Fully Pumped System with Two Pumps – no motorised valves

[hippio]

I have a conventional Warmflow U90HE boiler feeding a common flow manifold with 2 pumps, one via a cylinder stat for the conventional DHW and one for the CH circuit. There is no room stat fitted (some/most rads have thermostatic valves). The time controller is set to a gravity DHW mode – ie HW always on with heating.

I have seen from other posts how to convert to independent fully pumped by adding a relay, and am happy with how to do that and add a room stat.

My question is - is there any reason not to do this and would it bring any significant benefits or save running costs? I have read about possible problems with “competing pumps” and other balance issues but don’t quite understand it! At the moment it seems to take an age to heat the hot water cylinder up, with the pump running constantly on for more than an hour – is that normal? The cylinder stat does open eventually.

Also if a room stat is fitted should all radiator thermostatic valve then be kept fully open?

All help to keep my bills down gratefully received!

Thanks

 

[Dan Robinson]

Wire it up with hot water priority and use a separate timer and programmable room thermostat.

 

[dcawkwell]

Don't see a problem. Just add a room thermostat and it's fine.

I have seen a dual pump fitted on an oil boiler but don't know why anyone would have chosen to do that.
It is more usual to fit just the one pump and 3 port or 2 2port valves.

 

[picasso]

I have seen a dual pump fitted on an oil boiler but don't know why anyone would have chosen to do that.
It is more usual to fit just the one pump and 3 port or 2 2port valves.

so you dont need to install motorised valves.

example

 

[petit_pablo]

Pump plan's have their place, certainly easier to wire

 

[matt1e]

I'd wire it up as s-plan, would have to utilize a couple of relays or a flowswitch though

 

[stem]

My question is - is there any reason not to do this and would it bring any significant benefits or save running costs?

If you presently have TRV's on all of the radiators, then probably not much. The advantage with an electric interlock is that it will shut the boiler and pump down when the room thermostat is satisfied. What you save would depend upon how often this happens and that depends upon the design of your system and how well insulated your home is. If you only have TRV's, they will reduce the heat emitted into your rooms as they warm up and so the boiler will work less, but it will still fire up from time to time.

If your home is presently too warm for you and there is no control on the radiators then you would see some savings.

Also if a room stat is fitted should all radiator thermostatic valve then be kept fully open?

The TRV's are still required on the rest of the system, however, the radiator in the room where the thermostat is located should ideally not have a TRV, or if it does, it should be left on maximum. The other TRV's should be set according to the requirement of the room they are located in.

 

[Onetap]

At the moment it seems to take an age to heat the hot water cylinder up, with the pump running constantly on for more than an hour – is that normal? The cylinder stat does open eventually.

I think you have another pump in the boiler (you'd need to check the manual for the boiler to confirm that) so you're careering towards a system with 3 pumps.

Before you go barking up that particular tree, have you checked the boiler flow temperatures to the cylinder when there is a hot water demand?

The Warmflow U90HE is a condensing oil boiler, so it works most efficiently with low return temperatures. You can exploit this by having a lower heating flow temperature, for example with weather compensation.

The hot water cylinder requires a high flow temperature to heat the water up to the recommended 60 degC, most have a reheat time sized on (I think) 70 degC flow.

If it takes too long, then it's most probably inadequate flow temperature, inadequate flow rate or/and it's an old cylinder that has a low heat transfer surface area (short coil) and/or it's full of limescale.

Most boilers with weather compensation (& low flow temperatures) work with hot water priority, i.e., they run at a high temperature whilst there is a HW demand and shut off the flow to the heating. When the cylinder thermostat opens, it reverts to a lower flow temperature (and higher thermal efficiency) for the heating and shuts off the flow to the cylinder.

I'd suggest you check the flow temperature and the state of the cylinder before starting pipework surgery.

 

[hippio]

Thanks to you all for the very helpful and quick replies –

Matt1e – I think will go for an “S plan system” eventually and I have seen an old post (I think from you?) on how to do this with a relay.

Dan - Would a DHW priority be useful – I can’t see that this boiler has a facility to do this – could I get the same effect by programming HW outside CH times? Not sure how to do this otherwise.

Stem – thanks great advice on the radiators.

Onetap – thanks that is really interesting thinking. As far as I can tell (handbook) the U90HE is a simple condensing boiler – no pump (that option becomes UP90HE) and it doesn’t seem to have temperature compensation or Priority DHW temp increase either, unless someone knows otherwise.

Your ideas are really useful - the tank is I suspect old (1970s), poorly lagged (on the list of jobs) so I will check for limescale. The boiler output is set to a nominal 70 degrees with the cylinder stat at 150F (65C) which I intend to turn down to 140F (60C). This does not seem a huge difference, especially given the temp drop in the pipework. Also could the pump be running too fast? – it is physically the larger of the 2 and set to its higher speed - so that there may be little temp drop across the cylinder, although the bathroom towel rail is on this circuit too. You don’t have the equivalent of a lock shield value on a cylinder circuit do you?

I’m hoping to do most of the changes in wires (can do this) rather than pipes (not so good here!) but my remaining concern is I seem to be almost constanly heating the DHW when the heating is on, which seems a waste.

 

[Onetap]

The boiler output is set to a nominal 70 degrees with the cylinder stat at 150F (65C)....

That could be a large part of the problem. The cylinder will take much longer to reach 65 degC with the heating flow at 70 degC, than if it were at, say 82 degC. The last few degrees to the set point take longest.

In an extreme case, if the cylinder thermostat set point were higher than the flow temperature, the cylinder will always be 'ON'. If you add in the pipe heat losses, inaccuracies in an old thermostat, the cylinder thermostat being loose &/or with no heat transfer compound between it and the cylinder wall, etc., you may be close to that extreme.

In any event, I'd plan on replacing the cylinder if it is that old. The cylinder would have used an 82 deg C flow temperature. The cheapest cylinders from that era had short coils, with little heat transfer area. They looked the same externally and were often foisted on unsuspecting customers by unscrupulous installers.

Also could the pump be running too fast? – it is physically the larger of the 2 and set to its higher speed - so that there may be little temp drop across the cylinder, although the bathroom towel rail is on this circuit too. You don’t have the equivalent of a lock shield value on a cylinder circuit do you?

You can get two pumps 'fighting' one another, which is why it is best to avoid multiple pumps, unless you're familiar with low loss headers or buffer vessels. For example, I worked on a job in which there was a big CH pump set (duty & stand-by) adjacent to a smaller DHW cylinder pump set, with both pumping to & from the same flow and return headers. The DHW pump worked fine, until the CH pump was turned on; it was on 24/7 365 days a year. With both pumps running, the DHW pump produced exactly no flow. You could only get flow to the cylinder by turning off the DHW pumps and opening the valves on both DHW pumps; water then flowed to the cylinder, but backwards, from the return to the flow header. The cylinder took 12 hours to heat up and became dangerously hot overnight.
The fix involved a low-loss header system.


An S-plan system should be perfectly adequate.

There should be a lock-shield regulating valve (radiator valve) in the cylinder circuit since the pipework and cylinder coil should have little resistance compared with a radiator heating system. Most of the water would short-circuit through the cylinder circuit, starving the heating when both are on, unless you use a LS valve to add some resistance.

 

[hippio]

An S-plan system should be perfectly adequate.

There should be a lock-shield regulating valve (radiator valve) in the cylinder circuit since the pipework and cylinder coil should have little resistance compared with a radiator heating system. Most of the water would short-circuit through the cylinder circuit, starving the heating when both are on, unless you use a LS valve to add some resistance.

Thanks again. I was thinking to do "S plan" control but with the pumps not valves, just to get more control. I can't see that this will be much different from how it works from a plumbing sense now so shouldn't give any problems?

There are a couple of old wheel gate valves in each side of the HW circuit (which I dare not touch!) but no sign of any other restrictor valve of any sort. Is this just a hangover from a poorly thought conversion from gravity fed? The only alterations seem to be in the vicinity of the boiler.

So having started out thinking maybe just some control tweaks would help I probably need a really good professional to come and look, as I am not up for changing HW cylinders. I suspect that your comments are bang on which may help explain why we are going through oil at a scary rate - the house seems slow to heat up but slow to cool, which does not seem right! Thanks for getting me thinking about it all.

 

[hippio]

Just for anyone reading this at a later stage I rewired the pumps with a relay so as to be independent, and set the existing controller to match with a wireless stat. I also bit the bullet and had a new hot water tank fitted by a professional - although it was not clogged with limescale he though the coil was partially obstructed. Upshot of all these change is hot water now comes on for less than an hour a day to meet our needs, and we have used something like 25% less oil this year (I know it has been milder overall) so at least some if this saving is because I am not now pumping heat into the tank continually when the heating is on!

Thanks for all the help.