Unvented DHW tanks and controls
Discussion
Chaps, I have my new shiny boiler plumbed in and working a treat, so the next job is to move onwards in terms of controls and replacing the ancient dodgy cylinder....
I want to go unvented, principally to get shot of the loft tanks (which are enormous) so I can complete the insulation of the roof, and also to even up flow and pressures to taps / showers / whatevers around the house. Also, it's sodding tiny, so barely fills a bath, which is ridiculous.
Further, and this may or may not be a separate issue, it would be nice to be able to have complete thermal control over DHW provision - principally to have 60+ degrees for an hour morning and nnight, but 40 degrees the rest of the time - IE enough to wash up with at any given moment, but higher temps (thus the water goes "further") at peak times... Currently the heating and DHW is run off a pretty old simple time controller, though I have had a tank stat added to trigger the temps of the DHW cylinder. Cunning controller, or wiring up of two tank stats? Running it up hot will sort any Legionnaires problems too.
Having had a flick through previous threads on this, I have taken note of Ferg's comments on pressure/flow rates - in our case it would appear to not be a problem: we're on our own main, and the incomer is a colossal imperial thing you could drive a truck down. I forget what figures the plumber came out with, but it was sufficient to feed our 3kl accumulator and pressurised heating system... so I'd assume it'd be enough to deal with the addition of an unvented DHW cylinder.
Currently, all the cold water is fed direct off the mains, not from the loft tanks, so we have mains pressure at all sinks / baths / washing machine / dishwasher etc. Hot comes from the cylinder, and gravity fed to the taps, pumped to 3 showers and "gravity" (read: never quite makes it!) to the fourth.
So the question is - what tank?
Four showers
One bath
Five sinks upstairs
Four sinks downstairs
Future addition - new bath & shower room, second utility room with sinks
Would we still need the pumps for the showers? Oh, in the process, we'll rip out the current HW circuit - it's again full-on imperial copper running round blinking everywhere, and not one iota of it is lagged....
Is there one cunning tank out there that can handle dual-time/temp DHW heating, can have a solar panel or two attached at some point in the future, and isn't a Megaflow thing which my plumber seems militantly against!
Hope that's enough info, will add anything else that comes to mind.
I want to go unvented, principally to get shot of the loft tanks (which are enormous) so I can complete the insulation of the roof, and also to even up flow and pressures to taps / showers / whatevers around the house. Also, it's sodding tiny, so barely fills a bath, which is ridiculous.
Further, and this may or may not be a separate issue, it would be nice to be able to have complete thermal control over DHW provision - principally to have 60+ degrees for an hour morning and nnight, but 40 degrees the rest of the time - IE enough to wash up with at any given moment, but higher temps (thus the water goes "further") at peak times... Currently the heating and DHW is run off a pretty old simple time controller, though I have had a tank stat added to trigger the temps of the DHW cylinder. Cunning controller, or wiring up of two tank stats? Running it up hot will sort any Legionnaires problems too.
Having had a flick through previous threads on this, I have taken note of Ferg's comments on pressure/flow rates - in our case it would appear to not be a problem: we're on our own main, and the incomer is a colossal imperial thing you could drive a truck down. I forget what figures the plumber came out with, but it was sufficient to feed our 3kl accumulator and pressurised heating system... so I'd assume it'd be enough to deal with the addition of an unvented DHW cylinder.
Currently, all the cold water is fed direct off the mains, not from the loft tanks, so we have mains pressure at all sinks / baths / washing machine / dishwasher etc. Hot comes from the cylinder, and gravity fed to the taps, pumped to 3 showers and "gravity" (read: never quite makes it!) to the fourth.
So the question is - what tank?
Four showers
One bath
Five sinks upstairs
Four sinks downstairs
Future addition - new bath & shower room, second utility room with sinks
Would we still need the pumps for the showers? Oh, in the process, we'll rip out the current HW circuit - it's again full-on imperial copper running round blinking everywhere, and not one iota of it is lagged....
Is there one cunning tank out there that can handle dual-time/temp DHW heating, can have a solar panel or two attached at some point in the future, and isn't a Megaflow thing which my plumber seems militantly against!
Hope that's enough info, will add anything else that comes to mind.
If you are talking solar then 300 litres at least. With a solar coil, whether you connect now or not. Dual temperature is more difficult, but if you have two pockets on the cylinder and switch betwen them with a timer changeover it should be possible. Just bear in mind that by law the cylinder stat MUST be covered by a non re-settable oerheat stat AND be wired to a valve to positively cut off the primary flow.
Grant. Vaillant. Main.
Definitely not Megaflo.
Grant. Vaillant. Main.
Definitely not Megaflo.
40 degrees? Personally I would not want to be anywhere near water that had been sat in a hot water tank at that low a temperature - that's pretty much spot on for optimum growth of mesophiles (37°C).
You need to store water at <5°C in your CWSC, or at >63°C in your hot water tank to prevent the growth of these. Unfortunately, you can't store the hot water at a high enough temperature to start killing them off (>70°C), or you'll get limescale forming. All you can do then, is keep it high enough that they won't grow. Whether you have a separate low temperature tank, or let the temperature in your single tank drop during the day, you're going to be asking for pathogen based problems.
You need to store water at <5°C in your CWSC, or at >63°C in your hot water tank to prevent the growth of these. Unfortunately, you can't store the hot water at a high enough temperature to start killing them off (>70°C), or you'll get limescale forming. All you can do then, is keep it high enough that they won't grow. Whether you have a separate low temperature tank, or let the temperature in your single tank drop during the day, you're going to be asking for pathogen based problems.
jagnet said:
40 degrees? Personally I would not want to be anywhere near water that had been sat in a hot water tank at that low a temperature - that's pretty much spot on for optimum growth of mesophiles (37°C).
You need to store water at <5°C in your CWSC, or at >63°C in your hot water tank to prevent the growth of these. Unfortunately, you can't store the hot water at a high enough temperature to start killing them off (>70°C), or you'll get limescale forming. All you can do then, is keep it high enough that they won't grow. Whether you have a separate low temperature tank, or let the temperature in your single tank drop during the day, you're going to be asking for pathogen based problems.
Thank you for that, appreciate the input. I'd likewise thought just running it up to 60 once a day wouldn't be enough to kill icky things, but was assured 60-65 would be enough.You need to store water at <5°C in your CWSC, or at >63°C in your hot water tank to prevent the growth of these. Unfortunately, you can't store the hot water at a high enough temperature to start killing them off (>70°C), or you'll get limescale forming. All you can do then, is keep it high enough that they won't grow. Whether you have a separate low temperature tank, or let the temperature in your single tank drop during the day, you're going to be asking for pathogen based problems.
Okay, that kills that idea then... so I have to either have piping hot water 24/7 or only at set times. Damn.
Hereward said:
What system boiler have you just had fitted?
Wouldn't you be better going for 2 smaller cylinders, instead of one monster one? I have this arrangement and it works vey well. Also good to have a back-up one if/when the other one packs up.
This is what I'm wondering, but I'm unclear as to how to plan hooking them up - one thought was for a largeish one now, and then a secondary smaller one when the new suite is built.Wouldn't you be better going for 2 smaller cylinders, instead of one monster one? I have this arrangement and it works vey well. Also good to have a back-up one if/when the other one packs up.
It's a biomass boiler.
How would more than one tank work with regards to putting solar on?
Arthur Jackson said:
If you are talking solar then 300 litres at least. With a solar coil, whether you connect now or not. Dual temperature is more difficult, but if you have two pockets on the cylinder and switch betwen them with a timer changeover it should be possible. Just bear in mind that by law the cylinder stat MUST be covered by a non re-settable oerheat stat AND be wired to a valve to positively cut off the primary flow.
Grant. Vaillant. Main.
Definitely not Megaflo.
Why 300 at least for solar? To maximise any gains from shiny sunlight?Grant. Vaillant. Main.
Definitely not Megaflo.
I really wouldn't worry too much about savings made from keeping your tank at 65 v 40 degrees anyway.
Just doing some quick maths, assuming an average sized cylinder with a surface area of 2 sq.mtrs. Polystyrene (which is about the closest insulation material to that on a cylinder I can find off hand) has a thermal conductivity of 0.033 W/m K - and I'm assuming a layer 5cm thick (don't have one to hand I can measure).
So, unless I've messed up the calculations badly (maths was never my strong point) I reckon that at 65 degrees, you're losing 60 watts of heat per hour v 26 per hour at 40 degrees. Call the cost of energy 5p/kwh, the difference is 4p per day. I wouldn't even worry about putting it on a timer, as that'd save you all of ~1p per day.
But as I say, I could have messed that up totally. Just make sure hot water pipes out of the cylinder are properly lagged. It's amazing how much heat you can lose out of the horizontal vent pipe if you don't.
Just doing some quick maths, assuming an average sized cylinder with a surface area of 2 sq.mtrs. Polystyrene (which is about the closest insulation material to that on a cylinder I can find off hand) has a thermal conductivity of 0.033 W/m K - and I'm assuming a layer 5cm thick (don't have one to hand I can measure).
So, unless I've messed up the calculations badly (maths was never my strong point) I reckon that at 65 degrees, you're losing 60 watts of heat per hour v 26 per hour at 40 degrees. Call the cost of energy 5p/kwh, the difference is 4p per day. I wouldn't even worry about putting it on a timer, as that'd save you all of ~1p per day.
But as I say, I could have messed that up totally. Just make sure hot water pipes out of the cylinder are properly lagged. It's amazing how much heat you can lose out of the horizontal vent pipe if you don't.
Some initial thoughts as I'm involved with this sort of thing.
300 litre stainless steel cylinder running at "mains pressure" is enough for our four bathrooms etc.
The best solution for solar is to have another cylinder in series (which is what I have) - this becomes a preheat for the main cylinder. If this isn't possible you should have a solar coil fitted to the 300 litre one. - and if you are gong to use the biomass boiler to heat water this will need it's own coil.
There's huge amounts of rubbish talked about legionnaires disease. It would appear there is more chance of winning the lottery than catching it from a domestic hot water cylinder - note a significant proportion of houses have the bacteria in shower heads as water there is never more than 42. My thermostat is set the maximum allowed in the US, 49 degrees.
Only the pipes near the cylinder are worthwhile insulating, or the pipes that go to a regularly used sink - the heat loss from pipes to a shower for example is minuscule as they are used so rarely.
For anyone who is paranoid about water borne diseases - a better solution than high temperatures (which can be dangerous and wasteful of energy) is a UV light.
300 litre stainless steel cylinder running at "mains pressure" is enough for our four bathrooms etc.
The best solution for solar is to have another cylinder in series (which is what I have) - this becomes a preheat for the main cylinder. If this isn't possible you should have a solar coil fitted to the 300 litre one. - and if you are gong to use the biomass boiler to heat water this will need it's own coil.
There's huge amounts of rubbish talked about legionnaires disease. It would appear there is more chance of winning the lottery than catching it from a domestic hot water cylinder - note a significant proportion of houses have the bacteria in shower heads as water there is never more than 42. My thermostat is set the maximum allowed in the US, 49 degrees.
Only the pipes near the cylinder are worthwhile insulating, or the pipes that go to a regularly used sink - the heat loss from pipes to a shower for example is minuscule as they are used so rarely.
For anyone who is paranoid about water borne diseases - a better solution than high temperatures (which can be dangerous and wasteful of energy) is a UV light.
caziques said:
The best solution for solar is to have another cylinder in series (which is what I have) - this becomes a preheat for the main cylinder. If this isn't possible you should have a solar coil fitted to the 300 litre one. - and if you are gong to use the biomass boiler to heat water this will need it's own coil.
There's no way any amount of solar thermal panels are going to provide anywhere near enough hot water, so it'd have to have a connection to the boiler! The idea was just solar as a bit of "free" heat before the biomass did the lion's share of the work.caziques said:
There's huge amounts of rubbish talked about legionnaires disease. It would appear there is more chance of winning the lottery than catching it from a domestic hot water cylinder - note a significant proportion of houses have the bacteria in shower heads as water there is never more than 42. My thermostat is set the maximum allowed in the US, 49 degrees.
49 is the max allowed on the DHW cylinder? Blimey, the stat on our current one goes up to 75!caziques said:
For anyone who is paranoid about water borne diseases - a better solution than high temperatures (which can be dangerous and wasteful of energy) is a UV light.
My sister's place runs off fell water, and they have a UV filtration system, so that certainly makes sense.caziques said:
Only the pipes near the cylinder are worthwhile insulating, or the pipes that go to a regularly used sink - the heat loss from pipes to a shower for example is minuscule as they are used so rarely.
Really? So a 50m pipe run sat uninsulated isn't wasting heat, even when the shower is "only" run a couple of times a day? What about the convection of that water then dropping back into the tank as well?caziques said:
Only the pipes near the cylinder are worthwhile insulating, or the pipes that go to a regularly used sink - the heat loss from pipes to a shower for example is minuscule as they are used so rarely.
Really?
So a 50m pipe run sat uninsulated isn't wasting heat, even when the shower is "only" run a couple of times a day? What about the convection of that water then dropping back into the tank as well?
Hot water pipes going from the cylinder to appliances only get hot when water is running through them.
The actual heat loss during this time can be worked out, but as it only lasts for a few minutes a day the amount will be a few watts. However much insulation there is, by the next time the appliance is used the water will have cooled down - so the energy will be wasted in any case.
The cylinder itself and all pipes within a metre of it, plus if any pipes goes upwards (where convection or a thermosyphon will occur) should be insulated.
Solar hot water is energy storage. And getting the best out of solar is all about back up. Sticking a few panels on the roof and putting them into an existing cylinder will either mean the system will be inefficient (as you are trying to heat water that is already hot), or you will run out (as the back up system won't kick in early enough on a day with little sun). Also you have to cater for boiling and freezing.
I only solved my problem by fitting another cylinder - the first cylinder in series is solar only. This feeds through into another cylinder, which in my case is topped up with a hot water heat pump.
In summer the solar gets up to 60 (it's not a big system) and the heat pump does nothing. At the moment the solar gets up to about 35 - the heat pump takes it up to 49. Middle of winter the solar cylinder can be down to 10.
No worries about losing efficiency or running out of hot water.
We use about 300 litres of hot water a day - which would be about 13kW hrs of electricity. Average consumption over the last three years is 3.2kW hrs a day.
Only the pipes near the cylinder are worthwhile insulating, or the pipes that go to a regularly used sink - the heat loss from pipes to a shower for example is minuscule as they are used so rarely.
Really?
So a 50m pipe run sat uninsulated isn't wasting heat, even when the shower is "only" run a couple of times a day? What about the convection of that water then dropping back into the tank as well?
Hot water pipes going from the cylinder to appliances only get hot when water is running through them.
The actual heat loss during this time can be worked out, but as it only lasts for a few minutes a day the amount will be a few watts. However much insulation there is, by the next time the appliance is used the water will have cooled down - so the energy will be wasted in any case.
The cylinder itself and all pipes within a metre of it, plus if any pipes goes upwards (where convection or a thermosyphon will occur) should be insulated.
Solar hot water is energy storage. And getting the best out of solar is all about back up. Sticking a few panels on the roof and putting them into an existing cylinder will either mean the system will be inefficient (as you are trying to heat water that is already hot), or you will run out (as the back up system won't kick in early enough on a day with little sun). Also you have to cater for boiling and freezing.
I only solved my problem by fitting another cylinder - the first cylinder in series is solar only. This feeds through into another cylinder, which in my case is topped up with a hot water heat pump.
In summer the solar gets up to 60 (it's not a big system) and the heat pump does nothing. At the moment the solar gets up to about 35 - the heat pump takes it up to 49. Middle of winter the solar cylinder can be down to 10.
No worries about losing efficiency or running out of hot water.
We use about 300 litres of hot water a day - which would be about 13kW hrs of electricity. Average consumption over the last three years is 3.2kW hrs a day.
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