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Robert Tyrrell

Gas Engineer
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I've jut been out to a call where the heating system is pretty poor and I'd like some advice on how to remedy the situation. I have my own ideas but would like, and value other people comment/advice on the matter.

They have installed, a Viessman Vitodens 100 WB1B (35Kw) that is being used to heat 19 rads, and a 250ltr DHW cylinder.

The problem is that the DHW is ok but the rads aren't getting hot at all really. I can keep my hand on them all the time, even with the TRV's set to max.

I 'think' that the rads might be sludged up a bit as they're quite old by the look of them, and the DHW has to be on constantly so it's using power that the CH could be using however when I switched off the DHW, at the programmer, there was very little change at the rads. The pipework was getting hot in the boiler room and the display on the boiler was reading 68°. If I put the boiler into SERV mode the display would show 75° and the GR worked out at 35Kw.

I'm going to advise that they have a separate HO boiler installed for the DHW circuit and use the Viessman for the CH, after changing a few rads and giving the system a flush.

There is a ManaClean filter on the return btw but I don't think that the whole system was flushed when the boiler was installed, which was around 2.5 years ago.

If you have any further, helpful, suggestions then I would love to hear them.
 
Flow and return piped large enough ?
19 rads over 2 or 3 floors ??
Id have them at 28mm.
with 19 rads that boilers gonna be working hard.
 
How big is the property? May need 28mm and a low loss header.
If you're thinking of putting in an additional heat only boiler, is the gas meter big enough?
 
Flow and return piped large enough ?
19 rads over 2 or 3 floors ??
Id have them at 28mm.
with 19 rads that boilers gonna be working hard.

They come out of the boiler in 32mm, then after a couple of feet reduce to 28mm to disappear into the ceiling. The return is also 28mm until it gets back to the boiler, and yes - My first thought was that the boiler wasn't big enough.
That's why I'm suggesting an auxiliary boiler for the water.
 
How big is the property? May need 28mm and a low loss header.
If you're thinking of putting in an additional heat only boiler, is the gas meter big enough?
 
Flow and return piped large enough ?
19 rads over 2 or 3 floors ??
Id have them at 28mm.
with 19 rads that boilers gonna be working hard.

They come out of the boiler in 32mm, then after a couple of feet reduce to 28mm to disappear into the ceiling. The return is also 28mm until it gets back to the boiler, and yes - My first thought was that the boiler wasn't big enough.
That's why I'm suggesting an auxiliary boiler for the water.
 
In service mode did the rads get any hotter ? , it could be worth checking the temp difference between flow and return temp at the boiler . Not sure how long service mode is , you may have to set it a couple of times .
We all know the problems with the dreaded crunchy hoses .
 
How big is the property? May need 28mm and a low loss header.
If you're thinking of putting in an additional heat only boiler, is the gas meter big enough?
MP meter so probably. They already have a small combi running in the staff area on the other side of the building next to the gas meter cupboard.

I'm going to work out the pipe sizing over the weekend just in case that all need replacing.
 
In service mode did the rads get any hotter ? , it could be worth checking the temp difference between flow and return temp at the boiler . Not sure how long service mode is , you may have to set it a couple of times .

Temperature difference is quite big, but only that it's near to 20°, even in service mode.

We all know the problems with the dreaded crunchy hoses
Not sure what you mean by this?
 
Not sure what you mean by this?

Sorry I thought everyone knew lol , if its a gunged up system the poxy rubber hoses start to block and obviously inhibit flow , it depends on a lot of things mainly water quality and the Viessmanns DO NOT want or need inhibitor. Dont squeeze them when you are there just or the crud can come off and end up blocking plate , divertor etc. I love these boilers but hate them bloddy hoses . TD sounds ok though.
 
work the heat load side (by sizing kw of rads)

and if you can and its sized wrong which i think it is (if its an unvented cylinder theres the best part of 20kw taken)

stick another 35w 100w in with a low loss header, but your starting to get into commercial gas then (with 3 boilers) if you have it your fine
 
work the heat load side (by sizing kw of rads)

and if you can and its sized wrong which i think it is (if its an unvented cylinder theres the best part of 20kw taken)

stick another 35w 100w in with a low loss header, but your starting to get into commercial gas then (with 3 boilers) if you have it your fine
 
How hot is the return to the boiler?
Could the cylinder need balancing on the return to encourage the flow through the rads?
 
have you checked that the auto bypass is set correctly assuming there is one fitted? And are any of the radiators hot as it could be a balancing issue. Time the heating and hot water separately and the boileray be able to come with the demand. Try the simple\obvious remedies before delving in too deep.
 
How old is the pump? if the heating still doesn't heat that well with the hot water demand off then maybe the issue is either with the pump or the pipe size.

Just because the flow/return pipework is 35mm/28mm in the plant room doesn't mean its properly sized on the whole system.
 
How old is the pump? if the heating still doesn't heat that well with the hot water demand off then maybe the issue is either with the pump or the pipe size.

Just because the flow/return pipework is 35mm/28mm in the plant room doesn't mean its properly sized on the whole system.
 
Is that d boiler with hot water priority, the boiler will either heat d cylinder or heat rads but not both together.
If the heat is not lifting off boiler there may be an airlock on the flow.

Having said that I had a problem with this boiler last year.
It would heat the cylinder fine but when switched to heating mode, the boiler was hitting temperature fast but rads not really heating.
There is a two way diverter in boiler manifold, which goes 1 way for hw and d other for rads.
My boiler had a little debri stuck in d door so when heating rads some heat was going to cylinder killing d full circulation to rads. I had to change the manifold to sort d problem. Couldn't flush it out.
That may be your problem too
 
Is that d boiler with hot water priority, the boiler will either heat d cylinder or heat rads but not both together.
If the heat is not lifting off boiler there may be an airlock on the flow.

Having said that I had a problem with this boiler last year.
It would heat the cylinder fine but when switched to heating mode, the boiler was hitting temperature fast but rads not really heating.
There is a two way diverter in boiler manifold, which goes 1 way for hw and d other for rads.
My boiler had a little debri stuck in d door so when heating rads some heat was going to cylinder killing d full circulation to rads. I had to change the manifold to sort d problem. Couldn't flush it out.
That may be your problem too
 
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