Economy 7, immersion heater, indirect hot water from boiler?

[Adam Funk]

I have an Economy 7 electric tariff and a hot water cylinder with two sources of heat: a 3 kW electric element and a coil heated by the boiler. A few days ago I switched the central heating off, so I also switched off the indirect hot water from the boiler, since I thought it would be inefficient to run the boiler just for that. (The immersion heater was controlled by a timer that turned it on during the night-rate period and for a while during the day too.)

Yesterday we ran out of hot water, and I figured out that the mechanical timer that controls the electric heating element had stopped turning (I presume the motor has burnt out) in one of the "off" positions. For now, I've switched the power to the element back on at the timer and it works (all the time).

So here are my questions.

1. Is it in fact inefficient to run the boiler for indirect hot water when it's not running anyway to heat the radiators?

1. Is it even worthwhile to switch the immersion heater off some of the time during the higher-rate period? (The cylinder is insulated with a "jacket".) In other words, should I bother replacing the timer, or just connect the element directly to the switch?

2. Is it unsanitary to let the hot water cool off by switching the heater off for several hours during the day?

Thanks, Adam

[Jim Alexander]

but that's how everybody with boilers who haven't succumbed to the dubious advantages of combis run things. Anyway much less inefficient than a thermal power station and look at your energy prices, gas 2-3p/kWhr, electricity 9-10p/unit full rate, Economy 7 whatever it costs, your boiler efficiency rating whatever it is. I'm sure using your boiler for water heating is cheaper than Economy 7 electricity but do your own sums.

Jim A

[John Rumm]

Possibly[1], but it is still probably going to cost less than using electric.

[1] With a modern modulating boiler and/or a fast recovery cylinder then it will be very efficient. The least effective solution would be an old high power fixed rate boiler and a slow recovery cylinder - this would cause the boiler to cycle on and off frequently during reheating which wastes some energy each time the boiler is allowed to cool without dumping the heat into your hot water.

The key here is insulation - the better lagged it is the less it matters.

As long as it gets to 65 degrees at some point in the day you ought to be fine. The only problem might arise if it never got hot enough and you kept a tank of permanently tepid water.

[Adam Funk]

My boiler isn't particularly modern, and between the boiler and the cylinder are about 2.5 m of pipes (in each direction) that I can't insulate. I'll have to do some calculations with my kWh prices and a generous loss allowance!

How do I know if I have a fast or slow recovery cylinder?

I'm sure the cylinders that are foam-insulated when you buy them are better --- unfortunately mine wasn't (when I moved in; I didn't pick it out). What is the best "retrofit" cylinder insulation I can get?

[John Rumm]

What make and model is it?

You won't lose much in the grand scheme of things from the pipes in this case.

Chances are it is slow or slowish. Any markings on it? Also how long does it take to boiler to reheat it from cold?

On a slow recovery cylinder it probably will not be able to absorb heat at a rate of more than about 5kW. So if it were say 150L that would take

150 X 4200 X 35 = 22MJ to heat from say 5 to 40 degrees. If it is getting 5KJ / sec from the boiler it will take 4410 secs or over 70 mins to reheat.

A fast recovery will be able to take the full output of the boiler. So say it was 20kW that would bring the reheat from cold time down to under

20 mins.

You could box the whole thing in with 50mm of celotex or something similar if there is space.

[Adam Funk]

Well, it does look that way: comparing marginal unit prices, gas is

50% of the night electricity rate and 17% of the day electricity rate.

But almost 100% of each kWh of electricity bought (except for losses in the cables in my house --- if those weren't extremely low, I'd have a bigger problem!) whereas I can easily believed that my boiler is 70% efficient and the pipes and exchanger are also 70% efficient, which would make it break even with night electricity.

The comparison with the day rate, however, is a different matter.

[Roger Mills]

You could always replace the cylinder with a fast recovery foam insulated jobby - and kill two birds with one stone.

Assuming you *don't* do that, there's probably not much in it as far as the relative costs are concerned of heating the water either with gas or with off-peak electricity. But gas costs the same throughout the whole 24 hour period, whereas electricity doesn't. If you're in danger of running out of hot water, you could leave the gas HW heating on all the time provided you lag the cylinder well and provided that the HW circuit is pumped and controlled by a tank thermostat. [If it's gravity, it's a different ball game altogether!]

Another consideration - depending on the control system on your HW/CH system - is that you may be able to do some towel warming while the HW is being heated if you use the boiler. I have a Y-Plan system which uses a

3-port mid-position valve. In the summer, I do the following:

• Remove the actuator from the valve, but leave it electrically connected

• Manually set the 'wet' part of the valve to the mid position

• Turn off the CH at the programmer

• Manually turn off all radiators *except* two - one in the bathroom and one in the en-suite

Then, whenever the HW is being heated, these 2 radiators *also* get hot, and warm the towels draped over them.

[A simpler alternative is to programme the CH to come on for a couple of short periods each day, with the room stat set to max and all but the bathroom radiators turned off].

[Adam Funk]

I think I'll just improve the insulation around the cylinder.

It's pumped with a thermostat strapped onto the cylinder; fortunately it can switch the pump and HW valve on independently of the thermostat for the central heating.

[Roger Mills]

Good. In your position, I would definitely use the gas boiler rather than the immersion heater - if only to keep all the heating equipment exercised. All too often, when people turn it all off for the summer, they turn it on again in the autumn only to find that the pump has failed or that a control valve has seized up.

It also provides the option - via one or other of the tweaks I described earlier - of warming your towels at the same time.

[Adam Funk]

Corvec Maxiflame II.

That's an interesting idea, thanks.

[Adam Funk]

Good point. (I had to replace one of the valves a few years ago when it seized up, but I can't remember whether it was the CH or HW valve.)

[John Rumm]

From

Brand: Chaffoteaux Model name: Corvec Maxiflame Model qualifier: 2bf Model Identity no.: 4198046 First Manufactured: Last Manufactured: 1979 Boiler last updated on database 31/May/1999 SAP seasonal efficiency: 65.0% Efficiency band: Efficiency category: SAP Estimated SAP equation used: 0 Estimate of Annual Fuel Cost Power: 17.58kW Main type: Mounting: Regular Unknown Non-Condensing Fuel: Exposure: Flue: GAS Unknown Room-sealed:No Fan Burner control: Elec power firing: Unknown W Ignition: Elec power not firing: Permanent Pilot Light W

[Adam Funk]

I didn't know it was that old! Thanks for the information and the link.

[Adam Funk]

I would think it should be held at the "safe" temperature for a while, not too long before tapping it. But 65 C seems rather high to me --- are there any official recommendations on the safe temperature?

(BTW, I installed a new, digital timer, which has two advantages over the old one: no moving parts, and keeping the time and programme if the power is off. I'm also using electricity to heat the water during the E7 night period, and the boiler during the day. I haven't got round to improving the insulation on the cylinder yet. Thanks for the advice.)