reading this. We are in Dallas, TX and have a 3 year old Hydronic system using a Rinnai tankless water heater and an air exchanger that has an evaporator coil that has a lot of rust around the U shaped copper tubes that loop around the ends of the evaporator coil. What causes this and what is the proper way of treating the issue before it fails?

By: Jake

Jake — Tue, 03 Jan 2012 06:19:35 +0000

I've installed hundreds of
I've installed hundreds of combo system, mostly using the Polaris water heater. I always use potable water, even for in floor heating, never had a problem, works very well. Its the only system I would use in my own home.

By: Carl D. Clark

Carl D. Clark — Wed, 07 Dec 2011 01:36:39 +0000

David, Good point
David, Good point
I was looking at the diagram above showing 135 to 140 degree water being supplid to the air handler

By: David Butler

David Butler — Mon, 05 Dec 2011 23:14:13 +0000

Carl: A
Carl: A mixing valve is unnecessary in this application. Allison's load is less than 20k btu/hr. The Rinnai air handler has 28k btu/hr capacity @ 120F entering water temperature. Setting the heater's maximum temperature higher than necessary will lead to short cycles at peak load, which will cause temperature droop at the perimeter (see first article in "Related Articles" list above).

By: Carl D. Clark

Carl D. Clark — Mon, 05 Dec 2011 22:53:34 +0000

Allison,

Allison,
Don't forget the thermostatic mixing valve between the water heater and your household fixtures.... You don't want to scald anyone

By: David Butler

David Butler — Sat, 03 Dec 2011 07:18:02 +0000
Kevin: It
Kevin: It doesn't matter if a straight cooling coil is upstream or downstream of the hydronic coil. However, a heat pump coil should always be on the supply side of the hydronic coil else the warm air exiting the hydronic coil will boil the refrigerant. So with the Rinnai hydronic air handler, the heat pump coil is mounted in a separate enclosure on the return end. With a regular air handler, the hydronic coil is mounted in a separate enclosure on the supply end. Make sense?

I routinely specify heat pumps with hydronic supplemental heat in cold climate zones such as Montana and Minnesota. Unless you pay higher than average rates for electricity, a mid-efficiency heat pump costs less to operate than a high efficiency gas furnace or hydronic fan coil. In fact, the colder the climate, the longer the heating season and the more heating hours above the heat pump's thermal balance point. The only time I don't specify a heat pump as primary heat is when the marginal winter electric rate is higher than about 12 cents, or in areas where central air isn't needed. But in areas where natural gas isn't available, a heat pump will nearly always cost less to operate than the alternatives. The question in that case is how best to provide supplemental heat.

As for savings, it all depends on what you're comparing it to. Keep in mind that it costs about the same to heat with a hydronic fan coil as a conventional furnace with similar efficiency. Rinnai's Ultra line of water heaters are rated at up to 96%. So if gas costs $0.96/therm, then a 96% tankless water heater or forced air furnace will cost about $1.00 per therm plus about 5 or 6 cents per therm for blower energy (assuming ECM).

BTW, at 10 cents a kWh, a 15 SEER single stage heat pump costs about 73 cents/therm (10 x 29.3 / 4 COP). The COP already includes blower energy.

Finally, there are two reasons to consider a hydronic fan coil:
1) if the heat load is substantially less than the smallest capacity furnace, or
2) if the primary heat source is a heat pump and the seasonal supplemental load is large enough to justify the additional first cost (compared to electric strips).

By: Lee Handinbush

Lee Handinbush — Sat, 03 Dec 2011 06:49:47 +0000
There are alot of systems out
There are alot of systems out there like this originally with tank heaters. Now that the perceived higher efficiency of tankless is available but trust me in this application it is not. ANZI allows for water heaters in this combi application as long as there is at least one fixture drawing making this a open system. Biggest drawback especially with tankless in this app is the newer air handlers do have pump exercising and the downfall is it creates flow through tankless, thus it fires. So you have the tankless firing, cycling and heating when not desired, even during air conditioning season.

By: Kevin Virobik

Kevin Virobik — Sat, 03 Dec 2011 06:02:27 +0000
It's not a focus of this
It's not a focus of this particular article, but how would you tie A/C into your proposed hybrid system? Does the air handler care if it's pushing DHW/hydronic over the coils or refrigerant?

Would the hybrid system work in more extreme weather zones?

Have you done an analysis of the estimated cost savings?

Thanks for the thought-provoking article.

By: Kim DeForrest

Kim DeForrest — Sat, 03 Dec 2011 05:45:03 +0000
You may also want to look
You may also want to look into the warranty on the tankless water heater that you choose. Some models are not acceptable to be used in heating applications (whether closed or open loops) and may void warranty.

Most of the time they require that you utilize the commercial tankless models.

By: David Butler

David Butler — Sat, 03 Dec 2011 02:47:54 +0000
Thomas:
Thomas: Your point about ECM is correct. They save the most when operating against a low-static duct system. However, ECM's are still more efficient than PSC motors at higher static pressures.

Also, your point about "ramping up" is a bit misleading. Only variable speed blowers are designed to maintain constant CFM, a feature found on many, but not all variable speed furnaces and air handlers.

The term ECM is commonly used to refer to non-variable designs, even though variable blowers are based on the same EC motor core.

BTW, for the reason you said, I think the "constant CFM" often becomes a crutch for poor duct design and filter maintenance.