Circulating pump on flow or return?

tribesman

My Dad is renovating an old house with a direct gravity hot water system.

He's going to replace the cylinder with an indirect one and branch of the existing flow and return with a pumped radiator circuit. He's going to put a thermostat on the return side of the cylinder to control the circulation pump for the rads.

Fist of all, are there any gotchas to be aware of when doing this?

Also, should the circulation pump go on the flow or return side of the radiator circuit?

Thanks

Viking House

Put the pump on the return side of the cylinder because it is coder.
Put the thermostat on the flow side, it should come on when the heat reaches a certain temperature.

tribesman

Thanks,

If the thermostat goes on the flow side of the cylinder, am I right in thinking that the water in the cylinder will never get hot since the pump for the rads will kick in and take the heat away from the cylinder?

rooferPete

Hi,

Yes that can happen but the pump will only be on when the water is hot enough for the circuit, also you may find the cylinder will take the heat and return it to the boiler if for example the flow and return are in 1" but the branches to rads are reduced to 3/4 and 1/2".

You can overcome this by using a gate valve at the cylinder to balance the system.

.

Reyman

Viking House wrote:

Put the pump on the return side of the cylinder because it is coder.
Put the thermostat on the flow side, it should come on when the heat reaches a certain temperature.

Out of curiousity - why does the pump have to be on the 'cool' side of the system?

Does a gravity flow via the 1" cylinder circuit heat the water before the thermostat turns the pump on ?

If you don't use a thermostat but just let the pump switch on with the heating does it make much difference (this seems to be the way my system was put in !).

Roofer Pete: I'm not sure about putting a gate valve on the cylinder circuit ? I'd have thought there might be a safety issue as it is regarded as 'heat sink' if something goes wrong with the boiler.

tribesman

I should probably have mentioned that this will be heated by a solid fuel stove.

Reyman wrote:

If you don't use a thermostat but just let the pump switch on with the heating does it make much difference (this seems to be the way my system was put in !).

Because its a solid fuel we have to use a thermostat somewhere to turn on the pump. We can't just wire it in with the boiler. So your pump comes on with the boiler. Does this heat the water and rads simultaneously?

Rooferpete: The gravity circuit is 1" and the branch for the rads is a mixture of 3/4" and 1/2". Do you mean that because there is less resistance in the 1" circuit that water will flow through the cylinder even though the pump is on?

We had planned on putting the thermostat on the return side of the cylinder on the basis that the cylinder would heat first and then when that is fully up to temperature that the pump would kick in and heat the rads. Obviously it would be better if the two could be heated simultaneously.

Reyman wrote:

Roofer Pete: I'm not sure about putting a gate valve on the cylinder circuit ? I'd have thought there might be a safety issue as it is regarded as 'heat sink' if something goes wrong with the boiler.

My understanding here is that valves are no harm as long as they do not block the path between the boiler and the vent to the tank in the loft.

Reyman

tribesman wrote:

I should probably have mentioned that this will be heated by a solid fuel stove.

My understanding here is that valves are no harm as long as they do not block the path between the boiler and the vent to the tank in the loft.

A solid fuel burner --- No way would I put a gate valve on the cylinder circuit. It's your second line of protection if the vent gets blocked - you can't 'turn off' a solid fuel burner

I'd like to hear other views

rooferPete

Hi,

"My Dad is renovating an old house with a direct gravity hot water system."

The gate valve while it may sound crude is nothing more or less than a manually set thermostat that doubles to balancing the system.

Reyman,

The valve is fitted on the return of the coil from the cylinder, all safety is still in place because the expansion pipe for (safety) for solid fuel is on the flow.

I am interested to know at what point or time can the expansion pipe (flow) get blocked ?

If the main flow and return are fitted in 1" the same size as the coil, but the branches to the rads are in 3/4" and 1/2" the water will take the least line of resistance and flow through the cylinder because nobody can tell water that it's supposed to travel through the smaller bore pipes.

We are talking about a basic gravity fed solid fuel system, under no circumstances can a valve be fitted to the flow above the branch to the cylinder.

.

Reyman

Roofer Pete: My understanding is that you must always have an always open 'heat sink' for a solid fuel heating system.

The coil in the cylinder serves this purpose or else one or two radiators have to be fitted without valves so that they are always on and can dissipate any heat if there's an overheating situation.

I wouldn't rely solely on the vent from my memory of this type of heating

rooferPete

Hi Reyman,

Thanks, you had me worried for while, on solid fuel the flow must continue up into the attic where it takes on the role of an expansion pipe and is usually directed into the header tank.

Should you have a big fire on and the power fails causing the pump to stop working the water has the added safety of expanding into the header tank, as the water flows out of the system some can be lost as steam.

This should be replaced by a connection from the mains from the header tank into the return, a lot of systems have the water refilling via the expansion (flow) which in a real emergency is downright dangerous and all to save a few feet of copper.

I think that is what you mean by venting the system and making sure there is replacement water to prevent the boiler from exploding (unvented) or imploding (vented without replacement water).

.

Reyman

Hi Roofer Pete:

I'm still not convinced that a vent is sufficient protection in the event of a solid fuel system overheating and the radiators turned off.

I prefer the idea of a 'an always open' heat sink eg. an open cylinder coil or perhaps a bathroom radiator with no valve to dissipate the heat.

I used to have a reference on this from the IIRS - 'Guide to solid fuel installation' , which I can't lay my hands on now. And the IIRS seems to have disappeared, been closed down or is called something else now, so I can't source it .

I'll revert when I get a solid reference

tribesman

rooferPete wrote:

This should be replaced by a connection from the mains from the header tank into the return, a lot of systems have the water refilling via the expansion (flow) which in a real emergency is downright dangerous and all to save a few feet of copper.
.

Rooferpete: Would you mind explaining why this is dangerous?

rooferPete

Hi,

Just reading that quote and my grammer appears to need a bit of help, I believe it should read :

As water flows out of the system some can be lost as steam, this should be replaced from the header tank which is connected to the mains.

The connection from the header tank should be made to the return because there is a possibility that the replacement cold water will not get back to the boiler because the water in the flow is expanding and blowing off into the header tank.

Think of it as boiling water in a pipe rushing up to escape only to be blocked by cold water coming back down the same pipe, there is every chance that the force of the expanding water will prevent the cold water refilling the system, equally there is every chance the cold water will prevent the flow from expanding.

With luck on your side the flow will blow off at the safety valve, however that will not replace the water being removed from the system right beside the boiler.

At best you will have boiling water coming from the safety valve at force with water from the upstairs radiators refilling the boiler, at worst the boiler will empty and implode (usually crease) or burn out because their is no water flowing through it while the fire is burning.

Nearest I can explain why the replacement water should be on the return.

If you find the upstairs radiators require bleeding a lot it can be caused by the system venting or the pump set too high and the replacement water is not able to travel down the flow pipe.

.