Simplest Pressurized System with sidearm DHW

[Medman]

One of the issues I have been wondering about while changing over my system is the effectiveness of using the sidearm for DHW with the Simplest System.
Last year, with no storage, I flowed hot water in the loop to the house, through the sidearm, and back to the boiler at all times the boiler was hot. With the Simplest System I only want to flow water in the loop when a zone calls for heat. The rest of the time, heat will be flowing in the near-boiler piping to storage.
I'm thinking that with the sidearm's low rate of transfer the DHW zone will often be calling for heat, and thus not much heat will be going to storage.

Anyone know what the approximate transfer rates for sidearms are?
If I switch to a flat plate hx with a circ. on the DHW side will it work more efficiently?

Your input is appreciated. I have all the piping apart now, as I was fixing leaks and changing things over anyway. Should I look at switching to a flat plate hx? I'm a little concerned over cost.

 

[kabbott]

Any chance you can setup a loop from storage tank to side-arm that will thermosiphon?
I like the way my side-arm has performed so far--- keeps up with family of four. I run the loop from boiler through side-arm whenever boiler is hot like you.
I would like to change this so DHW can be heated from storage when boiler is not in use (shoulder seasons ) Trouble is I use open storage system, I
have not come up with a plan I am happy with YET. A separate pump for the side-arm is a poor plan IMO because of the low transfer rate the pump
will run far to much.

Tank in a tank (DHW tank in storage tank) Is an option with open storage, not easy to do with pressurized system.

 

[Gooserider]

I tend to agree that the sidearm might not be the most efficient approach, especially in the shoulder season when other zones might not be calling for heat as often. A pumped flat plate would probably do a bit better in that regard, but will require some additional juice and control circuitry. I think my choice might be to go for a coil in the DHW approach, or some other "tank-in-tank" variant, plumbed so that you get flow through the tank coil if any zone calls for heat, or so that the tank alone could put out a call if needed. Be sure to use flow checks and / or pipe configurations to prevent heat from thermosiphoning out of the tank. If sized appropriately, a tank coil can transfer a lot of heat quite quickly, and should keep the DHW tank well charged on zone calls, or charge quickly if the tank has to make it's own call.

The other key thing would be to design the DHW tank to super-heat up to whatever it will take out of the main loop, and then use a tempering valve to bring the faucet temp back down to safe levels - this will get you the maximum amount of hot water possible out of the tank before you need to worry about heating it back up.

Gooserider

 

[Medman]

I already have folw checks and the tempering valve in place. Last season I heated the tank to the max loop temp, which worked well.
The tank is too far removed from storage to thermosiphon. This is the issue; I do not want to flow boiler water 140' to the house and back again if no zones are calling for heat. Storage is at the boiler in my shop, and I want to keep the heat in the tank as long as possible.
I haven't looked into this, but is it possible to get a tank with a coil in it and electric elements?

 

[kabbott]

I already have folw checks and the tempering valve in place. Last season I heated the tank to the max loop temp, which worked well.
The tank is too far removed from storage to thermosiphon.

Sorry I thought the tank was in the house.

This is the issue; I do not want to flow boiler water 140' to the house and back again if no zones are calling for heat. Storage is at the boiler in my shop, and I want to keep the heat in the tank as long as possible.
I haven't looked into this, but is it possible to get a tank with a coil in it and electric elements?

This makes sense for the heating season when your zones will be calling for heat often. The trade off is on warmer days when your zones are not calling for
heat as often the DHW will heat via the electric. This is also when you are likely to have a fully charged storage tank.......heating nothing.The simplest is
not always so simple.This is why I went with P/S system, the cost of pumps is nothing compared to ease of control.

You are in a much colder climate than I am so this may not be an issue, especially if you have a large DHW tank.

I think you can get a tank with 1 or 2 coils and electric backup normally used for solar applications. The tank will be pricey!

 

[Gooserider]

I already have folw checks and the tempering valve in place. Last season I heated the tank to the max loop temp, which worked well.
The tank is too far removed from storage to thermosiphon. This is the issue; I do not want to flow boiler water 140' to the house and back again if no zones are calling for heat. Storage is at the boiler in my shop, and I want to keep the heat in the tank as long as possible.
I haven't looked into this, but is it possible to get a tank with a coil in it and electric elements?

Glad to know you already have much of the setup in place - I wasn't sure so I figured a bit of redundancy wouldn't hurt...

Far as I know you can get tanks w/ coils and electric elements pretty easily, as they are quite common in solar and some fossil burner setups, but as mentioned they tend to be pricey - partly because the people selling them can get away with it, and partly because it's a bit harder to manufacture that sort of tank.

Gooserider

 

[Medman]

Thanks for the input. My intent was to set up the DHW sidearm as a zone, so that when the tank sensor called for heat the loop circ would activate and heat would flow from boiler/tank to house, through sidearm, and back. I never thought to have the sidearm in series with the other heat zone as well, so that any time the house called for heat the water would flow through the sidearm also. This may be a way to go for now, and if it doesn't work I will have to look at a flat plate hx for DHW with another circ on the DHW side. A flat plate and small circ. is still cheaper than a new tank, I think.

I know that the DHW tank I have is routinely in the 160-170* range when the sidearm is running. 40 gallons of 160* water lasts a long time when tempering it to 120* at the outlet, and we never ran out of hot water last year. I guess I'll give this configuration a try for now and see how it goes.