instant water heater used for radiant

Ed_32

I am dealing with a customer who had a system designed for him, (not installed yet), that uses an instant water heater for domestic and radiant floor, they did spec a heat exchanger, but the water heaters lowest firing rate exceeds the total radiant load, and I believe will lead to short cycling. I suggested a storage water heater and heat exchanger but they don't want to lose the floorspace it would require. Any sugestions? Thanks, Ed.

Brad White_9

You are right, Ed

An instant hot water heater, especially without deep modulation and without a buffer tank, will short-cycle. What is the BTU load?

I would stick to your guns unless you can talk them into a smaller ModCon or other source. Maybe even a dedicated tank type DHW heater if the radiant load is in the 20-30 MBH range.

But as described, the system designed will cycle like Lance Armstrong biking to Sheryl Crow for make-up sex.

Ed_32

BTU

For the new addition they plan to use Qtrac, 4,360 btu. For the existing house staple up, 12,550 btu, and future addition 6,400 btu, probably Qtrac. It was engineered for 130 deg. water temp for all. there ready to install the Qtrac now, staple up in the near future, and the future addition in the next couple of years.

Brad White_9

Makes the case

for a 30 gallon lowboy heater, IMHO. Dedicated of course and if local codes allow. The gas demand will be much less too, not all at once.

Uni R_2

24K BTU?

Ed, if my load were that low, I'd get one of the Thermolec electric boilers with outdoor reset and not even bother with having gas. If that house ever managed to use 24K BTU for the full day it would still only cost about $3.50 at 15¢/KWH. It would be even less on a typical winter day. Most days a buck or two!

No CO risk, no annual cleanings, no explosion risk... well they may already have other gas appliances, but still that's not a lot of heatloss.

ALH_4

Electric

I talk about these all the time, but I feel they are underused. Laing EPR heaters are great, and designed for small heat loads like this.

-Andrew

Brad White_9

Nice item those Laings!

Had not seen them before. I would have mentioned electric but there is that "you pay 100% for 33% at 100% efficiency" equation.

My personal preference (engineer overcomplicating things right?) is ground source heat pump or some other electrical means with a higher COP... Not that it will ever pay for itself of course.

These Laings are nice- and definitely have an application.

Thanks

Mark Hunt

That's not


on their web site Andrew.

Have you used these before?

Mark H

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Unknown

Ed,

Where is this house?

25kBTUs/hr, in an area with a real heating season (Say, 5k plus Degree days), could make a decent case for a small mod/con boiler. If domestic is not a large draw on this unit, a Baxi or Trinity with a coil could handle it.

You are absolutely correct about the cycling in this application and my suspicion is once the heat exchanger and large non-ferrous pump are removed, you aren't going to cost much more for a much better setup.

Ed_32

Seattle, not many days at design temp, (28 deg)

Unknown

Tough call then. I'm seeing about 5k DD's a year there, roughly... low, but significant.. I'd push for the mod/con and settle for a tank water heater. Probably talking 150 gallons of natural gas/year difference or so in VERY rough ballpark numbers between the two.