low flow heater problems

D

Dan Mauser

Member
Hello!

I want to buy one hot box to serve both my carpet cleaner that has .25 to 2 gpm and pressure washing of 4-5 gpm. I spoke with 3 or 4 manufacturers, a couple of whom recommended I talk to a place that retails their hot boxes.

The retailer said there would be a few problems with a 350kbtu hot box serving such low flow rates compared to a 65kbtu heater. He said there would be a large fluctuation in outlet water temp (120-210F), and that this wouldn't happen at higher flow rates. But I don't understand how that can be.

Here's how I understand a hot box works:

1. Water temp 50, so hot box fires up, heat until 210 then shuts off burner

2. I spray hot water slowly with my carpet cleaner pump. Water temp gradually drops below thermostat off temperature (~190?) and burner comes on again and the cycle repeats.

Seems to me a high flow system would have more temp fluctuation.

Are there adjustable temperature settings for having the thermostat kick on and also to kick off? Like I could set it for 210 off and 190 on?

Guide me to deeper knowledge, please :)
Dan

PS: Thanks Ron for responding to my earlier post.
 
Hello Dan,

About a year ago I replaced my old stand alone boiler (750,000 Btu) with a 1,100,000 Btu unit. Of course the distributor said variable flow "No Problem!"

This first time I used it, it promptly super heated the water to well over 325 degrees!! That is as high as the temperature gauge goes that we installed!

Of course when I called the manufacturer and talked with their customer service department they said "We would never have suggested using that or any of our boilers for your application (variable flow)!! "Give the distributor a chance maybe they can make it work!"

We ended up putting in a different unit (Whitco) and the biggest difference is the use of a modulating fuel valve. The positioning of the valve and the design of their burner (this is a Lp system). We have never superheated, no sooting. Water temperature stays within a few degrees of set point most of the time, but we still have a high low range of about 65 degrees from set point. This unit is rated at 980,000 Btu.

I think most of these types of systems have a flow switch as well as a thermostat. When you have low flow the flow switch will allow the burner to light but as in our boiler it has about 400 feet of coil and the probe for the thermostat is at the end of the coil!

When you have a burner capable of heating much more than the flow the water will be very hot by the time it travels the distance of the coil.

Dave Olson
 
Thank you very much David!

I was about ready to forget getting a high pressure hot box and go with Little Giant strictly for carpet and tile cleaning. Then I spoke with a fellow who suggested have somehow reduce the size of the propane orifice for burner delivery. Sounds like something akin to your 'fuel modulating valve'. With this he said it could turn a 550 kbtu down to 50-100 kbtu. He felt there would be very little temp fluctuation. But you say you get 65F fluctuations still?

Take care,
Dan
 
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