Boilers, boilers..

[danw]

I'm seeking advice.

We're currently looking to replace our system boiler and hot water tank. The boiler and tank are too small for our house (detached, 30s, 3 beds, one bath, 3 downstairs rooms with small kitchen extension).

We've had a couple of heating engineers in and are receiving conflicting advice; as I am sure I will get here!

First chap said that a combi would be no good for our house. He said he would never use them and that they are no good for a house of our size. He also said that we would need to replace our old iron gas supply, even though it appears to be large enough.

Second man said that a combi would be fine, he recommended a floor-standing, Worcester Greenflow. Said we wouldn't need to replace the gas supply it would be fine. Said a wall mounted boiler would not be big enough. Having a floor standing boiler is less than ideal as it would take up much needed floor space in our small kitchen.

Third man says we would be fine with a wall mounted combi but said we'd need a new gas supply as old iron pipes a likely to rust and this can get inside the boiler. He also said they are prone to cracking and leaking.

What we would ideally like is: a combi, wall-mounted, with no new gas supply if possible. We are a small family (2 little kids), with not a lot of spare cash, and not a lot of room in our kitchen for the boiler.

I know that, ultimately, the decision is ours but if anyone has any advice for us it would be much appreciated.

thanks in advance, dan

[tarquinlinbin]

Gas pipe-the main thing is,is the diameter adequate enough to pass the correct flow rate without excessive pressure loss? if so then it shouldnt need replaceing if its in good condition. There may be minimal chance of dust/rust but its unlikely that this would bother the boiler as the flow would not be at a high rate and it would not be turbullent flow. Anyway its easy enough to put a filter on the gas inlet if the boiler doesnt already incorporate one.

The main issue with type of boiler is water delivery rate. Now then if you have a family house and have high water demands I'd suggest you stick with a system boiler and cylinder. Whether that cylinder is open vented or pressurised is up to you!.

Combi technology has improved greatly but for big family houses,they can still be tedious.

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[Mike Hibbert]

Personally (and I'm not a heating engineer!), I'd say your house would suit a combi perfectly. We have a 4 bed detatched and we run happily off a combi with no real problems at all. The only real advice I can give is to measure your flow of cold water, (in litres per min) and then make sure you get a combi that can heat roughly that much water. I guess what I mean is, if you only get 12L per min, there is no point buying a big floor standing combi, that will heat

20L/min. Equally, if you get 20L, then you are going to have problems with a 12L/min combi.

Obviously, this is all in my opinion only!

Cheers Mike

[Andy Hall]

There are a whole bunch of things here:

- If you are thinking about any form of direct mains fed hot water, then measuring the cold water rate at the cold tap in the kitchen is very important. If the supply is less than about 20 litres/min, you will be quite disappointed by the results in comparison with the existing storage system in terms of time to fill the bath. With two small kids, this will matter increasingly.

- An instantaneous boiler, be it a simple combi or one with storage like the floor standing WB Highflow (was that what you meant?) need a substantial gas supply. Normally in a run of copper tube, 22mm is used and on longer runs, 28mm. Remember that the iron pipe will have thicker walls and the inner diameter of something that looks to be around 22mm or more on the outside is not a lot larger than 15mm on the inside. The fitter should do the proper calculations, but it is possible that the pipe is inadequate. If you don't want to change the supply, this may rule out combi type boilers.

- The house size is not typically that important. With either a combi or a system boiler, in modern models the burner modulates, meaning reduces output to match required load. In a house of your size, it would be unlikely that the heating load would exceed the requirement of water heating unless you went for a peewilly little combi of

11-13lpm which you probably wouldn't want anyway.

- Bear in mind that combis have a flow rate specified in litres/min for a 35 degree temperature rise normally. If you consider that in the winter, cold mains temperature is often around 5 degrees, the effective bath water temperature being 40 degrees means that the specified flow rate becomes the total flow rate. For a 150 litre bath, the filling time can be frustrating if the boiler is too small.

- CPSU boilers like the WB Highflow help with this to a point, but only to the extent of the store. Once that runs out, the rate is reduced to that of the unaided boiler - around 12lpm. The manufacturer quotes 20lpm, but that includes the store behaviour and they don't specify how they made the measurement. Either way, it's possible to calculate how long it will take to deliver a certain volume of water at a certain temperature. Nevertheless, with the caveats, this might be a good choice. Have you thought about siting it in the airing cupboard instead of the cylinder? Is recovering the cylinder space important?

- There are larger wall mounted combi boilers with storage like the Alpha. However, at dimensions of 900x600x450, it is not going to look petite on your kitchen wall. Of course it needs a substantial gas supply.

- If you wanted the minimum impact solution, a replacement system boiler (condensing model) and a fast recovery perhaps slightly larger cylinder could well be a good option. You could choose a boiler that wouldn't require a supply upgrade and the HW performance could be improved substantially.

[Dave Plowman (News)]

The size of the house has little bearing on a combi or not - pretty well all are large enough to heat them. It's the speed of hot water delivery that may be a problem for baths, etc.

[Ed Sirett]

For a wall hung combi you have to ask yourself if you are going to be happy with around 10 litres/minute of HW flow. You may well be or you may well not be, the matter is one of taste and _must_ be answered before the rest of the questions.

I would expect that for your house around 15-20kW will be sufficient, what is the power of the current boiler? Did it ever heat the house?

A 'standard' (900-450mm) replacement unvented cylinder complying to current regs should provided enough HW for a bath every 20mins.

IME it is often not the capacity of the cylinder that is the problem but the reheat time.

If the gas pipe work is not rusting and adequate size it can be reused.

[Jeff]

FWIW - 4 bed house, 2 bathrooms + ensuite, just had a Alpha CD50 fitted, bath filling is on a par with the old cylinder system, its wall mounting and stuck in the loft on the gable wall - worth a look

Regards Jeff

[danw]

Thanks for all the replies.

Speed of hot water delivery is not a major problem for me. At present, the system boiler + tanks provides a very slow delivery of hot water and it's not really enough to fill our bath.

My assumption is that a combi would take it closer to the cold water pressure, though I may be wrong here.

It's likely that, once we have a shower fitted, baths will be a rare occurrence. So, waiting for one to fill wil not be a major problem.

Dan.

[danw]

Dumb question, but is measuring the water rate as simple as timing the time it takes to fill a known volume, then extraploting?

Yes, at present, the boiler and the hot water tank are in our smallest bedroom. We don't really want to place them in another bedroom and we don't have anywhere else we can put it. Unless we go for the loft but we have no brick wall in our loft, so that would increase expense we believe.

thanks, dan

[danw]

Pass

Did it ever heat the

I think it would just about mange it if we had thermostatic readioator valves. As it is, some rooms end up too hot, others too cold.

thank, dan

[Andy Hall]

Basically yes. Simply time how long it takes to fill a reasonable volume (e.g. marked bucket) and calculate from there.

One thing to realise is that it is not so much the pressure that matters but rather the flow rate, because that ultimately affects basic cold delivery rate.

For example, in an older property, and even some newer ones, the service pipe from the street may not be particularly large. This is because the assumption was that you would be filling a tank. Within the house, it's conventional to have 15mm for cold water mains from the rising point to the roof tank. Obviously you can do something about the latter because you would run a 22mm pipe to the combi and the same to the bath from there. However, depending on distance, a new service pipe can run to several £k.

None of this is necessarily a problem, but it's a basic check that people miss, assuming that because the supply now comes directly from the main it must be faster and better. Yes, the pressure may be higher on the back of your hand for low flow rates, but the flow rate may be poor. The water suppliers are only legally required to deliver 9 lpm which is pathetic.

Consider also that if you are likely to migrate to a shower, whether you like the feeling of pressure or also good flow rate. It's reasonably easy with a small shower head to produce a sensation of pressure rather like needles on the skin (even using an electric shower). However, you might or might not like that. Personally, I like both flow and a degree of pressure rather than a dribble, but I don't want to feel I'm pressure washing.

Another option is a heat bank. In your situation, a loft installation of one could be a possibility and it can be heated with a system boiler. With this, you can have mains pressure HW but without a large box in the kitchen. Have a look at

[Ed Sirett]

The boiler will have a maker, model and possible model number. Often the model number is a strong hint as to the power without looking for the rating plate.[1] If you find the rating plate it will have a lot of info the one I'd like to know is something like "input rating", "output rating", "Qin" or "Qout".

[1] Eg. Potterton Profile 50 means. 50k BTU/hr or about 15kW.

[Ed Sirett]

OK. So you'd like to be able to have a good shower. The presenting problem is that the bath filling rate is poor. (This could be due to a lot of reasons but a very plausible reason for this age of property is that some of the bathroom water is being delivered from the roof tank via the HW cylinder through very badly restricted iron pipes.)

You might find that replacing some of the pipes and possibly adding a pump serves you better in the medium term than a new boiler. With sensible use of plastic pipes this job may not be as expensive or as disruptive as you might imagine.

[Capitol]

The water suppliers are only legally required to

Bear in mind also that this flow rate varies according to the time of day.

Regards Capitol

[Nick]

Hi Ed,

I was wondering if you had seen any / many Wickes Combis, the 102,000 btu item ? It is a Halstead, possibly the Ace High, with a Wickes badge ?

Thanks,

Nick

[Ed Sirett]

The sheds and catalogues (Screwfux and TollStation etc.) tend to rebadge low end boilers to be sold on price.

I really must write the how to choose a boiler FAQ. The gist of it would be 1) "You get what you pay for (which might also be a name)." 2) "The cost of correct and legal fitting is so high relative to the unit that it does not pay to go to the cheap end.

For a few hundred more there are some good 35kW combis.

[Doctor Drivel]

Halstead boilers are decent. They are no Ravenheats.

[Doctor Drivel]

Oh NO. Not this garbage again. And you claim to be a professional as well. Wall hung combis can go up to 22 litres/min. To state that they are only fit for 10 litres/min is gross misinformation.

[Doctor Drivel]

It is now badged as the Potterton Promax. The better boilers have a Baxi badge.

Another one who has never heard of an in-line instant electric heater in the combi DHW draw-off.

[Doctor Drivel]

A decent boiler with a decent flowrate and a doddle to fit. The non-condensing version was fine with a two year guarantee. You CAN'T fit this boiler as it is not a condenser, unless the flue plume is nuisance.

Yes.