Controlling DHW to reduce gas consumption

[InertiaM]

Following on from the Controlling Appliances thread, I've been thinking about how to reduce gas consumption in our house.  We have a condensing boiler with Megaflo unvented system.

It occurs to me that in the summer months when central heating is not required, the DHW could be switched on later as all the heat generated by the boiler would be guaranteed to go to the DHW.  In addition, there are times when the actual temperature of the DHW could be lowered.

My idea is therefore to try and remove the thermostat from the Megaflo, and install a PT100 sensor with associated control system.  Has anyone installed a sensor instead of a thermostat before - and is it sensible / legal on an unvented system?

[Tigger]

>>Has anyone installed a sensor instead of a thermostat before.

I use the 'Thermostat' function on my TDC2 which uses the PT1000 sensor at the top of the hot water cylinder and I have to say it works a treat and I can control the target temp and hysteresis far more accurately with the TDC2 that I can with a regular cylinder stat.

As for it's sensibility/legality for use on an un-vented cylinder, I can't comment but if you wait a while, I'm sure somebody will come along that can

[JohnS]

I control my megaflo in summer by reducing the time the boiler comes on.  I have it on for 30 minutes a day.  10 in the morning and 20 early evening.  This is enough.  Simple and efective.  Using about 8kWh of gas a day on average, including the gas hob.

[spluger]

hi i'm using a Valiant Ecotech 615 with the vrc430 weather comp and v65 tank thingy which uses a temp prob that looks remarkably like the pt100

i have solar thermal but i've set the target temp of DHW to 50'C and boiler to only run between 6 and 9 pm
the prob sits halfway up the tank (vented)

using less than 1kwh gas per day inc cooking

but the 50'c water is plenty hot enough for baths/showers as cold water is warmer

i'd lose the morning water boost if tank temp is still high

also the cooler you run the water the more efficiency you'll get from your condensing boiler
same goes for the CH keep the water temp as low as you can

David

[craigski]

I control my megaflo in summer by reducing the time the boiler comes on.  I have it on for 30 minutes a day.  10 in the morning and 20 early evening.  This is enough.  Simple and efective.  Using about 8kWh of gas a day on average, including the gas hob.

I have similar megaflo system, but checking meter readings (1 m3 ~= 12kWh) Im using about 16kWh per day just for hot water. Boiler is a condensing type, so should be efficient.

Timer was set to 1 hour in morning and 1 hour in evening, I was assuming (wrongly?) once the megaflow had heated, there would be no demand for more heat from boiler.

[brackwell]

spluger,
Have you tried running at 45C or lower.

!6 Kwh is awful lot just for DHW  - thats like a electric instant shower on for 2 hrs !

[craigski]

Shouldn't DHW be at least 50 degC to kill bacteria, eg legionella?


I agree 16kWh is a lot compared to the 8kWh previously posted. I will try to manually tweak my system by adjusting flow (currently 70degC) and HW 'ON' time on the timer. As I said, I think I wrongly assumed that it didnt matter how long the HW was 'ON' for, as I was assuming the thermostat on megaflo would take care of demand.

[spluger]

boiler set to 45'C today

will post how wife friendly that is when we get some dull days

as for the legionella issue
tank is copper, pipes are copper, bugs dont like copper only your local friendly travellers
sunny days = 70'C+ tank top and 60'C+ tank bottom

and in winter i'll run the DHW a bit hotter

and to be fair my gas usage has dropped from £50 mth 2 years ago to £23 mth now and thats with gas prices going up and up and up and up and up

still should be less this year as new boiler

just gotta work on the leccy


David

[craigski]

as for the legionella issue
tank is copper, pipes are copper, bugs dont like copper

www.pbpsa.com/eng/downloads/legionella.pdf suggests the little bugs can survive on copper over time. Would it not be best to heat to 50degC to be on safe side?

[brackwell]

Legionella can be found in stagnant water at warm temp but is killed of by occasionally heating to 60C say once a week but in moving water the chances are very low.  No health inspector type dare tell you it is ok to run lower so you make your own decisions in life. I dont know of any case where legionella has been contracted from a domestic hot water system.

The extract quoting after 300 days ... what relevance is that.

I run mine 40 - 45C in the summer but it is a little bit more difficult to control this low in winter when the CH is on as well,but then it does not matter as any losses are heating the house anyway.

Running this low really cuts the tank losses and the fuel bill.

[Stuart]

spluger I have a Ecotech 615 too, what sort of how water tank have you got? we have a 114L thermal store. Usually run the gasser at 55 before shower time when we get no sun. No control system here yet, did buy a timer controller but let myself get distracted!

Stuart

[spluger]

Stuart

i went the whole hog and got the weather comp VRC430 and the VR65 which controls the 3way valve and has a temp prob (ptc type) for the tank
the tank is a 210ltr open vent standard foam insulated tank in insulated(ish) airing cupboard buried in old blankets etc

i mounted the tamp sensor halfway up the tank just pushed between insulation and tank side

solar sensors are top and bottom as standard

using the valiant control gear means the boiler can modulate depending on the temp of tank for DHW and outside V inside temp for CH
this means that it nearly always keeps the boiler in its condensing mode where a condensing boiler is most efficient


we use 2 showers a day
wife in morning
me at night after work
sometimes i shower in morning to if a really hot night
and sometimes a bath or two a week for wee fella
or a Junior shower

so far having the boiler run in the evening only between 6-9 pm works great
when its dull boiler will run for 5-20mins and thats it as tank then stays upto temp (unless a late bath is pulled)

i also balanced all my Rads with a 20'C temp drop as recommended by Valiant (took bloody hours) so will see what diff this makes this winter as boiler fitted mid June.

as i under stand it without these controlls the boiler will run hotter. you could try turning the CH temp down on the boiler as IIRC the boiler does not know if your heating DHW or CH and would try to heat the water to that temp (dial on front of boiler)

hope that helps


David

[craigski]

Thanks guys, you have convinced me Im not going to get ill if I reduce HW temp.

Can you let me know where you are setting the temp to 45C?

I have 2 places I can set temp, the boiler thermostat (this is currently nearly Max, about 70degC, on the advise of the engineer that serviced the boiler) and also the stat on the megaflo, that is mounted 1/3 of total height from base.

I assume the boiler stat needs to be set higher than the megaflo stat?

Also water at top of megaflow will be hotter than where the stat is mounted?

I have a fluke IR digital thermometer, so I can take rough readings on any of the copper pipe work.

I can monitor my gas usage once I have made some adjustments and report back on any savings, currently using 16kWh per day for similar HW usage as David (2 showers per day, 1 or 2 baths per week).

Im also wondering if the CH/HW pump should be slowed when just HW circuit is being used? Its currently set to middle setting '2'.

I did start another thread about insulating copper pipes, Im not sure how much heat is leaking from the system. Any heat that is leaking is being wasted, as the system is in non-living space in the cooler attached garage.

http://www.navitron.org.uk/forum/index.php/topic,14548.0.html

[djh]

Legionella can be found in stagnant water at warm temp but is killed of by occasionally heating to 60C say once a week but in moving water the chances are very low.  No health inspector type dare tell you it is ok to run lower so you make your own decisions in life. I dont know of any case where legionella has been contracted from a domestic hot water system.

Legionella doesn't care whether water is moving or stagnant! How on earth would it know? The major risk is from large systems such as cooling towers and commercial/industrial air-conditioning, which are most definitely moving water. The case that was mentioned in that leaflet was a water fountain. Stagnant? But you're right that it takes time for the bacteria to multiply and water standing in a tank allows

The lack of reported domestic cases is partly due to the difficulty of diagnosing such cases. But you're right that the risk is generally low and especially for regular users of a particular system (you build up immunity). It's most risk for visitors and those with weak immune systems. You're also right that heating the water to 60 C for about half-an-hour a week will keep it under control.

The extract quoting after 300 days ... what relevance is that.

Do you replace your tank every year then? The 800 days (not 300, read carefully) is about the growth of a biofilm on the surface of the tank and plumbing that then allows bacteria to grow more easily. So after a couple of years the biofilm will have grown and the environment in the tank is less hostile for bacteria.

The legionella risk is frequently overblown and misunderstood, and I don't want to come across as paranoid, but neither is the risk non-existent.

[brackwell]

craigski,

I dont think its too critical at what temp you run your boiler but i believe it is critical at what temp you run your megaflo.   The most cost effective regime i believe is to run your boiler for a fixed time, to be found by trial and error, But say 30 mins in the morning timed so that it finishes the same time as the first shower etc is required. This will bring the water to a temp so that it satisfies that days use.  At the end of the day the temp needs to be 40C plus a bit if you have long cold pipe runs.  You will have to experiment to refine the principle to suit your water demand. Remember if you have visitors to make adjustments!  Try not to have the boiler "cycling" and keep it to once a day if you can as there are losses to just heating up the boiler and pipes etc.  I think this will halve you gas consumption but if not you need to start serious lagging or perhaps you should do that anyway.

djh,
No doubt you right but as i said you have to make your own decisions in life - that is life.

Ken