Does DHW Priority Interfer with Heating?

[DavidNJ]

It seems that some tankless water heaters can be used for space heating and that most boilers ofter domestic hot water priority.

As I understand it, the domestic hot water priority turns the full attention of the boiler to producing hot water. In a system with an indirect water tank that would reheat the tank. In a tankless system wouldn't it have to run continuously until the domestic hot water demand is satisfied?

In either case, it seems that the hot water requirement could keep the furnace from providing space heating for a significant period. Is this a problem that exists in practice?

One manufacturer's hydronic fan coil, the fan coil include the circulation pump:

http://hvac-talk.com/vbb/attachment.php?attachmentid=115982&stc=1&d=1281331402

[snupytcb]

most controls have a timer so if the domestic runs too long it will switch back to heat. i have never had a problem.

[Rickrod3]

In a tankless system wouldn't it have to run continuously until the domestic hot water demand is satisfied?

Yes

In either case, it seems that the hot water requirement could keep the furnace from providing space heating for a significant period. Is this a problem that exists in practice?

Running domestic HW priority on indirect water heaters in my experiance has not been a problem.

The Drawing you provided does not have any info as to the control set up.
I would think that it would prioritize domestic hot water.
If someone took a 10 or 20 min shower and, you had no heat for that period of time I doubt that anyone would notice. Unless it was below design temperature for your area.

[DavidNJ]

In a tankless system wouldn't it have to run continuously until the domestic hot water demand is satisfied?

Yes

In either case, it seems that the hot water requirement could keep the furnace from providing space heating for a significant period. Is this a problem that exists in practice?

Running domestic HW priority on indirect water heaters in my experiance has not been a problem.

The Drawing you provided does not have any info as to the control set up.
I would think that it would prioritize domestic hot water.
If someone took a 10 or 20 min shower and, you had no heat for that period of time I doubt that anyone would notice. Unless it was below design temperature for your area.

Depending on how close the system is, design temps would imply 100% utilization. A 20 minute shower (not uncommon in this house) would mean a 33% reduction in capacity. Even if the boiler was over sized, the air handlers, hot water coils, pipes, and circulation pumps would also have to be over sized to take advantage of it.

Or, is a storage tank like those used on chillers or for indirect hot water used to store heated water for heating? Presumably a water heater sized tank (which was also recommended as a possible chiller tank) with water in the pipes could have 10-20 minutes worth of heating. In my research so far, I've never heard of a storage tank on the heating side; did I miss it?

[genduct]

Dave, How long would it take for your home to drop 1 degree, 2 degrees?
How long would you be under that shower if the water temp dropped 10 degrees?
I think you had your answer all along

[pacnw]

it would depend on the control of any valves in the system.

if there is an electronic, Taco, Honeywell or the like, control box then probably yes.

if no such similar control then probably not, just a lower temp water supplied to the air handler causing your issue.

[DavidNJ]

Dave, How long would it take for your home to drop 1 degree, 2 degrees?
How long would you be under that shower if the water temp dropped 10 degrees?
I think you had your answer all along


If the design heat loss is 125mbtu/hr, 20 minutes would be around 40mbtu. Like running a 10-ton AC for 20 minutes, wouldn't? The air itself doesn't have much heat capacitance. Wouldn't it depend on the capacitance of the building?

Maybe the issue is that a 20 minute shower would use 20-30 gallons of 120F water, much less than the tank and only representing 18mbtu of heat. The furnaces seem to be over sized to handle the DHW needs. Dedicated DHW heaters are 60-80mbtu. High efficiency ones are 100-120mbtu. Tankless can be 200mbtu. It would only need 6 minutes of a 175mbtu furnace or 4 minutes of a 250mbtu furnace to replace that.

Filling a whirlpool would still just drain the tank requiring 35mbtu or about 12 minutes of DHW.

If the pipes held 30-50 gallons, then their heat capacitance would roughly be equal to the loss.

Is that how it works? Leveraging the storage tank, over sizing the boiler relative to design heating loads, and utilizing the capacitance of water in the pipes?


it would depend on the control of any valves in the system.

if there is an electronic, Taco, Honeywell or the like, control box then probably yes.

if no such similar control then probably not, just a lower temp water supplied to the air handler causing your issue.

Don't have an issue yet...trying to ensure there isn't an issue because of something I don't understand. There are lots of things I don't understand.

How would the electronic controls behave differently?