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# DHW Priority

Member Posts: 303
The supply water temp here is usually down in the 40 degree range, so a 100 degree rise is easy to figure if you want 140 degree DHW. I've been taught that a 100 degree rise at 5 gpm requires 225,000 btu/hr. So, unless you have 225,000 btu/hr available, then you have to slow down the gpm's to bring the required demand inside the curve. I spend alot of time educating homeowners that have been promised "all the hot water they could ever want" to the facts of life. They just need to lower their want level!

• Member Posts: 6,928

Yet another "interesting" situation in the super-insulated house heated by a Bradford-White "water heater" with space heating coil.

There's a 70-gallon whirlpool tub.

Water heater stores 75 gallons with 60 mbh gross input. Temp setting of water heater planned to be 130F year-round.

1/2" filler for the tub, so I'll estimate 15 minutes to fill.

At 100F tub temp with 60F rise (40F incoming water temp), that's 70 * 60 * 8.33 = 34,986 btu required to fill the tub.

At stated 78% recovery efficiency, the water heater can add about 11,700 btus in these 15 minutes leaving 23,286 btus that must come from the water in the tank itself.

Assuming the water in the tank begins at 130F, the tank temp will drop about 23,286 / 8.33 / 75 = 37.27F degrees to about 93F. (Yes, I know 93F is less than 100F but it just means that the initial fill water has to be slightly warmer than 100F...)

Well and fine EXCEPT that the system is ALSO providing space heating. In "normal" circumstances with the constantly circulating full TRVd system maintaining space temp I expect no more than 3,000 btus required for space heating in these 15 minutes.

Obviously, the available supply temp for space heating will fall considerably while the tub is filling, thus significantly reducing emitter output, so let's say 1,500 btus go to space heating in this period.

1,500 / 8.33 / 75 = 2.4F reduction in tank temp on top of the 37.27F drop from filling the tub = a tank at about 90F after the tub has filled.

Thus, I'll need about 40 * 8.33 * 75 = 24,990 BTUs to return the tank to 130F. Again, the water heater can provide about 46,800 btu/hr @ 78% recovery efficiency.

24,990 + 1,500 btu space heating deficit during tub filling a total btu deficit of about 26,500.

For each 15 minutes of the recovery period, I can transfer about 11,700 BTUs. With an assumed space heating load of 3,000 btus per 15 minutes, that means full recovery of the tank in less than 30 minutes after the tub is filled.

All well and fine using these assumptions, but what if conditions are different?

Say there's an additional DHW load (like clothes washing) during the recovery period or say for some reason some TRVs have been set higher during the period--or even that the tank BEGAN at a temp significantly lower than the expected 130F due to being at the bottom of the differential (you know how "rough" the differential of water heater gas valves are...) In any of these cases I can see at least a possibility of insufficient energy to fill the whirlpool.

Control for this system at present amounts to nothing more than TRVs and a warm-weather shutdown for the space heating circulator.

Should I include custom DHW priority? I know how to do it in a fail-safe manner, but we're talking two decent temp sensors, two electronic relays and two conventional relays. Worthwhile? Am I over-analyzing?

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• Member Posts: 929
priority

Wow, you got my head spinning. I do think you are overanalysing this situation,but it is good to see someone care enough to figure it all out. I like domestic priority. I don't like using a water heater for a boiler, but I know it's done everyday, somewhere. Can't you use a simple priority controller like a TACO or ARGO or whatever? Don't know your layout, but it seem that would be the easiest. Where are you in MO? Was there a few years ago visiting friends in Kirksville.
• Member Posts: 284
DHW

Raise standby temp, add tempering valve/mixing valve as needed? (>140F required for Legionella protection?)
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