Appropriate Relay for 4500W Water Heater?

Steamback

I'm installing a radiant heating system which will use 2x120 gallon hot water heaters as the heat source. These tanks are going to be heated with solar and will get up to 160-170F, but we'll also have to keep them at about 115-120 as backup heat using the internal elements.

The problem is the radiant is a low mass wood floor system and heat calculations show that sometimes I'll need a radiant supply temperature of 130 up to maybe even 140. There's no way I want to leave the water tanks at 140 all winter to satisfy all heating loads, and I don't see the client wanting to manually adjust the tank aquastats every time it gets too cold outside.

I'm looking at ways I can use a boiler control with outdoor reset to vary water temperature from say 110 to 140 in the water tanks as outdoor temperature varies. The cheapest I have seen that I think will work is a Tekmar D 256. Or at least I think it should.
http://www.tekmarcontrols.com/products/hvac-systems/256.html

But I'll need a 30 Amp 240VAC relay for each heater, activated by the boiler control contact closure on whatever boiler control with outdoor reset I end up using. Are relays like this my best option?
http://www.mouser.com/ds/2/357/199 Spec Sheet-165.pdf

I see I can get relays like above in 24VAC input coil voltage, but see requirement like minimum 1 Amp at 24VAC. Do common 24 VAC doorbell transformers supply 1 Amp? And, do HVAC relay boards like an Argo AR822 with built in transformers supply 1 Amp?

Thanks for any input on how to do this.

ratio

What you want to switch that kind of load is a contactor, not a relay. While they are available with 24V coils, you could be looking at 50-75 VA inrush current, each. I'd give some thought to 120V coils on the contactors, even if that means an additional control relay.

EBEBRATT-Ed

I agree with @ratio a 30 amp contactor with a 120 volt coil. You can use a relay with a 24 volt coil to break the 120 volt circuit to the contactor. 4500 watt element will draw bout 19 amps at 240 volt

ratio

Personally, I'd use contactors like this, controlled by something like this, switched at 120 (or 208/240) but there's certainly cheaper ways to do it, and quite possibly better ways—I don't do lots of hydronics, this is just a quickie. If you find low volt controls that are just the right thing, you could use a RIB relay, but TBH nearly any 24VAC relay should be fine for that kind of pilot duty.

Zman

To add a layer of complexity, your hot water heaters each have 2 elements. They are setup so that the top element needs to satisfy before the bottom will kick in. Normally they don't ever both run at the same time, the stock wiring will not support that. You will need to decide how that will sequence as well.

vibert_c

@glynnbebee Please elaborate further on what you are trying to accomplish. What are the other heating loads?
A door bell transformer is designed for a duty cycle of .5 seconds. You can't use this unit in your proposed service. More information please.
vibert_c

hot_rod

If you look under the hood of the modern electric boilers, or electric tankless heaters they use small solid state relays to control the elements, this allows you to modulate output inexpensively.

JUGHNE

FWIW doorbell transformers are only 10 to 16 volts. Although I did find one 16 volt running a 24 volt gas valve.
My go to contactors for 240 has always been AC contactors. Most rated 40 amps of inductive load. I use the leftover defrost timer boxes for mounting them inside.

Steamback

Great. Thanks everyone. I did some reading online about Contactors and agree it's a good way to control the elements on the water tanks.

Regarding Zman's comment, these tanks are actually solar storage tanks with single 4500W elements each, not 2 each.

Regarding the transformer concerns, I'm guessing it's fine to use the 24V supplied by an Argo board and use the contact closure on the Outdoor Reset Board only, assuming the boiler control I get doesn't supply it's own 24V.

Regarding Vibert_c's question, I attached a very basic diagram. The only other heating loads are the DHW needs, hence a basic inability to just leave the water tanks unheated for maximum solar temperature gain on the tanks. If I didn't have the need to combine the DHW and solar radiant pre-heating, the need for Contactors and Modulation of the water tank temperature wouldn't be an issue.

I know in theory I should be looking at extra heating equipment (and adding more cost) to make a more ideal system, but this is what I inherited, so I'm starting at what might be the best compromise (modulating tank temp based on outside temp). I sold and installed the solar preheating package before the final heating system was figured-out...the heating system was going to be done by a different contractor. Now I find myself installing the heating system and am working with what we have...2 big water tanks.

The owner has a HUGE PV-electric array and has a big energy surplus, so electric heating for DHW and space heating is no big deal. I'm thinking my next option to look at is adding an electric boiler as a backup on the radiant loop, setting it to 140, and not heating the water tanks at all so that we get the best solar hot water efficiency. But that opens up a few cans of worms, like needing to worry about the times when the tanks might be colder than the radiant return loop, hence the system as shown would actually be robbing some heat from the radiant loop via the plate exchanger. And I'd also need to probably add another small tank for DHW, possibly heated through the electric boiler, which would require a tank with a heat exchanger and some controls. This is really the reason I decided to start with water tank temp modulation first as an idea...

Anyway, thanks for all the help.

Steamback

PS - @hot_rod, Do you mean if that if I used the correct electric boiler on the Radiant loop to provide the backup space heating, then I could interface outdoor reset directly to that electric boiler? Would that require finding a boiler that had integrated outdoor reset, or using an external control with its' own outdoor reset?

This diagram I attached now is much more what I have been thinking is necessary given the situation...solar tanks remain unheated but boost radiant loop temps before the backup heater as much as possible. Since the client has a big PV array and excess electric via net-meetering, I'm not shy about suggesting they use a separate electric water heater for DHW.

I know some are wondering why this isn't a drainback solar system. Long story, but yes I would have preferred drainback. Luckily I have had great luck with Steamback solar thermal for years now, and see it as a very viable alternative from an installer's point of view.

Steamback

PS - hot_rod suggested that I could find an electric boiler which could modulate by temperature outside. Does this require I find the right electric boiler, or any electric boiler could be used?

I have been thinking that the attached picture is more like what I should be building. It adds cost, but the efficiency should be a lot greater even after considering initial investment...right?

hot_rod

Many of the electric boilers either come standard with ODR or they have a model with ODR available. Here is one I recently used.

http://www.thermo2000.com/pdf/en-US/specs/MiniBthUltra-En.pdf

Thermolec is another Canadian brand and ElectroMN out of Minnesota, Siesco is also a modulating, I've used all of these brands over the years.

Any electric boiler could be converted into a ODR modulating, a control to run it and some solid state relays to operate the elements, probably cheaper to buy a model with the control built in, modulating electric boilers are much less $$ compared to gas or LP.

JUGHNE

140 for radiant floor heat is pretty warm, would you have a mixing valve to feed that tubing?

hot_rod

There are a bunch of combi system piping arrangements in here. If they show gas fired boiler backup, electric boilers could be substituted, same control and piping logic. Your drawing is a bit more complicated than need be.

https://www.caleffi.com/sites/default/files/coll_attach_file/idronics_6_0.pdf