Cold water running hot

mackerski

Hi folks,

Perhaps somebody has encountered this before. I live in a duplex townhouse with no attic. Therefore my water tanks are in the hot press, stacked above each other, hot below, cold above. Pressure to taps is delivered by pump. The classic apartment plumbing setup AFAIK.

I find that once my central heating is in operation (which also has the effect of heating the max amount of water in my hot water cistern), the cold water from the tank (bathroom sinks) begins to run warm or hot. I'm assuming that this is because of conduction from the hot water in the tank below, but surely there should be insulation to guard against this. It seems a shocking waste of energy (and it's not nice rinsing your mouth with hot water).

Is this a flawed installation, or does this kind of messing just go with the territory?

Dermot

Mellor

If both tanks are in the hot press, when the CH is on the the cold is going to warm up over time as the air in the hot press will heat up through convection.

crosstownk

Mellor wrote:

If both tanks are in the hot press, when the CH is on the the cold is going to warm up over time as the air in the hot press will heat up through convection.

Yes, it will have some heating effect on the water above. But the cold water should not be 'hot' - luke warm at best - surely the insulated hot water cylinder should retain its heat. I'm not at all familiar with the set-up in a duplex house - so maybe I'm mistaken. But the OP is right, it is a waste of expensive energy. Especially as water requires quite a large amount of energy to heat.

DublinDilbert

Hi,

Do you have an electric emersion?? do you use it?

if the thermostat is broken on an electric emersion, it will cause the hot tank to boil, when this happens the hot water cylinder will "vent"... it will generaly "vent" into the cold water storage tank, if this happens for long enough it will cause the cold water storage tank to get warmer and warmer...

it may not be what's happening, just a theory... i think you mentioned that this happens when the central heating is on, is there any chance the emerssion is turning on at the same time??

vallo

Is there a chance that your hot tank is overheating and that an overflow from the cylinder is feeding into your cold tank - that is how many houses are plumbed but I don't know much about apartment plumbing.
If it is a pumped system, there is probably an expansion chamber on your hwc which would rule this out.
A simpler possibility is that the water in your tank is really cold, but it passes through pipes that are adjacent to the hot central heating pipes and that just this water is being warmed. If this was the case, it would only run warm for a few minutes.
If the hwc really is heating your cold water tank then you need to get yourself a new cylinder with better insulation!

Lex Luthor

vallo wrote:

Is there a chance that your hot tank is overheating and that an overflow from the cylinder is feeding into your cold tank

this is what I would have thought also

mackerski

Some good stuff here. Let me address some of it, and the rest I can look into. The tank does have an immersion heater, though I only use it in seasons where the central heating isn't in use. From recollection, this problem may be happening when the immersion is in "bath" mode (with no central heating on), though that's not for definite. If so, it would put a dent in the theory that it's hot heating pipes to blame. Though I can test this by lifting the lid on the cold tank and testing the temperature (before the water could hit any hot pipes).

The notion of a broken stat is a nice one for an immersion-based problem, though it doesn't really apply here. However, can anyone tell me what's supposed to stop the central heating from overheating the water? It that a simple case of the circulating heating water no longer being any hotter than the tank water?

Dermot

crosstownk

mackerski wrote:

However, can anyone tell me what's supposed to stop the central heating from overheating the water? It that a simple case of the circulating heating water no longer being any hotter than the tank water?

Dermot

The central heating boiler will have a stat fitted (usually unadjustable) to it which will limit the temperature of the water circulating through the system. For the sake of economy, you should have a thermostat fitted to your HWC which will shut of the CH supply once a certain temperature in the HWC has been acheived.

I would imagine that in a duplex the DHW expansion pipe is shorter than in a regular house so the chances are that if you have very hot water in the HWC it will 'expand' or 'vent' into the cold storage tank a lot sooner.

Something like this may alleviate the problem > http://content.honeywell.com/uk/homes/Catalogue/Heating%20Controls/2.9%20L641.pdf You will also need a 2 port motorised valve like this > http://content.honeywell.com/uk/homes/Catalogue/Heating%20Controls/2.19%20V4043.pdf The parts will be less than €60 from C&F Quadrant in Ballyfermot - labour depends on your plumber.

DublinDilbert

Hi Dermot,

When we were talking about the thermo-stat being broken, we were talking about an internal thermo-stat, thats fitted to the immersion element, and will cut the power to the immersion element when the water temperture in the tank reaches 60 degrees or so...

The water coming out of your gas boiler should not be warmer than 60/70 degrees, if it was any hotter than this it could cause some one to have a bad burn off a radiator.

mackerski

DublinDilbert wrote:

Hi Dermot,

When we were talking about the thermo-stat being broken, we were talking about an internal thermo-stat, thats fitted to the immersion element, and will cut the power to the immersion element when the water temperture in the tank reaches 60 degrees or so...

The water coming out of your gas boiler should not be warmer than 60/70 degrees, if it was any hotter than this it could cause some one to have a bad burn off a radiator.

Sure, I got that. We can exclude the immersion for now (though I'm still not convinced it doesn't have its own problems). But I'm trying to reason why the tank water won't get overheated by the central heating. So far, I'm assuming that it's down to the fact that the boiler water at 60 degrees can't raise the tank water to any higher temperature, placing an upper limit on the energy usage and removing the requirement for a stat. But that still doesn't explain the symptom...

Dermot

mackerski

Right, an update: the water is already hot in the cold water cylinder. So any explanations relating to heating pipes outside the hot press aren't relevant. It's either simple conduction (crappy insulation) or some kind of preventative discharge from the hot tank.

Time for more digging...

Dermot