Ping Andy Hall re flowswitches

Sorry about the ping, but, quoting from an earlier Andy post

"You could use a flow switch (DPS have them for 22mm) and wire it in series with a bronze pump or alternatively just use a single sided shower pump."

I've been unable to follow up this reference (could be there's too many DPS's out there)

Could you clarify, please?

Also, if you've a moment, would the Superduty 45l cylinder do for a bath in me bungalow? If it's enough for a bathful it would do for me. My present tank is a 900x400 96?L.

I would guess a bathe to be 200L, but don't know anything about the hot/cold mix for a useful temperature

TIA

mike

Reply to
mike ring
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Select the products drop down and flow switches are on there.

45 litres seems perilously small. Are you just looking at one bath and nothing else?

It is very difficult to estimate because there are many variables that all inter-relate.

The cylinder will store water at 60 degrees. In the worst case you would be mixing with cold water at around 5 degrees and you want the resulting water to be at around 45 degrees, maybe a bit more for a bath to begin with. I don't have the formula to work that out accurately in front of me, but in that scenario you would get maybe 70 litres of bath temperature water assuming no heat were going to be added.

However, with the fast recovery cylinder it will be, but only to the capacity of the boiler at the absolute outside. You would be taking heat out of the cylinder faster than it goes in (especially with a pump) so once the cylinder is empty of water that was already heated, you are relying on the setup to effectively work like a combi. This would mean dropping the flow right down to maintain the temperature.

It could work out OK, but I think it could be a bit marginal depending on how much water you like to have in the bath.

Could you do some tests with the present cylinder and figure out how the bath is filling? Obviously the cold water temperature is higher right now.

If you really want to go down that small, DPS used to make a heatbank based on a small cylinder like this. Because the water in these is stored at 80 degrees and heats the cold water through a heat exchanger, more energy is stored and you can get effectively a third more effective storage or reduce the cylinder size to 3/4 for the same water producing capacity.

However, this is still sailing close to the wind IMHO. However, I don't like situations where I have to wait for HW or turn down the tap because the water is in effect being instantly heated. If you can accept that and want to save a little bit of space it could be worth considering.

.andy

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Reply to
Andy Hall

According to my calculations, if you mix 45 litres at 60degC with water at

5degC to achieve a mixed temperature of 45degC, you end up with only about 62 litres

This is about thirteen and a half gallons in real money. Isn't the usual assumption that a decent bath requires 30 gallons?

Reply to
Set Square

Yes I agree.

I found the formula and it is:

Vc (Tf - Tc) = Vh ( Th - Tf)

where

Vc = volume of cold Tf = final temperature Tc = cold temperature Vh = volume of hot Th = hot temperature

So this one works out to:

Vc ( 45 -5) = 45 (60 - 45)

Vc = (45 x 15) / 40 = approx 17 litres

Add the volume of hot and the total is around 62 litres as you say.

I don't mix units because it confuses, but did have in my mind about

150 litres which is in the area of 35 of your old gallons.

It's harder to work out the effect of the boiler adding in heat because this will be going on while the cylinder is being emptied. That is a rather more complex calculation.

However, I don't think it makes up the shortfall before the temperature of the hot drops. One could just leave the hot on only and just let the bath fill until the required volume is reached, but it will take longer and the required volume at required temperature may never actually happen.

.andy

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Reply to
Andy Hall

100 litres is now the aveage of a modern bath.

If the water is stored at 80C and blended down to 50-55C using a Superduty with insertion probe cyl stat and an average 15kW boiler with a DHW priority using a blending valve on the coil primaries to ensure only 80C temperature water enters the cylinder coil, a 45 litre would just manage a bath. The boiler would be adding around 5-6 litres/min to the cylinder draw-off as it is re-heating.

Reply to
IMM

Travis Perkins do the Telford Typhoon quick recovery cylinder at £99. Make your system to a DHW priority and you will have nice baths. A 45 litre quick recovery cylinder is eqiv to about a 100 litre British Standard cylinder. A

45 litre will just scrape through in providing a bath, but the system has to be setup right to give all its heat to the cylinder and react fast enough.

Typical Usage

BS - Quick recovery

114 - 45 gives a standard single bath and gravity shower. 120 - 80 gives a standard single bath and pumped power shower. 206 - 120 gives twin bathrooms.
Reply to
IMM

You are assuming cold water to be 5C. As the cold will come from a cold storage tank that would have gained heat from the house. Realistically this

5C should be 15C. 5C is the temp of the mains in mid-winter.
Reply to
IMM

Footbath maybe.

Perhaps at 100 litres, but not at anything worthwhile.

.andy

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Reply to
Andy Hall

I said at the outset that this is for the worst case, and obviously a system like this should be designed for that.

I don't believe for one moment that it is realistic to assume that the cold water tank temperature is going to be 15 degrees in the winter.

Even if one assumes that it is insulated in the recommended way of insulation all round but not underneath, the heat gain through a ceiling is not that large.

Assuming a very simple calculatio on the basis of heat transfer through plasterboard from a hallway or landing at 18 degrees into a space where the tank is of 5 degrees and a space under the tank of 1 square metre, and a U value of 1.6 for the plasterboard:

Heat transfer is 13 x 1.6 = :21W

Assume a small roof tank of 115 litres (25 gallons).

To raise the temperature of the water from 5 degrees to 15 degrees as you suggest would require

115 x 4200 x (15 - 5) joules

= 4,830,000

At a rate of 21W, this will take 230,000 seconds or approximately 64 hours.

However, the heat transfer in practice will not be 21W because this will reduce as the water in the tank warms up so it would take rather longer. If the house isn't heated to achieve 18 degrees below the tank it will take longer still.

Therefore the suggestion of 15 degrees being achieved is way over optimistic unless one only takes a bath every 10 days or so. Of course, you may do that......

Even if one were to assume your example and take it that the cold water temperature is 15 degrees; referring back to the original calculation we get a cold water volume of

45 x(60 - 45)/30

which is 22.5 litres

Add this to the 45 litres of hot and you have 67.5

It doesn't make any notable difference.

.andy

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Reply to
Andy Hall

These figures are grossly over-optimistic as already demonstrated.

.andy

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Reply to
Andy Hall

No. A modern bath.

Yes. A modern bath.

Reply to
IMM

A cold water tank insoide trhe insulation envelope will be around 15C.

Put it this way. A house with no insulation whatsoever consumes about 5 times more gas to heat it.

A tank does not empty from 15C and replenished with 5C water in one go. The tank is sized above the cylinder capacity and with more to spare. Only about 32/3 of the water would be replaced by 5C water. Assume a tank over a cylinder cupboard, which is the recommended position, lagged all around and nothing under, then this 18C , would be more like 25C, a form of heat recovery.

Reply to
IMM

Your amateur demo was flawed, as usual. You are correct in one way, the figures are for a 40 minute re-heat, before Part L, which is 30 minutes (that's if all conditions are correct in boiler temp and flow through the coil). The 114 litre is now more like 96-100, as I stated. And the quick recovery has to be DHW priority with boiler on full temp.

Reply to
IMM

Is this one that defies the laws of physics like many of your ideas?

The bottom of a standard 1700mm bath measures approximately 1150mm x

450mm or thereabouts.

If you put 100 litres of water in it, you will get a depth (assuming it were rectangular) of 19cm.

Like I said. A footbath.

Even if you sit in the thing and Archimedes comes to your assistance, it isn't a decent bath.

.andy

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Reply to
Andy Hall

Take your reservations up with Twyfords then. Many modern baths are shaped for a person. Wide one end, narrow the other. This reduces water consumption.

Reply to
IMM

It won't be. I demonstrated that that is not possible.

Which has nothing whatever to do with this.

If you do that, it makes the situation worse because it would take even longer for the larger volume of cold water to gain heat from the house.

It would be precisely what was drawn off. If the cold tap to the bath is also taken from the roof tank, then the effect is even worse.

You're clutching at straws. It makes very little difference.

The period taken to acquire enough heat to make any difference is measured in days. Even that assumes that the rate of heat transfer is constant. In fact, it will tail off as the water warms up.

Even with all of that, and even if the cold water tank did attain 15 degrees, it only adds a few litres of bath temperature water anyway.

.andy

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Reply to
Andy Hall

I don't have any reservations, other than to point out that 100 litres of water does not make for a worthwhile bath.

Even Twyfords, good as I am sure their products are, are not able to defy physics.

.andy

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Reply to
Andy Hall

It isn't flawed at all. I clearly stated the assumptions.

Reheat is not the issue in a very small cylinder. It is what is going to happen to the water temperature when it runs out, which it will do very quickly.

If you turn the hot tap on full, and work on the basis of about 25 litres per minute from the tap, which is reasonable for a bath tap on

22mm from a tank one floor above, the cylinder will be empty of hot water in under two minutes.

In that time if you have a 30kW boiler, running at 100% efficiency and

100% transfer rate (three big ifs), enough energy will have been transferred to raise the temperature of the incoming water by 19 degrees.

This is way short of the energy needed to maintain final bathwater temperature and at 5 degrees cold water temperature you are adding water at 25 degrees. You aren't achieving 45 degrees unless the mains water temperature is 25 degrees and that only happens in pretty hot weather.

45 litres is too small......

.andy

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Reply to
Andy Hall

In a modern shaped bath it does.

What has physics got to do with it?

Reply to
IMM

Realistically

You thought you did.

When there is no insulation under the cold tank...

Once up to temp you require the water replenished to get up to temp. It will never get to 5C as the higher water temp will raise its temp.

100 litres in all.

What straws are these? It makes a lot of difference to have 25C directly under the cold water storage tank.

And even more when 20C.

Reply to
IMM

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