How to make a latching timer for the hot water?

[EFLImpudence]

I completely missed that one - apologies.


Should these systems, then, not have a solution to the problem?

 

[Mikefromlondon]

Check if I understood your requirement:

You want a 24hr timer and thermostat combination , where the timer comes on once a day at a specific time, for a very short period just to knock the heating system on, then when the water temperature reaches 52c, the system switches off until it gets kicked in again by the timer next day.

Simple solution is to uses a 24hr timer that kicks in a relay that latches on through the thermostat, if thermostat is already at 52c then it won't need to kick in, as the thermostat would be open contacts, so no heating would kick in, if the thermostat is closed i.e. water temp below 52c, the relay comes on and latches on via one of its own contacts, until the thermostat knocks it out and there after it won't kick in until the next day at the same time.

An override switch (push switch) can be used to kick in manually if the water tempt is below 52c, use a mains neon to indicate status.
diagram coming in case you are confused. The system will only latch on if thermostat is below 52c, The push button must be rated to accept full rating, so bear in mind this switch must also be able to handle mains voltage and current.
Earthing is not shown for simplicity.

(Very similar to what John D described but without the drawback of heating on for 5 minutes even if the water was already at 52c, the trick is in arranging wiring and not having to use a two pole relay or another series thermostat)

You can use the Manual Push button when for example your hot water has all been used up and the thermostat contacts are now closed asking for heat, but the timer may not have come on yet and is considerably long way from its set time and you need more hot water, so pressing this manual button momentarily will latch relay on, start heating your water again, and later on when the timer actually comes on, and if the water is colder than 52c, it will start heating again, but if it is still above 52c, then the relay will not energise and after 1 minute the time will expire and no further heating would kick in until next day kicked in by the timer or manual button.

 

[ericmark]

When brewing I want to keep 40 pints of beer at a set temperature, as start the beer is often too hot, only by a few degrees, but put a fermenter of beer at 24°C in the brew freezer and set the required temperature to 19°C with the sensor on the fermenter under a sponge so not affected by the air temperature as the unit will normally fire up twice to do this, air temperature drops to 8°C at first run, then the unit switches off, and the beer continues to thermo syphon inside the fermenter and the temperature is sensed as still being high, and it fires up a second time this time cooling air to around the 14°C after which it seems to then get into a set on/off time according to ambient temperature.

I would expect the same to be true with heating, with beer the heater is very small around 8W so it does not happen with heating, but if doing it at same rate as cooling one must expect any heat exchanger to have some delay transferring the energy. It will depend on surface area of the coil transferring the energy, but it is likely it needs to run more than once to transfer the energy to maintain the temperature, the all important thing is it all hits 60°C for at least one hour, at 85°C peak it is likely this does happen, but there is an advantage in keeping the peak temperature low, so it is now common to set the storage tank cooler so not as much heat is lost.

There will be a ideal run time, as said you want it to run for around 20 minutes at a time, rather than 5 minute bursts, but also you want it to always be above 60°C so no bugs multiplying in the water, so using some thing like the InkBird 308 set to maximum differential with maximum delay and no timer as such, will ensure a reasonable run time and maintaining the water over 60°C. The InkBird Instructions seem to say the compressor delay is only active on cooling, The user manual of MH1210A seems to say the delayed start works both heating and cooling, although I have not tested. The E1000 controller is replacing the STC-1000 and has touch controls, however I think again delay is only on cooling like the InkBird with no delay on heating.

The MH1210A temperature controller at under £7 from Ebay will likely do the job, but does need putting in a box

however one must ask why does the system need altering, one would have expected to find a device like this already built in?

 

[cjard]

Should I have known that?

I'm not sure; I did say in one of my posts, before you first posted:

The boiler (a heat pump) seldom takes more than an hour to produce a tank of hot water, but I don't want a nearly hot tank at 1pm to cause the heat pump to cycle on and off ...

But This might have been missed between your successive postings?

 

[EFLImpudence]

Yes, I did miss it. I assumed it was am immersion. Sorry.

 

[EFLImpudence]

It's been stated already but surely these systems should incorporate something to prevent the pump being activated more frequently than is desirable.

 

[cjard]

however one must ask why does the system need altering, one would have expected to find a device like this already built in?

In my DHW system? It's a bit old school compared to the Mitsubishi that my father in law has. The heat pump has a command unit with a thermostat in that will do central heating/ufh.. the heat pump unit itself modulates, and in ch mode basically supplies 22 degree water by upping the temp of whatever the return is, to 22. In DHW mode it works as hard as it can to generate heat but strives for a 7 degree differential between flow and return. As it approaches 60 degrees return it dials down (but efficiency is pretty hammered by then anyway). Vaillant had an optional extra timer relay for DHW where it would run the heat pump in DHW for at most an hour then flick over to immersion to finish off but I didn't buy this, and I think it's a bit primitive. As such, vaillant don't have any sort of sophisticated hot water control- they just expect you to wire a time clock to their relay in series with the thermostat.
I suppose in a way I've recreated their system, or rather you guys have, but with a bit of refinement- my timer is two channel so daily I'll take the tank to 52 and hen once a week I'll have the other channel activate for an hour and run the immersion to hit 65 degrees to cook the legionella

And it'll all still be cheaper to run than the fancy pants Mitsubishi next door

 

[cjard]

It's been stated already but surely these systems should incorporate something to prevent the pump being activated more frequently than is desirable.

Ah, yes, that was what Eric mark was driving at (and your query to e same). Missed those, sorry

In CH, the heat pump will only activate the compressor max 3 times per hour, but in my tests with my light switch control it will respond to a call for DHW every time it's asked. It does at least cease running the compressor as the temp approaches 60 on the return, but really I'm actuallly hoping (once I've wired up these Chinese thermometers to various parts of he cylinder to get a view on how it stratifies) to see if I can get away with having the cylinder at more like 40 to 45 rather than 55; lower standing losses etc.. and the heat pump is more efficient at lower temps

I'd also love to get onto economy 10 but that seems Damn impossible these days

 

[ericmark]

You seem to be aware of the legionella and need to cook them, we had it at the batching plant I was working at, used hot water to get the correct setting time for concrete, it was mixed with cold at the batching plant and should have been in excess of 65°C in the main holding tank, however for what ever reason it did not hit the temperature for long enough.

This means you have to consider if the once a week raise is enough to kill, rather than just inhibit their reproduction? Only needs one small area which did not heat enough, I would consider the way to be safe is to always keep at 65°C from Wikipedia:-

• Above 70 °C (158 °F) – Legionella dies almost instantly
• At 60 °C (140 °F) – 90% die in 2 minutes (Decimal reduction time (D) = 2 minutes)
• At 50 °C (122 °F) – 90% die in 80–124 minutes, depending on strain (Decimal reduction time (D) = 80–124 minutes)
• 48 to 50 °C (118 to 122 °F) – can survive but do not multiply
• 32 to 42 °C (90 to 108 °F) – ideal growth range
• 25 to 45 °C (77 to 113 °F) – growth range
• Below 20 °C (68 °F) – can survive, even below freezing, but are dormant

Looking at that you want over 50°C at all times, and 70°C once a week, it may be easier to have a immersion heater to give it the once a week boost? We were sure the temperature was high enough, what caused the problem was the hot water was tapped into to feed some high pressure washers used to clean down the wagons, this resulted in the water being finely atomised, so could be breathed in. Some where in the system was a cool spot, where it could survive.

The other method is the UV and circulating pump, may be the more modern systems have that built in?

 

[SimonH2]

At my last place (candle manufacturer) we used water for heating and cooling the moulds - we had something like up to 20,000 gallons of lukewarm water in storage - and hence regular dosing & testing for legionella control. At one point (as the story was told to me), the factory manager questioned the cost and "do we need to spend that ?". The maintenance manager only had to say "Forum 28" and got an instant signoff on the order - Forum 28 being not very far from us.