Water Heater

[Richard A Downing]

My kitchen is a very long way from the combi-boiler in my bungalow. Getting hot water at the tap means running that tap for quite a while

- it's a waste of water generally, and the hot water left in the pipe in particular.

I was thinking that perhaps I could fit a local water heater for the sink - bearing in mind that the dishwasher is recent model with good ratings and uses a cold fill, so the occasional washing up and hand washing is about all that it will be used for.

I found this:

I'm a bit concerned about the 'unvented'. Does this fall foul of the Unvented Cylinder rules (installer registerd, annual inspection etc...)?

R.

[Tim Lamb]

In message , Richard A Downing writes

I did this a few years back and it certainly stops a lot of

*complaints*.

I can't answer your question but I do have a different one. Mine was a cheapo job from Screwfix. The instructions, roughly translated from the Italian I think, were adamant that the supply must be from the cold system. This is a hard water area and our hot water is fed from a softener. It seemed daft to me to clog the heater up with lime scale so I tried to find out the reasoning. Nobody seemed to know.

There must be sufficient pipe volume for expansion and there must be an over pressure relief valve. Ours is a vented hot water system so......

regards

[Ian Stirling]

I wondered about something different. Maybe a 5l container, in a thermos flask. Hot water pipe in, hot water pipe out. You run the tap once, and then the container is full for the next few times you use it. Work best with very hot water, and a mixer tap I suppose.

[Doctor Drivel]

You can install a secondary circulation pump on combi, to keep the draw-off pipe hot all the time.

Fit a secondary circulation loop back to the combi's inlet with a dedicated pump, or a normal CH bronze pump or Grundfos Booster pump, if more pressure is required to operate the combi's burner. Most modern combi's will operate at low pressures these days. Check valves must be used to prevent the pump pumping down the mains.

Before the combi, the inlet could be a length of 28mm, preferably two in parallel. being a part of the secondary circulation loop, this stores hot water that will also prevent a cold spot when the combi is going through the firing sequence. The draw-off/secondary loop must be well lagged.

Have a pipe stat on the loop near the boiler to keep the draw-off temp to around 40C, so it will not be extracting heat from the combi continuously.

[Ed Sirett]

Not if the vessel is less than 15 litres (which is will be). The annual inspection is not mandatory only for the warranty.

[Doctor Drivel]

.....and insurance companies. They will not pay out if an unvented is not serviced.

[Christian McArdle]

Nope. It is under 15l, so doesn't count under the unvented cylinder rules. It appears to be ideal for your purposes.

Christian.

[Doctor Drivel]

A pumped secondary circulation loop is the best as it gives instant hot water at all taps, does not use expensive electricity to heat hot water and probably cheaper too.

[Christian McArdle]

The problem is that would cycle the combi burner, which could be quite inefficient. With only very occasional use of water for hand washing, I suspect that the electric heater will be more efficient. It would also be much easier to install, as it doesn't require a new pipe to be laid and decorated over. If the original pipe wasn't insulated (and it probably isn't), you'll need 2 new pipes. A secondary loop without insulation will be extremely inefficient, as it will cycle with much greater frequency and have greater heat loss to the building in summer. (In winter, heat loss in itself would be acceptable, provided the pipe runs are in the heated zone).

Christian.

[David Hansen]

On Wed, 11 Jan 2006 11:56:08 -0000 someone who may be "Doctor Drivel" wrote this:-

In some circumstances.

At the expense of standing heat losses and electricity consumption. As has been said, the existing hot water pipes are unlikely to be insulated well, so two new ones will be necessary. I do find it interesting that one of the advantages of an instant boiler is the elimination of standing losses, yet standing losses in the hot water main are being advocated.

Electricity is expensive if not used intelligently. Small water heaters for sinks which are used occasionally is an intelligent use of electricity.

For, "occasional washing up and hand washing", I doubt this very much. For a large heavily used kitchen sink then things would be different.

[Doctor Drivel]

Only to occasionally run the pump. Standing losses are minimised by heavy pipe insulation.

Why two? Just fit a pipe back to the combi forming the loop and insulate all the darw-off pipework,

Hot water main? The hot water draw-off pipe. You can eliminate standing looses completely by having an instantaneous water heater at each tap if you want.

A combi eliminates standing heat losses from a cylinder. Heat losses on the draw-off pipe is the same for both. It is always best to insulate draw off pipes from a combi or cylinder to the taps anyhow.

Not when a big gas heater is available that can heat the water at 1/4 of the price.

All ads up.

[David Hansen]

On Wed, 11 Jan 2006 13:29:21 -0000 someone who may be "Doctor Drivel" wrote this:-

Assuming one can get at the existing pipework to insulate it. often not too easy.

Precisely. That was what the original question was about and you typed against that idea.

Only for that heat to be lost continuously from the pipework.

[Richard A Downing]

I'm grateful for all the contributions on this thread, Doctor Drivel set me thinking, and others have reassured me on the legal/safety points.

It's helping me make up my mind. I think that I will be installing the heater for the Kitchen Sink, the nearest other hot tap is 17 metres away, and then another 15 metres to the Combi-boiler. The boiler is at one end of the L-shaped bungalow and the kitchen sink is at the other end.

This means that a pumped secondary circuit would be 35 metres long (70M if you look at it as a loop), and this would be in the roof (only practical route) above the insulation layer. Even with insulated pipe this is going to represent a large loss. It's also quite hard to install as the house is extended and there are two cavity walls to go though in the roof void. Clearly in many situations, particularly in multi-story buildings and where the heat losses are inside, the secondary loop makes good sense.

By not running a hot water pipe to the kitchen I can terminate it at the bathroom only 15M from the boiler, reducing losses. The rising main comes in at the Kitchen sink so there is a very simple installation there.

I have Building Control visiting tomorrow, so I'll raise it with them - I might as well let them tell me 2Jags view.

R.

[Doctor Drivel]

Cycling is cut down to minimum by the pipe stats and heavy pipe insulation. Not an concern. Some combis have a 'keep warm' mode which keep the plate hot, which cycle far more than a secondary circulation loop.

It only does one tap and it is not more efficient at all. A loop does 'all' taps. To have instant hot water at all taps you would need an electric heater at each tap, and that is then big bucks.

It can't be as it is 3 to 4 time the price to heat water using it.

Depends on the situation. In many cases it may just require a few floor boards lifted and carpets rolled back.

Two?

You insulate the loop; simple.

One thing I have gleend over the years, is that in a mains pressure system the customer will accept low flows at taps as long as the water is 'instantly' at the taps. If the flow is low and they have to wait a few minutes for hot water they get pissed off quickly. In one recently, the pressure was 2.2 bar and via a 1/2" water main pipe (would not upgrade due to new block drive).

A full bore valve was fitted at the maintap and a dedicated pipe taken to a heat bank. At the stop c*ck a tee and a pipe to the cold taps, with gate vale to restrict cold water flows. Also a gate vale in the odd pipe line to restrict flow and in-line isolators turned down at basins, dishwashers and w/machines (vital with these) etc. A secondary loop was fitted and they really appreciated the instant hot water not noticing the flows were lower. In reality you don't need high flows on most appliances, only at baths and showers. Dishwashers and w/machines can virtually trickle in; an extra 2 minutes in run time makes not difference. The water system was balanced to give priority to the two showers, which performed very well.

[Doctor Drivel]

One at each tap is 'very' expensive.

You have been told that need not be the case of the loop is insulated. Pay attention at the back.

[Doctor Drivel]

No. 19mm foam insulation will really keep the heat in. You could also drill through the joists and thread 15mm plastic pipe through and use this as the loop return. Then this can be covered by the existing insulation.

Heat loss will not be that much of a problem, and installation simple as most is in a large loft. With the hot water dead legs you have I would 'definately' put in a secondary circulation loop supplying all taps.

The BCO? Their knowledge of water systems and heating is limited. They know a lot about concrete though.

[Tony Bryer]

The OP explicitly said that "occasional washing up and hand

energy going into the water being used will be very small. In this situation an undersink 10/15L heater at £150 or so is just what is required, not the expense and complication of a loop with circulating pump etc. The very fact that you advocate a pumped loop to overcome the dead leg shows that losses from standing pipework, even if insulated, are not insignificant.

[Christian McArdle]

We are in winter, but not a cold day. The loft temperature is 10C. The DHW temp is set to 60C. The incoming main is 5C.

Assuming that with 35m pipework, the heat loss will be 50 x (2pi x 35 x .042)/(40/15) = 173W.

So, over the course of a 16 hour day (assuming system is retired for the night), you will use 2.8kWh a day just keeping the pipes warm.

To heat 1 litre of water will take:

4187 * 1 * 55 = 230.2kJ.

or 0.064kWh.

So for usage of x litres, the combi secondary looped system will use:

0.064x + 2.8 units a day

The electric system will use

0.064x

Adjusting for price, at say 3 times the cost,

0.192x

Now break even will be at

0.064x + 2.8 = 0.192x

x = 2.8/0.128 = 21.9 litres.

Therefore, if you use less than 21.9 litres to wash your hands daily, then the electric heater will be cheaper to run.

This assumes that the losses in the combi from being kept unnecessarily warm will be similar to the losses from the insulated 10 litre tank.

As for installation costs, the cost of the pipework and insulation is comparable to that of the heater. The installation costs for the electric heater are a fraction.

Christian.

[Doctor Drivel]

What expense? £80 for Grundfos booster will do and then a cheap pipe stat and a couple of check valves and 15mm plastic pipe, cheap time clock and insulation, that will do 'all' taps. Complication? It is simple. The water heater cost £116, then pipe and fittings and electrics. That only does 'ONE' tap. All logic is against is electric heater in this particular house.

19mm insulation will loose little energy. Also the dead legs on this house to all taps are excessive, that a loop 'has' to be used.

[Doctor Drivel]

Your assumptions on heat loss are way out. Also this 'expensive' eclectic heater only does one tap. This house 'needs' a secondary loop because of the excessive dead leg in the draw-off. The loop will do all taps and cost no more than an expensive electric heater. You are flogging a dead horse.