o/t backboilers on woodburners

[briany]

As I said, I omitted the heat leak rads & open vent to expansion tank - from the diagram, not the system!
I've also got pressure relief valves on the outputs from both appliances. The central heating loop will be isolated from the heating appliances in the event of a power outage - no more hot water into that loop. The appliance loops can drive water through the gravity circuit incorporating heat leak rads, venting into the attic expansion tank if necessary, and pressure relief valves as a failsafe.

 

[MdB]

Sounds like it is something you have thought about a lot

 

[Flyfisher]

Thanks for sharing that Briany, it's more or less the principle I've been thinking of using. Please report back when you have commissioned the system!

While I understand (or think I do) the necessity of preventing the active boiler driving heat into the non-active boiler, I'm less sure about why this would actually happen anyway in a gravity system. In such a system, the active boiler will heat its water, which will rise by convection and thus circulate through the heat-exchanger coil in the DHW cylinder. The DHW tank water will thus be heated (as intended), as will the water in the heat-exchanger coil connected to the inactive boiler. However, the water in the inactive boiler circuit will not circulate through convection because it is only being heated at its highest point, i.e. within the DHW cylinder. Based on this, I don't see how the inactive boiler could receive heated water.

But I'm not a plumbing or heating expert so I'm not saying it can't happen, only that I don't understand how it could.

 

[Aidan]

I'm in the throes of a similar dilemma in my house.

You only need one zone valve on each circuit since you can't get water flowing out of the circuit if there's no flow in. If they're plumbed in, I'd leave them, but I'm 99% sure the system will work with no ill affects with just one zone valve. One valve would maintain the system pressure, as opposed to the two valves isolating the radiator circuit and causing low/negative pressures as the water contracts.

I think you will have problem with;
a) reverse circulation through the cylinder coil when the central heating pump is running. The water can flow through both the boiler and the cylinder, so you'll get a mixed flow temperature by mixing water from both routes.

b) the pump thermostats starting the pump, pump runs, cold return water enters boiler, pump stops. It may burn out a motor if it happens repeatedly.

c) the boiler return temperature will get too low on starting the heating from cold.

Is the twin coil cylinder a vented hot water storage cylinder?

 

[briany]

Hmmm, well spotted, Aidan. I should have posted on here before installing the valves. I'm not worried about pressure buildup, as the loop is vented. Anyway, I can lock two of the valves open permanently and disconnect them from the leccy so they become redundant.

The reverse circulation I'd thought about, but am kind of hoping for the best on... Surely you couldn't have the water being drawn from both routes? That's essentially trying to suck the water out of both ends of a pipe simultaneously.
What I had thought (and hope) that since there would be an existing circulation flow ongoing from the boiler side, this is the path of least resistance?

I hadn't though about the on/off pump issue. Any suggestions? How about a delay timer that keeps the pump running for a set time, say 30 mins/an hour? One can get a timed relay switch that can be set for a range of timings quite easily.

Yes, the twin coil cylinder is vented. Vents, valves and more vents. I'm a bit paranoid about the whole exploding boiler thing. If it doesn't work, ah well, the whole exercise kept me occupied for a while and I learned a lot. Going boom, on the other hand, isn't fun.

B

 

[Penners]

I have a question on briany's system as set out in the diagram above. It's asked out of complete ignorance, but here goes:-

I notice that both primary circuits pump heated water from the boiler into the top of their respective coils. Is this correct? Instinctively I feel that the hot water should go into the bottom of the coil, so that the heat source is low down in the cylinder, where the cold water is. Otherwise, won't you get exaggerated layering of hot water at the top and cold at the bottom?

 

[Jon Maine]

It is quite standard to run hot into the upper end of the heat exchanger.

 

[MdB]

I guess it means that you get hot water at the top of the tank as quickly as possible when it refills.

 

[briany]

It's definitely counter-intuitive to run a hot water feed into the highest point directly, but that's what the internet (WMBO) told me to do. My understanding is that as the hot water cools, it descends through the coils within the cylinder, rather than warmer water having to push cooler water up the coils. Overall it must provide the most efficient flow.

 

[Aidan]

The radiator loop is vented; it shouldn't be. There is only a need for a primary vent at the boiler (the DHW storage cylinder has a separate vent). The nearby cold feed connection acts as a point of no pressure change, the pressure throughout the pumped circuit will change when the pump is on or off. If you have a second vent, you will probably get pumping over & air being sucked in.

You will definitely get water flowing both ways. The gravity flow through the boiler is generated by the difference in densities of the flow and return water and (from memory) the available pressure is typically of the order of a couple of hundred pascals, depending on the heights and temperatures. The pressure generated by a CH pump is typically about 10K pascals and will overwhelm any gravity circulation. An injection tee might help, but I've never used one and don't know the theory.

A timer would stop the pump on/off/on. There is also a hazard that the radiator circuit could take so much heat from the boiler that the return temperatre would drop below 50 degC or so. This would cause condenstaion at the return connection and rapid corrosion. This happens during start-up, but the condensate evaporates when the system reaches normal temperature. The radiator circuit could keep the return temperature too low continuously. I think there's also a similar concern about condensing tars in the flue system of a solid fuel boiler, creating a risk of a chimney fire.