Heating - auto-fill

[Onetap]

Thermal store, I believe. Unusual; ignore most of the above. Make sure that immersion heater is off. Can't stop.

[Mike Buckley]

Now I'm more confused.

And there is no immersion heater.

[Onetap]

in principal to this;

's full of boiler water at 80 degC or so, as against a hot water storage cylinder that contains potable hot water at 60 degC. That's why your CH pump draws water from it.

It heats domestic hot water by passing it through an internal pipe coil. The blue thing is a mini-expansion vessel. Hot water should be at mains pressure (the main advantage), although there may be a pressure reducing valve.

Someone will ID it shortly and you can get the manufacturer's details on-line. The round black thing (middle, bottom) is an immersion heater. HOneywell NF-50 isn't a boiler I've heard of. Any pics of that?

[Onetap]

Ideal Classic NF50? The last reliable Ideal made, allegedly.

[Mike Buckley]

Yup, it's stickered both as a Honeywell and an Ideal Classic.

[Mike Buckley]

Interesting. No makers label I can see. All sounds expensive if it goes wrong.

Does all that explain why no pressure gauge anywhere?

The thermostat on the side of the cylinder is set at 70 degrees or so (left it in position from when we moved in).

It can't be that unusual, at least not round here. Our house is one of a massive estate built[1] by the same builder (David Wilson homes) around

17 years ago.

Any idea what the mini-earthquakes might have been?

[1] Gamston, Nottingham.

[Mike Buckley]

Eh? So if the cylinder is cold there's no hot water? An internal coil suggests that the cylinder needs to be hot before any water can be heated.

Might explain a general lack of hot water after a longish shower when the tank is massive.

[Onetap]

Yes. A heat bank, I think is the correct term; thermal stores have plate heat exchangers. The tank in the loft is probably a 4 or 8 gallon feed and expansion tank. You should be able to sketch a schematic to figure it out. I'm not sure what the other pump does) unless there is a plate heat exchanger. There's usually a thermostatic mixing valve on the hot outlet because it can come out excessively hot. They don't go well with condensing boilers, so it may become redundant when the boiler expires and you need a condensing boiler.

idea about the earthquakes, it sounds like water boiling, pump failing, but it would take a while on site to figure that out.

[Onetap]

I just realised; one pump sends water through the boiler while it's running until the thermostat on the cylinder shuts it down. The other would send water around the heating system while the timeclock is calling for it, until the room temperature sensor is satisfied. I don't know why it would be marked as hot water unless I've got it wrong.

It would be worthwhile to lower the thermostats to see if one stops the earthquakes.

It doesn't need a pressure gauge, it's open vented.

I'd hazard a guess that the boiler pump is defective or there's inadequate flow for another reason, and the water is boiling on it's way through the boiler. There's not much else to go wrong with it.

Check there's water in the tank, it should just cover the outlet when the system is cold, allowing space for expansion. Check whether inhibitors have been added, there should be a sticker on the boiler.

The radiators would need venting if the top is cold.

[Mike Buckley]

Tried lowering the thermostats both on the cylinder and the boiler, initially had no effect but not had any earthquakes for 24 hours now. I'm not 100% sure the boiler was lit while the earthquakes were happening, they were certainly terminated by a definite "click" from the airing cupboard but I never got to find out where that was coming from.

How to check water in tank? No sight glass. No stickers on boiler for inihibitors, but I've checked all rads and they're hot to the top.

Thanks for the info.

[Onetap]

No, I meant the loft tank. It probably has a plastic drop-on lid. I also meant turn down the thermostats if the rumbling happens again. You may hear the click if you do operate them. The heat store cylinder may be heated by cheap-rate electricity by the immersion heater. The click could be a contactor opening to shut down the immersion heater..

[Mike Buckley]

I hope not, will check

Cold water tank as far as I know is just there to balance the pressure as per usual, has a drop on lid.