Use a multipoint and a combi boiler together ?

[Steve Doncaster]

Hi, my first post to a group - please can anyone help with the following:

I have a Worcester multipoint boiler (fitted about a year ago to replace a very similar looking Main Severn multipoint boiler) upstairs in the bathroom. It supplies a wash basin and bath upstairs and a kitchen sink and washing machine downstairs.

I am having central heating installed based around an Ideal Isar HE24 condensing combination boiler fitted in the bathroom, supplying 7 radiators and DHW. The installation of this is almost complete with just the connections for gas and water to the boiler and its hot water outlets to be connected. The plumber is planning to disconnect the multipoint boiler and divert its gas supply and water feed to the new boiler and to connect the new boiler's DHW feed to the point that the multipoint currently supplies.

I am wondering if I could keep the multipoint boiler in use to feed the washing machine and probably the downstairs sink (and later, a yet to be fitted, downstairs wash hand basin) and so have the new combi boiler supply just the upstairs bath and wash basin (and later, a yet to be installed, shower).

The reason I am considering this is that I would like DHW upstairs (particularly to a shower), not to have its flow rate or temperature unduly influenced by either the washing machine requiring hot water or hot water being used at the downstairs sink.

Is my idea workable? I realise that the cold water supply might be an issue that could effect hot water flow rate/temperature if it is not of sufficient pressure but I have no idea how to judge this. Also the main gas supply may not be able to feed enough gas to maintain both boilers at full capacity; again I don't know how to judge this. The new boiler is supplied with gas via 22mm pipe all the way and the multipoint has 22mm to it except for the last 2 meters which is 15mm - this could be changed to 22mm all the way.

So, is my idea workable, effective or overkill that would have problems that I haven't seen?

In case these figures help: The multipoint has a max rated output of 21.4 kW with 4 to 12 litres/ min flow for a 25 degree temperature rise (control set to minimum) and

2 to 6.1 litres per minute flow for a 50 degree rise (control set to maximum).

The Isar HE24 has max DHW output of 23.4 kW (80 000 Btu/hour) with 9.6 litres per minute for a 35 degree temperature rise. DHW Specific rate

11.2 litres per minute (whatever that means).

One other problem, it's now Sunday 11th March 2007 and the plumber is coming on Monday to finish the installation and (maybe), remove the multipoint boiler.

Any advice would be very helpful and most welcome.

Regards, Steve.

[John Rumm]

In principle yes.

Find whichever cold tap gives you the best flow rate and time it to fill a bucket of known capacity. Since you have two relatively low powered heaters, a main that can supply 25lpm ought to be more than adequate.

A standard domestic supply/meter is usually rated to supply 60kW, so you are inside that so long as you don't have other gas using appliances to consider that you have not mentioned.

Depending on your water mains capacity it may not totally free you from temperature fluctuations at the shower (although if it is thermostatic that will help). If for example the washing machine starts to fill, it may reduce the available flow rate to the "other" water heater which may cause a rise in temperature. (note that the combi will not include any flow regulation - the water temperature setting justs sets a maximum temp for the water - draw it faster than it can heat to that temperature and it will just supply cooler water. The multipoint however may regulate the flow to maintain a constant temperature. (The combi will be able to modulate its gas rate at low demands to limit the temperature)

[Andrew Gabriel]

I would definately try to keep the Multipoint. An issue with combis is no backup hot water when they die. Well, you currently do have. I have a Main Medway Super, and another useful thing about it is it still works during power cuts, as it has no electricity supply. It's also incredibly simple, and so less likely to go wrong and probably easier to fix in most cases.

It's probably 30kW input. That's what the Mains are, IIRC.

So that's probably 26kW input.

I don't know what the limit of a domestic gas supply is, but I'm running a 25kW Keston and a 30kW Main and a gas cooker on mine (grill @ 7kW is highest load), and it maintains the pressure OK. Mine are all on a 28mm feed from the meter, reducing to separate

15mm feeds for last couple of feet in most cases. Separate pipework back to the meter would be better, if that's sensible in your case. The plumber should be able to do the calcs for what pipework sizes are required.

[Steve Doncaster]

(> --

Hi, thanks for that John. I think you are saying that if my mains water supply is sufficient then I should not suffer temperature fluctuations at the shower. I'll get a bucket and time the cold water flow. I just hope that the plumber is broad minded enough to see my viewpoint if I decide to keep the multipoint. The installation is being done at no cost to me due to a grant and so he might feel I have no say in the matter. It would save him the work of removing the multipoint and making good where the flue is. Thanks, Steve.

[Steve Doncaster]

Thank you Andrew, two very good points about when the combi breaks down and the advantage during a power cut. So far I am now keen to keep the multipoint. Cheers, Steve.

[Andy Hall]

There's certainly no reason not to do so since you have them.

The extent to which there won't be some level of interaction in terms of flow through one affecting the other will depend on the water supply and the pipework, and where there is a shower, the thermostat.

As far as the gas part is concerned, there are specific installation and measurement rules that the fitter will be required to follow. This might mean upgrading the pipework from the meter to where the 22mm pipes to boiler and multipoint go their separate ways. That all depends on pipe lengths and number of bends etc. There is a requirement for the max. pressure drop that is permitted from meter to appliance, so you might have to be prepared to pay for any additional gas pipework that was not envisioned. Shouldn't be a lot though.

[Steve Doncaster]

meter to the bathroom floor level. From that is a 15mm feed to the downstairs gas hob and I think, a 15mm feed (about 2 meters) to the multipoint. The plumber was planning to upgrade that feed to 22mm to feed the combi (and take the multipoint out). I could maybe get him to extend the 22mm gas pipe where it enters the bathroom, to have 2 x 22mm feeds, one to each boiler - do you think that this is what I should do? Running an additional 22mm pipe into the bathroom would be a problem and I'm guessing couldn't increase flow as there is only one

22mm pipe from the gas meter into the house. Also, I'm not in much of a position to negotiate with the plumber as the combi installation is a grant job whereby I don't pay. Regards, Steve.

[John Rumm]

I won't say it will eliminate fluctuations at the shower, but it will help - it will depend a bit on the mains performance and the controls implemented by the multipoint.

For example, you may have the boiler set to supply HW at 50. It will be able to do this - but only up to a point. With ground water at say 5 in the winter you will need a 45 degree rise to achieve this. The boiler can supply heat at 1.44MJ / Min maximum. You want a 45 degree rise, so you get a maximum of 7.6L of water at that temp from the boiler.

Say you set your shower to deliver 8 lpm of hot water, the temperature from the boiler would fall to 42 degrees - that might be about right for your shower without adding much cold. Something else now makes a demand on the cold main causing the flow rate to the shower to fall to 7 lpm. The temp will now rise to the limit set by the boiler HW stat.

The better the mains performance the less impact other demands will have on the shower.

One downside to bear in mind is the extra servicing costs of two heaters vs one.

If you want to cleaver you *may* be able to blend the output of both heaters to produce a single higher flow rate hot supply, but that starts getting complicated especially since you have two different makes and types of kit.

You are the customer, and it is "your" grant money - so I can't see that being a good excuse for the plumber.

[David Hansen]

On 11 Mar 2007 08:44:56 -0700 someone who may be "Steve Doncaster" wrote this:-

How many metres between bathroom and kitchen?

Could the old boiler be removed and set aside for later installation downstairs, or left disconnected but blanked off? Given that the plumber is coming to complete a grant job he may well want to finish what is specified, but might be sweet talked into gentle removal or leaving it.

[Steve Doncaster]

high-ceilinged (guess approx 3 meters). The washing machine is in a utility room (ok, outhouse), attached to the kitchen and currently fed from the multipoint in the bathroom with maybe 5 meters of 15mm hot water pipe. I don't want to remove the multipoint as it would leave a large hole in the wall where the flu is. I could have it blanked off but having it plumbed in later would mean more upheaval and a new plumber less acquainted with the pipe runs etc. Regards, Steve

[David Hansen]

On 11 Mar 2007 11:16:12 -0700 someone who may be "Steve Doncaster" wrote this:-

No long pipe runs then.

Could the multipoint heater be fitted in the outhouse (former washouse peresumably), ideally between the kitchen and washing machine?

Presumably that is what is intended. Is it a big bathroom?

New showers and basins will, mean upheaval. Hot water is best generated near the point of use, to minimise losses. Given that the new boiler is in the best use of the old one is to generate hot water near the kitchen.

There are a number of, "I wouldn't start from here", possibilities, but these are not helpful.

[Steve Doncaster]

for the near future and so the upheaval for those is not a current concern. The wash hand basin will, alongside a WC, be fitted in the utility room where hot and cold and a waste are currently available. I could move the multipoint but there is no need other than to shorten the hot water run to the washing machine (but I'll insulate that as a help) - it currently sits on the bathroom wall, immediately above the downstairs sink which I plan it to continue to feed. The bathroom is large and having two boilers (2 meters apart), on one of its walls isn't an issue. It's normal that I don't set off from the best starting point, such is life. Regards, Steve.

[John Rumm]

meter to the bathroom floor level. From that is a 15mm feed to the downstairs gas hob and I think, a 15mm feed (about 2 meters) to the multipoint. The plumber was planning to upgrade that feed to 22mm to feed the combi (and take the multipoint out). I could maybe get him to extend the 22mm gas pipe where it enters the bathroom, to have 2 x 22mm feeds, one to each boiler - do you think that this is what I should do? Running an additional 22mm pipe into the bathroom would be a problem and I'm guessing couldn't increase flow as there is only one

22mm pipe from the gas meter into the house. Also, I'm not in much of a position to negotiate with the plumber as the combi installation is a grant job whereby I don't pay. Regards, Steve.

He may be able to upgrade to 28mm instead...

[Owain]

Another possibility (could be done at a later date) is to run an indirect hot water cylinder from the 'radiator' part of the combi.

Owain

[Ed Sirett]

The only major obstacles I can think of:

The gas supply fro the two appliances will more or less have to be independant and pipes all the way back to the meter.

The location of the two appliances obviously uses up extra space but it might also be that one of the appliances has a flue location which is a problem for the other.

Otherwise, if that's what you want to do then the customer is always right...

[Steve Doncaster]

here:

Hide quoted text -

Hi Ed, thank you for your input. Both boilers are now wall mounted and flued - no problems space or proximity wise. There is no way I could run 2 x 22mm pipes from the gas meter to the boilers as firstly it's about 15 to 20 meters, would run under two floors and as far as I know, the meter doesn't cater for two outlets of 22mm. But mainly, anything like that makes it all too big a job. Thanks, Steve.

[Steve Doncaster]

Hi Owain, thanks but not an area I want to go into. I just hoped I could leave the existing multipoint in situ and share the DHW load between it and the new combi without much planning, angst or expense - nor any steep learning curves. Regards, Steve.

[Steve Doncaster]

Thank you for every one's ideas. However, the plumber said his company would not allow both boilers to be installed together so the combi and

7 rads are installed and working and the multipoint is now in storage. Any one like to buy a nearly new Worcester Multipoint gas boiler complete with all fittings, flue etc? Regards, Steve.