System short cycling

Krash

My system is used to heat my whole, 2,000 sq. ft. home, plus domestic water. The system works fine when used to heat the whole house. However, I will be alone in the house during the winter months from now on, so I am trying to heat only two small rooms. But that causes the heater to short cycle. On for about a minute, then off for about a minute.

This is a split system with the heating side, and fresh water side separated by a heat exchanger. That heat exchanger is connected to a 199,000 A.O. Smith propane heater by about 3 feet of tubing. The heater requires about 15 degrees or more difference between the output and input temperatures or it shuts the heater off until the inlet temperature cools.

When the whole house is being heated, the return temperature of fluid is enough to keep the heat exchanger cool enough to allow the system to operate normally. But with only two small rooms connected, the heat exchanger is getting overheated allowing the return water to the heater to rise too quickly.

My thinking is that I need some way to cool the water returning to the heater, maybe by a buffer tank or cooling coil of some sort? Has anyone come across this situation, and has a fix?

Any help would be appreciated!

Jamie Hall

When a system has a boiler which is as wildly oversized as yours is going to be, the only real solution is a buffer tank, and the bigger the better.

hot_rod

So a 199,000 water heater is the hydronic heat source, via a plate HX? So 75 gallon or larger tank? That tank should buffer a small heat load.

If the system is zoned with zone valves a delta P circulator would reduce flow, automatically through the heat exchanger as zones open and close. What size circulator is on the A and B side of the plate HX?

Krash

A 75 gallon tank would be very hard to fit in the space available.

Could I use some sort of coil instead? The system is against a wall of an unheated garage which is very cold. If a coil could be located in that garage, would that reduce the temperature of the return to the heater?

Jamie Hall

Krash said:

A 75 gallon tank would be very hard to fit in the space available.

Could I use some sort of coil instead? The system is against a wall of an unheated garage which is very cold. If a coil could be located in that garage, would that reduce the temperature of the return to the heater?

Ah... well, yes it probably would. You would be heating the garage, and to the extent that you did, that could reduce the return temperature. However, most times you want your hard earned dollars to be spent heating the places where you are living, no? You would be solving a pretty much non-problem (short cycling the boiler) by wasting the heat -- and money.

Not sure that's a good bargain.

pecmsg

Whats going to happen in the unheated rooms? How are you going to protect the pipes?

hot_rod

Do you have all the settings adjusted in the boiler control? There should be multiple settings to reducing cycling. What is the model of your boiler? Attach a picture 

Krash

The boiler is actually a water heater. 199,000 btu, By A.O. Smith (Takagi). I have talked to the manufacturer who assures me that the return must be 15 degrees lower than the output, or the unit stops heating. There is no adjustment.

Perhaps the only practical solution is the go back to heating more rooms?

The entire system uses glycol, so there is little chance of reaching it freezing point.

hot_rod

Yeah, that is not the best appliance for the application. If you had and actual boiler it would have adjustments to address short cycling, derate, step fire, time delay, outdoor reset etc

Probably the only fix is 30 gallons of buffer, or more. Or replace it with a real hydronic boiler.

ryanwc

Certainly heat the house rather than heating the garage. 

Dave Carpentier

Do the other rooms have their own thermostats and individual zone valves ?

EdTheHeaterMan

My thinking is that I need some way to cool the water returning to the heater, maybe by a buffer tank or cooling coil of some sort? Has anyone come across this situation, and has a fix?

You could use the unheated rooms to cool the return water somewhat. Just spitballing here

ryanwc

As long as the rest of the house is insulated and sealed, it’s your supersized, open air buffer tank. Not sure why you’d buy another one.