Air in radiant heat system questions

WarmJames

I have an infloor radiant heat system I installed about 30 years ago, that works great.
It uses a 200K btu natural gas copper fin boiler.
I also have a lot of excess solar power so I wanted to supplement the gas boiler with some electric heat.
So I plumbed in a 4.5kw spa heater element in series with the gas boiler.
And the system works, however, I can hear the electric element boiling some water (hising) and I was wondering if this could be introducing air into the system?
So my questions are, is this a concern and aside from loss of heat transfer efficiency what is the problem with air inside the system?
I do of course have an air seperator.
I see there are commercial electric boilers available, how do they deal with this?
Thanks for your thoughts in advance
James

EdTheHeaterMan

My guess might be that the air is actually made up of steam bubbles Not Air Bubbles. Do you have a limit control on the electric resistance heater to stop the electrical power from overheating the water and making steam?

WarmJames

No I dont, I tried installing a triac style dimmer, and it worked but it threw off so much noise in the line that all audio devices picked up the hum like crazy. Then I tried a timer and cycled it on for 1 second and off for 1 second and this seems to work but obviously reduces efficiency by 50%.
What kind of limit control are you thinking about?
Are steam bubbles bad for the system?
And if so is it ferrous metal corrosion we are worried about or what?

kcopp

What is the element in? In line? how is it controlled?
You may be better off having it in a tank.

hot_rod

Got enough flow moving across the element? If it is in a small container you may not have enough flow to scrub away the heat?
Also the elements quiet down when they some mineral build up.

look at the ratings on the triac to see if and what resistive load it is designed for.

cycling the element on off isn’t changing the efficiency, it’s still near 100%

The cycling is just changing the heat input %

Consider parallel piping.  In series some of your electrical energy is going up the flue of that unfired copper boiler. Possible a lot if it doesn’t have a flue damper. The unfired boiler becomes a cooling tower😗

WarmJames

its inline, like here:
https://www.spaguts.com/heater-assy-4-5kw-220v-flo-thru-vita-spas-17-63in-x-2-0in-26-1008-6s-k-spl-1771?gclid=EAIaIQobChMInPaix6uO_QIVOButBh2KhQRHEAQYAiABEgLx2PD_BwE

and controlled with a PID temp controller

WarmJames

hot_rod said:

Got enough flow moving across the element? If it is in a small container you may not have enough flow to scrub away the heat?
Also the elements quiet down when they some mineral build up.

look at the ratings on the triac to see if and what resistive load it is designed for.

cycling the element on off isn’t changing the efficiency, it’s still near 100%

The cycling is just changing the heat input %

Consider parallel piping.  In series some of your electrical energy is going up the flue of that unfired copper boiler. Possible a lot if it doesn’t have a flue damper. The unfired boiler becomes a cooling tower😗

The triac was rated for 80 Amps so plenty of capacity,
I could also go with smaller heating element I suppose but prefer large as I can.
The electric element is after the gas boiler water flow which does have a operable flue damper.
Would having the heating element inside of a tank help? (is this what commercial electric boilers do?)
The water flow rate would be the same either way..

EdTheHeaterMan

Looking at this picture there appears top be a location for the limit under the wing nut. (HaHa We used to call a friend in grade school WingNut) Perhaps there are limit controls that fit there, just to reduce the chance of overheating the spa heater.

WarmJames

Yes I have installed a PT100 temp probe connected to a PID temp controller driving a Solid state relay so it turns off the element when it reaches ~136F

EdTheHeaterMan

Looking at the spa parts website the limit control or thermostats appear to be wired back to the spa control using low voltage. That may be a problem to integrate into this application.

Perhaps an electric water heater lower thermostat may be a better choice. Not really sure how to do that though. What is the differential and how fast does it react to the temperature change? Answers to those questions may render that option insufficient for your purpose. Or it may be the perfect choice. Getting the flow rate and the kW adjusted and properly dialed in, is the key to getting this right. The Limit device should only be a safety backup.

hot_rod

since since that has 2” connections and a small diameter tube indicates you want so good flow through it. A spa pump might move 15 gpm or more through a heater like that
what is the flow rate through your system? Is it zoned?  I’ll bet 2 gpm would be enough flow to move 15,000 btu and keep from boiling on the element surface.
4500x3.41= btu

Usually you have a flow switch on an electric heater like that. Element only runs when the pump is running

Even with the electric element after the gas boiler, you are still flowing through the boiler and losing a % if the energy you just payed for.

why do you need a triac control at all? Seems like that 4.5 Kw is a tiny fraction of the 200,000 gas boiler, can’t you use full output? A simple snap disc water heater control could act as both temperature control and high limit safety

ive homemade a few small electric boilers over the years

Here is one of my latest an element in the bottom of a hydraulic separator

And yes I did burn up 1 element when the circulator quit and had no flow across the element. This is where I discovered some flow is required to protect the 4.5 kw element 

If you had a 6 gallon or more tank capacity you could probably run without any flow. That how electric water heaters work👀

Those small 2.5 gallon water heaters can only get away with 13- 1500w elements before the get too hot, frommore trial and error research😂

WarmJames

I do have it wired in with the gas boiler flow switch so it cant run without water flow.
I dont think temp regulation is a problem on my setup
if these are steam bubbles like EdTheHeaterMan is suggesting then maybe I dont have a problem..
if introducing steam bubbles into the system isnt a problem (they re-integrate themselves into the water down the water flow line) then maybe my concerns are unfounded?

hot_rod

If you have a good air puger, no need to be concerned. Best would be a microbubble type purger just downstream of that heater, it will catch even the smallest bubbles, probably on first pass.

What type of radiant do you have that you are running 136F?

WarmJames

I do have a micro bubble air purger like this
https://www.resideo.com/us/en/pro/products/water/hydronic-solutions/components-accessories/air-vents/1-1-2-in-npt-connection-supervent-air-eliminator-pv150-u/
downstream of both boilers.

You think 136 is too low?

I run the boiler heating loop at 136F then the 3 zone pumps have mixing valves set 105F which makes the floors ~80F which is about right as I am in northern California and its pretty temperate here.
Very well insulated house, seems to work..

hot_rod

That makes sense that you have a mixing valve. I assumed you were sending 136 to the floor. That air purger should keep the system air free