A Hot Water heat question and hypothetical solution...

Jamie Hall

As most of you are aware, I'm a steam guy. Not so much hot water. However, one of the buildings I care for has hot water heat, and the owner wants to save money on the heating bills. Can't blame her.

The situation as it stands. A moderately old, but not old enough to change, hot water boiler, oil fired. Running about 80% last time it was cleaned. It is controlled by an aquastat. The thermostat controls the one and only circulating pump. No primary/secondary on this one! It is not a condensing boiler.

The problem is that if the house temperature is set below about 50, there is a good chance that one of the radiator return lines will freeze up while the pump is off. It has, and it was a colossal bore to get it flowing again.

The system is piped for parallel flow, not reverse return.

For various reasons -- some of them better than others and mostly related to the quirks of an older house (try 1789 for the original...) but some to the desires of the owner the heat has to be kept on at a low level -- while the domestic water in much of the house can be drained, not all of it can, and it sometimes is desired to open the house in the dead of winter (I did once last winter, at 15 below...).

So... my idea is to modify the system to be primary/secondary and keep the secondary running at a constant flow (zoning could be done, but seems to be overkill in the application). Probably use an outdoor reset to control the temperature of the constant circulation supply (as I envision it, the ODR would fire the boiler and the primary loop to maintain), but with a low temperature aquastat on the return from the secondary side, which would also fire the primary and the boiler if it started to drop too low -- say below about 40 -- as a backup.

I really don't want to go to antifreeze if I can help it.

Am I completely crazy? Or only sort of crazy? And if the latter, suggestions will be most gratefully accepted... For that matter, suggestions will be gratefully accepted even if I am completely crazy...

STEVEusaPA

Why not just some heat tape?

Jamie Hall

STEVEusaPA said:

Why not just some heat tape?

I'll take a look at that -- but the pipe that froze last winter is buried in a wall. An outside wall, of course...

HVACNUT

That's a good idea, and is often used. The better ODR controls have freeze protection to run the secondary circ. Most cut in at 40*, but can be adjusted.

Noel

Constant circ with a thermostat controlling the burners mimics gravity hot water and works very well. Because of the large volume, I would skip the ODR control, because the water temp is going to vary based on indoor temp quite nicely, Jamie.

TomS

A real simple solution is a device called Therm Guard. Its a small device with two wires which connect across your thermostat. You can program it to come on at any number of 15 minute intervals for periods of 1 minute or more. So you program it what you feel comfortable with - say to come on every 30 minutes for a period of two minutes. http://www.bearmountaindesign.com/

Boon

ThermGuard says it turns the boiler on, which I don't think you want. Freeze protecting/Exercising the pump would save energy compared to constant circ.

I remember seeing a zone control with a freeze protection option that ran the circ once or twice an hour for a few minutes - and now I can't find it so maybe I'm making that up but I don't think so.

Leon82

> @Boon said:
> ThermGuard says it turns the boiler on, which I don't think you want. Freeze protecting/Exercising the pump would save energy compared to constant circ.
>
> I remember seeing a zone control with a freeze protection option that ran the circ once or twice an hour for a few minutes - and now I can't find it so maybe I'm making that up but I don't think so.

The taco sr503 has a jumper for it

JUGHNE

Jamie, here is an old school idea you might like:
Intermatic Model C8865, repeat cycle timer, SPDT 20 amp contacts.
Put 120VAC to the motor and the timer runs constantly.
Install trip pins, (60 slots available) to run your existing pump.
Install outdoor Tstat set at maybe 35-40, in series to run the pump for however many minutes per hour as you wish.
Maybe 10 minutes twice an hour when ODT calls.

The timer NO contacts and outdoor Tstat in series connect in parallel across whatever pump control you have on the boiler now.
All line voltage. Just an idea.

Gordy

I agree with @Noel comment. Also @JUGHNE idea will save some juice.

Jamie Hall

Thanks guys! See what I can rig up here -- probably along the lines of @Noel 's comment, and I have one of the Intermatic's @JUGHNE mentions to throw into the pot I'll let you all know what I come up with -- and how well it works.

Harvey Ramer

You could also install a 2 stage ETC with the probe measuring one of the coldest spots in the piping. Set it to 35 or whatever you are comfortable with, and a 3 or 4 degree differential. First stage activates the pump, second stage activates the boiler. Simple and efficient. Just have to get it placed right.