hot water calorifier tank has burst running off back boiler what did I do wrong?! Need advise!

[Happymeals]

I have a morso squirrel with back burner which we are running a calorifier 94L and small towel rail off. We found hot water to be scalding while the room wasn’t that warm. Stove was brought second and Realised we had quite a bad leak in the back boiler that must have been leaking inside the log burner. We were having to top up the system everyday with water. Would this have made a difference to the heat in the room? 

 

Anyway the hot water tank has now burst at the welded seam even though we had two pressure relief valves on it. We have no idea why our system ran too hot? We were using the stove sensibly running it with a shovel of coal at a time not over loading it. Should we have had more heat sinks in the system? 

 

We are wondering how to proceed.

thinking of temp using the log burner unconnected and dry?  Is this dangerous?

Don’t know wither to try again with another back boiler or to just remove and try and think of a better way for heating and hot water.

[Tony Brooks]

I assume the "hot water tank" is a calorifier.

 

The boiler water should only flow through a coil in the calorifier so if a seam burst  its more likely to be domestic water pressure related. I also don't think you will have 2 PRVs on the domestic circuit but may have one on that circuit and one on the boiler. One of those PRVs might be a mixing valve.

 

Likely suspects would be having no expansion vessel on the domestic hot water, faulty domestic pump pressure switch, PRV jammed shut.

 

I am not going to comment on running the log burner without water in the back boiler because depending upon the stove it may ruin the back boiler. If that is a risk you are willing to take have  a low fire with open  connections on the back boiler so any water in the boiler can evaporate.

 

A water leak into the fire would reduce the heat it produced but by how much depends upon the size of leak.

 

If the domestic water got too hot I suspect the stove may have been run too high, it has an air leak into the stove, or it ran away.

 

 

 

 

[Happymeals]

Hey thanks for your reply Tony.

yes sorry hot water tank - calorifier.

It was gravity fed so no pump and we had an expansion tank on the domestic hot water. We did have two PRV on the tank itself as we installed ourselves so seems so strange they both failed? One pressure and one thermo I think? We do have a pump for the cold water which obviously replaces water in the hot water tank but that was working fine. 
 

I don’t think there was an air leak into the stove as it was always a bit hard to get going and never burned that well (may be due to the damp leak).

 

it seems so strange as to why it failed we even got someone to come and look at it before it burst and he said it looked fine. The water temp in the calorifier was obviously too hot which built up pressure but no idea why it burst before the expansion tank and PRV were activated? The system was brand new so shouldn’t of had anything blocking water flow

[Tracy D'arth]

Something does not ring true here. 2 prvs, one thermo? one pressure? suggests a sealed system to me.  Expansion tank connected to what?  What do you mean on the domestic hot water?

[Tony Brooks]

If you fitted the PRV did you ensure the pressure rating was within the specifications for the calorifier? I would expect it to be less than 3 bar and maybe only 2.5 bar.

 

I was talking about the DOMESTIC water pump, not a central heating pump.

 

I think your understanding of how the systemS work together may be a bit lacking. I don't understand why you would put a "thermo" PRV on a domestic water system. I can understand why you would fit a thermostatic mixing valve on the hot outlet so hot and cold water are blended to supply a safe temperature. I can understand why you might fit one to the coil circuit connected to the back boiler but with a suitable expansion/vent pipe I don't see the need for one.

 

What pressure is the PRV on the hot water outlet? Do you have an expansion vessel on the domestic hot water system? Is there a non-return valve at the calorifier cold inlet? Do you have an accumulator? Although that should not have any effect on your problem if there is a NRV.

 

I am beginning to suspect the whole thing may not be correctly plumbed. Can you post a diagram and/or some photos of the calorifier and surrounds.

5 minutes ago, Tracy D'arth said:

Something does not ring true here. 2 prvs, one thermo? one pressure? suggests a sealed system to me.  Expansion tank connected to what?  What do you mean on the domestic hot water?

 

Yes, something very odd - it he has fitted a sealed system he must like living with a bomb. I am beginning to suspect we may be dealing with a house type installation by someone who simply does not understand.

[Rose Narrowboats]

I have seen successful pressurised installations on boats, but I can see no advantage and lot more complication/disaster potential compared to a conventional open to atmosphere system.

 

One towel rail is unlikely to be enough to absorb the heat output from a Squirrel boiler which is approximately 2kw. Once the calorifier is up to temperature then it is absorbing no heat, so you need to be able to absorb the entire boiler through the towel rail, pipework and radiators.

 

Where did the hot water tank/calorifier come from? If it really was a domestic hot water tank it's no surprise it split.

 

You will damage the boiler if you use it dry. I've heard it said that filling with sand will allow to use it, but I've never done it myself to a boiler I've subsequently put water back into, so try that at your own risk.

Edited February 21, 2021 by Rose Narrowboats

[Jen-in-Wellies]

Is it a proper marine calorifier that you had, or a household hot water tank? Marine cauliflowers are designed to work at higher water pressures than open vented domestic tanks. They are more expensive than a domestic tank, so people are tempted to use the wrong sort. Did the calorifier have an expansion tank connected to it? This is not the same as an accumulator. No exppansion tank will lead to the pressure spiking in the cauliflower. The PRV is supposed to bleed off any excess pressure, but if these fail, then that can split the tank. It is bad practice anyway as it shortens the tanks life.

Jen

Edited February 22, 2021 by Jen-in-Wellies

[Ex Brummie]

What Jen said. Also, if not a marine calorifier, many water supply pumps will overpressurise  it.

[Tony Brooks]

Just realised the heat sink for the stove is nowhere near large enough once the calorifier is hot. The back boiler output is far too high for the OP's needs. he must be running his stove hard to try to warm the boat so is getting the domestic water very hot but I very much doubt that is the cause of the calorifier failure.

 

Plumb a few rads onto the gravity flow system and then you will have a better balance between the heat dump for the back boiler and the desired direct stove output.