TT Excellence DHW piping suggestions?

MarkPFalade

Got a project where 100,000 BTUH will heat the house but 3 GPM is not enough for DHW. My thought is to install a 25 gal storage tank to add some capacity. I'm drawing a bit of a blank on piping it though. I figure CW in to the TT, TT outlet to inlet on tank, tank outlet to house, recirc line with pump from tank outlet to CW on TT. I'm assuming I'll need a check valve in that line. My concern is when the pump is running and the DHW is in use the pump will dead head. Any thoughts? Any other/better ways to pipe this?



Thanks!

Robert O'Brien

Why not

just  use a Solo and indirect?

Mark Eatherton

Dead head it...

Dead heading pumps is actually more common than most people think.



Is it a good idea? No, but in some cases, like yours, and circ return systems, it is necessary.



Is it DANGEROUS? Not in a residential setting.



Dan has a story about dead heading a pump and having it blow up on him. THat was not a fractional horse power motor. It was a rather LARGE pump that was imparting a LOT of mechanical energy into a small amount of water that got super heated, and flashed to steam. The chances of this occurring on a small fractional horsepower circulator are slim to none.



Will it damage the pump or shorten its' life expectancy? Probably not.



I once had a job that I inherited where there was a RFH system and 4 fan coil units serving a rather large, highly glazed area. The consumer said that in the four years that she had lived there that the floor had never felt warm. Upon investigation, I found a Taco 007 serving the radiant manifold that had been running during the heating season for four years. Only problem was, no one had opened the mini valves on the 8 port manifold, so no circulation was occurring. The pump bearings were wetted due to bubble compression, but it couldn't move water due to the closed valves at the manifold. I opened the valves, purged the circuits, and the little 07 picked up and started working right away.



So, in short (guess it's too late for that eh...) as long as the pump has water in its bearing for lubrication, and it won't be stagnated for too long, it is not a big deal. It won't lock up, won't burn the impeller off the end of the shaft, and won't explode.



Go ahead with your plans. The only thing I would add is an aquastat at the tank to shut the pump down when the tank is at temp. No sense wasting electricity.



ME

MarkPFalade

Thanks ME

I'm glad to hear that because way back in the memory banks I recall doing something similar with a tankless and thinking about this one got me wondering about that one, but now I can relax. ;-)



Price is the only reason I'm considering going with the 110 and storage. it's the old story, "We're going to sell the house and don't want to spend too much:" I figure a 177 and indirect would raise the cost a fair amount so I'm trying to work around it for the guy. The house is 3000 sq ft. 100 ft long and 30 ft wide if you can believe that. Something like 27 windows on the rear long wall. Kind of a strange one.  

SpeyFitter

Tank upstream?

A few other options to consider

1) Upstream of the TT - put an electric hot water tank set at a very low temp (if possible) like 80 degrees then use the TT to bump it up.

2) Upstream of the TT, have a S/S indirect that the boiler heats up as a seperate heating load as a preheat of the domestic water- again set it for 80 degrees, for periods of extreme demand.

3) Upstream of the TT, have a solar storage tank, but this one might not be as feasible or as reliable depending on the climate and the output of the solar.

Now having said all of the above, I don't know this boiler, but will it only allow 3 GPM to flow through it's heat exchanger regardless, or will it only produce 3 GPM of reasonable temp hot water through it's heat exchanger?

At least with the above options you don't need a seperate pump, assuming your bathrooms aren't too far from your hot water capacity.

MarkPFalade

Thanks Scott,

I should clarify, it will only produce 3 gpm of 120* water continouosly. The piping is 3/4" so it will allow much more through it but is not capable of heating any more than 3 gpm to a usable temp, so as soon as someone opens that kitchen sink faucet while you're taking a shower things will begin to get a bit chilly.